02 Ra41202en10gla0 Lte Air Interface v03

8/20/2019 02 Ra41202en10gla0 Lte Air Interface v03

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LTE Air Interface

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1 ©Nokia Siemens Networks RA41202EN10GLA0

LTE RPESSLTE Air Interface



LTE Air Interface

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2 © Nokia Siemens Networks RA41202EN10GLA0

Nokia Siemens Networks Academy

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LTE Air Interface

3


Module Objectives

After completing this module, the participant should be able to:

• Understand the basics of the OFDM transmission technology• Explain how the OFDM technology avoids the Inter Symbol Interference

• Recognise the different between OFDM & OFDMA

• Identify the OFDM weaknesses

• Review the key OFDM parameters

• Analyze the reasons for SC-FDMA selection in UL

• Describe the LTE Air Interface Physical Layer

• Calculate the Physical Layer overhead

• Identify LTE Measurements

• List the frequency allocation alternatives for LTE• Review the main LTE RRM features

• Identify the main voice solutions for LTE



LTE Air Interface

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Module Contents

• OFDM Basics

• OFDM & Multipath Propagation: The Cyclic Prefix

• OFDM versus OFDMA• OFDM Weaknesses

• OFDM Key Parameters

• SC-FDMA

• LTE Air Interface Physical Layer

• Physical Layer Overhead

• LTE Measurements

• Frequency Variants• RRM Overview

• VoIP in LTE



LTE Air Interface

5


Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

6


The rectangular Pulse

Advantages:

+ Simple to implement: there is no complexfilter system required to detect such pulsesand to generate them.

+ The pulse has a clearly defined duration.This is a major advantage in case of multi-path propagation environments as it simplifieshandling of inter-symbol interference.

Disadvantage:

- it allocates a quite huge spectrum. However

the spectral power density has null pointsexactly at multiples of the frequency fs = 1/Ts.This will be important in OFDM.

time

a m p l i t u d e

Ts f s =

1

T s

Time Domain

frequency f/f s

s p e c t r a l p o w e r d e

n s i t y Frequency Domain

f s

Fourier

Transform

Inverse

Fourier

Transform



LTE Air Interface

7


TDMA

f

t

f

• Time Division

FDMA

f

f

t

• Frequency Division

CDMA

f

t c o d e

s

f

• Code Division

OFDMA

f

f

t

• Frequency Division

• Orthogonal subcarriers

Multiple Access Methods User 1 User 2 User 3 User ..

OFDM is the state-of-the-art and most efficient and robust air interface



LTE Air Interface

8


OFDM Basics

• Transmits hundreds or even thousands of separately modulated radiosignals using orthogonal subcarriers spread across a wideband channel

Orthogonality:

The peak ( centre

frequency) of one

subcarrier …

…intercepts the

‘nulls’ of the

neighbouring

subcarriers

15 kHz in LTE: fixed

Total transmission bandwidth



LTE Air Interface

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OFDM Basics

• Data is sent in parallel across the set of subcarriers, each subcarrier onlytransports a part of the whole transmission

• The throughput is the sum of the data rates of each individual (or used)subcarriers while the power is distributed to all used subcarriers

• FFT ( Fast Fourier Transform) is used to create the orthogonal subcarriers. Thenumber of subcarriers is determined by the FFT size ( by the bandwidth)

Power

frequency

bandwidth



LTE Air Interface

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OFDM versus coventional FDM

• OFDM allows a tight packing of small carrier - called the subcarriers - into a given

frequency band.

P o w e r D e n s i t y

P o w e r D e n s i t y

Frequency (f/fs) Frequency (f/fs)

Saved

Bandwidth

At the edges of this band there might be some guard bands required to protectsystems on adjacent bands from out-of-spectrum emissions by the OFDM system.



LTE Air Interface

11


Fast Fourier Transform (FFT)

time frequency

T

1/T

FFT

time frequency

T

0

FFT

• FFT is a method for calculating the Discrete Fourier Transform (DFT) and it is andfundamental element in OFDM

• IFFT = Inverse FFT.

• FFT/IFFT allows to move between time & frequency domain representations.

• FFT & IFFT are blocks included in an OFDMA system:

– FFT in the Receiver

– IFFT in the Transmitter



LTE Air Interface

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LTE is standardised in the 36-series of 3GPP

Release 8:

TS 36.1xx Equipment requirements (terminals, eNodeB)

TS 36.2xx Layer 1 (physical layer) specifications

TS 36.3xx Layer 2 and 3 specifications

TS 36.4xx Network signalling specifications

TS 36.5xx User equipment conformance testing

LTE is standardised in the 36-series of 3GPP

Release 8:

TS 36.1xx Equipment requirements (terminals, eNodeB)

TS 36.2xx Layer 1 (physical layer) specifications

TS 36.3xx Layer 2 and 3 specifications

TS 36.4xx Network signalling specifications

TS 36.5xx User equipment conformance testing

Physical layer specifications:

TS 36.201 Physical layer; General description

TS 36.211 Physical channels and modulation

TS 36.212 Multiplexing and channel codingTS 36.213 Physical layer procedures

TS 36.214 Physical layer; Measurements

Physical layer specifications:

TS 36.201 Physical layer; General description

TS 36.211 Physical channels and modulation

TS 36.212 Multiplexing and channel codingTS 36.213 Physical layer procedures

TS 36.214 Physical layer; MeasurementsFrequency

eNodeB

Subcarriers

OFDM

A

OFDM ASC-

FDMA

SC-

FDMA

LTE Air Interface Specifications

The LTE radio interface is standardised in the 36-series of 3GPP Release 8. The

detailed physical layer structure is described in 5 physical layer specifications.



LTE Air Interface

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Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

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Multi-Path Propagation & Inter-Symbol Interference (ISI)

BTSBTS

Time 0 Ts

+

d1(Direct path)

d3

d2

d1< d2 < d3

Time 0 Tt Ts+Tt

Tt

ISIInter Symbol Interference

In order to understand why it is necessary to use a cyclic prefix, let us consider atypical multipath propagation environment. In our example, there is the directpropagation path between the base station and mobile device, a second path with a

small delay, and a third path with a large delay. The replicas of the transmitted signalare received with different delays, causing the multipath delay spread of the radiochannel.



LTE Air Interface

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Multi-Path Propagation & the Guard Period

2

time

TSYMBOL

Time Domain

1

3

time

TSYMBOL

time

TSYMBOL

Tg

1

2

3

Guard Period (GP)

Guard Period (GP)

Guard Period (GP)

(Direct path)

•The cancellation of inter-symbol interference makes more complex the hardwaredesign of the receivers.

•One of the goals of future radio systems is to simplify receiver design and thus therectangular pulse is the first choice.

•Inter-symbol interference originating from the pulse form itself is simply avoided bystarting the next pulse only after the previous one finished completely, thereforeintroducing a Guard Period (Tg) after the Pulse.

•There is no inter-symbol interference between symbols as long as the multi-pathdelay spread (e.g. delay difference between first and last detectable path) is lessthan the guard period duration Tg.



LTE Air Interface

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Tg: Guard period duration

ISI: Inter-Symbol Interference

Propagation delay exceeding the Guard Period

1

2

34

time

TSYMBOLTime Domain

time

time

Tg

1

2

3

time

4

Delay spread > Tg

ISI

The Guard Period should be designed such that it is always longer than the multipath

delay spread, in order to avoid inter-symbol interference between successive OFDM

symbols.

Note that in the example of this slide, the Guard Period is too short, so there will be

inter-symbol interference!



LTE Air Interface

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The Cyclic Prefix OFDM symbol

OFDM symbol

OFDM symbol

OFDM symbol

Cyclic

prefix

Part of symbol

used for FFT

processing in the

receiver

• In all major implementations of the OFDMA

technology (LTE, WiMAX) the Guard Period

is equivalent to the Cyclic Prefix CP.

• This technique consists in copying the last

part of a symbol shape for a duration of

guard-time and attaching it in front of the

symbol (refer to picture sequence on the

right).

• CP needs to be longer than the channel

multipath delay spread (refer to previous

slide).

• A receiver typically uses the high correlation

between the CP and the last part of the

following symbol to locate the start of thesymbol and begin then with decoding.



LTE Air Interface

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The OFDM Signal

•The OFDM signal is made of multiple subcarriers.

•The distance between the center frequencies of the subcarriers is exactly theinverse of the Symbol period (Ts). Bigger Ts means subcarriers will allocated closerand more subcarriers could be allocated on a given spectrum bandwidth.

•An OFDM symbol is the combination of “n” subcarrier Symbol being produced inparallel at the same time.



LTE Air Interface

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Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

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OFDM

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Plain OFDM

time

s u b c a r r i e r

...

...

...

...

...

...

...

...

...

1 2 3 common info

(may be addressed via

Higher Layers)

UE 1 UE 2 UE 3

• OFDM stands for Orthogonal Frequency Division

Multicarrier • OFDM: Plain or Normal OFDM has no built-in

multiple-access mechanism.

• This is suitable for broadcast systems like DVB-T/H

which transmit only broadcast and multicast signals

and do not really need an uplink feedback channel

(although such systems exist too).

• Now we have to analyze how to handle access of

multiple users simultaneously to the system, each

one using OFDM.



LTE Air Interface

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OFDMA®

1

1

1

.

.

.

2

.

.

.

3

.

.

.

.

.

.

.

.

.

Orthogonal Frequency

Multiple Access

OFDMA®

time

...

...

...

...

...

...

...

...

...

1

1

1 1

2

22

2 2

3 33 3 3

1

s u b c a r r i e r

1

1 1 1

111

3 3 3

33 3 3 3

3

Resource Block (RB)

1 2 3 common info

(may be addressed via

Higher Layers)

UE 1 UE 2 UE 3

OFDMA® stands for Orthogonal Frequency Division

Multiple Access

• registered trademark by Runcom Ltd.

• The basic idea is to assign subcarriers to users based on their

bit rate services. With this approach it is quite easy to handle

high and low bit rate users simultaneously in a single system.

• But still it is difficult to run highly variable traffic efficiently.

• The solution to this problem is to assign to a single users so

called resource blocks or scheduling blocks.

• such block is simply a set of some subcarriers over some

time.

• A single user can then use 1 or more Resource Blocks.

•OFDMA was introduced with 802.16 (WiMAX) WirelessMAN-OFDMAfor the downlink.

•802.16d uses such a mechanism with variable block sizes. The firstOFDM symbols in each frame are used to indicate which user getswhich blocks with which size.

•EUTRAN will use a similar system, but with fixed block sizes and theassignment mechanism is not specified yet (2007-08).



LTE Air Interface

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Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

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Inter-Carrier Interference (ICI) in OFDM

• The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.

• If the receiver’s frequency slips some fractions from the subcarriers center frequencies,

then we encounter not only interference between adjacent carriers, but in principle betweenall carriers.

• This is known as Inter-Carrier Interference (ICI) and sometimes also referred to as

Leakage Effect in the theory of discrete Fourier transform.

• One possible cause that introduces frequency errors is a fast moving Transmitter or

Receiver (Doppler effect).



LTE Air Interface

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f 0 f 1 f 2 f 3 f 4

P

I3

I1I4I0

I C I = I n t e r - C a r r i e r I n t e r f e r e n c e

Leakage effect due to Frequency Drift: ICI

Two effects begin to work:1. -Subcarrier 2 has no longer its

power density maximum here -

so we loose some signal

energy.

2.-The rest of subcarriers (0, 1, 3

and 4) have no longer a null

point here. So we get some

noise from the other subcarrier.

•If we have a little frequency drift between transmitter and receiver, then wedecode the symbol of subcarrier 2, for instance, a little bit offset from its truecenter frequency.

•The result is a lower signal to noise ratio by a decreased signal level and anincreased noise level. This is the Inter-Carrier Interference effect forOFDM.

•To limit the influence of the ICI on OFDM systems completely by hardwarewe would have to have receivers and transmitters with under 0.1 ppmfrequency stability. This would drastically increase the cost and complexity ofhardware.

•Thus quite a big part of the OFDM software in the receiver deals withfrequency correction using the cyclic prefix, but also reference or pilot signalssent with the signal.



LTE Air Interface

25


Doppler in OFDM and Loss of Orthogonality

• Doppler effect (shift): Change in frequency of a wave due to the relative motion ofsource and receiver.

• Symbols are distorted in the time domain

▪ Frequency shifts make symbol detection inaccurate

▪ MCS schemes with high number of bits per subcarrier are not suitable for MSsmoving at high speed

▪ More difficult to support high data rates

▪ Doppler only impacts SINRs at the higher range i.e. > 20dB

It reduces orthogonality

• The frequency domainsubcarriers are shifted causinginter-carrier interference (ICI)

• Frequency shift in thesubcarriers limits the SNR values

• The nulls of interferers andpeaks of signals will not coincide

ICI in the absence of orthogonality



LTE Air Interface

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Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

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Subcarrier types

Data subcarriers: used for data transmission

– Reference Signals:

▪ used for channel quality and signal strength estimates.

▪ They don’t occupy a whole subcarrier but they are periodically embedded in thestream of data being carried on a data subcarrier.

Null subcarriers (no transmission/power):

▪ DC (centre) subcarrier : 0 Hz offset from the channel’s centre frequency

▪ Guard subcarriers: Separate top and bottom subcarriers from any adjacentchannel interference and also limit the amount of interference caused by thechannel. Guard band size has an impact on the data throughput of the channel.

Guard (no power)

DC (no

power)

data

Guard (no power)



LTE Air Interface

28


OFDMA Parameters in LTE

• Channel bandwidth: DL bandwidths ranging from 1.4 MHz to 20 MHz

• Data subcarriers: the number of data subcarriers varies with thebandwidth

– 72 for 1.4 MHz to 1200 for 20 MHz



LTE Air Interface

29


OFDMA Parameters in LTE

• Frame duration: 10ms created from slots and subframes.

• Subframe duration (TTI): 1 ms ( composed of 2 x 0.5 slots)

• Subcarrier spacing: Fixed to 15kHz ( 7.5 kHz defined for MBMS)• Sampling Rate: Varies with the bandwidth but always factor or

multiple of 3.84 to ensure compatibility withWCDMA by using common clocking

Frame Duration

Subcarrier Spacing

Sampling Rate ( MHz)

Data Subcarriers

Symbols/slot

CP length

1.4MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

10 ms

15 kHz

Normal CP=7, extended CP=6

Normal CP=4.69/5.12 μs, extended CP= 16.67μs.

1.92 3.84 7.68 15.36 23.04 30.72

72 180 300 600 900 1200

10ms

Fixed 15kHz: reduces the complexity of a system supporting multiple channel bandwidths

MBMS: Multimedia Broadcast Multicast system

To ensure that all signals are received correctly, the receiver sampling rate must beslightly higher than the bandwidth of the signal used to carry it (i.e. for a channelbandwidth of 1.75MHz the sampling rate should be 2 MHz)



LTE Air Interface

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Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

31


Peak-to-Average Power Ratio in OFDMA

The transmitted power is the sum of the

powers of all the subcarriers

• Due to large number of subcarriers, the

peak to average power ratio (PAPR)

tends to have a large range

• The higher the peaks, the greater the

range of power levels over which the

transmitter is required to work.

• Not best suited for use with mobile

(battery-powered) devices



LTE Air Interface

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SC-FDMA in UL

• Single Carrier Frequency Division Multiple Access: Transmission technique used for Uplink

• Variant of OFDM that reduces the PAPR:

– Combines the PAR of single-carrier system with themultipath resistance and flexible subcarrierfrequency allocation offered by OFDM.

– It can reduce the PAPR between 6…9dB comparedto OFDMA

– TS36.201 and TS36.211 provide the mathematicaldescription of the time domain representation of anSC-FDMA symbol.

• Reduced PAPR means lower RF hardware

requirements (power amplifier)

S C -F DMA

OF DMA



LTE Air Interface

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SC-FDMA and OFDMA Comparison (1/2)

• OFDMA transmits data in parallel across multiple subcarriers

• SC-FDMA transmits data in series employing multiple subcarriers

• In the example:

– OFDMA: 6 modulation symbols ( 01,10,11,01,10 & 10) are transmitted per

OFDMA symbol, one on each subcarrier

– SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA symbol using

all subcarriers per modulation symbol. The duration of each modulation

symbol is 1/6th of the modulation symbol in OFDMA

OFDMA SC-FDMA

SC-FDMA: If data rate increases-> more bandwidth is needed to transmit moresymbols.

•When data rate changes, more symbols per slot are transmitted. As the bandwidthincreases the symbol duration decreases.

•For double data rate the amount of FFT inputs in transmitter doubles (as well astotal BW) and symbol duration is halved



LTE Air Interface

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SC-FDMA and OFDMA Comparison (2/2)

Visually, the OFDMA signal is clearly multi-carrier and the SC-FDMA signal looksmore like single-carrier, which explains the “SC” in its name. Note that OFDMA andSC-FDMA symbol lengths are the same at 66.7 μs; however, the SC-FDMA symbol

contains N “sub-symbols” that represent the modulating data.It is the parallel transmission of multiple symbols that creates the undesirable highPAR of OFDMA. By transmitting the N data symbols in series at N times the rate, theSC-FDMA occupied bandwidth is the same as multi-carrier OFDMA but —crucially— the PAR is the same as that used for the original data symbols.



LTE Air Interface

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Uplink Air Interface TechnologySC-FDMA

• User multiplexing in frequency domain (in OFDMA the user multiplexing is insub-carrier domain)

• One user always continuous in frequency

• Smallest UL bandwidth, 12 subcarriers: 180 kHz (same for OFDMA in DL)

• Largest UL bandwidth: 20 MHz (same for OFDMA in DL)

– Terminals are required to be able to receive & transmit up to 20 MHz, depending onthe frequency band though

User 2 f

User 1 f

f

Receiver



LTE Air Interface

36


Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

37


LTE Physical Layer - Introduction

FDD

..

..

..

..

Downlink Uplink

Frequency band 1

Frequency band 2

.. ..Single frequency

band

TDD

• It provides the basic bit transmission functionality over air

• LTE physical layer based on OFDMA DL & SC-FDMA in UL

– This is the same for both FDD & TDD mode of operation

• There is no macro-diversity in use

• System is reuse 1, single frequency network operation is feasible

– no frequency planning required

• There are no dedicated physical channels anymore, as all resourcemapping is dynamically driven by the scheduler



LTE Air Interface

38


LTE Physical Layer Structure – Frame Structure (FDD)

10 ms frame

0.5 ms slot

s0 s1 s2 s3 s4 s5 s6 s7s18 s19…..

1 ms sub-frame

SF0 SF1 SF2 SF9…..

sy4sy0 sy1 sy2 sy3 sy5 sy6

0.5 ms slot

SF3

SF: SubFrame

s: slot

Sy: symbol

• FDD Frame structure ( also called Type 1 Frame) is common to both UL & DL

• Divided into 20 x 0.5ms slots – Structure has been designed to facilitate short round trip time

- Frame length = 10 ms

- FDD: 10 sub-frames of 1 ms for UL & DL

- 1 Frame = 20 slots of 0.5ms each

- 1 slot = 7 (normal CP) or 6 OFDM

symbols (extended CP)

In FDD, there is a time offset between uplink and downlink transmission.



LTE Air Interface

39


LTE Physical Layer Structure – Frame Structure (TDD)

SF

#0

SF

#0

. . .

f

time

UL/DL

carrier

radio frame 10 ms

subframe

D w P T S

D w P T S

G P G P

U p P T S

U p P T S

SF

#2

SF

#2SF

#4

SF

#4SF

#0

SF

#0

. . .

D w P T S

D w P T S

G P G P

U p P T S

U p P T S

SF

#2

SF

#2SF

#4

SF

#4

subframe

half frame

DwPTS: Downlink Pilot time Slot

UpPSS: Uplink Pilot Time Slot

GP: Guard Period to separate between UL/DL

Downlink Subframe

Uplink Subframe

Frame Type 2 (TS 36.211-900; 4.2)

• each radio frame consists of 2 half frames

• Half-frame = 5 ms = 5 Sub-frames of 1 ms• UL-DL configurations with both 5 ms & 10 ms DL-to-UL switch-point periodicity are supported

• Special subframe with the 3 fields DwPTS, GP & UpPTS; length of DwPTS + UpPTS +GP = 1

subframe

• DL / UL ratio can vary from 1/3 to 8/1 according to service requirements of the carrier

Synchronous Code Division Multiple Access (or SCDMA): low chip rate mode ofWCDMA



LTE Air Interface

40


Subframe structure & CP length

Short cyclic prefix:

Long cyclic prefix:

Copy= Cyclic prefix

= Data

5.21 s

16.67 s

• Subframe length: 1 ms for all bandwidths

• Slot length is 0.5 ms

– 1 Subframe= 2 slots

• Slot carries 7 symbols with normal CP or 6 symbols with long CP

– CP length depends on the symbol position within the slot:

▪ Normal CP: symbol 0 in each slot has CP = 160 x Ts = 5.21μs;remaining symbols CP= 144 x Ts = 4.7μs

▪ Extended CP: CP length for all symbols in the slot is 512 x Ts = 16.67µs

Ts:

‘sampling time’ of the overall channel

basic Time Unit = 32.5 nsec

Ts =1 sec

Subcarrier spacing X max FFT size

Subcarrier spacing= 15kHz; max. FFT size= 2048



LTE Air Interface

41


Resource Block and Resource Element

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

Subcarrier 1

Subcarrier 12

1 8 0 K H z

1 slot 1 slot

1 ms subframe

R B

ResourceElement

• Physical Resource Block PBR or Resource Block RB:

– 12 subcarriers in frequency domain x 1 slot period in time domain

– Capacity allocation based on Resource Blocks

Resource Element RE:

– 1 subcarrier x 1 symbol period

– theoretical min. capacity allocation unit

– 1 RE is the equivalent of 1 modulationsymbol on a subcarrier, i.e. 2 bits(QPSK), 4 bits (16QAM), 6 bits (64QAM).



LTE Air Interface

42


Physical Resource Blocks

....

12 subcarriers

Time

Frequency

0.5 ms slot

1 ms subframe

or TTI

Resource

block

During each TTI,

resource blocks for

different UEs are

scheduled in the

eNodeB

During each TTI,

resource blocks for

different UEs are

scheduled in the

eNodeB

• In both the DL & UL direction, datais allocated to users in terms of

resource blocks (RBs).

• a RB consists of 12 consecutive

subcarriers in the frequency domain,

reserved for the duration of 0.5 ms

slot.

• The smallest resource unit a

scheduler can assign to a user is a

scheduling block which consists of

two consecutive resource blocks

•Depending on the required data rate and the scheduling decision done in theeNodeB, each UE may or may not be assigned resource blocks during eachtransmission time interval of 1 ms.

• In downlink, the resource blocks may be located adjacently in the frequencydomain, or in a distributed fashion for added frequency diversity.•In downlink, resource blocks can carry several types of channels and must alsocarry certain reference and synchronisation signals.



LTE Air Interface

43


LTE Channel Options

Bandwidth options: 1.4, 1.6, 3, 3.2, 5, 10, 15 and 20 MHzBandwidth options: 1.4, 1.6, 3, 3.2, 5, 10, 15 and 20 MHz

Subcarriers in frequency domain (15 kHz or 7.5 kHz subcarrier spacing)

Channel bandwidth

(MHz)

Number of

subcarriers

Number of resourceblocks

1.4

72

6

3

180

15

5

300

25

10

600

50

15

900

75

20

1200

100

Each channel bandwidth offers a certain number of subcarriers, or - expressed inanother way - a certain number of resource blocks. From the table you can easilysee that a resource block always occupies 12 subcarriers.



LTE Air Interface

44


DL Physical Resource Block

....

12 subcarriers

Time

0.5 ms slot

1 ms subframe

or TTI

DL reference

signal

• Reference signals position in time

domain is fixed (symbol 0 & 4 / slot for

Type 1 Frame) whereas in frequencydomain it depends on the Cell ID

• Reference signals are modulated to

identify the cell to which they belong.

• This signal, consisting of a known

pseudorandom sequence, is required for

channel estimation in the UEs. (like

CPICH in WCDMA).

• Note that in the case of MIMO

transmission, additional reference

signals must be embedded into the

resource blocks.



LTE Air Interface

45


DL Physical Channels

• PDSCH: Physical Downlink Shared Channel – carries user data, L3 Signalling, System Information Blocks & Paging

• PBCH: Physical Broadcast Channel – for Master Information Block only

• PMCH: Physical Multicast Channel – for multicast traffic as MBMS services

• PCFICH: Physical Control Format Indicator Channel – indicates number of OFDM symbols for Control Channels = 1..4

• PDCCH: Physical Downlink Control Channel – carries resource assignment messages for DL capacity allocations & scheduling

grants for UL allocations

• PHICH: Physical Hybrid ARQ Indicator Channel – carries ARQ Ack/Nack messages from eNB to UE in respond to UL transmission

There are no dedicated channels in LTE, neither UL nor DL.

PCFICH: carriers the number of symbols for the PDCCH

Transport channels:

BCH – broadcast channel, fixed transport formatDL-SCH – downlink shared channel, used for transmission of downlink data in LTE,supports DRX

MCH – multicast channel (support MBMS, semi-static transport format andscheduling, can be coordinated for multi-cell transmission)

PCH – paging channel, supports DRX to save battery power

See 3GPP TS 32.211 V8.1.0

PBCCH: transmitted once every 10 ms. Frame

PDSCH, PCFICH and PHICH share capacity in the first three symbol periods

PDSCH and PMCH are mapped into the available spece in the resource blocks



LTE Air Interface

46


UL Physical Channels

• PUSCH: Physical Uplink Shared Channel – Transmission of user data, L3 & L1 signalling (L1 signalling: CQI, ACK/NACKs, etc.)

• PUCCH: Physical Uplink Control Channel – Carries L1 control information in case that no user data are scheduled in this subframe

(e.g. H-ARQ ACK/NACK indications, UL scheduling request, CQIs & MIMO feedback).

– These control data are multiplexed together with user data on PUSCH, if user data are

scheduled in the subframe

• PRACH: Physical Random Access Channel – For Random Access attempts; SIBs indicates the PRACH configuration (duration;

frequency; repetition; number of preambles - max. 64)



LTE Air Interface

47


UL Physical Resource Block: DRS & SRS

....

12 subcarriers

Time

0.5 ms slot

1 ms subframe

or TTI

Frequency

Sounding Reference

Signal on last OFDM

symbol of 1 subframe;

Periodic or aperiodic

transmission

Sounding Reference

Signal on last OFDM

symbol of 1 subframe;

Periodic or aperiodic

transmission

Demodulation

Reference Signal in

subframes that carry

PUSCH

Demodulation

Reference Signal in

subframes that carry

PUSCH

Note: when the

subframe contains

the PUCCH, the

Demodulation

Reference Signal is

embedded in a

different way

Note: when the

subframe contains

the PUCCH, the

Demodulation

Reference Signal is

embedded in a

different way

• The Demodulation Reference

Signal is transmitted in the third

SC-FDMA symbol (counting

from zero) in all resource blocksallocated to the PUSCH

carrying the user data.

• This signal is needed for

channel estimation, which in

turn is essential for coherent

demodulation of the UL signal

in the eNodeB.

• The Sounding Reference

Signal SRS provides UL

channel quality information as a

basis for scheduling decisions

in the base station. This signal

is distributed in the last SC-

FDMA symbol of subframes

that carry neither PUSCH nor

PUCCH data.

PUCCH: Physical UL Control Channel



LTE Air Interface

48


b0 b1

QPSK

Im

Re10

11

00

01

b0 b1b2b3

16QAM

Im

Re

0000

1111

Im

Re

64QAM

b0 b1b2b3 b4 b5

• 3GPP standard defines the following options: QPSK,

16QAM, 64QAM in both directions (UL & DL) – UL 64QAM not supported in RL10

• Not every physical channel is allowed to use any

modulation scheme:

• Scheduler decides which form to use depending on carrier

quality feedback information from the UE

Modulation Schemes

QPSK:

2 bits/symbol

16QAM:

4 bits/symbol

64QAM:

6 bits/symbol

QPSKPDCCH,

PCFICH

Physicalchannel

Modulation

PDSCH QPSK,

16QAM,

64QAM

PMCH QPSK,

16QAM,

64QAM

PBCH QPSK

PHICH BPSK

PUSCH QPSK,

16QAM,

64QAM

PUCCH BPSK

and/or

QPSK



LTE Air Interface

49


Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

50


DL Reference Signal Overhead

Reference Signal (RS)

- If 1 Tx antenna*: 4 RSs per PRB

- If 2 Tx antenna*: there are 8 RSs per PRB

- If 4 Tx antenna*: there are 12 RSs per PRB

Example below: Normal CP (84 RE) & 2 Tx antenna*, DL RS overhead = 8 / 84 = 9.52 %

* with 1/2/4 Antenna PortsPRB: Physical Resource Block



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51


Synchronization Signals Overhead

Primary Synchronization Signal (PSS)- occupies 144 Resource Elements per frame (20 timeslots); i.e. (62 subcarriers + 10

empty Resource Elements) x 2 times/frame

Example: Normal CP, 10 MHz bandwidth; PSS overhead = 144 / (84 × 20 × 50) = 0.17 %

Secondary Synchronization Signal (SSS) – Identical calculation to PSS; same overhead as for PSS

2 3 4 5 7 8 9 10

1 2 3 4 5 6 7

1 2 3 4 5 6

10ms Radio frame

1ms Subframe SSS

PSS0.5ms = 1 slot

Normal CP

Extended CP

PSS & SSS frame + slot

structure in time domain

(FDD case)

checking for SSS

at 2 possible positions

CP length

checking for SSS

at 2 possible positions

CP length



LTE Air Interface

52


The combination of PDCCH, PCFICH & PHICH occupies the first 1, 2 or 3 symbols per TTI*

Resource Elements

reserved for

Reference Symbols

(2 antenna port case)

Control Channel

Region (1-3 OFDM symbols*)

1 2 s u b c a r r i e r s

F r e q u e n c y

TimeData Region

One subframe (1ms)

PDCCH, PCFICH & PHICH overhead (1/2)

* up to 4 OFDM symbols in case of 1.4 MHz bandwidth

12 x7x2 = 12 x7 reflects the number of RE per RB and x2 reflects there are 2 RBsper TTI



LTE Air Interface

53


PDCCH, PCFICH & PHICH overhead (2/2)

The number of RE occupied per 1 ms TTI is given by (12 × y – x), where:

• y depends upon the number of OFDM symbols per TTI (1, 2 or 3*) occupied byControl Channels

• x depends upon the number of RE already occupied by the Reference Signal

• x = 2 for 1 Tx antenna (Antenna Port)

• x = 4 for 2 Tx antennas (Antenna Ports)

• x = 4 for 4 Tx antennas (Antenna Ports) when y = 1

• x = 8 for 4 Tx antennas (Antenna Ports) when y = 2 or 3

Example: in the case of normal CP, 2 Antenna Ports & 3 OFDM symbols occupied by Control

Channels:

Control Channel Overhead = (12 × 3 - 4) / (12 × 7 × 2) = 19.05%

* up to 4 OFDM symbols in case of 1.4 MHz bandwidth



LTE Air Interface

54


PBCH Overhead

Occupies (288* – x) Resource Elements (REs) per 20 timeslots per transmit antenna

The value of x depends upon the number of REs already occupied by the Reference Signal:

x = 12 for 1 Tx antenna, x = 24 for 2 Tx antennas & x = 48 for 4 Tx antenna

- Example: normal CP, 2 Tx antennas, 10 MHz bandwidth;

PBCH Overhead = (288 – 24) / (84 × 20 × 50) = 0.31%


Repetition Pattern of PBCH = 40 ms

one radio frame = 10 ms

PBCH

* PBCH uses central 72 Subcarrier over 4 OFDM symbols in Slot 1



LTE Air Interface

55


UL Demodulation Reference Signal Overhead (1/2)

Demodulation ReferenceSignal (DRS)

• The DRS is sent on the 4th

OFDM symbol of each RBoccupied by the PUSCH.

PUCCH

PUCCH

PUSCH

Slot= the whole band

Reference signal: 12 RE (per RB) x (50-2) RBs not dedicated to PUCCH /(84 x50)=13.14%



LTE Air Interface

56


Example:For 1.4 MHz Channel Bandwidth, the PUCCH occupies 1 RB per Slot.

The number of RE per RB is 84 when using the normal CP.

This means the DRS overhead* is: ((6-1) × 12)/(6 × 84) = 11.9 %

Channel BW PUCCH RB/slot DRS Overhead*

1.4 MHz 1 ((6-1) × 12) / (6 × 84) = 11.9 %

3 MHz 2 ((15-2) × 12) / (15 × 84) = 12.38 %

5 MHz 2 ((25-2) × 12) / (25 × 84) = 13.14 %

10 MHz 4 ((50-4) × 12) / (50 × 84) = 13.14 %

15 MHz 6 ((75-6) × 12) / (75 × 84) = 13.14 %

20 MHz 8 ((100-8) × 12) / (100 × 84) = 13.14 %

UL DRS Overhead (2/2)

* for normal CP



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57


PRACH Overhead

PRACH

• PRACH uses 6 Resource Blocks in the frequency domain.

• The location of those resource blocks is dynamic. Two parameters from RRC layer define it:

– PRACH Configuration Index: for Timing, selecting between 1 of 4 PRACH durationsand defining if PRACH preambles can be send in any radio frame or only in evennumbered ones

– PRACH Frequency offset: Defines the location in frequency domain

• PRACH Overhead calculation: 6 RBs * RACH Density / (#RB per TTI) x 10 TTIs per frame

– RACH density: how often are RACH resources reserved per 10 ms frame i.e. for RACHdensity: 1 (RACH resource reserved once per frame)

Channel BW PRACH Overhead

1.4 MHz (6 × 1) / (6 × 10) = 10 %

3 MHz (6 × 1) / (15 × 10) = 4 %

5 MHz (6 × 1) / (25 × 10) = 2.40 %10 MHz (6 × 1) / (50 × 10) = 1.20 %

15 MHz (6 × 1) / (75 × 10) = 0.8 %

20 MHz (6 × 1) / (100 × 10) = 0.6 %



LTE Air Interface

58


PUCCH Overhead

PUCCH

• Ratio between the number of RBs used for PUCCH and the total number of RBs in frequency

domain per TTIChannel BW PUCCH RB/slot PUCCH Overhead

1.4 MHz 1 1 / 6 = 16.67 %

3 MHz 2 2 / 15 = 13.33 %

5 MHz 2 2 / 25 = 8 %

10 MHz 4 4 / 50 = 8 %

15 MHz 6 6 / 75 = 8 %

20 MHz 8 8 / 100 = 8%

Time

T o t a l U L

B a n d w i t h

PUCCH

PUCCH

PUSCH

1 subframe = 1ms

F r e q u e

n c y


PUSH UCI: Aperiodic CQI reports and ACK/NACK



LTE Air Interface

59


Physical Layer Overhead Example

Example of overhead:

• DL 2Tx – 2RX

• UL 1TX - 2RX

• PRACH in every frame

• 3 OFDM symbols for PDCCH



LTE Air Interface

60


Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

61


LTE Measurements

Physical layer measurements have not been extensively discussed in the LTE

standardization. They could change.

Intra LTE measurements ( from LTE to LTE)• UE measurements

– CQI measurements

– Reference Signal Received Power (RSRP)

– Reference Signal Received Quality ( RSRQ)

• eNB measurements – Non standardized (vendor specific): TA, Average RSSI, Average SINR, UL CSI,

detected PRACH preambles, transport channel BLER

– Standardized: DL RS Tx Power, Received Interference Power, Thermal Noise Power

Measurements from LTE to other systems

• UE measurements are mainly intended for Handover. – UTRA FDD: CPICH RSCP, CPICH Ec/No and carrier RSSI

– GSM: GSM carrier RSSI – UTRA TDD: carrier RSSI, RSCP, P-CCPCH

– CDMA2000: 1xRTT Pilot Strength, HRPD Pilot Strength

CSI: Channel State Information (received power per PRB)

TA: Timing Advance

List of detected preambles: The eNB shall report a list of detected PRACHpreambles to higher layers. Higher layer utilize this info for the RACH procedure

Transport BLER: The ACK/ NACKs for each transmission of the HARQ process arereported to the MAC. Based on these ACK/NACKs the higher layers compute theBLER for RRM issues.

TA: The eNB needs to measure the initial timing advance (TA) of the uplink channelsbased on the RACH preamble

Average RSSI: Measured in UL by eNB. It can be used as a level indicator for the ULpower control. The RSSI measurements are all UE related and shall be separatelyperformed for ( TTI intervals)

· UL data allocation (PUSCH)

· UL control channel (PUCCH)

• Sounding reference signal (SRS)

Average SINR: In UL the eNB measures SINR per UE. The average SINR can beused as a quality indicator for the UL power control

UL CSI: channel state information per PRB for each UE. The CSI shall be thereceived signal power averaged per PRB.



LTE Air Interface

62


UE Measurements: RSRP & RSRQ

RSRP (Reference Signal Received Power)

• Average of power levels (in [W]) received across all Reference Signal symbolswithin the considered measurement frequency bandwidth.

• UE only takes measurements from the cell-specific Reference Signal elements ofthe serving cell

• If receiver diversity is in use by the UE, the reported value shall be equivalent tothe linear average of the power values of all diversity branches

RSRQ ( Reference Signal Received Quality)

• Defined as the ratio N ×RSRP/(E-UTRA carrier RSSI), where N is the number ofRBs of the E-UTRA carrier RSSI measurement bandwidth. The measurements inthe numerator and denominator shall be made over the same set of resourceblocks

Note: 3GPP has open issues on these e.g. measurement bandwidth on RSSI

Seems that it has been removed: E-UTRA Carrier Received Signal StrengthIndicator, comprises the total received wideband power observed by the UE from allsources, including co-channel serving and non-serving cells, adjacent channel

interference, thermal noise etc.



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63


eNodeB Measurements

DL Reference Signal Transmitted Power

• Average of power levels (in [W]) transmitted across all Reference Signal symbols

within the considered measurement frequency bandwidth• Reference point for the DL RS TX power measurement: TX antenna connector

• The DL RS TX power signaled to the UE is not measured, it is just an eNB internalsetting

Received Interference Power:

• Received interference power, including thermal noise, within one PRBs bandwidth

Thermal noise power: No x W

• Thermal noise power within the UL system bandwidth (consisting of variable # ofresource blocks)

– ‘No’: white noise power spectral density on the uplink carrier frequency and ‘W’ : denotesthe UL system bandwidth.

• Optionally reported with the Received Interference Power

• Reference point: RX antenna connector

• In case of receiver diversity, the reported value is the average of the power in thediversity branches

Thermal noise power and Received Interference Power are measured for the sameperiod of time.



LTE Air Interface

64


Module Contents

• OFDM Basics




• SC-FDMA





• VoIP in LTE



LTE Air Interface

65


LTE Frequency Variants in 3GPP – FDD

1

2

3

4

5

7

8

9

6

2x25

2x75

2x60

2x60

2x70

2x45

2x35

2x35

2x10

824-849

1710-1785

1850-1910

1920-1980

2500-2570

1710-1755

880-915

1749.9-1784.9

830-840

Total[MHz] Uplink [MHz]

869-894

1805-1880

1930-1990

2110-2170

2620-2690

2110-2155

925-960

1844.9-1879.9

875-885

Downlink [MHz]

10 2x60 1710-1770 2110-2170

11 2x25 1427.9-1452.9 1475.9-1500.9

1800

2600

900

US AWS

UMTS core

US PCS

US 850

Japan 800

Japan 1700

Japan 1500

Extended AWS

Europe Japan Americas

788-798 758-768

777-787 746-756

Japan 800

US700

2x10

2x1013

12 2x18 698-716 728-746

14 US700

US700

Band 15 – 16: reserved

815 – 830 860 – 875

704 – 716 734 – 746

2x15

2x1217

18

* „digital dividend“

US700

E-UTRAband

UHF (TV)*832 – 862 791 – 821

830 – 845 875 – 890

2x30

2x1519

20

Japan 800



LTE Air Interface

66


LTE Frequency Variants - TDD

33

34

35

36

37

39

40

38

1x20

1x60

1x15

1x20

1x40

1x60

1x100

1x50

1910 - 1930

1850 - 1910

2010 - 2015

1900 - 1920

1880 - 1920

1930 - 1990

2300 - 2400

2570 - 2620

Total[MHz]

Frequency[MHz]

UMTS TDD 1

UMTS TDD 2

US PCS

US PCS

US PCS

Euro midle gap 2600

China TDD

China TDD

E-UTRAband



LTE Air Interface

67


Module Contents

• OFDM Basics


• OFDM versus OFDMA• OFDM Key Parameters

• OFDM Weaknesses

• SC-FDMA





• VoIP in LTE



LTE Air Interface

68


RRM building blocks & functionsOverview

Scope of RRM:

• Management & optimized utilization of the

radio resources:

• Increasing the overall radio network capacity

& optimizing quality

•Provision for each service/bearer/user an

adequate QoS (if applicable)

RRM located in eNodeB

NSN LTE RRM Framework consists of RRM building blocks, RRM functions andRRM algorithms.

L3 RRM:

ICIC: Selects certain parts of the Frequency Spectrum of the LTE Carrier.Exclusively for PDSCH and PUSCH on Cell Basis. Remaining channels not affected.

DRX/DTX algorithm: To support provisioning of measurement gaps for Inter-RAT-HOand DRX/DTX mode in later product releases. Not supported in RL09.

Differences with RRM WCDMA:

•Softer and Soft handovers are not supported by the LTE system

•LTE requirements on power control are much less stringent due to the different nature of LTEradio interface i.e. OFDMA (WCDMA requires fast power control to address the “Near-Far”problem and intra-frequency interferences)

•On the other hand LTE system requires much more stringent timing synchronization for

OFDMA signals.



LTE Air Interface

69


LTE RRM: Scheduling (1/4)

• Motivation

– Bad channel condition avoidance

OFDMA

The part of total available

channel experiencing bad

channel condition (fading)can be avoided during

allocation procedure.

CDMA

Single Carrier transmission

does not allow to allocate

only particular frequencyparts. Every fading gap

effects the data.



LTE Air Interface

70


Scheduler (UL/DL) (2/4)

• Cell-based scheduling (separate scheduler per cell)

• Scheduling on TTI basis (1ms)

• Resource assignment in time and frequency domain (UL/DL)

• Proportional Fair (PF) resource assignment among UEs

• Priority for SRB (Signalling Radio Bearers) over DRB (Data Radio Bearers)

• Priority handling (UL/DL) for

• Random Access procedure

• Signalling

• HARQ re-transmission

• Uplink:

• Scheduler controls UEs & assigns appropriate grants per TTI

• Channel unaware UL scheduling based on random frequency allocation

(Channel-aware UL scheduling foreseen for RL30 & it will be SW licensed)

• Downlink:• Channel aware DL scheduling - Frequency Domain Packet Scheduling (FDPS) -

based on CQI with resources assigned in a fair manner

RL09



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71


Downlink Scheduler (3/4)Algorithm

• Determine which PRBs are available (free) and can

be allocated to UEs

• Allocate PRBs needed for common channels like

SIB, paging, and random access procedure (RAP)

• Final allocation of UEs (bearers) onto PRB.

Considering only the PRBs available after the

previous steps

– Pre-Scheduling: All UEs with data available for

transmission based on the buffer fill levels

– Time Domain Scheduling: Parameter

MAX_#_UE_DL decides how many UEs are

allocated in the TTI being scheduled

– Frequency Domain Scheduling for Candidate

Set 2 UEs: Resource allocation in Frequency

Domain including number & location ofallocated PRBs

Evaluation of available resources (PRBs/RBGs)

for dynamic allocation on PDSCH

Resource allocation and scheduling

for common channels

DL scheduling of UEs:

Scheduling of UEs/bearers to PRBs/RBGs

Start

End

Pre-Scheduling:

Select UEs eligible for scheduling

-> Determination of Candidate Set 1

Time domain schedulingof UEs according to simple criteria

-> Determination of Candidate Set 2

Start

End

Frequency domain scheduling

of UEs/bearers

-> PRB/R BG allocation to UEs/bearers

RL09

Feature ID(s): LTE45

SIB: System Information Broadcast

MAX_#_UE_DL depends on the bandwidth: 7UEs (5 MHz), 10UEs (10MHz) and 20UEs (20MHz)



LTE Air Interface

72


Uplink Scheduler (4/4)Algorithm

• Evaluation of the #PRBs that will be assigned to UEs

• Available number of PRBs per user: resources are assigned via PRB groups (group of

consecutive PRBs)Time domain:

• Max_#_UE_UL which can be scheduled per TTI time frame is restricted by an O&M

parameter and depends on the bandwidth: 7 UEs (5 MHz), 10 UEs (10MHz) and 20 UEs

(20MHz)

Frequency Domain:

• Uses a random function to assure equal distribution of PRBs over the available frequency

range (random frequency hopping)

a) b)

RL09


Example of allocation in frequency domain:

Full Allocation: All available PRBs are assigned to

the scheduled UEs per TTI

Fractional Allocation: Not all PRBs are assigned.Hopping function handles unassigned PRBs as if

they were allocated to keep the equal distribution

per TTI

PRBs allocated for PRACH, PUCCH are excluded for PUSCH scheduling



LTE Air Interface

73


LTE RRM: Link Adaptation by AMC (UL/DL) (1/6)

• Motivation of link adaptation: Modify the signal transmitted to and by a

particular user according to the signal quality variation to improve the system

capacity & coverage reliability.

• It modifies the MCS (Modulation & Coding Scheme) & the transport block size

(DL) and ATB (UL)

• If SINR is good then higher MCS can be used -> more bits per byte ->

more throughput.

• If SINR is bad then lower MCS should be used (more robust)

• Flexi Multiradio BTS performs the link adaptation for DL on a TTI basis

• The selection of the modulation & the channel coding rate is based:• DL data channel: CQI report from UE

• UL: BLER measurements in Flexi LTE BTS


RL09

Optimizing air interface efficiency

Adaptive Transmission Bandwidth (ATB): Calculates maximum

numb er of PRBs that UL SCH can assigned to a particular UEtaking into account UE QoS profile and available UE power

headroom



LTE Air Interface

74

74 © N oki a S ie me ns Ne tworks

Link Adaptation / AMC for PDSCH (2/6)

Procedure:

• Initial MCS is provided by O&M

(parameter INI_MCS_DL) & is setas default MCS

• If DL AMC is not activated (O&M

parameter ENABLE_AMC_DL) the

algorithm always uses this default

MCS

• If DL AMC is activated HARQ

retransmissions are handled

differently from initial transmissions

(For HARQ retransmission the

same MCS has to be used as for

the initial transmission)

• A MCS based on CQI reporting

from UE , shall be determined for

the PRBs assigned to UE as

indicated by the DL scheduler

yes

no

no

RL09



LTE Air Interface

75


Link Adaptation / AMC for PUSCH (3/6)

Functionality

• UL LA is active by default but can be deactivated by O&M parameters. If notactive, the init ial MCS is used all the time

• UE scope

• Two parallel algorithms adjust the MCS to the radio channel conditions:

– Inner Loop Link Adaptation (ILLA):

▪ Slow Periodic Link adaptation (20-500ms) based on BLER measurementsfrom eNodeB (based on SINR in future releases)

– Outer Loop Link Adaptation (OLLA): event based

▪ In case of long Link Adaptation updates and to avoid low and high BLERsituations, the link adaptation can act based on adjustable target BLER:

- “Emergency Downgrade” if BLER goes above a MAX BLERthreshold (poor radio conditions)

- “Fast Upgrade” if BLER goes below of a MIN BLER threshold(excellent radio conditions)

RL09



LTE Air Interface

76


Downlink – fast

▪ 1 TTI

– channel aware▪ CQI based

– MCS selection▪ 1 out of 0-28

– output▪ MCS

▪ TBS

– up to 64QAM support

Uplink

– slow periodical▪ ~30ms

– channel partly aware▪ average BLER based

– MCS adaptation▪ +/- 1 MCS correction

– output▪ MCS

▪ ATB

– up to 16 QAM support

Comparison: DL & UL Link adaptation for PSCH (4/6)

MCS: Modulation & Coding Scheme

TBS: Transport Block Size

ATB: Automatic Transmission Bandwidth

Adaptive Transmission Bandwidth (ATB): Responsible for definingmaximum number of PRBs that can be assigned to a particular UEby UL SCH



LTE Air Interface

77


Outer Link Quality Control (OLQC) (5/6)

Feature: CQI Adaptation (DL)

• CQI information is used by the scheduler & link adaptation in such a way that a certain

BLER of the 1st HARQ transmission is achieved

• CQI adaptation is the basic mean to control Link Adaptation behaviour and to remedy UE

measurement errors

• Only used in DL

• Used for CQI measurement error compensation

– CQI estimation error of the UE

– CQI quantization error or

– CQI reporting error

• It adds a CQI offset to the CQI reports provided by UE. The corrected CQI report isprovided to the DL Link adaptation for further processing

• CQI offset derived from ACK/NACK feedback

RL09

Optimize the DL performance




LTE Air Interface

78


Support of aperiodic CQI reports (6/6)

Functionality

• Aperiodic CQI reports scheduled in addition to periodic reports

– Periodic CQI reports on PUCCH – Aperiodic CQI reports on PUSCH

Description

• Controlled by the UL scheduler

– Triggered by UL grant indication (PDCCH)

• Basic feature


Benefits

• Not so many periodic CQIs on PUCCH

needed

• Allow frequent submission of more detailed

reports (e.g. MIMO, frequency selective

parts)

RL10



LTE Air Interface

79


LTE RRM: Power Control (1/4)

Downlink:• There is no adaptive or dynamic power control in DL but semi-static power

setting

• eNodeB gives flat power spectral density (dBm/PRB) for the scheduled

resources:

– The power for all the PRBs is the same

– If there are PRBs not scheduled that power is not used but the power of the

remaining scheduled PRBs doesn’t change:

▪ Total Tx power is max. when all PRBs are scheduled. If only 1/2 of the PRBs are

scheduled the Tx power is 1/2 of the Tx power max ( i.e. Tx power max -3dB)

• Semi-static: PDSCH power can be adjusted via O&M parameters

– Cell Power Reduction level CELL_PWR_RED [0...10] dB attenuation in 0.1 dB steps

Improve cell edge behaviour, reduce inter-cell interference & power consumption

RL09




LTE Air Interface

80


Power Control (2/4)

Uplink:

• UL PC is a mix of Open Loop Power Control & Closed Loop Power Control:

• Closed Loop PC component f(i): Makes use of feedback from the eNB. Feedback are TCP

commands send via PDCCH to instruct the UE to increase or decrease its Tx power

Improve cell edge behaviour, reduce inter-cell interference and power consumption

RL09

Feature ID(s): LTE27&LTE28

])}[()()()())((log10,min{)( _ 010 dBmi f i PL j j P i M P i P TF PUSCH PUSCH CMAX PUSCH +∆+⋅++= α

• UL Power control is Slow power control:

– No need for fast power control as in 3G:

if UE Tx power was high it incremented

the co-channel for other UEs.

– In LTE all UEs resources are orthogonalin frequency & time

TPC: Transmit Power Control

WCDMA: If UE Tx power was high it increased the co-channel interference for otherUEs

Open loop suffers from errors in UE path loss measurement and tx power settingwhereas closed loop PC is less sensitive to errors

Control over power spectral density, not absolute power. The power is changed bythe UL scheduler by varying the bandwidth granted. The power per Hz remainsconstant.



LTE Air Interface

81


Power Control (3/4)

Uplink (cont.):

• UL PC is a mix of Open Loop Power Control & Closed Loop Power Control:

• PCMAX: max. UE Tx power according to UE power class; e.g. 23dBm for class 3

• MPUSCH: # allocated PRBs. The UE Tx Power is increased proportionally to the # of allocated

RBs. Remaining terms of the formula are per RB

• P0_PUSCH: eNB received power per RB when assuming path loss 0 dB. Depends on α

• α: Path loss compensation factor. Three values:

– α= 0, no compensation of path loss

– α= 1, full compensation of path loss (conventional compensation)

– α≠ { 0 ,1 } , fractional compensation

• PL: DL Path loss calculated by the UE• Delta_TF: increases the UE Tx power to achieve the required SINR when transmitting a

large number of bits per RE. It links the UE Tx power to the MCS.

RL09

Feature ID(s): LTE27&LTE28

])}[()()()())((log10,min{)( _ 010 dBmi f i PL j j P i M P i P TF PUSCH PUSCH CMAX PUSCH +∆+⋅++= α

PL calculated by UE using a

combination of RSRPmeasurements and knowledge ofthe RS transmit power(broadcasted in SIB2)Power control does not control the absolute UE Tx. power but the Power SpectralDensity (PSD), power per Hz, for a device The PSDs at the eNodeB from differentusers have to be close to each other so the receiver doesn’t work over a large range

of powers.Different data rates mean different tx bandwidths so the absolute Tx power of the UEwill also change. PC makes that the PSD is constant independently of the txbandwidth



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Conventional & Fractional Power Control (4/4)

• Conventional PC schemes:

– Attempt to maintain a constant SINR at the receiver

– UE increases the Tx power to fully compensate for increases in the path loss

• Fractional PC schemes:

– Allow the received SINR to decrease as the path loss increases.

– UE Tx power increases at a reduced rate as the path loss increases. Increases in

path loss are only partially compensated.

– [+]: Improve air interface efficiency & increase average cell throughputs by reducing

Intercell interference

• 3GPP specifies fractional power control for the PUSCH with the option to disable it &

revert to conventional based on α

Conventional PowerControl: α=1

If Path Loss increases by

10 dB the UE Tx power

increases by 10 dB

Fractional Power

Control: α { 0 ,1}

If Path Loss increases

by 10 dB the UE Tx

power increases by <

10 dB

UE TxPower UE Tx

Power

ULSINR

ULSINR



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83


LTE RRM: Radio Admission Control (RAC)

Objective: To admit or reject requests for establishment of Radio Bearers (RB) on a

cell basis

• Based on number of RRC connections and number of active users per cell

– Non QoS aware

– Both can be configured via parameters

▪ RRC connection is established when the SRBs have been admitted & successfully

configured

▪ UE is considered as active when a Data Radio bearer (DRB) is established

– Upper bound for maximum number of supported connections depends on the

BB configuration of eNB :

▪ RL10: support for 200, 400 & 800 active users respectively in 5, 10 & 20 MHz

▪ RL20: up to 840 active users in 20MHz

• Handover RAC cases have higher priority than normal access to the cell

• RL09: All RRC connection setup request are admitted by default to avoid RAC complexity

RL09

At reception of the HO request message the RAC decides in an ‘all-or-nothing’manner on the admission / rejection of the resources used by the UE in the sourcecell (prior to HO). 'All-or-nothing' manner means that either both SRB AND (logical)

DRB are admitted or the UE is rejected. RL09 all SRB are admitted.

SRB: between UE and eNB



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84


LTE RRM: MIMO / Antenna Control (1/5)Transmit diversity for 2 antennas

Benefit: Diversity gain, enhanced cell coverage

• Each Tx antenna transmits the same stream of data with Receiver gets replicas ofthe same signal which increases the SINR.

• Synchronization signals are transmitted only via the 1st antenna

• eNode B sends different cell-specific Reference Signals (RS) per antenna

• It can be enabled on cell basis by O&M configuration

• Processing is completed in 2 phases:

• Layer Mapping: distributing a stream of data into two streams

• Pre-coding: generation of signals for each antenna port

RL09

• Additional antenna specific coding is applied to the signals before transmission toincrease the diversity effect.

Transmit diversity is open loop (it doesn’t take any advantage of any feedback fromthe UE as weights are fixed). It is simpler to implement and doesn’t have theoverhead generated by the feedback information. Tx diversity is the solution for openloop spatial multiplexing when transferring a single code word.



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85


S1

S2

Spatial multiplexing (MIMO) for 2 antennas (2/5)

Benefit: Doubles peak rate compared to 1Tx antenna

• Spatial multiplexing with 2 code words

• Supported physical channel: PDSCH

Two code words

(S1+S2) are

transmitted in

parallel to 1 UEdouble peak

rate

Layer

Mapping

L1

L2

Precoding

Map onto

Resource

Elements

×

Map onto

Resource

Elements

OFDMA

OFDMA

Modulation

Modulation

Code word

1

Code word

2

×

Scale

×

×

W2

W1

• 2 code words

transferred whenchannel conditions

are good

• Signal generation is similar to Transmit

Diversity: i.e. Layer Mapping & Precoding

• Can be open loop or closed loop depending

if the UE provides feedback

The cyclic delay operation for the second antenna causes a linear phase shift alongthe frequency dimension. Thus, summing the cyclically delayed signal in the receiverand the un-delayed signal from the first antenna causes a frequency selective fading

pattern

UE provides feedback in terms of:

CQI

Rank Indication (RI) – number of layers to use

Precoding Matrix Indicator (PMI) – set of weights to apply during precoding



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Precoding (3/5)

• Precoding generates the signals for each antenna port

• Precoding is done multiplying the signal with a precoding matrix selected from a predefinedcodebook known at the eNB and at the UE side

• Closed loop: UE estimates the radio channel, selects the best precoding matrix (the one thatoffers maximum capacity) & sends it to the eNB

• Open loop: no need for UEs feedback as it uses predefined settings for Spatial Multiplexing& precoding

Pre-coding codebook for 2 Tx antenna case



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DL adaptive open loop MIMO for 2 antennas (4/5)

Benefit: High peak rates (2 code words) & good cell edge

performance (single code word)

• 2 TX antennas

• Dynamic selection between

• Transmit diversity

• Open loop spatial multiplexing with 2

code words

• Supported physical channel: PDSCH

• Dynamic switch considers the UE specific

link quality

• Enabled/disabled on cell level (O&M)

• If disabled case either static spatial

multiplexing or static Tx diversity can

be selected for the whole cell (all UEs)

2 code words (A+B) are

transmitted in parallel to 1 UE

which doubles the peak rate

1 code word A is

transmitted via 2

antennas to 1 UE;

improves the LiBu

A

B

A


RL09

Note: DL adaptive closed loop

MIMO has been moved to RL20

LiBu: Link Budget

Note: CQI adaptation needs to be supported/enabled ;Tx diversity needs to besupported/enabled. MIMO is currently non adaptive

Release 10: UE radio capabilities are considered

Performance counter for transmission mode usage is supported per cell



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MIMO, DL channels & RRM Functionality (5/5)

Available MIMO options vs. channel type

• Options for Transmit Diversity (2 Tx):

– Control Channels

– PDSCH

• Options for Dual Stream (SM):

– Only DL PDSCH

• MIMO is SW feature

Channel can be configured to use MIMO mode

Channel cannot be configured to use MIMO mode

In UL, Flexi eNodeB has 2Rx Div. :

• Maximum Ratio Combining

Benefit: increase coverage by

increasing the received signal

strength and quality

RRM MIMO Mode Control Functionality

• Refers to switch between:

Tx Diversity (single stream)

MIMO Spatial Multiplexing (double stream)

1x1 SISO / 1x2 SIMO

• Provided by eNB only for DL direction



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LTE RRM: Connection Mobility Control (1/3)Handover Types

• Intra-RAT handover

– Intra eNodeB – Inter eNodeB

Data forwarding over X2• High performance for 15…120 km/h

• Optimized performance for 0…15 km/h

HO in case of no X2 interface configured between Serving eNB & Target eNB: HO via

S1 interface – RL20

• Inter-RAT Handover

– PS domain only

– RL20: LTE WCDMA

– RL30: LTE CDMA2000

– RL40: WCDMA LTE

– Not assigned: LTE GSM; GSM LTE



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Intra frequency handover via X2 (2/3)

• Basic Mobility Feature

• Event triggered handover based

on DL measurements (ref.

signals)

• Network evaluated HO decision

• Operator configurable

thresholds for• coverage based &

• best cell based handover

• Data forwarding via X2

• Radio Admission Control (RAC)gives priority to HO related

access over other scenarios S1

S1 X2

MMES-GWP-GW

RL09


A reliable and lossless mobility



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Inter RAT Handover to WCDMA (3/3)

• Only for multimode devices supporting

LTE & WCDMA

• Event triggered Handover based on DL

measurement Reference Signal Received

Power (RSRP)

• Operator configurable RSRP threshold

• Inter-RAT HO measurements only

activated if there is not Intra-frequency

neighbour cell

• Network evaluated HO decision

• eNB broadcasts IRAT cell selection

information

• best target WCDMA cell may be selected

when above the threshold• eNB initiates Handover via EPC

MMES-GW

P-GW

SGSNRNC

S1Iub

LTE

WCDMA

RL20



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Module Contents

• OFDM Basics


• OFDM versus OFDMA• OFDM Key Parameters

• OFDM Weaknesses

• SC-FDMA





• VoIP in LTE



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VoIP in LTE

Paging in

LTE

2G/3G RAN

MMEE-UTRAN

MSC-S MGW

CS call setup in

2G/3G

CS Fallback handover

• Voice is still important in LTE

• CS voice call will not be possible in LTE since there is no CS core interface

• Voice with LTE terminals has a few different solutions• The first voice solution in LTE can rely on CS fallback Handover where LTE

terminal will be moved to 2G/3G to make CS call

• The ultimate LTE voice solution will be VoIP + IMS (not RL10)

RL20

IP Multimedia Subsystem, a set of specifications from 3GPP for delivering IPmultimedia to mobile users

VoIP: supported in RL20



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Single Radio Voice Call Continuity (SR-VCC)

LTE VoIP

3G CS voice

LTE VoIP

3G CS voice 3G CS voice 3G CS voice

Single Radio Voice Call

Continuity (SR-VCC)

Options for voice call continuity when running out of LTE coverage

• 1) Handover from LTE VoIP to 3G CS voice – Voice Handover from LTE VoIP to WCDMA CS voice is called SR-VCC

– No VoIP needed in 3G

• 2) Handover from LTE VoIP to 3G VoIP

– VoIP support implemented in 3G

RL30


http://slidepdf.com/reader/full/02-ra41202en10gla0-lte-air-interface-v03 95/95

LTE Air Interface


LTE Voice Evolution

Broadband LTE

introduction

MGW MSS

LTE

HSPA & I-HSPA

2G/3G

CS/PS

Increased radio

efficiency for voice

service

LTE

HSPA & I-HSPA

2G/3G

Full IMS centric

multimedia servicearchitecture

LTE

HSPA & I-HSPA

PS

E v o l u t i o n t o I M S

V o I P

s o l u t i o n

I n t r o d u c e

N V S V o I P

s o l u t i o n

NVS

IMSMGW MSSNVS

CS/PS

Data only LTE Fast track LTE VoIP IMS multimedia

• CS fallbackhandover

• VoIP

• SR-VCC

• VoIP

• SR-VCC

Timing of phases to be fixed

NVS: NVS: NSN Voice Server

IMS: IP Multimedia Server

02 Ra41202en10gla0 Lte Air Interface v03

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Transcript of 02 Ra41202en10gla0 Lte Air Interface v03