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PROJECT REPORT
Entitled
“PERFORMANCE ANALYSIS OF LTE PHYSICAL LAYER USING
SYSTEM VUE”
Submitted in partial fulfillment ofthe requirement
For the Degree of
: Presented & Submitted By:
PRAVAT KARKI (Roll No.U09EC410)
BIPLAV BHURTEL (Roll No. U09EC428)
KISHOR BHANDARI (Roll No. U09EC429)
AVI GUPTA (Roll No. U09EC435)
B. TECH. IV (Electronics & Communication) 8th Semester
: Guided By:
Prof. (Ms.) SHILPI GUPTA
Associate Professor, ECED.
(MAY - 2013)
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
SardarVallabhbhai National Institute of Technology
Surat-395 007, Gujarat, INDIA.
SardarVallabhbhai National Institute of Technology
Surat-395 007, Gujarat, INDIA.
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
This is to certify that the B. Tech. IV (8th Semester) PROJECT REPORT entitled “Performance Analysis of LTE Physical Layer using System Vue” is presented & submitted by Candidates Mr. Pravat Karki, Biplav Bhurtel, Kishor Bhandari, Avi Gupta, bearing Roll No.U09EC410,U09EC428,U09EC429,U09EC435 respectively in the fulfillment of the requirement for the award of B. Tech. degree in Electronics & Communication Engineering.
They have successfully and satisfactorily completed their Project Exam in all respect. We, certify that the work is comprehensive, complete and fit for evaluation.
Prof. (Ms.) Shilpi Gupta Prof. P.K. SHAH
Project Guide Head of the Dept., ECED
Assistant Professor Associate Professor
SEMINAR EXAMINERS:
Name Signature with date
1. Prof. (Ms.) Jigisha N. Patel
2.Prof. Mehul C. Patel
3.Mrs. Kirti Inamdar
DEPARTMENT SEAL
MAY-2013.
ACKNOWLEDGEMENT
This Project was not possible to be completed without the help and able guidance of many
great people. We, with profound veneration and reverence would like to thank our guide Prof.
(Ms) SHILPI GUPTA (ECED,SVNIT) for her amiable attitude and motivation .She provided
constant guidance and support throughout semester for this project preliminary despite her tight
academic schedule.
We too take the opportunity to express our gratitude to our affable peers and venerable staff
members for the encouragement they have shown towards the work of ours and for their direct
and indirect assistance.
We would also like to express our appreciation to all the people who have been supporting us for
our project on “Performance Analysis of LTE physical layer using System Vue” and paved the
way to a better completion of this Project.
ABSTRACT
Technological advancement aims to make wireless communication efficient in terms of data
speed and QoS of service. Despite commercial 3G networks are starting to be fully operational
and High Speed Data Packet Access (HSDPA) is on its way to be deployed, operators and
manufacturers are already in a race towards 4G technologies.The road to 4G has a mandatory
milestone in Long Term Evolution (LTE) as it is a promising technology which will allow
backwards compatibility besides a higher performance. LTE, which is mainly deployed in a
macro/microcell layout, provides improved system capacity and coverage, high peak data rates,
low latency, reduced operating costs, multi-antenna support, flexible bandwidth operation and
seamless integration with existing systems.
In this project report, we present an overview of the techniques being used for LTE and explain
different technological advancement scenario of Long Term Evolution (LTE).For this we have
implemented and observed the OFDM modulation or Multiplexing Scheme that is basis of
implementation of physical layer of LTE.This report presents anintroduction to the performance
evaluation of LTE downlink physical layer according to the latest 3GPP specifications and
describes the various parameters which are important to analyze the performance of the LTE
Network. Our work aims to make a comprehensive investigation of the maximum data
throughput under different conditions and scenarios and calculate the Bit Error rates for the same
conditions by implementation in the System Vue software. Accordingly we have implemented
MIMO and SISO schemes of LTE and Measured Throughput and BER Under different
Modulation Schemes.
i
Table of Contents
List of Figures ……………………………………………………………………………….......iii
List of Tables…………………………………………………………………………………….vii
CHAPTER – 1 LTE Introduction………………………………………………………………...1
1.1 Historical Context …………………………………………………………………....1
1.2 Introduction of LTE in Mobile Radio………………………………………………..2
1.3 Requirements and Targets for the Long Term Evolution…………………………….3
1.4 System Performance Requirements………………………………………………......4
1.5 Multi carrier Technology…………………………………………………………......9
1.6 Multiple Antenna Technology…………………………………………………….…10
CHAPTER – 2 LTE BASIC CONCEPTS…………………………………………………........11
2.1 Single Carrier Modulation and Channel Equalization…………………………..…...11
2.2 OFDM…………………………………………………………………………....…..14
2.3 OFDMA……………………………………………………………………………...16
2.3.1 Comparison of OFDMA with Packet-Oriented Protocols……………..…..16
2.3.2 OFDMA and LTE Generic Frame Structure………………………….…...18
2.4 MIMO and MRC……………………………………………………………..….…..19
2.5 SC-FDMA……………………………………………………………………….…..21
CHAPTER – 3 LTE Physical Layer …………………………………………..………………...24
3.0.1 Generic Frame Structure …………………………………………………….….…24
3.1 Downlink………………………………………………………………….…….…....24
3.1.1 Modulation Parameters………………………………………………………..…...25
3.1.2 Downlink Multiplexing…………………………………………………….……....26
3.1.3 Physical Channels………………………………………………………….….…...26
3.1.4 Physical Signals…………………………………………………………….….…..28
ii
3.1.5 Transport Channels…………………………………………………………….......30
3.1.6 Mapping Downlink Physical channels to Transport channels……………….…….31
3.1.7 Downlink Channel Coding……………………………………………….…….….32
CHAPTER – 4 OFDM……………………………………...……………………………………33
4.1 OFDM for LTE……………………………………………………………..….…….33
4.2 OFDM architecture ……………………………………………………………...…..34
4.3 FFT Implementation……...……………………………………………………….…36
4.4 OFDMA BASICS……………………………………………………………………37
CHAPTER – 5 OFDM IMPLEMENTATION………………………………………….……....40
5.1 OFDM Block Diagram……………………………………………………………....40
5.2 Components used in OFDM Implementation …………………………...………….41
5.3 Measurement of signal at different points…………………………………………...56
CHAPTER –6 PERFORMANCE ANALYSIS FOR LTE……………………………………...62
6.1 Quantitative factors in LTE …………………………………………………………62
6.2 Implementation and Results For Different Schemes………………………..……….63
6.3 Results and Analysis…………………………………………………………………73
CHAPTER-7 CONCLUSION………………………………………………………………..…74
REFERENCES …………………………………………………………………………….……75
ACRONYM……………………………………………………………………………………..76
iii
List of figures
Fig 1.1.Approximate timeline of the mobile communications standards landscape…………...…3
Fig 2.1 Multipath Caused by Reflections off Objects Such as Buildings and Vehicles…………12
Fig 2.2 Multipath-Induced Time Delays Result in ISI………………….............…….…………12
Fig 2.3 Longer Delay Spreads Result in Frequency Selective Fading……………..………........12
Fig 2.4 Transversal Filter Channel Equalizer…………………………………………..…..........13
Fig 2.5 OFDM Eliminates ISI via Longer Symbol Periods and a Cyclic……………………….14
Fig 2.6 FFT of OFDM Symbol Reveals Distinct Subcarriers…………………..……………….15
Fig Preamble and Header………………………………………………………………………..16
Fig 2.8 LTE Generic Frame Structures……………………………………………….…............18
Fig 2.9 MRC/MIMO Operation Requires Multiple Transceivers………………………..….......19
Fig. 2.10 MRC Enhances Reliability in the Presence of AWGN and Frequency
Selective Fading………………………………………………………………………………..20
Fig 2.11 Reference Signals Transmitted Sequentially to Compute Channel Responses for
MIMO Operation……………………………………………………..………………..………..21
Fig 2.12 SC-FDMA and OFDMA Signal Chains Have a High Degree of
Functional Commonality…...………………………………………………………………..….22
Fig 2.13 SC-FDMA Subcarriers Can be Mapped in Either Localized or
DistributedMode…...……………………………………………………………...……….….....23
Fig 3.1 LTE Generic Frame Structures………………………………………………..…………24
Fig 3.2 Resource Elements Mapping of Reference Signals………………………………..….…29
iv
Fig 3.3 Mapping DL Transport Channels to physical channels ……………………………..…..31
Fig 4.1 .Effect of channel on signals with short and long symbol duration………………….….34
Fig 4.2 Simplex Point-to-point transmission using OFDM………………………………….….34
Fig 4.3 OFDM Cyclic Prefix (CP) insertion………………………………………………….….35
Fig. 4.4 cyclic extension and windowing of OFDM………………………………………….….35
Fig 4.5 OFDM Transmitter…………………………………………………………………........36
Fig 5.1 : OFDM Block Diagram…………………………………………………………………40
Fig. 5.2 Parameter change box of a RandomBits block………………………………………….41
Fig.5.3 Data sink block…………………………………………………………………………..41
Fig. 5.4 Parameter box of a Complex Symbol Mapper…………………………………………42
Fig.5.5 Parameter block for OFDM Subcarrier Multiplexing block…………………………....43
Fig.5.6 Parameter box ofa Complex Fast Fourier Transformation block…………………….….44
Fig.5.7 Parameter box of an OFDM guard interval insertion block……………………………..45
Fig. 5.8 Parameter box of a Complex to Envelope converter block…………………………….45
Fig.5.9 Parameter box of an Add Noise Density to Input Block………………………………...46
Fig. 5.10 An envelope to complex converter block……………………………………………..47
Fig. 5.11 Parameter box of an OFDM guard interval removal block…………………………....48
Fig. 5.12 A Complex Fast Fourier Transform block…………………………………………….49
Fig. 5.13 Parameter box of an OFDM subcarrier demultiplexing block………………………..50
Fig.5.14 Parameter box is a Complex Symbol Demapper/Slicer block………………………....51
Fig.5.15 Parameter box of a Delay block………………………………………………………..52
v
Fig. 5.16 Parameter box of a Bit and Frame Error Rate Measurement Block……………….….53
Fig 5.17 OFDM Block…………………………………………………………………………...53
Fig 5.18 .Output of Random bit Generator………………………………………………………54
Fig 5.19: Scatterplot diagram for 16 QAM………………………………………………………54
Fig 5.20: Polar plot after sub carrier allotment…………………………………………………..55
Fig 5.21: Output of FFT………………………………………………………………………….55
Fig 5.22: OFDM Signals after Guardband Insertion…………………………………………….56
Fig 5.23: OFDM signal after noise addition……………………………………………………..56
Fig 5.24: Spectrum at receiver side………………………………………………………………57
Fig 5.25: Scatter plot after Demodulation………………………………………………………..57
Fig 5.26: Output Signal…………………………………………………………………………..58
Fig 6.1 Basic Block for calculating BER of LTE SISO Scheme…………………………….…..60
Fig 6.2 BER vs SNR plot of LTE SISO Scheme………………………………………………..61
Fig 6.3 BLER vs SNR plot of LTE SISO Scheme……………………………………………....61
Fig 6.4 Block Diagram for calculating MIMO BER with QPSK Modulation……………….…62
Fig 6.5 : BER vs SNR Graph of MIMO LTE System…………………………………………...62
Fig 6.6 Block Diagram for MIMO BER plot with 16 QAM Modulation……………………….63
Fig 6.7 :BER vs SNR Graph of MIMO LTE System with 16 QAM Modulation……………....63
Fig 6.8 Basic blocks of SISO Throughput Calculation…...………………………………….….64
Fig 6.9 Throughput vs SNR plot of LTE SISO Scheme……………………………………........64
Fig 6.10 SISO Throughput fraction vs SNR…………………………………………………......65
Fig 6.11 Block Diagram for plotting MIMO Throughput for QPSK Modulation………....…….66
Fig 6.12 Throghputvs SNR plot MIMO scheme with QPSK
Modulation………………….…....................................................................................................66
vi
Fig 6.13 MIMO Throughput Fraction vsSNR for QPSK Modulation……………………..........67
Fig 6.14 Block Diagram for plotting MIMO Throughput for 16 QAM Modulation…………….68
Fig 6.15 MIMO Throughput vsSNR for 16QAM modulation………………………………….68
Fig 6.16 MIMO Throughput Fraction VSSNRfor 16QAM modulation………………………..69
vii
LIST OF TABLES
Table 1: Available Downlink Bandwidth is divided into Physical Resource Blocks………....14
Table 2: Downlink OFDM Modulation Parameters…………………………...……………....23
Table 3: Cyclic Prefix Duration……………………………………………………………….23
Performance Analysis Of LTE Physical layer using System Vue
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CHAPTER 1 LTE INTRODUCTION
1.1 Historical Context
The Long Term Evolution of UMTS is one of the latest steps in an advancing series of mobile
telecommunications systems. Arguably, at least for land-based systems, the series began in
1947 with the development of the concept of cells by Bell Labs, USA. The use of cells enabled
the capacity of a mobile communications network to be increased substantially, by dividing the
coverage area up into small cells each with its own base station operating on a different
frequency.[1]
The early systems were confined within national boundaries. They attracted only a small
number of users, as the equipment on which they relied was expensive, cumbersome and
power-hungry, and therefore was only really practical in a car.
The first mobile communication systems to see large-scale commercial growth arrived in the
1980s and became known as the ‗First Generation‘ systems. The First Generation used
analogue technology and comprised a number of independently developed systems worldwide
(e.g. AMPS (Analogue Mobile Phone System, used in America), TACS (Total Access
Communication System, used in parts of Europe), NMT (Nordic Mobile Telephone, used in
parts of Europe) and J-TACS (Japanese Total Access Communication System, used in Japan
and Hong Kong)).[1]
Global roaming first became a possibility with the development of the ‗Second Generation‘
system known as GSM (Global System for Mobile communications), which was based on
digital technology. The success of GSM was due in part to the collaborative spirit in which it
was developed. By harnessing the creative expertise of a number of companies workingtogether
under the auspices of the European Telecommunications Standards Institute (ETSI), GSM
became a robust, interoperable and widely accepted standard.
Fuelled by advances in mobile handset technology, which resulted in small, fashionable
terminals with a long battery life, the widespread acceptance of the GSM standard exceeded
initial expectations and helped to create a vast new market. The resulting near-universal
Performance Analysis Of LTE Physical layer using System Vue
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penetration of GSM phones in the developed world provided an ease of communication never
previously possible, first by voice and text message, and later also by more advanced data
services. Meanwhile in the developing world, GSM technology had begun to connect
communities and individuals in remote regions where fixed-line connectivity was nonexistent
and would be prohibitively expensive to deploy.
This ubiquitous availability of user-friendly mobile communications, together with increasing
consumer familiarity with such technology and practical reliance on it, thus provides the
context for new systems with more advanced capabilities. In the following section, the series of
progressions which have succeeded GSM is outlined, culminating in the development of the
system known as LTE – the Long Term Evolution of UMTS (Universal Mobile
Telecommunications System).
1.2 Introduction of LTE in Mobile Radio
In contrast to transmission technologies using media such as copper lines and optical fibers, the
radio spectrum is a medium shared between different, and potentially interfering, technologies.
As a consequence, regulatory bodies – in particular, ITU-R (International Telecommunication
Union – Radio communication Sector) [1], but also regional and national regulators play a key
role in the evolution of radio technologies since they decide which parts of the spectrum and
how much bandwidth may be used by particular types of service and technology. This role is
facilitated by the standardization of families of radio technologies – a process which not only
provides specified interfaces to ensure interoperability between equipment from a multiplicity
of vendors, but also aims to ensure that the allocated spectrum is used as efficiently as possible,
so as to provide an attractive user experience and innovative services.
The complementary functions of the regulatory authorities and the standardization
organizations can be summarized broadly by the following relationship:
Aggregated data rate = bandwidth × spectral efficiency
On a worldwide basis, ITU-R defines technology families and associates specific parts of the
spectrum with these families. Facilitated by ITU-R, spectrum for mobile radio technologies is
identified for the radio technologies which meet ITU-R‘s requirements tobe designated as
Performance Analysis Of LTE Physical layer using System Vue
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members of the International Mobile Telecommunications (IMT) family. Effectively, the IMT
family comprises systems known as ‗Third Generation‘ (for the first time providing data rates
up to 2 Mbps) and beyond.
Figure 1.1.Approximate timeline of the mobile communications standards landscape[12].
1.3 Requirements and Targets for the Long Term Evolution
Discussion of the key requirements for the new LTE system led to the creation of a formal
Study Item‘ in 3GPP with the specific aim of ‗evolving‘ the 3GPP radio access technology to
ensure competitiveness over a ten-year time-frame. Under the auspices of this Study Item, the
requirements for LTE were refined and crystallized, being finalized in June 2005.
They can be summarized as follows:
• reduced delays, in terms of both connection establishment and transmission latency;
• increased user data rates;
• increased cell-edge bit-rate, for uniformity of service provision;
• reduced cost per bit, implying improved spectral efficiency;
Performance Analysis Of LTE Physical layer using System Vue
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• greater flexibility of spectrum usage, in both new and pre-existing bands;
• simplified network architecture;
• seamless mobility, including between different radio-access technologies;
• reasonable power consumption for the mobile terminal.
It can also be noted that network operator requirements for next generation mobile systems
were formulated by the Next Generation Mobile Networks (NGMN) alliance of network
operators, which served as an additional reference for the development and assessment of the
LTE design.
1.4 System Performance Requirements
Improved system performance compared to existing systems is one of the main requirements
from network operators, to ensure the competitiveness of LTE and hence to arouse market
interest. In this section, we highlight the main performance metrics used in the definition of the
LTE requirements and its performance assessment.
Improved system performance compared to existing systems is one of the main requirements
from network operators, to ensure the competitiveness of LTE and hence to arouse market
interest. In this section, we highlight the main performance metrics used in the definition of the
LTE requirements and its performance assessment.
It can be seen that the target requirements for LTE represent a significant step from the capacity
and user experience offered by the third generation mobile communications systems which
were being deployed at the time when the first version of LTE was being developed. As
mentioned above, HSPA technologies are also continuing to be developed to offer higher
spectral efficiencies than were assumed for the reference baseline. However, LTE has been able
to benefit from avoiding the constraints of backward compatibility, enabling the inclusion of
advanced MIMO schemes in the system design from the beginning, and highly flexible
spectrum usage built around new multiple access schemes.
Performance Analysis Of LTE Physical layer using System Vue
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Peak Rates and Peak Spectral Efficiency
For marketing purposes, the first parameter by which different radio access technologies are
usually compared is the peak per-user data rate which can be achieved.
This peak data rate generally scales according to the amount of spectrum used, and, for MIMO
systems, according to the minimum of the number of transmit and receive antennas. The peak
data rate can be defined as the maximum throughput per user assuming the whole bandwidth
being allocated to a single user with the highest modulation and coding scheme and the
maximum number of antennas supported. Typical radio interface overhead (control channels,
pilot signals, guard intervals, etc.) is estimated and taken into account for a given operating
point. For TDD systems, the peak data rate is generally calculated for the downlink and uplink
periods separately. This makes it possible to obtain a single value independent of the
uplink/downlink ratio and a fair system comparison that is agnostic of the duplex mode. The
maximum spectral efficiency is then obtained simply by dividing the peak rate by the used
spectrum allocation.
The target peak data rates for downlink and uplink in LTE Release 8 were set at 100 Mbps and
50 Mbps respectively within a 20 MHz bandwidth,7 corresponding to respective peak spectral
efficiencies of 5 and 2.5 bps/Hz. The underlying assumption here is that the terminal has two
receive antennas and one transmit antenna. The number of antennas used at the base station is
more easily upgradeable by the network operator, and the first version of the LTE specifications
was therefore designed to support downlink MIMO operation with up to four transmit and
receive antennas.
When comparing the capabilities of different radio communication technologies, great
emphasis is often placed on the peak data rate capabilities. While this is one indicator of how
technologically advanced a system is and can be obtained by simple calculations, it may not be
a key differentiator in the usage scenarios for a mobile communication system in practical
deployment. Moreover, it is relatively easy to design a system that can provide very high peak
data rates for users close to the base station, where interference from other cells is low and
techniques such as MIMO can be used to their greatest extent. It is much more challenging to
Performance Analysis Of LTE Physical layer using System Vue
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provide high data rates with good coverage and mobility, but it is exactly these latter aspects
which contribute most strongly to user satisfaction.
In typical deployments, individual users are located at varying distances from the basestations,
the propagation conditions for radio signals to individual users are rarely ideal, and the
available resources must be shared between many users. Consequently, although the claimed
peak data rates of a system are genuinely achievable in the right conditions, it is rare for a
single user to be able to experience the peak data rates for a sustained period, and the envisaged
applications do not usually require this level of performance.
A differentiator of the LTE system design compared to some other systems has been the
recognition of these ‗typical deployment constraints‘ from the beginning. During the design
process, emphasis was therefore placed not only on providing a competitive peak data rate for
use when conditions allow, but also importantly on system level performance, which was
evaluated during several performance verification steps. System-level evaluations are based on
simulations of multicell configurations where data transmission from/to a population of mobiles
is considered in a typical deployment scenario. The sections below describe the main metrics
used as requirements for system level performance.
Cell Throughput and Spectral Efficiency
Performance at the cell level is an important criterion, as it relates directly to the number of cell
sites that a network operator requires, and hence to the capital cost of deploying the system. For
LTE , it was chosen to assess the cell level performance with full-queue traffic models (i.e.
assuming that there is never a shortage of data to transmit if a user is given the opportunity) and
a relatively high system load, typically 10 users per cell.
The requirements at the cell level were defined in terms of the following metrics:
• Average cell throughput [bps/cell] and spectral efficiency [bps/Hz/cell];
• Average user throughput [bps/user] and spectral efficiency [bps/Hz/user];
Performance Analysis Of LTE Physical layer using System Vue
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• Cell-edge user throughput [bps/user] and spectral efficiency [bps/Hz/user] (the metric used for
this assessment is the 5-percentile user throughput, obtained from the cumulative distribution
function of the user throughput).
For the UMTS Release 6 reference baseline, it was assumed that both the terminal and the base
station use a single transmit antenna and two receive antennas; for the terminal receiver the
assumed performance corresponds to a two-branch Rake receiver with linear combining of the
signals from the two antennas. For the LTE systems, the use of two transmit and receive
antennas was assumed at the base station. At the terminal, two receive antennas were assumed,
but still only a single transmit antenna. The receiver for both downlink and uplink is assumed to
be a linear receiver with optimum combining of the signals from the antenna branches.
Voice Capacity
Unlike full queue traffic (such as file download) which is typically delay-tolerant and does not
require a guaranteed bit-rate, real-time traffic such as Voice over IP (VoIP) has tight delay
constraints. It is important to set system capacity requirements for such services – a particular
challenge in fully packet-based systems like LTE which rely on adaptive scheduling. The
system capacity requirement is defined as the number of satisfied VoIP users, given a particular
traffic model and delay constraints. The details of the traffic model used for evaluating LTE can
be found. Here, a VoIP user is considered to be in outage (i.e. not satisfied) if more than 2% of
the VoIP packets do not arrive successfully at the radio receiver within 50 ms and are therefore
discarded. This assumes an overall end-to-end delay (from mobile terminal to mobile terminal)
below 200 ms. The system capacity for VoIP can then be defined as the number of users present
per cell when more than 95% of the users are satisfied
Mobility and Cell Ranges
LTE is required to support communication with terminals moving at speeds of up to 350 km/h,
or even up to 500 km/h depending on the frequency band. The primary scenario for operation at
such high speeds is usage on high-speed trains – a scenario which is increasing in importance
across the world as the number of high-speed rail lines increases and train operators aim to offer
an attractive working environment to their passengers. These requirements mean that handover
Performance Analysis Of LTE Physical layer using System Vue
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between cells has to be possible without interruption –in other words, with imperceptible delay
and packet loss for voice calls, and with reliable transmission for data services.
These targets are to be achieved by the LTE system in typical cells of radius up to 5 km, while
operation should continue to be possible for cell ranges of 100 km and more,to enable wide-
area deployments.
Broadcast Mode Performance
The requirements for LTE included the integration of an efficient broadcast mode for high rate
Multimedia Broadcast/Multicast Services (MBMS) such as mobile TV, based on a Single
Frequency Network mode of operation . The spectral efficiency requirement is given in terms
of a carrier dedicated to broadcast transmissions –i.e. not shared with unicast transmissions.In
broadcast systems, the system throughput is limited to what is achievable for the users in the
worst conditions. Consequently, the broadcast performance requirement was defined in terms of
an achievable system throughput (bps) and spectral efficiency (bps/Hz) assuming a coverage of
98% of the nominal coverage area of the system. This means that only 2% of the locations in
the nominal coverage area are in outage – where outage for broadcast services is defined as
experiencing a packet error rate higher than 1%. This broadcast spectral efficiency requirement
was set to 1 bps/Hz [10]. While the broadcast mode was not available in Release 8 due to
higher prioritization of other service modes, Release 9 incorporates a broadcast mode
employing Single Frequency Network operation on a mixed unicast-broadcast carrier.
User Plane Latency
User plane latency is an important performance metric for real-time and interactive services. On
the radio interface, the minimum user plane latency can be calculated based on signaling
analysis for the case of an unloaded system. It is defined as the average time between the first
transmission of a data packet and the reception of a physical layer acknowledgement. The
calculation should include typical HARQ8 retransmission rates (e.g. 0–30%). This definition
therefore considers the capability of the system design, without being distorted by the
scheduling delays that would appear in the case of a loaded system. The round-trip latency is
obtained simply by multiplying the one-way user plane latency by a factor of two. LTE is also
required to be able to operate with IP-layer one-way data-packet latency across the radio access
Performance Analysis Of LTE Physical layer using System Vue
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network as low as 5 ms in optimal conditions. However, it is recognized that the actual delay
experienced in a practical system will be dependent on system loading and radio propagation
conditions.
Control Plane Latency and Capacity
In addition to the user plane latency requirement, call setup delay was required to be
significantly reduced compared to previous cellular systems. This not only enables a good user
experience but also affects the battery life of terminals, since a system design which allows a
fast transition from an idle state to an active state enables terminals to spend more time in the
low-power idle state. Control plane latency is measured as the time required for performing the
transitions between different LTE states.
1.5 Multi carrier Technology
Adopting a multicarrier approach for multiple access in LTE was the first major design choice.
After initial consolidation of proposals, the candidate schemes for the downlink were
Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple WCDMA, while the
candidate schemes for the uplink were Single-Carrier Frequency-Division Multiple Access (SC-
FDMA), OFDMA and Multiple WCDMA. The choice of multiple-access schemes was made in
December 2005, with OFDMA being selected for the downlink, and SC-FDMA for the uplink.
Both of these schemes open up the frequency domain as a new dimension of flexibility in the
system.
This resulting flexibility can be used in various ways:
Different spectrum bandwidths can be utilized without changing the fundamental system
parameters or equipment design;
Transmission resources of variable bandwidth can be allocated to different users and
scheduled freely in the frequency domain;
Fractional frequencies re-use and interference coordination between cells is facilitated.
Performance Analysis Of LTE Physical layer using System Vue
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Extensive experience with OFDM has been gained in recent years from deployment of digital
audio and video broadcasting systems such as DAB, DVB and DMB. This experience has
highlighted some of the key advantages of OFDM, which include:
Robustness to time-dispersive radio channels, thanks to the subdivision of the wideband
transmitted signal into multiple narrowband subcarriers, enabling inter-symbol
interference to be largely constrained within a guard interval at the beginning of each
symbol
Low-complexity receivers, by exploiting frequency-domain equalization
Simple combining of signals from multiple transmitters in broadcast networks
1.6 Multiple Antenna Technology
The use of multiple antenna technology allows the exploitation of the spatial-domain as another
new dimension. This becomes essential in the quest for higher spectral efficiencies. With the
use of multiple antennas the theoretically achievable spectral efficiency scales linearly with the
minimum of the number of transmit and receive antennas employed, at least in suitable radio
propagation environments. Multiple antenna technology opens the door to a large variety of
features, but not all of them easily deliver their theoretical promises when it comes to
implementation in practical systems. Multiple antennas can be used in a variety of ways, mainly
based on three fundamental principles.
Diversity gain: Use of the spatial diversity provided by the multiple antennas to improve the
robustness of the transmission against multipath fading.
Array gain: Concentration of energy in one or more given directions via precoding or beam
forming. This also allows multiple users located in different directions to be served
simultaneously (so-called multi user MIMO).
Spatial multiplexing gain: Transmission of multiple signal streams to a single user on multiple
spatial layers created by combinations of the available antennas.
Performance Analysis Of LTE Physical layer using System Vue
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CHAPTER 2 LTE BASIC CONCEPTS
Before jumping into a detailed description of the LTE PHY, it‘s worth taking a look at some of
the basic technologies involved. Many methods employed in LTE are relatively new in cellular
applications. These include OFDM, OFDMA, MIMO and Single Carrier Frequency Division
Multiple Access (SC-FDMA). LTE employs OFDM for downlink data transmission and SC-
FDMA for uplink transmission. OFDM is a well-known modulation technique, but is rather
novel in cellular applications. A brief discussion of the basic properties and advantages of this
method is therefore warranted. When information is transmitted over a wireless channel, the
signal can be distorted due to multipath. Typically (but not always) there is a line-of-sight path
between the transmitter and receiver. In addition, there are many other paths created by signal
reflection off buildings, vehicles and other obstructions as shown in Figure 2.1. Signals
travelling along these paths all reach the receiver, but are shifted in time by an amount
corresponding to the differences in the distance travelled along each path.
2.1 Single Carrier Modulation and Channel Equalization
To date, cellular systems have used single carrier modulation schemes almost exclusively.
Although LTE uses OFDM rather than single carrier modulation, it‘s instructive to briefly
discuss how single carrier systems deal with multipath-induced channel distortion. This will
form a point of reference from which OFDM systems can be compared and contrasted. The
term delay spread describes the amount of time delay at the receiver from a signal traveling
from the transmitter along different paths. In cellular applications, delay spreads can be several
microseconds. The delay induced by multipath can cause a symbol received along a delayed
path to ―bleed‖ into a subsequent symbol arriving at the receiver via a more direct path. This
effect is depicted in Figure 2.1-1 and is referred to as inter-symbol interference (ISI). In a
conventional single carrier system symbol times decrease as data rates increase. At very high
data rates (with correspondingly shorter symbol periods), it is quite possible for ISI to exceed
an entire symbol period and spill into a second or third subsequent symbol.
Performance Analysis Of LTE Physical layer using System Vue
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Figure 2.1 Multipath Caused by Reflections Off Objects Such as Buildings and Vehicles[13]
Figure 2.2 Multipath-Induced Time Delays Result in ISI [13]
It‘s also helpful to consider the effects of multipath distortion in the frequency domain. Each
different path length and reflection will result in a specific phase shift. As all of the signals are
combined at the receiver, some frequencies within the signal passband undergo constructive
interference (linear combination of signals in-phase), while others encounter destructive
interference (linear combination of signals out-of-phase). The composite received signal is
distorted by frequency selective fading. (See Fig. 2.2)
Figure 2.3 Longer Delays Spreads Result in Frequency Selective Fading [13]
Performance Analysis Of LTE Physical layer using System Vue
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Single carrier systems compensate for channel distortion via time domain equalization. This is a
substantial topic by itself, and beyond the scope of this paper. Generally, time domain
equalizers compensate for multipath induced distortion by one of two methods:
1. Channel inversion: A known sequence is transmitted over the channel prior to sending
information. Because the original signal is known at the receiver, a channel equalizer is able to
determine the channel response and multiply the subsequent data-bearing signal by the inverse
of the channel response to reverse the effects of multipath.
2. CDMA systems can employ rake equalizers to resolve the individual paths and then
combine digital copies of the received signal shifted in time to enhance the receiver signal-to-
noise ratio (SNR). In either case, channel equalizer implementation becomes increasingly
complex as data rates increase. Symbol times become shorter and receiver sample clocks must
become correspondingly faster. ISI becomes much more severe possibly spanning several
symbol periods.
Figure 2.4: Transversal Filter Channel Equalizer [13]
The finite impulse response transversal filter (see Fig. 2.4) is a common equalizer topology. As
the period of the receiver sample clock (Δ) decreases, more samples are required to compensate
for a given amount of delay spread. The number of delay taps increases along with the speed
and complexity of the adaptive algorithm. For LTE data rates(up to 100 Mbps) and delay
spreads (approaching 17 μsec), this approach to channel equalization becomes impractical. As
we will discuss below, OFDM eliminates ISI in the time domain, which dramatically simplifies
the task of channel compensation.
Performance Analysis Of LTE Physical layer using System Vue
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2.2 OFDM
Unlike single carrier systems described above, OFDM communication systems do not rely on
increased symbol rates in order to achieve higher data rates. This makes the task of managing
ISI much simpler. OFDM systems break the available bandwidth into many narrower sub-
carriers and transmit the data in parallel streams. Each subcarrier is modulated using varying
levels of QAM modulation, e.g. QPSK, QAM, 64QAM or possibly higher orders depending on
signal quality. Each OFDM symbol is therefore a linear combination of the instantaneous
signals on each of the sub-carriers in the channel. Because data is transmitted in parallel rather
than serially, OFDM symbols are generally MUCH longer than symbols on single carrier
systems of equivalent data rate.
There are two truly remarkable aspects of OFDM. First, each OFDM symbol is preceded by a
cyclic prefix (CP), which is used to effectively eliminate ISI. Second, the sub-carriers are very
tightly spaced to make efficient use of available bandwidth, yet there is virtually no interference
among adjacent sub-carriers (Inter Carrier Interference, or ICI). These two unique features are
actually closely related. In order to understand how OFDM deals with multipath distortion, it‘s
useful to consider the signal in both the time and frequency domains.
To understand how OFDM deals with ISI induced by multipath, consider the time domain
representation of an OFDM symbol shown in Fig 2.5. The OFDM symbol consists of two major
components: the CP and an FFT period (TFFT). The duration of the CP is determined by the
highest anticipated degree of delay spread for the targeted application. When transmitted
signals arrive at the receiver by two paths of differing length, they are staggered in time as
shown in Fig. 2.5.[6]
Figure 2.5: OFDM Eliminates ISI via Longer Symbol Periods and a Cyclic Prefix [13]
Performance Analysis Of LTE Physical layer using System Vue
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Within the CP, it is possible to have distortion from the preceding symbol. However, with a CP
of sufficient duration, preceding symbols do not spill over into the FFT period; there is only
interference caused by time-staggered ―copies‖ of the current symbol. Once the channel
impulse response is determined (by periodic transmission of known reference signals),
distortion can be corrected by applying an amplitude and phase shift on a subcarrier-by-
subcarrier basis. Note that all of the information of relevance to the receiver is contained within
the FFT period. Once the signal is received and digitized, the receiver simply throws away the
CP. The result is a rectangular pulse that, within each subcarrier, is of constant amplitude over
the FFT period. The rectangular pulses resulting from decimation of the CP are central to the
ability to space subcarriers very closely in frequency without creating ICI. Readers may recall
that a uniform rectangular pulse (RECT function) in the time domain results in a SINC function
(sin(x) / x) in the frequency domain as shown in Fig. 2.6. The LTE FFT Period is 67.77 μsec.
Note that this is simply the inversion of the carrier spacing (1 /Δf). This results in a SINC
pattern in the frequency domain with uniformly spaced zero-crossings at 15 kHz intervals—
precisely at the center of the adjacent subcarrier. It is therefore possible to sample at the center
frequency of each subcarrier while encountering no interference from neighboring subcarriers
(zero-ICI).[6]
Figure 2.6 FFT of OFDM Symbol Reveals Distinct Subcarriers [13]
Performance Analysis Of LTE Physical layer using System Vue
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2.3 OFDMA
OFDMA is employed as the multiplexing scheme in the LTE downlink. Perhaps the best way to
describe OFDMA is by contrasting it with a packet-oriented networking scheme such as
802.11a. In 802.11a, Carrier-Sense Multiple Access (CSMA) is the multiplexing method.
Downlink and uplink traffic from the fixed access point (AP) to mobile user stations (STAs) is
by means of PHY layer packets. As explained below, OFDMA makes much more efficient use
of network resources.
2.3.1 Comparison of OFDMA with Packet-Oriented Protocols
Like 3GPP LTE, IEEE 802.11a uses OFDM as the underlying modulation method. However,
802.11a uses CSMA asthe multiplexing method. CSMA is essentially a listen-before-talk
scheme. For example, when the AP has queued traffic for a STA, it monitors the channel for
activity. When the channel becomes idle, it begins to decrement an internal timer that is
randomized within a specified window. The timer will continue to be decremented as long as
the network remains idle. When the timer reaches zero, the AP will transmit a PHY layer packet
of up to 2000 bytes addressed to a particular STA (or all STAs within the cell in the case of
broadcast mode). The randomized back-off period is designed to minimize collisions, but it
cannot eliminate them entirely.[7]
Figure 2.7Conventional Packet Oriented Networks Like IEEE 802.11a Precede Each Data
Transmission with a PHY Layer Preamble and Header [13]
Each 802.11a PHY packet utilizes all of the PHY layer bandwidth for the duration of the
packet. Consider the 802.11a PHY packet format shown in Figure 2.7. Each 802.11a packet has
Performance Analysis Of LTE Physical layer using System Vue
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a data payload of varying length from 64 to 2048 bytes. If the packet transmission is successful,
the receiving station transmits an ACK. Unacknowledged packets are assumed to be dropped.
Note that each packet is preceded by a PHY preamble which is 20 µsec in duration. The
purposes of the PHY preamble are:
• Signal detection
• Antenna diversity selection
• Setting AGC
• Frequency offset estimation
• Timing synchronization
• Channel estimation
The address of the intended recipient is not in the PHY preamble. It is actually in the packet
data and is interpreted at the MAC layer. From a networking perspective, the packet-oriented
approach of 802.11a has the advantage of simplicity. Each packet is addressed to a single
recipient (broadcast mode not withstanding). However, the randomized backoff period of the
CSMA multiplexing scheme is idle time and therefore represents an inefficiency. The PHY
preamble is also network overhead and further reduces efficiency, particularly for shorter
packets. The typical real-world efficiency of an 802.11a system is approximately 50 percent. In
other words, for a network with a nominal data rate of 54 Mbps, the typical throughput is about
25 – 30 Mbps. Some of the inefficiencies can be mitigated by abandoning the CSMA
multiplexing scheme and adopting a scheduled approach to packet transmission. Indeed,
subsequent versions of the 802.11 protocol include this feature. Inefficiencies due to dedicated
ACK packets can also be reduced by acknowledging packets in groups rather than individually.
In spite of potential improvements, it remains difficult to drive packet-oriented network
efficiency much beyond 65 to 70 percent. Further, because each packet completely consumes
all network resources during transmission and acknowledgement, the AP can provide addressed
(non-broadcast) traffic to user terminals only on a sequential basis. When many users are active
within the cell, latency can become a significant problem. Clearly, the objective of cellular
Performance Analysis Of LTE Physical layer using System Vue
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carriers is to create as much network demand as possible for a wide variety of traffic that
includes voice, multimedia, and data. Efficiency and low latency are therefore paramount. As
we will see in the following section, OFDMA is superior to packet-oriented schemes in both of
these critical dimensions.
2.3.2 OFDMA and the LTE Generic Frame Structure
OFDMA is an excellent choice of multiplexing scheme for the 3GPP LTE downlink. Although
it involves added complexity in terms of resource scheduling, it is vastly superior to packet-
oriented approaches in terms of efficiency and latency. In OFDMA, users are allocated a
specific number of subcarriers for a predetermined amount of time. These are referred to as
physical resource blocks (PRBs) in the LTE specifications. PRBs thus have both a time and
frequency dimension. Allocation of PRBs is handled by a scheduling function at the 3GPP base
station (eNodeB).[7]
Figure 2.8 LTE Generic Frame Structures [13]
In order to adequately explain OFDMA within the context of the LTE, we must study the PHY
layer generic frame structure. The generic frame structure is used with FDD. Alternative frame
structures are defined for use with TDD. However, TDD is beyond the scope of this paper.
Alternative frame structures are therefore not considered. As shown in fig. 2.8, LTE frames are
10 msec in duration. They are divided into 10 subframes, each subframe being 1.0 msec long.
Each subframe is further divided into two slots, each of 0.5 msec duration. Slots consist of
either 6 or 7 ODFM symbols, depending on whether the normal or extended cyclic prefix is
employed.
Performance Analysis Of LTE Physical layer using System Vue
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Table 1: Available Downlink Bandwidth is Divided into Physical Resource Blocks [13]
The total number of available subcarriers depends on the overall transmission bandwidth of the
system. The LTE specifications define parameters for system bandwidths from 1.25 MHz to 20
MHz as shown in Table 1. A PRB is defined as consisting of 12 consecutive subcarriers for one
slot (0.5 msec) in duration. A PRB is the smallest element of resource allocation assigned by
the base station scheduler.
2.4 MIMO and MRC
The LTE PHY can optionally exploit multiple transceivers at both the basestation and UE in
order to enhance link robustness and increase data rates for the LTE downlink. In particular,
maximal ratio combining (MRC) is used to enhance link reliability in challenging propagating
conditions when signal strength is low and multipath conditions are challenging. MIMO is a
related technique that is used to increase system data rates.
Figure 2.9 MRC/MIMO Operation Requires Multiple Transceivers [13]
Figure 2.9a shows a conventional single channel receiver with antenna diversity. This receiver
structure uses multiple antennas, but it is not capable of supporting MRC/MIMO. The basic
Performance Analysis Of LTE Physical layer using System Vue
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receiver topology for both MRC and MIMO is shown in Figure 2.9b. MRC and MIMO are
sometimes referred to as ―multiple antennas‖ technologies, but this is a bit of a misnomer. Note
that the salient difference between the receivers shown in Figures 2.9a and 2.9b is not multiple
antennas, but rather multiple transceivers. With MRC, a signal is received via two (or more)
separate antenna/transceiver pairs. Note that the antennas are physically separated, and
therefore have distinct channel impulse responses.
Channel compensation is applied to each received signal within the baseband processor before
being linearly combined to create a single composite received signal. When combined in this
manner, the received signals add coherently within the baseband processor. However, the
thermal noise from each transceiver is uncorrelated. Thus, linear combination of the channel
compensated signals at the baseband processor results in an increase in SNR of 3 dB on average
for a two-channel MRC receiver in a noise-limited environment.
Fig. 2.10 MRC Enhances Reliability in the Presence of AWGN and Frequency Selective Fading
[13]
Aside from the improvement in SNR due to combining, MRC receivers are robust in the
presence of frequency selective fading. Recall that physical separation of the receiver antennas
results in distinct channel impulse responses for each receiver channel. In the presence of
frequency selective fading, it is statistically unlikely that a given subcarrier will undergo deep
fading on both receiver channels. The possibility of deep frequency selective fades in the
composite signal is therefore significantly reduced. MRC enhances link reliability, but it does
not increase the nominal system data rate. In MRC mode, data is transmitted by a single
Performance Analysis Of LTE Physical layer using System Vue
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antenna and is processed at the receiver via two or more receivers. MRC is therefore a form of
receiver diversity rather than more conventional antenna diversity. MIMO, on the other hand,
does increase system data rates. This is achieved by using multiple antennas on both the
transmitting and receiving ends.
Figure 2.11 Reference Signals Transmitted Sequentially to Compute Channel Responses for
MIMO Operation [13]
In order to successfully receive a MIMO transmission, the receiver must determine the channel
impulse response from each transmitting antenna. In LTE, channel impulse responses are
determined by sequentially transmitting known reference signals from each transmitting
antenna as shown in Figure 2.11.
2.5 SC-FDMA
LTE uplink requirements differ from downlink requirements in several ways. Not surprisingly,
power consumption is a key consideration for UE terminals. The high PAPR and related loss of
efficiency associated with OFDM signaling are major concerns. As a result, an alternative to
OFDM was sought for use in the LTE uplink. Single Carrier – Frequency Domain Multiple
Access (SC-FDMA) is well suited to the LTE uplink requirements. The basic transmitter and
receiver architecture is very similar (nearly identical) to OFDMA, and it offers the same degree
of multipath protection. Importantly, because the underlying waveform is essentially single-
carrier, the PAPR is lower.
Performance Analysis Of LTE Physical layer using System Vue
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Figure 2.12 SC-FDMA and OFDMA Signal Chains Have a High Degree of Functional
Commonality [13]
The block diagram of Figure 2.12 shows a basic SC-FDMA transmitter / receiver arrangement.
Note that many of the functional blocks are common to both SC-FDMA and OFDMA, thus
there is a significant degree of functional commonality between the uplink and downlink signal
chains. The functional blocks in the transmit chain are:
1. Constellation mapper: Converts incoming bit stream to single carrier symbols (BPSK,
QPSK, or 16QAM depending on channel conditions)
2. Serial/parallel converter: Formats time domain SC symbols into blocks for input to FFT
engine
3. M-point DFT: Converts time domain SC symbol block into M discrete tones
4. Subcarrier mapping: Maps DFT output tones to specified subcarriers for transmission. SC-
FDMA systems either use contiguous tones (localized) or uniformly spaced tones (distributed)
as shown in Figure 2.13. The current working assumption in LTE is that localized subcarrier
mapping will be used. The trades between localized and distributed subcarrier mapping are
discussed further below.
5. N-point IDFT: Converts mapped subcarriers back into time domain for transmission
6. Cyclic prefix and pulse shaping: Cyclic prefix is pre-pended to the composite SC-FDMA
symbol to provide multipath immunity in the same manner as described for OFDM. As in the
case of OFDM, pulse shaping is employed to prevent spectral regrowth.
Performance Analysis Of LTE Physical layer using System Vue
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7. RFE: Converts digital signal to analog and up-convert to RF for transmission
In the receive side chain, the process is essentially reversed. As in the case of OFDM, SC-
FDMA transmissions can be thought of as linear summations of discrete subcarriers. Multipath
distortion is handled in the same manner as in OFDM systems (removal of CP, conversion to
the frequency domain, then apply the channel correction on a subcarrier-by- subcarrier
basis).Unlike OFDM, the underlying SC-FDMA signal represented by the discrete subcarriers
is—not surprisingly—single carrier. This is distinctly different than OFDM because the SC-
FDMA subcarriers are not independently modulated. As a result, PAPR is lower than for
OFDM transmissions. Analysis has shown that the LTE UE RFPA can be operated about 2 dB
closer to the 1-dB compression point than would otherwise be possible if OFDM were
employed on the uplink [2].
Figure 2.13 SC-FDMA Subcarriers Can be Mapped in Either Localized or Distributed Mode
[13]
As mentioned above, SC-FDMA subcarriers can be mapped in one of two ways: localized or
distributed as shown in Figure 2.13. However, the current working assumption is that LTE will
use localized subcarrier mapping. This decision was motivated by the fact that with localized
mapping, it is possible to exploit frequency selective gain via channel-dependent scheduling
(assigning uplink frequencies to UE based on favorable propagation conditions).
Performance Analysis Of LTE Physical layer using System Vue
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CHAPTER 3 LTE PHYSICAL LAYER
The capabilities of the eNodeB and UE are obviously quite different. Not surprisingly, the LTE
PHY DL and UL are quite different. The DL and UL are treated separately within the
specification documents. Therefore, the DL and UL are described separately in the following
sections.
3.0.1 Generic Frame Structure
One element shared by the LTE DL and UL is the generic frame structure. As mentioned
previously, the LTE specifications define both FDD and TDD modes of operation. This paper
deals exclusively with describing FDD specifications. The generic frame structure applies to
both the DL and UL for FDD operation.
Figure 3.1 LTE Generic Frame Structure [13]
LTE transmissions are segmented into frames, which are 10 msec in duration. Frames consist of
20 slot periods of 0.5 msec. Sub-frames contain two slot periods and are 1.0 msec in duration.
[8]
3.1 Downlink
The LTE PHY specification is designed to accommodate bandwidths from 1.25 MHz to 20
MHz OFDM was selected as the basic modulation scheme because of its robustness in the
presence of severe multipath fading. Downlink multiplexing is accomplished via OFDMA. The
DL supports physical channels, which convey information from higher layers in the LTE stack,
and physical signals which are for the exclusive use of the PHY layer. Physical channels map to
Performance Analysis Of LTE Physical layer using System Vue
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transport channels, which are service access points (SAPs) for the L2/L3 layers. Depending on
the assigned task, physical channels and signals use different modulation and coding
parameters.
3.1.1 Modulation Parameters
OFDM is the modulation scheme for the DL. The basic subcarrier spacing is 15 kHz, with a
reduced subcarrier spacing of 7.5 kHz available for some MB-SFN scenarios. Table 3.2
summarizes OFDM modulation parameters.[8]
Table 2 Downlink Ofdm Modulation Parameters[13]
Depending on the channel delay spread, either short or long CP is used. When short CP is used,
the first OFDM symbol in a slot has slightly longer CP than the remaining six symbols, as
shown in Table 3. This is done to preserve slot timing (0.5 msec). [8]
The cyclic prefix is actually a copy of the last portion of the data symbol appended to the front
of the symbol during the guard interval. By adding a cyclic prefix, the channel can be made to
behave as if the transmitted waveforms were from time minus infinite, and thus ensure
orthogonality, which essentially prevents one subcarrier from interfering with another (called
intercarrier interference, or ICI). This is accomplished because the amount of time dispersion
from the channel is smaller than the duration of the cyclic prefix. After discovering the process
for OFDM, a cyclic prefix has been proposed for other modulations to improve the robustness
to multipath.
Performance Analysis Of LTE Physical layer using System Vue
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Table 3 Cyclic Prefix Duration[13]
Note that the CP duration is described in absolute terms (e.g. 16.67 μsec for long CP) and in
terms of standard time units, Ts. Ts is used throughout the LTE specification documents. It is
defined as Ts = 1 / (15000 x 2048) seconds, which corresponds to the 30.72 MHz sample clock
for the 2048 point FFT used with the 20 MHz system bandwidth.
3.1.2 Downlink Multiplexing
OFDMA is the basic multiplexing scheme employed in the LTE downlink. OFDMA is a new-
to-cellular technology and is described in detail in Section 2.3 above. As described in Section
2.3, groups of 12 adjacent subcarriers are grouped together on a slot-by-slot basis to form
physical resource blocks (PRBs). A PRB is the smallest unit of bandwidth assigned by the base
station scheduler. A two dimensional (time and frequency) resource grid can be constructed to
represent the transmitted downlink signal. Each block in the grid represents one OFDM symbol
on a given subcarrier and is referred to as a resource element. Note that in MIMO applications,
there is one resource grid for each transmitting antenna.
3.1.3 Physical Channels
Three different types of physical channels are defined for the LTE downlink. One common
characteristic of physical channels is that they all convey information from higher layers in the
LTE stack. This is in contrast to physical signals, which convey information that is used
exclusively within the PHY layer.
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LTE DL physical channels are:
• Physical Downlink Shared Channel (PDSCH)
• Physical Downlink Control Channel (PDCCH)
• Common Control Physical Channel (CCPCH)
Physical channels are mapped to specific transport channels as described in Section 3.1.5
below. Transport channels are SAPs for higher layers. Each physical channel has defined
algorithms for:
• Bit scrambling
• Modulation
• Layer mapping
• CDD precoding
• Resource element assignment
Layer mapping and pre-coding are related to MIMO applications. Basically, a layer corresponds
to a spatial multiplexing channel. MIMO systems are defined in terms of N transmitters x N
receivers. For LTE, defined configurations are 1x 1, 2 x 2, 3 x 2 and 4 x 2. Note that while there
are as many as four transmitting antennas, there are only a maximum of two receivers and thus
a maximum of only two spatial multiplexing data streams. For a 1 x 1 or a 2 x 2 system, there is
a simple 1:1 relationship between layers and transmitting antenna ports.
However, for a 3 x 2 and 4 x 2 system, there are still only two spatial multiplexing channels.
Therefore, there is redundancy on one or both data streams. Layer mapping specifies exactly
how the extra transmitter antennas are employed. Precoding is also used in conjunction with
spatial multiplexing. Recall that MIMO exploits multipath to resolve independent spatial data
streams. In other words, MIMO systems require a certain degree of multipath for reliable
operation. In a noise-limited environment with low multipath distortion, MIMO systems can
actually become impaired.[8]
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Physical Downlink Shared Channel
The PDSCH is utilized basically for data and multimedia transport. It therefore is designed for
very high data rates. Modulation options therefore include QPSK, 16QAM and 64QAM. Spatial
multiplexing is also used in the PDSCH. In fact, spatial multiplexing is exclusive to the
PDSCH. It is not used on either the PDCCH or the CCPCH.
To guard against propagation channel errors, convolutional turbo coder is used for forward
error correction. The data is mapped to spatial layers according to the type of multi-antenna
technique.
Physical Downlink Control Channel
The PDCCH conveys UE-specific control information. Robustness rather than maximum data
rate is therefore the chief consideration. QPSK is the only available modulation format. The
PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first
slot of a subframe.
Common Control Physical Channel
The CCPCH carries cell-wide control information. Like the PDCCH, robustness rather than
maximum data rate is the chief consideration. QPSK is therefore the only available modulation
format. In addition, the CCPCH is transmitted as close to the center frequency as possible.
CCPCH is transmitted exclusively on the 72 active subcarriers centered on the DC subcarrier.
Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol
within the slot and l refers to the subcarrier. CCPCH symbols are mapped to resource elements
in increasing order of index k first, then l.
3.1.4 Physical Signals
Physical signals use assigned resource elements. However, unlike physical channels, physical
signals do not convey information to/from higher layers. There are two types of physical
signals:
Performance Analysis Of LTE Physical layer using System Vue
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• Reference signals used to determine the channel impulse response (CIR)
• Synchronization signals which convey network timing information
Reference Signals
Reference signals are generated as the product of an orthogonal sequence and a pseudo-random
numerical (PRN) sequence. Overall, there are 510 unique reference signals possible. A
specified reference signal is assigned to each cell within a network and acts as a cell-specific
identifier.
Figure 3.2 Resource Element Mapping of Reference Signals[13]
As shown in Figure 3.2, reference signals are transmitted on equally spaced subcarriers within
the first and third-from-last OFDM symbol of each slot. UE must get an accurate CIR from
each transmitting antenna. Therefore, when a reference signal is transmitted from one antenna
port, the other antenna ports in the cell are idle. Reference signals are sent on every sixth
subcarrier. CIR estimates for subcarriers that do not bear reference signals are computed via
interpolation
Performance Analysis Of LTE Physical layer using System Vue
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Changing the subcarriers that bear reference signals by pseudo-random frequency hopping is
also under consideration.
Synchronization Signals
Synchronization signals use the same type of pseudo-random orthogonal sequences as reference
signals. These are classified as primary and secondary synchronization signals, depending how
they are used by UE during the cell search procedure. Both primary and secondary
synchronization signals are transmitted on the 72 subcarriers centered around the DC subcarrier
during the 0th and 10th slots of a frame (recall there are 20 slots within each frame).
3.1.5 Transport Channels
Transport channels are included in the LTE PHY and act as service access points (SAPs) for
higher layers. Downlink Transport channels are:[6]
Broadcast Channel (BCH)
• Fixed format
• Must be broadcast over entire coverage area of cell
Downlink Shared Channel (DL-SCH)
• Supports Hybrid ARQ (HARQ)
• Supports dynamic link adaption by varying modulation, coding and transmit power
• Suitable for transmission over entire cell coverage area
• Suitable for use with beam forming
• Support for dynamic and semi-static resource allocation
• Support for discontinuous receive (DRX) for power save
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Paging Channel (PCH)
• Requirement for broadcast over entire cell coverage area
• Mapped to dynamically allocated physical resources
Multicast Channel (MCH)
• Requirement for broadcast over entire cell coverage area
• Support for MB-SFN
• Support for semi-static resource allocation
3.1.6 Mapping Downlink Physical Channels to Transport Channels
Transport channels are mapped to physical channels as shown in Figure 3.3. Supported
transport channels are:[6]
1. Broadcast channel (BCH)
2. Paging channel (PCH)
3. Downlink shared channel(DL-SCH)
4. Multicast channel (MCH)
Figure 3.3 Mapping DL Transport Channels to physical channels[13]
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Transport channels provide the following functions:
• Structure for passing data to/from higher layers Under Consideration
• Mechanism by which higher layers can configure the PHY
• Status indicators (packet error,DL Physical Channels CCPCH PDSCH PDCCHCQI
etc.) to higher layers.
• Support for higher-layer peer-to-peer signalling
3.1.7 Downlink Channel Coding
Different coding algorithms are employed for the DL physical channels. For the common
control channel (CCPCH), modulation is restricted to QPSK. The PDSCH uses up to 64 QAM
modulation. For control channels, coverage is the paramount requirement. Convolutional
coding has been selected for use with the CCPCH, though a final determination regarding code
rate has not yet been made. On the PDSCH, higher-complexity modulation is employed to
achieve the highest possible downlink data rates. The PDSCH uses QPSK, 16QAM, or 64QAM
depending on channel conditions. As a result, coding gain is emphasized over latency. Rate 1/3
turbo coding has been selected for the PDSCH.
The cyclic prefix is actually a copy of the last portion of the data symbol appended to the front
of the symbol during the guard interval. By adding a cyclic prefix, the channel can be made to
behave as if the transmitted waveforms were from time minus infinite, and thus ensure
orthogonality, which essentially prevents one subcarrier from interfering with another (called
intercarrier interference, or ICI). This is accomplished because the amount of time dispersion
from the channel is smaller than the duration of the cyclic prefix. After discovering the process
for OFDM, a cyclic prefix has been proposed for other modulations to improve the robustness
to multipath.
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CHAPTER 4 OFDM
4.1 OFDM for LTE
The choice of an appropriate modulation and multiple-access technique for mobile wireless data
communications is critical to achieving good system performance. In particular, typical mobile
radio channels tend to be dispersive and time-variant, and this has generated interest in
multicarrier modulation.
In general, multicarrier schemes subdivide the used channel bandwidth into a number of
parallel sub channels. Ideally the bandwidth of each sub channel is such that they are, ideally,
each non-frequency selective (i.e. having a spectrally flat gain); this has the advantage that the
receiver can easily compensate for the sub channel gains individually in the frequency domain.
Orthogonal Frequency Division Multiplexing (OFDM) is a special case of multicarrier
transmission where the non-frequency-selective narrowband sub channels, into which the
frequency-selective wideband channel is divided, are overlapping but orthogonal. This avoids
the need to separate the carriers by means of guard-bands, and therefore makes OFDM highly
spectrally efficient. The spacing between the sub channels in OFDM is such that they can be
perfectly separated at the receiver. This allows for a low complexity receiver implementation,
which makes OFDM attractive for high rate mobile data transmission such as the LTE
downlink.
It is worth noting that the advantage of separating the transmission into multiple narrowband
sub channels cannot itself translate into robustness against time-variant channels if no channel
coding is employed. The LTE downlink combines OFDM with channel coding and Hybrid
Automatic Repeat request (HARQ) to overcome the deep fading which may be encountered on
the individual sub channels. These aspects lead to the LTE downlink falling under the category
of system often referred to as ‗Coded OFDM‘ (COFDM). The primary advantage of OFDM
over single-carrier schemes is its ability to cope with severe channel conditions (for example,
attenuation of high frequencies in a long copper wire, narrowband interference and frequency-
selective fading due to multipath) without complex equalization filters
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Figure 4.1 .Effect of channel on signals with short and long symbol duration.[12]
4.2 OFDM architecture
Fig 4.2 Simplex Point-to-point transmission using OFDM[12]
Figure shows the block diagram of a simplex point-to-point transmission system using OFDM
and FEC coding. The three main principles incorporated are as follows:
1. The IDFT and the DFT are used for, respectively, modulating and demodulating the data
constellations on the orthogonal SCs . These signal-processing algorithms replace the
banks of I/Q-modulators and demodulators that would otherwise be required. Note that at the
input of the IDFT, N data constellation points {xi,k} are present, where N is the number of DFT
points. (i is an index on the SC; k is an index on the OFDM symbol). These constellations can
be taken according to any phase shift keying (PSK) or QAM signaling set (symbol mapping).
The N output samples of the IDFT, being in TD, form the baseband signal carrying the data
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symbols on a set of N orthogonal SCs. In a real system, however, not all of these N possible
SCs can be used for data Usually, N is taken as an integer to the power of two, enabling the
application of the highly efficient (inverse) FFT algorithms for modulation and demodulation.
2. The second key principle is the introduction of a cyclic prefix as a GI, whose length should
exceed the maximum excess delay of the multipath propagation channel . Due to the cyclic
prefix, the transmitted signal becomes periodic, and the effect of the time-dispersive multipath
channel becomes equivalent to a cyclic convolution, discarding the GI at the receiver. Due to
the properties of the cyclic convolution, the effect of the multipath channel is limited to a point
wise multiplication of the transmitted data constellations by the channel TF, or the FT of the
channel IR; that is, the SCs remain orthogonal This conclusion will also follow from the
derivation of the system model. The only drawback of this principle is a slight loss of effective
transmit power, as the redundant GI must be transmitted. Usually, the GI is selected to have a
length of one tenth to a quarter of the symbol period, leading to an SNR loss of 0.5 to 1 dB .
Fig 4.3 OFDM Cyclic Prefix (CP) insertion.[12]
Fig. 4.4 cyclic extension and windowing of OFDM[12]
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The equalization (symbol demapping) required for detecting the data constellations is an
element wise multiplication of the DFT output by the inverse of the estimated channel TF
(channel estimation). For phase modulation schemes, multiplication by the complex conjugate
of the channel estimate can do the equalization. Differential detection can be applied as well,
where the symbol constellations of adjacent SCs or subsequent OFDM symbols are compared
to recover the data.
3. FEC coding and (FD) interleaving are the third crucial idea applied. The frequency-selective
radio channel may severely attenuate the data symbols transmitted on one or several SCs,
leading to bit errors. Spreading the coded bits over the bandwidth of the transmitted system, an
efficient coding scheme can correct for the erroneous bits and thereby exploit the wideband
channel‘s frequency diversity. OFDM systems utilizing error-correction coding are often
referred as coded OFDM (COFDM) systems. The complex equivalent baseband signals
generated by digital signal processing are in-phase/quadrature (I/Q)–modulated and up-
converted to be transmitted via an RF carrier. The reverse steps are performed by the receiver.
Synchronization is a key issue in the design of a robust OFDM receiver. Time and frequency
synchronization are paramount, respectively, to identify the start of the OFDM symbol and to
align the modulators‘ and the demodulators‘ local oscillator frequencies. If any of these
synchronization tasks is not performed with sufficient accuracy, then the orthogonality of the
SCs is(partly) lost. That is, ISI and ICI are introduced.
4.3 FFT Implementation
Figure 4.5 OFDM Transmitter[12]
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As seen in OFDM block diagram the signal to be transmitted is defined in the frequency
domain.
A Serial to Parallel (S/P) converter collects serial data symbols into a data block Sk = [Sk [0] ,
Sk [1] , . . . , Sk [M 1]]T of dimension M, where the subscript k is the index of an OFDM
symbol (spanning the M sub-carriers).here M parallel data string can be differently modulated
or same modulation can be applied on it which results in complex vector Xk = [Xk [0] , Xk [1] ,
. . . , Xk [M 1]]T. Zero padding is done that makes processed carrier greater than modulated
subcarrier (N>M). To avoid ISI completely, the CP length G must be chosen to be longer than
the longest channel impulse response to be supported. The CP converts the linear (i.e.
aperiodic) convolution of the channel into a circular (i.e. periodic) one which is suitable for
DFT processing. The output of the IFFT is then Parallel-to-Serial (P/S) converted for
transmission through the frequency- selective channel. At the receiver, a highly efficient FFT
implementation may be used to transform the signal back to the frequency domain. i.e, the
reverse operations are performed to demodulate the OFDM signal. Assuming that time- and
frequency-synchronization is achieved, a number of samples corresponding to the length of the
CP are removed, such that only an ISI-free block of samples is passed to the DFT.
4.4 OFDMA BASICS
The additional tasks that the OFDMA receiver needs to cover are time and frequency
synchronization. Synchronization allows the correct frame and OFDMA symbol timing to be
obtained so that the correct part of the received signal is dropped (cyclic prefix removal). Time
synchronization is typically obtained by correlation with known data samples – based on, for
example, the reference symbols – and the actual received data. The frequency synchronization
estimates the frequency offset between the transmitter and the receiver and with a good estimate
of the frequency offset between the device and base station, the impact can be then
compensated both for receiver and transmitter parts. The device locks to the frequency obtained
from the base station, as the device oscillator is not as accurate (and expensive) as the one in the
base station.
Even if in theory the OFDMA transmission has rather good spectral properties, the real
transmitter will cause some spreading of the spectrum due to imperfections such as the clipping
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in the transmitter. Thus the actual OFDMA transmitter needs to have filtering similar to the
pulse shape filtering in WCDMA. In the literature this filtering is often referred as windowing.
An important aspect of the use of OFDMA in a base station transmitter is that users can be
allocated basically to any of the sub-carriers in the frequency domain. This is an additional
element to the HSDPA scheduler operation, where the allocations were only in the time domain
and code domain but always occupied the full bandwidth. The possibility of having different
sub-carriers to allocated users enables the scheduler to benefit from the diversity in the
frequency domain, this diversity being due to the momentary interference and fading
differences in different parts of the system bandwidth. The practical limitation is that the
signaling resolution due to the resulting overhead has meant that allocation is not done on an
individual sub-carrier basis but is based on resource blocks, each consisting of 12 sub-carriers,
thus resulting in the minimum bandwidth allocation being 180 kHz. When the respective
allocation resolution in the time domain is 1 ms, the downlink transmission resource allocation
thus means filling the resource pool with 180 kHz blocks at 1 ms resolution. Note that the
resource block in the specifications refers to the 0.5 ms slot, but the resource allocation is done
anyway with the 1 ms resolution in the time domain. This element of allocating resources
dynamically in the frequency domain is often referred to as frequency domain scheduling or
frequency domain diversity. Different sub-carriers could ideally have different modulations if
one could adapt the channel without restrictions. For practical reasons it would be far too
inefficient to try either to obtain feedback with 15 kHz sub-carrier resolution or to signal the
modulation applied on a individual sub-carrier basis. Thus parameters such as modulation are
fixed on the resource block basis.
The OFDMA transmission in the frequency domain thus consists of several parallel subcarriers,
which in the time domain correspond to multiple sinusoidal waves with different frequencies
filling the system bandwidth with steps of 15 kHz. This causes the signal envelope to vary
strongly compared to a normal QAM modulator, which is only sending one symbol at a time (in
the time domain). The momentary sum of sinusoids leads to the Gaussian distribution of
different peak amplitude values. This causes some challenges to the amplifier design as, in a
cellular system; one should aim for maximum power amplifier efficiency to achieve minimum
power consumption.
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There is a signal with a higher envelope variation (such as the OFDMA signal in the time
domain) requires the amplifier to use additional back-off compared to a regular single carrier
signal. The amplifier must stay in the linear area with the use of extra power back-off in order
to prevent problems to the output signal and spectrum mask. The use of additional back-off
leads to a reduced amplifier power efficiency or a smaller output power. This either causes the
uplink range to be shorter or, when the same average output power level is maintained, the
battery energy is consumed faster due to higher amplifier power consumption. The latter is not
considered a problem in fixed applications where the device has a large volume and is
connected to the mains, but for small mobile devices running on their own batteries it creates
more challenges.
An OFDMA system is also sensitive to frequency errors. The basic LTE sub-carrier spacing of
15 kHz facilitates enough tolerance for the effects of implementation errors and Doppler Effect
without too much degradation in the sub-carrier orthogonality.
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CHAPTER 5 OFDM IMPLEMENTATION
5.1 OFDM Block Diagram
It shows the Block Diagram of the OFDM System including transmitter, receiver and channel.
Fig 5.1: OFDM Block Diagram
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5.2 Components used in OFDM
1.RandomBits (Random Bit Generator)
The RandomBits block generates a random bit sequence in which the probability of a 0 bit is
ProbOfZero and the probability of a 1 bit is 1 – ProbOfZero. Parameters can be set in the
parameter box as shown in “Fig.5.2” where the RandomBits block is shown on the right top
corner.
Fig. 5.2 Parameter change box of a RandomBits block.
2.S INK ( DAT A S I NK)
Fig.5.3 Data sink block.
This block is an output data drop down box which provides three choices on how the
simulation data will be stored:
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1. DataSet: simulation data is stored in a dataset; data in the dataset can be plotted in
graphs/tables as well as post processed in equation pages.
2. File: simulation data is written into one (or more) file(s).
3. Both: simulation data is stored in a dataset as well as written into one (or more)
file(s).
3.Mapper (Complex Symbol Mapper)
Fig. 5.4 Parameter box of a Complex Symbol Mapper.
Mapper block supports different types of modulation: BPSK, QPSK, PSK8, PSK16, QAM16,
QAM32, QAM64, QAM128, QAM256 or can be user defined. Input to this block is Boolean
bit sequence and output is complex symbols. These parameters can be set in the parameter
box shown in the “Fig. 5.4”. Mapper groups consecutive bits as specified by the
BITORDER parameter in the input to form a symbol value which is mapped to a complex
valued constellation point that is output.
A constellation point is a pair of real values (I,Q) that is expressed on the output as I + jQ.
Later in the modulation chain, I modulates the inphase part of the carrier, and Q modulates
the quadrature part of the carrier over a symbol period. Each modulation type has its
constellation and symbol length. For QPSK, PSK8, and PSK16 the mapping from bits to
symbols is using Gray encoding. For QAM16, QAM32, QAM64, QAM128, and QAM256,
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Gray encoding is used inside each quadrant. When ModType is specified to
USER_DEFINED, a custom constellation is defined with Mapping Table. The input symbol
is mapped directly to a constellation point as a 0 based index into Mapping Table.
4.OFDM subcarrier multiplexing
Fig.5.5 Parameter block for OFDM Subcarrier Multiplexing block
Input to this block is multiple complex signals to be placed in subcarriers and there are two
outputs to this block: a) OFDM frequency domain format ready for applying IDFT and b)
EVM (Error Vector Magnitude) reference subcarriers. Both the outputs are in complex form.
The “Fig5. 5 ” shows a set of example values of the parameters to be set. This model
multiplexes different types of signals in frequency-domain to corresponding subcarrier
locations. The Output is ready to be processed by the Inverse Digital Fourier Transformation
(IDFT). Each branch of the input bus may be data of an OFDMA user or pilots.
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5.FFT_Cx (Complex Fast Fourier Transform)
Input and output to this block is complex signals. This model computes the DFT (Discrete
Fourier Transform) of the input signal using a mixed radix FFT (Fast Fourier Transform)
algorithm. At every execution of this model, Size complex samples are read from the input.
This block of Size samples is zero padded (if Size < FFTSize) to create a block of FFTSize
Samples. The block of FFTSize samples is then processed by a mixed radix FFT algorithm to
produce FFT_Size equally spaced samples that is the DFT of the input signal. The Direction
parameter specifies whether a forward or inverse FFT will be performed
Fig.5.6 Parameter box of a Complex Fast Fourier Transformation block
The FreqSequence parameter specifies the order in which the frequency values are written to
the output (forward FFT case) or read from the input (inverse FFT case). The “Fig. 5.6” shows
a set of examples values for the parameters and defaults values.
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6.OFDM_GuardInsert (OFDM guard interval insertion)
Fig.5.7 Parameter box of an OFDM guard interval insertion block.
This model inserts guard intervals to OFDM symbols. The inputs are consecutive OFDM
time-domain signals from IDFT (Inverse Digital Fourier Transformation) module. Both the
input and output are complex signals. Both prefix and postfix can be added to input signal.
The stuff signal may be cyclic shift (extension) of an IDFT period or zeros. Different guard
intervals may be added to different OFDM symbols. The “Fig. 5.7” shows an example
values of parameters along with their default values.
7.CxToEnv (Complex to Envelope)
Fig. 5.8 Parameter box of a Complex to Envelope converter block.
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The CxToEnv block converts the complex signal at input to a complex envelope signal at
output using the characterization frequency associated with the complex envelope signal at
input fc. Thus there can be two input signals, a complex signal and a complex envelope
signal. This block reads 1 sample from both inputs and writes 1 sample to output. Output to
this block is a complex envelope signal. The input fc is optional. CxToEnv is a modulator
whose output obtains it’s I and Q values from the input and its carrier frequency from fc. If
the fc input is not a complex envelope signal, then the output will be made a real signal and
the imaginary part of input will be ignored.
8.Add NDensity (Add Noise Density to input)
Fig.5.9 Parameter box of an Add Noise Density to Input Block.
Both the input and output to this block is envelope signals. This model adds noise to the input
signal. At every execution, it reads 1 sample from the input and writes 1 sample to the output.
If NDensityType is set to Constant noise density, then the noise added is white Gaussian.
The noise density is specified in the NDensity parameter. Although the units for this
parameter are power units the value is interpreted as power spectral density, that is, power
per frequency unit (Hz). The total noise power added to the input signal is NDensity × BW,
where NDensity is the noise power spectral density in Watts/Hz and BW is the simulation
bandwidth in Hz. If NDensityType is set to Noise density vs freq, then the spectral profile
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of the noise added can be specified using the NDensityFreq (values need to be in Hz) and
NDensityPower (values need to be in dBm/Hz) array parameters. Here used , Additive white
Gaussian noise is a channel model in which the only impairment to communication is
linear addition of wideband or white noise with a constant spectral density(expressed as
watts per hertz of bandwidth )and Gaussian distribution of amplitude.
9.EnvToCx (Envelope to Complex)
Fig. 5.10 An envelope to complex converter block.
Input to this block is an envelope and there are two outputs, a complex signal and its
characteristic frequency. EnvToCx decomposes input into a complex envelope and its
characteristic frequency. For every input sample, one sample is written to both outputs. If
input is a real baseband signal (v), then the output is real and set to the input value (v), and fc
is a complex envelope signal set to 0+j*0 with a zero characteristic frequency. If input is a
complex envelope signal (i+j*q with non-zero characterization frequency f1), then the output
is a complex envelope set to the input value (i+j*q with non-zero characterization frequency
f1), and fc is a complex envelope signal set to 0+j*0 with non-zero characteristic frequency
set to f1. The parameter box for this model is similar to that of a Complex to Envelope
converter as shown in “Fig. 5.10”.
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10.OFDM_GuardRemove (OFDM guard interval removal)
Fig. 5.11 Parameter box of an OFDM guard interval removal block.
Both the input and output signals to this block is complex signals. This model removes the
guard interval of the OFDM symbol. It outputs OFDM time-domain signals to DFT (Digital
Fourier Transformation) module directly. It can remove different guard intervals for different
OFDM symbols. It can work in two modes: CyclicShift and Zeros for cyclic guard interval
and zero padding guard interval removal. The mode can be specified in parameter
GuardStuff. In CyclicShift mode, it can remove cyclic guard interval. Parameter CIRAdjust
can be specified to get the DFT signals with cyclic delay, which can shift the CIR (channel
impulse response) in the time domain.In Zeros mode, it can remove zero padding guard
interval. Parameter CIRLength is the non-oversampled samples number covered by
maximum multipath delay. In zero padding mode, it needs to add the CIRLengthOS samples
that follow DFT window to the first CIRLengthOS samples in DFT window. So the input
begins with the CIRLengthOSth
sample and ends in the CIRLengthOSth
sample of
nextOFDM symbol.
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11.FFT_Cx (Complex Fast Fourier Transform)
Fig. 5.12 A Complex Fast Fourier Transform block.
This block is the same as the one used for IFFT purpose. Input and output to this block is
complex signals and parameter box is shown is “Fig. 6(f)”. In the block used previously, the
direction parameter was set as “inverse” to obtain IFFT of signals. But there the direction
parameter is set as “forward” to obtain the FFT of the signal. As mentioned earlier, this
model computes the DFT (Discrete Fourier Transform) of the input signal using a mixed
radix FFT (Fast Fourier Transform) algorithm. At every execution of this model, Size
complex samples are read from the input. This block of Size samples is zero padded (if Size <
FFTSize) to create a block of FFTSize samples. The block of FFTSize samples is
then processed by a mixed radix FFT algorithm to produce FFT_Size equally spaced
samples that is the DFT of the input signal. The Direction parameter specifies whether
a forward or inverse FFT will be performed. The FreqSequence parameter specifies the
order in which the frequency values are written to the output (forward FFT case) or read from
the input (inverse FFT case)
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12. OFDM_SubcarrierDemux (OFDM subcarrier demultiplexing)
Fig. 5.13 Parameter box of an OFDM subcarrier demultiplexing block.
The input signal is OFDM frequency domain signals from DFT and the output is the signals
demultiplexed from input subcarriers. This model de-multiplexes different type of signals
from OFDM symbols in frequency-domain. The parameters of this model is similar to that
of OFDM_SubcarrierMux and should be set accordingly.
13. D EMA P P E R ( COM P LEX S YMBOL DEMAPP E R )
As in the mapper block, this demapper block also supports different types of modulation
schemes like, BPSK, QPSK, PSK8, PSK16, QAM16, QAM32, QAM66, QAM128, and
QAM256 or can be user defined. And just opposite to the mapper block, the input to this block
is a complex symbol sequence and the output is boolean bit sequence. Demapper inputs a
complex value, finds the nearest constellation node for the input, and outputs both the
constellation node and the symbol value for the constellation node in a bit sequence specified
by the BitOrder parameter. Thus this block has two outputs
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Fig.5.14 Parameter box as a Complex Symbol Demapper/Slicer block.
For each input, one constellation node is output at Node and depending on the ModType
parameter Symbol Length number of bits is output at Bits. A constellation value is a pair of
real values (I,Q) that is expressed on the input as I + jQ. Earlier in the modulation chain, I
modulated the inphase part of the carrier, and Q modulated the quadrature part of the carrier
over a symbol period. The output symbols are assumed to be Gray coded. When ModType is
specified to USER_DEFINED, a custom constellation is defined with MappingTable. The
output symbol is mapped directly to a constellation point as a 0 based index into
MappingTable. The parameters are similar to that of a mapper block and have to be set
accordingly.
14. Delay.
Input and output to this block can be signals of anytype. This model introduces a delay of N
samples to the input signal. For every input, there is one output. The initial N output samples
have a null value. For scalar signals, a null value is 0. For matrix signals, a null value is a
matrix with the same size as the input matrix and with all its elements set to 0.
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Fig.5.15 Parameter box of a Delay block.
15. BER_FER (Bit and Frame Error Rate Measurement)
There are two inputs to this block, a reference bit stream and a test bit stream. Both the inputs
are of integer type. The BER_FER model can be used to measure the BER (bit error rate) and
FER (frame error rate) of a system. In some systems, FER is referred to as PER (packet error
rate) or BLER (block error rate). The input signals to the reference (REF) and test (TEST)
inputs must be bit streams. The bit streams must be synchronized, otherwise the BER/FER
estimates are wrong. The Start parameter defines when data processing starts. The end of
data processing depends on the settings of the Stop and EstRelVariance parameters: If
EstRelVariance is 0.0, then data processing ends when Stop is reached. If EstRelVariance is
greater than 0.0, then data processing ends when EstRelvariance is met or when Stop is
reached. In this case, Stop acts as an upper bound on how long the simulation runs just in
case the simulation takes too long for EstRelVariance to be met. In this mode of operation,
messages are printed in the simulation log showing the value of estimation relative variance
as the simulation progresses. The EstRelVariance parameter can be used to control the
quality of the BER estimate obtained. The lower the value of EstRelVariance the more
accurate the estimate is. The BitsPerFrame parameter sets the number of bits in each frame.
A frame is considered to be in error if at least one of the bits in the frame is detected
incorrectly.
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Fig. 5.16 Parameter box of a Bit and Frame Error Rate Measurement Block.
5.3 Measurement of signal at different points.
Fig 5.17 OFDM Block
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1. Input signal
The input signal as the random bits is provided by the Random bit generator.The figure shows
the amplitude vs. time curve for the input signal
Fig 5.18 .Output of Random bit Generator
2. Output of the mapper
The mapper maps the input bits to the corresponding symbols according the maaping scheme
specified.The figure shows the mapping scheme for the 16 QAM mapping.
Fig 5.19: Scatterplot diagram for 16 QAM
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3. Output of the subcarrier MUX
It shows the polar plot of the subcarrier
Fig 5.20: Polar plot after sub carrier allotment
4. Output of FFT
Fig 5.21: Output of FFT
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5. After Guard Band Insertion
Cyclic prefix is added to prevent the ISI in OFDM
Fig 5.22: OFDM Signals after Guardband Insertion
6. After Noise addition
Noise is added to simulate the noise present in the diffrenet wireless environments.
Fig 5.23: OFDM signal after noise addition
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7. Spectrum at Receiver Side
The output spectrum after demodulation and guard band removal is shown in the figure.
Fig 5.24: Spectrum at receiver side
8. Scatter plot after Demodulation
The Constellation plot at the receiver side contains the same input plot with little variation of
position due to noise or other variations.
Fig 5.25: Scatter plot after Demodulation.
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9. Output Signal
The output of the mapper is taken to obtain the output signal. The demapper converts the
corresponding symbols into continous bit stream.
Fig 5.26: Output Signal.
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CHAPTER 6
PERFORMANCE ANALYSIS FOR LTE
6.1 Quantitative factors in LTE
1. BER
In digital transmission, the number of bit errors is the number of received bits of a data
stream over a communication channel that have been altered due
to noise, interference, distortion or bit synchronization errors.The bit error rate or bit error
ratio (BER) is the number of bit errors divided by the total number of transferred bits during a
studied time interval. BER is a unitless performance measure, often expressed as a percentage.
The bit error probability is the expectation value of the BER. The BER can be considered as an
approximate estimate of the bit error probability. This estimate is accurate for a long time
interval and a high number of bit errors.The packet error rate (PER) is the number of incorrectly
received data packets divided by the total number of received packets. A packet is declared
incorrect if at least one bit is erroneous. The BER may be improved by choosing a strong signal
strength (unless this causes cross-talk and more bit errors), by choosing a slow and
robust modulation scheme or line coding scheme, and by applying channel coding schemes such
as redundant forward error correction codes.
2. BLER
Block Error Rate (BLER) is used in LTE/4G technology to know the in-sync or out-of-sync
indication during radio link monitoring (RLM). This is number of erroneous blocks / Total no of
Received Blocks. Normal in-sync condition is 2% of BLER and for out-of-sync is 10%.
3.THROUGHPUT
Throughput or network throughput is the average rate of successful message delivery over a
communication channel. This data may be delivered over a physical or logical link, or pass
through a certain network node. The throughput is usually measured in bits per second (bit/s or
bps), and sometimes in data packets per second or data packets per time slot.
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6.2 Implementation and Results For Different Schemes
1 .SISO BER
The block diagram for calculating the BER of the SISO Scheme is shown in the figure .The ber
is found out by comparing the received signal with the original input signal
Figure 6.1 Basic Block for calculating BER of LTE SISO Scheme
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Figure 6.2 BER vs SNR plot of LTE SISO Scheme
Figure 6.3 BLER vs SNR plot of LTE SISO Scheme
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2.MIMO BER
The diagram for finding the BER of the MIMO systems is shown in the figure.
Figure 6.4 Block Diagram for calculating MIMO BER with QPSK Modulation
Fig6.5 :BER vs SNR Graph of MIMO LTE System
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Figure 6.6 Block Diagram for MIMO BER plot with 16 QAM Modulation
Fig6.7 :BER vs SNR Graph of MIMO LTE System with 16 QAM Modulation
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3.SISO THROUGHPUT
Block diagram for implementation of the lte SISO systems is given in the figure.Constant noise
is added for simulation of the AWGN noise
Figure 6.8 Basic blocks of SISO Throughput Calculation
Figure 6.9 Throughput vs SNR plot of LTE SISO Scheme
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Figure 6.10 SISO Throughput fraction vs SNR
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4.MIMO THROUGHPUT
The block diagram for the implementation of the 2x2 MIMO scheme with HARQ erorr
correction is shown in the figure.
Figure 6.11 Block Diagram for plotting MIMO Throughput for QPSK Modulation
Figure 6.12 Throughput vs SNR plot MIMO scheme with QPSK Modulation
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Figure 6.13 MIMO Throughput Fraction vs SNR for QPSK Modulation
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Figure 6.14 Block Diagram for plotting MIMO Throughput for 16 QAM Modulation
Figure 6.15 MIMO Throughput vs SNR for 16QAM modulation
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Figure 6.16 MIMO Throughput Fraction vs SNR for 16QAM modulation
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6.3 Result and Analysis.
As compared from the MIMO and SISO Graph ,For SNR =13.3db.
Throughput for QPSK Modulation for SISO =0.6mbps
Throughput for QPSK Modulation for MIMO =9.5mbps
The throughput for the MIMO system increases due to the use of multiple antennas.
As seen from the figure in case of MIMO for SNR=15db
Throughput for QPSK Modulation =10.6 Mbps
Throughput for 16 QAM Modulation =15 Mbps
As the higher modulation schemes are used the throughput of the system increases .
From the graph, for SNR = 15db
BER of the SISO System for QPSK Modulation=nearly 1e-3
BER of the MIMO System for QPSK Modulation= 1e-6
As the number of antennas increases the quality of the channel increases due to diversity and
the BER Performance is improved.
For the MIMO Systems, For SNR= 25 db
BER of the MIMO System for QPSK Modulation= 1e-6
BER of the MIMO System for 16 QAM Modulation= 1e-3
As the higher modulation schemes are used The BER performance is degraded due to the less
redundancy in the higher modulation schemes.
So we recommend the use of Adaptive Modulation Schemes for balancing the need of the higher
throughput and the good signal quality and MIMO schemes for improving both throughput and
BER Performance.
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CHAPTER 7 CONCLUSION
The 3GPP Long Term Evolution is the latest evolution of the wireless communication systems.
LTE is part of the UMTS standards but includes many changes and improvements identifiedby
the 3GPP consortium. The goal of LTE is to increase the data throughput and the speed of
wireless data using a combination of new methods and technologies like OFDM and MIMO
technics. The LTE downlink transmission is based on Orthogonal Frequency Division Multiple
Access (OFDMA).
In this project work, an effective study, analysis and evaluation of the LTE downlink
performance with 2x2MIMO techniques in comparison with the traditional SISO system has
been carried out. The performance is evaluated with respect to two definitive metrics namely
Throughput and BER,. In our research we analyze that for a fix value of SNR, the BER increases
for high order modulation (16-QAM and 64-QAM) used in LTE system. On the other hand, the
lower order modulation scheme ( QPSK) experience less BER at receiver thus lower order
modulations improve the system performance in terms of BER and SNR. If we consider the
bandwidth efficiency of these modulation schemes, the higher order modulation accommodates
more data within a given bandwidth and is more bandwidth efficient as compare to lower order
modulation. Thus there exists a tradeoff between BER and bandwidth efficiency among these
modulation schemes used in LTE. We also observed the increase in throughput with the use of
multiple antenna i.e. MIMO.
To obtain high performance under all the changing conditions use of adaptive Modulation and
Coding is suggested and use of MIMO systems is suggested for either increasing the reliability
of the network or to increase the throughput.
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References
[1]. 3GPP TR 25.913 - v7.3.0, Requirements for EUTRA and EUTRAN,
http://www.3gpp.org/ftp/Specs/archive/25%5Fseries/25.913/
[2]. Van Nee and Prasad, OFDM for Wireless Multimedia Communications, Artech House
Publishers,ISBN 0-890006-530-6, 2000
[3]. T Doc #R1-060023, Cubic Metric in 3GPP-LTE, Motorola, Helsinki, January 2006
[4. 3GPP TS 36.300 – v8.0.0, E-UTRA and E-UTRAN Overall Description,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.300/
[5]. 3GPP TS 36.201 – v1.0.0, LTE Physical Layer – General Description,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.201/
[6.] 3GPP TS 36.211 – v1.0.0, Physical Channels and Modulation,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.211/
[7]. 3GPP TS 36.212 – Multiplexing and Channel Coding,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.212/
[8]. 3GPP TS 36.213 – v1.0.0, Physical Layer Procedures,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.213/
[9]. 3GPP TS 36.214 – v0.1.0, Physical Layer – Measurements,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.214/
[10]. 3GPP TS 36.300 v8.0.0, E-UTRA and E-UTRAN Overall Description; Stage 2,
http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.300/
[12]. Ericssion white paper entitled LTE –a 4G solution
http://www.ericsson.com/res/docs/whitepapers/wp-4g.pdf/
[13]. Free scale white paper entitled Overview of the 3GPP Long Term Evolution Physical layer
[14]. Seminar Ausgewählte Kapitel der Nachrichtentechnik, WS 2009/2010LTE:
Der Mobilfunk der ZukunftScheduling & HARQ.
http://www.lmk.lnt.de/fileadmin/Lehre/Seminar09/Ausarbeitungen/Ausarbeitung_Schrage.pdf/
[15]. System Vue 2011 Electronic Design Software From Agilent Techonolgies
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Acronym
16QAM Sixteen point quadrature amplitude modulation
3GPP Third Generation Partnership Project
64QAM Sixty Four point quadrature amplitude modulation
ACK Acknowledgement
AGC Automatic gain control
AP Access point
ARQ Automatic repeat request
BCH Broadcast channel
BPSK Binary phase shift keying
BW Bandwidth
CCPCH Common control physical channel
CDD Cyclic delay diversity
CDMA Code Division Multiple Access
CIR Channel impulse response
CP Cyclic prefix
CQI Channel quality indication
CSMA Carrier sense multiple access
DC Direct current
DFT Discrete Fourier transform
DL Downlink
DL-SCH Downlink-shared channel
DRX Discontinuous receive
eNodeB Enhanced Node B
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FDD Frequency division duplexing
FFT Fast Fourier transform
GMSK Gaussian minimum shift keying
GT Guard time
HARQ Hybrid automatic repeat request
HSDPA High Speed Downlink Packet Access
HSUPA High Speed Uplink Packet Access
ICI Inter carrier interface
IDFT Inverse discrete Fourier transform
IEEE Institute of Electrical and Electronics Engineers
IFFT Inverse fast Fourier transform
MBMS Multimedia broadcast multicast service
MB-SFN Multicast/broadcast single frequency network
MCH Multicast channel
MIMO Multiple Input Multiple Output
MRC Maximal ratio combining
NACK Not acknowledgement
OFDM Orthogonal Frequency Division Multiplexing
PAPR Peak-to-average power ratio
PCH Paging channel
PDCCH Physical downlink control channel
PDSCH Physical downlink shared channel
PHY Physical layer
PRACH Physical random access channel
PRB Physical resource block
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PRN Pseudo random numerical sequence
PSK Phase shift keying
PUCCH Physical uplink control channel
PUSCH Physical uplink shared channel
QAM Quadrature amplitude modulation
QPSK Quadrature phase shift keying
RACH Random access channel
RFE Radio front end
RFPA Radio frequency power amplifier
S/P Serial-to-parallel
SAP Service access point
SC-FDMA Single Carrier – Frequency Division Multiple Access
SNR Signal-to-noise ratio
STA Station
TDD Time Division Duplexing
UE User equipment
UL Uplink
UL-SCH Uplink – shared channel