LTE Air Interface Overview

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TM51154EN04GLA2 Nokia Solutions and Networks 2014

Nokia AcademyLTE Air Interface OverviewLTE/EPS Fundamentals Course

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2 TM51154EN04GLA2 Nokia Solutions and Networks 2014

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Nokia Solutions and Networks 2014

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Module ObjectivesAfter completing this module, the participant should be able to:

Understand the basics of the OFDM transmission technology. Explain different methods for Multiplexing the access with OFDM. Analyze the reasons for SC-FDMA selection in UL. Discuss about LTE/EUTRAN Subcarriers, the Frame Structure,

Resource Block and the Modulation options. List the frequency allocation alternatives for LTE. Describe the basics of MIMO. Identify maximum bit rates for LTE. Distinguish different LTE UE categories. Describe the basics for HARQ. List the Radio Resource Management (RRM) task in LTE/EUTRAN.

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Module Contents

Orthogonal Frequency Division Multiplexing OFDM Multiple Access OFDM implementation in LTE/EUTRAN SC-FDMA LTE/EUTRAN Radio Frames OFDM Resource Block Modulation Schemes in LTE/EUTRAN LTE/EUTRAN Frequency Variants MIMO DL & UL Peak Bit Rates LTE UE Categories HARQ Radio Resource Management

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TDMA

f

t

f

Time Division

FDMA

f

f

t

Frequency Division

CDMA

f

t

f

Code Division

OFDMA

f

f

t

Frequency Division Orthogonal subcarriers

Wireless Access Technology User 1 User 2 User 3 User ..

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

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Multiple Access

1 2 3 4 5

2

12345

4 2

1

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1

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5

5

3

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24

1

Pow

er

Frequency

TDMATime Division

Multiple Access,

2G e.g. GSM, PDC

FDMAFrequency

DivisionMultiple Access1G e.g. AMPS,

NMT, TACS

CDMA

Code DivisionMultiple Access3G e.g. UMTS,

CDMA2000

1 2 3UE 1 UE 2 UE 3 4 UE 4 UE 55

OFDMAOrthogonalFrequency

DivisionMultiple Access

e.g. LTE

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OFDM Basics

Orthogonal Frequency Division Multiplexing (OFDM) is a digital encoding and modulation technique

The channel bandwidth is divided into lower bandwidth subcarriers Each subcarrier operates at a different, equally-spaced center

frequency Bits are modulated and transmitted simultaneously on each data

subcarrier during a symbol time LTE uses OFDMA in the DL and SC-FDMA in the UL

Channel

Subcarriers

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OFDM Basics

- Data is sent in parallel across the set of subcarriers, each subcarrier only transports 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

Power

frequency

bandwidth

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Multi-Carrier Modulation

Multiple carriers in parallel (Subcarriers).

frequency

Serial-to-Parallel

Converter

011001011100101001011101

011 001 011 100 101 001 011 101

Subcarriers

Guard Bands

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Multi-Carrier Modulation

The center frequencies must be spaced so that interference between different carriers, known as Adjacent Carrier Interference ACI, is minimized; but not too much spaced as the total bandwidth will be wasted.

frequencyf0 f1 f2 fN-1fN-2

ACI = Adjacent Carrier Interference

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OFDM: Orthogonal Frequency Division Multi-Carrier

OFDM allows a tight packing of small carrier - called the subcarriers - into a given frequency band.

Pow

er De

nsi

ty

Pow

er De

nsi

ty

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

SavedBandwidth

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OFDM Basics Transmits hundreds or even thousands of separately modulated

radio signals using orthogonal subcarriers spread across a wideband channel

Orthogonality:

The peak ( centre frequency) of one subcarrier

intercepts the nulls of the neighbouring subcarriers

Total transmission bandwidth

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OFDM and Multiple Access

Up to here we have only discussed simple point-to-point or broadcast OFDM.

Now we have to analyze how to handle access of multiple users simultaneously to the system, each one using OFDM.

OFDM can be combined with several different methods to handle multi-user systems:

1.-Plain OFDM

3.-Orthogonal Frequency Division Multiple Access OFDMA

2.-Time Division Multiple Access via OFDM

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1.- Plain OFDM

Plain OFDM: 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).

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Plain OFDM

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subc

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1 2 3 common info(may be addressed viaHigher Layers)

UE 1 UE 2 UE 3

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2.- Time Division Multiple Access via OFDM

Time Division Multiple Access via OFDM: The simplest model to implement multiple access handling is by putting a time multiplexing on top of OFDM.

The disadvantage of this simple mechanism is, that every user gets the same amount of capacity (subcarriers) and it is thus rather difficult to implement flexible (high and low) bit rate services.

Furthermore it is nearly impossible to handle highly variable traffic (e.g. web traffic) efficiently without too much higher layer signaling and the resulting delay and signaling overhead.

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1 2 3 common info(may be addressed viaHigher Layers)

UE 1 UE 2 UE 3

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2.- Orthogonal Frequency Division Multiple Access OFDMA

Orthogonal Frequency Division Multiple Access OFDMA: is a 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 one or more Resource blocks.

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Orthogonal FrequencyMultiple Access

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Resource Block (RB)1 2 3 common info(may be addressed via

Higher Layers)UE 1 UE 2 UE 3

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Time Division Multiple Accesson OFDM

time

subc

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OFDMA is registered trademark of Runcom Technologies Ltd.

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Resource Block (RB)1 2 3 common info(may be addressed via HL)UE 1 UE 2 UE 3

Different Methods for OFDM Multiple Access

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Channel Bandwidth

1.4 MHz

3 MHz

5 MHz

10 MHz

15 MHz

20 MHz

LTE defines channel sizes from 1.4 MHz to 20 MHz

Channel

Channel

Channel

Channel

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OFDM Challenges

High Peak-to-Average Power Ratio (PAPR) of the transmitted signal: this results in requirements for expensive linear power amplifiers.

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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

Peak-to-Average Power Ratio in OFDM

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- 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 the

multipath resistance and flexible subcarrier frequency allocation offered by OFDM.

It can reduce the PAPR between 69dB compared to OFDMA

TS36.201 and TS36.211 provide the mathematical description of the time domain representation of an SC-FDMA symbol.

- Reduced PAPR means lower RF hardware requirements ( power amplifier)

SC-FD

MA

OFD

MA

SC-FDMA in UL

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SC-FDMA and OFDMA Comparison

- 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 and 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.

OFDMA SC-FDMA

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Comparing OFDMA & SC-FDMA

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Channel Direction

Downlink (DL) is always from the eNodeB to the UEs Uplink (UL) is always from the UE(s) to the eNodeB

eNodeB

UE1

UEn

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LTE FDD and TDD Modes

Uplink Downlink

Bandwidthup to 20MHz

Duplex Frequencyf

t Bandwidthup to 20MHz

GuardPeriod

f

t

Uplink

Downlink

Bandwidthup to 20MHz

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TDD vs. FDD

Downlink Downlink

Uplink

Uplink

FDD TDD

Time

Frequency

Throughput

DL DLUL UL

Only this is needed

Wasted

We get what we need

Downlink throughput is also affected

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LTE Radio Frames, Slots and Subframes FDD mode

The basic EUTRAN Radio Frame is 10 ms long. The EUTRAN Radio Frame is divided into 20 slots, each one 0.5 ms long. Always two slots together form a subframe. The subframe (1 ms) is the

smallest time unit the scheduler assigns to physical channels. In case of FDD there is a time offset between uplink and downlink

transmission.

Slot#0

Slot#0

Slot#1

Slot#1

Slot#2

Slot#2

Slot#3

Slot#3

Slot#16Slot#16

Slot#17Slot#17

Slot#18Slot#18

Slot#19Slot#19

. . .

Slot#0

Slot#0

Slot#1

Slot#1

Slot#2

Slot#2

Slot#3

Slot#3

Slot#16Slot#16

Slot#17Slot#17

Slot#18Slot#18

Slot#19Slot#19

. . .

f

DL carrier

UL carrier

radio frame 10 ms

radio frame 10 ms

subframe 0 subframe 1 subframe 8 subframe 9

subframe 0 subframe 1 subframe 8 subframe 9DL/U

L Ti

me

offs

et

time

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LTE Radio Frames, Slots and Subframes TDD mode

If TDD mode is used, subframe 0 and subframe 5 must be downlink, all other subframes can dynamically be used as uplink or downlink period.

Slot#0

Slot#1

Slot#2

Slot#3

Slot#16

Slot#17

Slot#18

Slot#19

. . .

f

time

UL/DLcarrier

radio frame 10 ms

subframe 0 subframe 1 subframe 5 subframe 9

. . .

Downlink Subframe Uplink Subframe

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LTE Physical Layer Structure Frame Structure

- FDD Frame structure is common to both uplink and downlink. - Divided into 20 x 0.5ms slots Structure has been designed to facilitate short round trip time

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

- Frame length =10 ms- FDD: 10 ms sub-frame for UL and

10 ms sub-frame for DL- 1 Frame = 20 slots of 0.5ms each- 1 slot = 7 ( normal CP) or 6

symbols ( extended CP)

SF: SubFrames: slotSy: symbol

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LTE Slot

The LTE Slot carries: 7 symbols with short cyclic prefix 6 symbols with long prefix

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Cyclic Prefix

Extended Symbol Time

TCP

Cyclic Prefix

TCP accounts for multipath delay (distance) Cyclic Prefix copies signal from the end of the symbol time and

attaches in front of the symbol time Normal TCP is 4.67 s Extended TCP is 16.67 s

Symbol Time

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Multi-Path Propagation and Inter-Symbol Interference

Inter Symbol Interference

BTSTime 0 Ts

+

Time 0 Tt Ts+Tt

Tt

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Multi-Path Propagation and the Guard Period2

time

TSYMBOLTime Domain

1

3

time

TSYMBOL

time

TSYMBOL

Tg

1

2

3

Guard Period (GP)

Guard Period (GP)

Guard Period (GP)

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f0 f1 f2 f3 f4

15 kHzLTE/EUTRAN Air Interface

LTE uses a 15 kHz subcarrier spacing (fs). Therefore the Symbol duration (Ts) is 66.67s.

This corresponds to bandwidths from 1.4 MHz, 3 MHz, 5 MHz,10 MHz, 15MHZ and up to 20 MHz.

Its an operators choice how many subcarriers (bandwidth) a cell should get.

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OFDM Key ParametersIn LTE not all the available channel bandwidth (e.g. 20 MHz) will be used. For the transmission bandwidth typically 10% guard band is considered (to avoid the out band emissions).If BW = 20MHz Transmission BW = 20MHz 2MHz = 18 MHz the number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers

TransmissionBandwidth [RB]

Transmission Bandwidth Configuration [RB]

Channel Bandwidth [MHz]

Reso

urce

block

Ch

an

nel

edge

Ch

an

nel

edge

DC carrierActive Resource Blocks

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OFDMA Parameters- Channel bandwidth: Bandwidths ranging from 1.4 MHz to 20 MHz- Data subcarriers: They vary with the bandwidth

72 for 1.4MHz to 1200 for 20MHz

Guard (no power)

DC (no power)data

Guard (no power)

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Physical Resource Blocks In both the downlink and

uplink direction, data is allocated to users in terms of resource blocks (RBs).

A resource block consists of 12 consecutive subcarriers in the frequency domain, that are reserved for the duration of one 0.5 millisecond time slot.

The smallest resource unit a scheduler can assign to a user is a scheduling block which consists of two consecutive 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

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OFDM Resource Block for LTE/EUTRAN

EUTRAN combines OFDM symbols in so called resource blocks (RB).

A single resource block is always 12 consecutive subcarriers during one slot (0.5 ms):

12 subcarriers * 15 kHz= 180 kHz It is the task of the scheduler to assign

resource blocks to physical channels belonging to different users or for general system tasks.

A single cell must have at least 6 resource blocks (72 subcarriers) and up to 100 are possible (1200 subcarriers).

frequency

time

Subcarriers

Subframe1ms

SubcarrierBandwidth

15kHz

Ban

dwid

th18

0kH

z

Slot Slot

Res

ou

rce

Blo

ck

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OFDM resource Grid for LTE/EUTRAN

frequency

time

Slot = 0.5 ms

12 su

bcar

riers

6 or 7 Symbols/slot

OFDM Symbol

Scheduling Resource Block(SRB)

OFDM symbols are arranged in a 2 dimensional matrix called the resource grid: One axis of the grid is the subcarrier index The other axis is the time.

Each OFDM symbol has its place in the resource grid.

Subframe = 1 ms

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Resource Block and Resource Element

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

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

Subcarrier 12

180

KHz

1 slot 1 slot1 ms subframe

Capacity allocation is based on Resource Blocks

Resource Element ( RE): 1 subcarrier x 1 symbol

period Theoretical minimum

capacity allocation unit. 1 RE is the equivalent of 1

modulation symbol on a subcarrier, i.e. 2 bits for QPSK, 4 bits for 16QAM and 6 bits for 64QAM.

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 60 1 2 3 4 5 6 0 1 2 3 4 5 60 1 2 3 4 5 6 0 1 2 3 4 5 60 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 60 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 60 1 2 3 4 5 6 0 1 2 3 4 5 60 1 2 3 4 5 6 0 1 2 3 4 5 6

Physical Resource Block or Resource Block (PRB or RB)

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OFDMA and SC-FDMA in LTE

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OFDMA Parameters- Frame duration: 10ms created from slots and subframes- Subframe duration ( TTI): 1 ms ( composed of 2 x 0.5slots)- Subcarrier spacing: Fixed to 15kHz ( 7.5 kHz defined for MBMS)

Varies with the bandwidth but always factor or multiple of 3.84 to ensure compatibility with

WCDMA by using common clocking

Frame Duration

Subcarrier Spacing

Resource Block

Data Subcarriers

Symbols/slot

CP length

1.4MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz10 ms

15 kHz

Normal CP=7, extended CP=6

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

6 15 25 50 75 100

72 180 300 600 900 1200

10ms

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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 60 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 60 1 2 3 4 5 6 0 1 2 3 4 5 6

1 slot 1 slot

1 ms subframe

OFDM resource Grid for LTE/EUTRAN

Reference symbols helps the UE to keep the synchronization with the network over the air interface, both in term of time and frequency synchronization.

Subcarrier 1

Subcarrier 12

180

KHz

OFDM Symbols/ Time Domain

Reference Symbols

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b0 b1QPSK

Im

Re10

11

00

01

b0 b1b2b316QAM

Im

Re

0000

1111

Im

Re

64QAMb0 b1b2b3 b4 b5

Each OFDM symbol even within a resource block can have a different modulation scheme. EUTRAN defines the following options: QPSK, 16QAM, 64QAM.Not every physical channel will be allowed to use any modulation scheme:Control channels to be using mainly QPSK. In general it is the scheduler that decides which form to use depending on carrier quality feedback information from the UE.

Modulation Schemes for LTE/EUTRAN

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LTE Modulation Techniques

Modulation techniques supported: BPSK 1 bit per symbol QPSK 2 bits per symbol16QAM 4 bits per symbol64QAM 6 bits per symbol

BPSK used for preambles DL traffic uses QPSK, 16QAM, 64QAM UL traffic uses QPSK, 16QAM, (64QAM optional)

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Modulation

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Downlink Peak Bit Rate 2x2 MIMO (2 antennas for TX, 2 Antennas for RX) 64QAM Control & Reference symbol overhead 14.8% 172 Mbps in 20 MHz and 86 Mbps in 10 MHz

Resource blocks 6 15 25 50 100Subcarriers 72 180 300 600 1200

Modulation coding 1.4 MHz 3.0 MHz 5.0 MHz 10 MHz 20 MHzQPSK 1/2 Single stream 0.9 2.2 3.6 7.2 14.416QAM 1/2 Single stream 1.7 4.3 7.2 14.4 28.816QAM 3/4 Single stream 2.6 6.5 10.8 21.6 43.264QAM 3/4 Single stream 3.9 9.7 16.2 32.4 64.864QAM 4/4 Single stream 5.2 13.0 21.6 43.2 86.464QAM 3/4 2x2 MIMO 7.8 19.4 32.4 64.8 129.664QAM 1/1 2x2 MIMO 10.4 25.9 43.2 86.4 172.864QAM 1/1 4x4 MIMO 20.7 51.8 86.4 172.8 345.6

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Resource blocks 5 14 24 49 99Subcarriers 60 168 288 588 1188

Modulation coding 1.4 MHz 3.0 MHz 5.0 MHz 10 MHz 20 MHzQPSK 1/2 Single stream 0.7 2.0 3.5 7.1 14.316QAM 1/2 Single stream 1.4 4.0 6.9 14.1 28.516QAM 3/4 Single stream 2.2 6.0 10.4 21.2 42.816QAM 1/1 Single stream 2.9 8.1 13.8 28.2 57.064QAM 3/4 Single stream 3.2 9.1 15.6 31.8 64.264QAM 1/1 Single stream 4.3 12.1 20.7 42.3 85.564QAM 1/1 V-MIMO (cell) 8.6 24.2 41.5 84.7 171.1

Uplink Peak Bit Rate

Single stream transmission with 64QAM assumed Reference symbol overhead 14.3% 85 Mbps in 20 MHz and 42 Mbps in 10 MHz

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LTE UE Categories

Qualcomm first chipset has 50 Mbps downlink and 25 Mbps uplink

All categories support 20 MHz 64QAM mandatory in downlink, but not in uplink (except Class 5) 2x2 MIMO mandatory in other classes except Class 1

Class 1 Class 2 Class 3 Class 4 Class 510/5 Mbps 50/25 Mbps 100/50 Mbps 150/50 Mbps 300/75 MbpsPeak rate DL/UL

20 MHzRF bandwidth 20 MHz 20 MHz 20 MHz 20 MHz

64QAMModulation DL 64QAM 64QAM 64QAM 64QAM

16QAMModulation UL 16QAM 64QAM 16QAM 16QAM

YesRx diversity Yes YesYes Yes

1-4 txBTS tx diversity

OptionalMIMO DL 2x2 4x42x2 2x2

1-4 tx 1-4 tx 1-4 tx 1-4 tx

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3GPP LTE SpectrumBand MHz Uplinks MHz Downlink MHz Region or typical name1 2x60 1920-1980 2110-2170

FDD

UMTS core, 2.1GHz2 2x60 1850-1910 1930-1990 US PCS, 1900MHz 3 2x75 1710-1785 1805-1880 1800MHz4 2x45 1710-1755 2110-2155 US AWS5 2x25 824-849 869-894 850MHz; US, Korea, APAC, MEA, Africa7 2x70 2500-2570 2620-2690 2.6GHz8 2x35 880-915 925-960 GSM 9009 2x35 1749-1784 1844-1879 Japan 170010 2x60 1710-1770 2110-2170 Extended AWS11 2x20 1427.9-1447.9 1475.9-1495.9 Japan 150012 2x17 699-716 729-746 US 700 MHz Lower (Band A,B,C)13 2x10 777-787 746-756 US 700 MHz Upper (Band C) Verizon14 2x10 788-798 758-768 US 700 MHz Upper (Band D+)17 2x12 704-716 734-746 US 700 MHz Lower (Band B, C) AT&T18 2x15 815-830 860-875 Japan 800 new19 2x15 830-845 875-890 Japan 800 new20 2x30 832-862 791-821 800MHz; European Digital Dividend band21 2x15 1448-1463 1496-1511 Japan (upper 1500)22 2x80 3410-3490 3510-3590 3,5 GHz band FDD 23 2x20 2000-2020 2180-2200 US S-band24 2x34 1626.5-1660.5 1525-1559 US L-band25 2x65 1850-1915 1930-1995 US ext. 190026 2x35 814-849 859-894 Korea, US: Extended 850 27 2x17 807-824 852-869 Latin America, 85028 2x45 703-748 758-803 APAC 700; mainstream33 1x20 1900-1920

TDD

UMTS core TDD34 1x15 2010-2025 UMTS core TDD35 1x60 1850-1910 US (possible TDD alternative to FDD)36 1x60 1930-1990 US (possible TDD alternative to FDD)37 1x20 1910-1930 US38 1x50 2570-2620 2.6GHz - TDD part, China, Europe, Lat.Am39 1x40 1880-1920 China UMTS TDD40 1x100 2300-2400 China TDD, APAC ,MEA, RUS,.41 1x194 2496-2690 US TDD42 1x200 3400-3600 TDD global43 1x200 3600-3800 TDD global44 1x90 703-803 APAC700; alternative

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MIMO MIMO stands for Multiple Input Multiple Output. It is a key technology to increase a channels capacity by using multiple transmitter

and receiver antennas. The very basic ideas behind MIMO have been established already 1970 , but have

not been used in radio communication until 1990.

Air Interface

Transmission antennas

Reception antennas

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MIMO

MIMO is currently used in 802.11n, 802.16d/e to increase the channel capacity.

LTE supports 2x2 and 4x4 MIMO configurations.

Two kinds of MIMO techniques:

Multistream transmission (also known as spatial multiplexing) MIMO Transmit Diversity (or space-time coding) MIMO.

TX RX

Tx RxMIMOChannel

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Examples of MIMO Usage

Spatial multiplexing

Transmission diversity

Typically, close to the eNodeB Spatial multiplexing could be used to improve the throughputAt the cell edge Transmission diversity could be used to improve the coverage

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Dynamic MIMO mode

Depending on Radio Conditions: switch between Diversity and Spatial Multiplexing

SpatialMultiplex

Diversity

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Scope of RRM Management and optimized utilization of the (scarce) radio resources: Provision for each service/bearer/user an adequate QoS (if applicable) Increasing the overall radio network capacity and optimizing quality

RRM is located in eNodeB

LTE-UE

Evolved Node B

(eNB)

X2

LTE-Uu

eNB

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56 TM51154EN04GLA2 Nokia Solutions and Networks 2014Presentation / Author / Date

Radio Admission Control ( RAC)

Objective: To admit or to reject the 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- Operator configures both max. number of established RRC connections and

max. number of active users per cell. RRC connection is established when the SRBs have been admitted and

successfully configured. UE is considered as active when Radio bearer is established

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LTE vs. R99 Scheduling

NodeB Rel. 99

eNodeB LTE

Fast pipe is shared among UEs

Dedicated pipe for every UE

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Scheduler Types

A variety of scheduling strategies is available.Examples are:- Round-RobinNo quality indication is taken into consideration. The resources are mainly shared in an equal

manner.

- Max C/I.The UE with the best channel conditions gets the highest priority. The cell throughput is

maximised. Starvation of UEs with channels of low quality may be a disadvantage.- Proportional Fair.This algorithm defines priorities based on the quality and the averaged scheduled rate. - QoSDifferent strategies exist to get QoS related information integrated.E.g. Depending on the priority of the service and/or the UE, RT/NRT service type. a scheduling

weight can be introduced.

Combinations of the different types can also be applied.

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59 TM51154EN04GLA2 Nokia Solutions and Networks 2014Presentation / Author / Date

Link Adaptation by AMC (UL/DL)

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 and coverage reliability.

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 use ( more robust)

Flexi Multimode BTS performs the link adaptation for DL and UL on a TTI basis

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Handover Types

E-UMTS micro cells

Intra-frequency HO(intra eNB)

intra-frequency HO(inter eNB, inter MME)

1a

interfrequency HO

other RAT

E-UMTS macro cell

intersystem HOtriggered by e.g. - coverage of E-UMTS- service load

1b

32

intersystem HOtriggered by other RAT

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RRM differences between LTE & UMTS

The main difference reflects decentralized RRM control moved to the edge of E-UTRAN (RRM resides at eNB) as opposed to the centralized RRM control in UMTS (RNC entity performs most RRM functions).

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 LTE radio 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.

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Appendix

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Radio Protocols Architecture

MAC

RLC

PDCP

Physical Layer

RRC

L1

L2

L3

Radio Bearer

Logical Channel

Transport Channels

Control Plane User Plane

Physical Channels

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Radio Protocols Architecture (1/2)

The EUTRAN radio protocol model specifies the protocols terminated between UE and eNB. The protocol stack follows the standard guidelines for radio protocol architectures (ITU-R M1035) and is thus quite similar to the WCDMA protocol stack of UMTS.

The protocol stack defines three layers: the physical layer (layer 1), data link and access layer (layer 2) and layer 3 hosting the access stratum and non-access stratum control protocols as well as the application level software (e.g. IP stack).

physical layer: The physical layer forms the complete layer 1 of the protocol stack and provides the basic bit transmission functionality over air. In LTE the physical layer is driven by OFDMA in the downlink and SC-FDMA in the uplink. FDD and TDD mode can be combined (depends on UE capabilities) in the same physical layer. The physical layer uses physical channels to transmit data over the radio path. Physical channels are dynamically mapped to the available resources (physical resource blocks and antenna ports). To higher layers the physical layer offers its data transmission functionality via transport channels. Like in UMTS a transport channel is a block oriented transmission service with certain characteristics regarding bit rates, delay, collision risk and reliability. Note that in contrast to 3G WCDMA or even 2G GSM there are no dedicated transport or physical channels anymore, as all resource mapping is dynamically driven by the scheduler.

MAC (Medium Access Control): MAC is the lowest layer 2 protocol and its main function is to drive the transport channels. From higher layers MAC is fed with logical channels which are in one-to-one correspondence with radio bearers. Each logical channel is given a priority and MAC has to multiplex logical channel data onto transport channels. In the receiving direction obviously demultiplexing of logical channels from transport channels must take place. Further functions of MAC will be collision handling and explicit UE identification. An important function

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Radio Protocols Architecture (2/2)RLC (Radio Link Control): Each radio bearer possesses one RLC instance working in either of the

three modes: UM (Unacknowledged), AM (Acknowledged) or TM (Transparent). Which mode is chosen depends on the purpose of the radio bearer. RLC can thus enhance the radio bearer with ARQ (Automatic Retransmission on reQuest) using sequence numbered data frames and status reports to trigger retransmission. Note that it shall be possible to trigger retransmissions also via the HARQ entity in MAC. The second functionality of RLC is the segmentation and reassembly that divides higher layer data or concatenates higher layer data into data chunks suitable for transport over transport channels which allow a certain set of transport block sizes.

PDCP (Packet Data Convergence Protocol): Each radio bearer also uses one PDCP instance. PDCP is responsible for header compression (ROHC RObust Header Compression; RFC 3095) and ciphering/deciphering. Obviously header compression makes sense for IP datagram's, but not for signaling. Thus the PDCP entities for signaling radio bearers will usually do ciphering/deciphering only.

RRC (Radio Resource Control): RRC is the access stratum specific control protocol for EUTRAN. It will provide the required messages for channel management, measurement control and reporting, etc.

NAS Protocols: The NAS protocol is running between UE and MME and thus must be transparently transferred via EUTRAN. It sits on top of RRC, which provides the required carrier messages for NAS transfer.

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FDD | TDD - Layer 1( DL: OFDMA, UL: SC-FDMA )

Medium Access Control (MAC)

Physical Channels

Transport Channels

RLC

Control)

RLC(Radio Link

Control)

PDCP

Convergence Protocol)

PDCP(Packet Data


RLC

Control)

RLC(Radio Link

Control)

PDCP


PDCP(Packet Data


RLC

Control)

RLC(Radio Link

Control)

PDCP


PDCP(Packet Data


RLC

Control)

RLC(Radio Link

Control)

Convergence

PDCP(Packet Data


RLC

Control)

RLC(Radio Link

Control)

Convergence

PDCP(Packet Data


Logical Channel

(E-)RRC(Radio Resource Control)

IP / TCP | UDP | Application Layer

Radio Bearer

ROHC (RFC 3095)

Security

Segment./Reassembly

ARQ

Scheduling /Priority Handling

HARQ

De/Multiplexing

CRC

Coding/Rate Matching

Interleaving

Modulation

Resource Mapping/MIMO

NAS Protocol(s)(Attach/TA Update/)

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LTE Air Interface Overview

Documents

Transcript of LTE Air Interface Overview