What affects LTE Cell throughput

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AIRCOM LTE Webinar Series:

What affects LTE Cell throughput

2 © 2013 AIRCOM International Ltd

About the Presenters

Graham Whyley – Lead Technical Trainer

AIRCOM Technical Master Trainer since 2005

Currently responsible for all LTE training course creation and delivery

Over 20 years of training experience at companies including British Telecom and Fujitsu

Contact us at [email protected]

Adam Moore – Learning & Development Manager

With AIRCOM since 2006

Member of CIPD


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Agenda-What affects LTE Cell throughput

Maximizing the data rate and spectral efficiency are the main targets in LTE cellular systems.

Transport Block Size

Codewords

LTE UE categories

What effects Cell throughput


What affects Cell throughput

L1/L2

IP

UDP

GTP-U

eNode B

L1

MAC

RLC

PDCP

Relay

L1

MAC

RLC

PDCP

UE

IP

Application

TCP/UDP

DATA

DATA

DATA

DATA

DATA

DATA


User Plane

Application Application Application Rate

TCP overhead UDP

Real Time Non Real Time

TCP overhead UDP


IP overhead IP overhead

RLC layer will concatenate or segment the data coming from PDCP layer into correct block size

RLC

PDCP

overhead RLC

PDCP

overhead


WHAT IS A TRANSPORT BLOCK

RLC HEADER

RLC HEADER

RLC

MAC MAC HEADER

TRANSPORT BLOCK

RLC

MAC

IP TCP /UDP


User Plane

Application Application Application Rate

TCP overhead UDP


TCP overhead UDP


IP overhead IP overhead

RLC

PDCP

overhead RLC

PDCP

overhead

L1

MAC

UE

overhead

overhead L1

MAC

UE

overhead

overhead

MAC layer selects the modulation and coding scheme configures the physical layer

QPSK 2 bits

16QAM 4 bits

64QAM 6bits

Different coding Rates


LTE UE categories Normal Cyclic Prefix

7 symbols = 0.5 ms

Freq

uen

cy D

om

ain

12

su

bca

rrie

rs =

18

0 k

Hz

Time Domain

Resource Element 2 bits 4 bits 6 bits


Now how many bits are

transferred in this 1ms

transport block size?

Modulation and coding scheme (MCS): The MCS index (0…31) is used by the base station to signal to the terminal the modulation and coding scheme to use for receiving or transmitting a certain transport block. Each MCS index stands for a certain modulation order and transport block size index


RRC Connection Reconfiguration

Message

Since the size of transport block is not fixed

MCS Index

|UE ID/RNTI Type |C-RNTI | |Subframe Number |2 | |UE ID/RNTI Value |'8627'H || |Transport Block Indicator |single TB info | |Modulation Order DL 1 |QAM64 | |New Data Indicator DL 1 |new data | |Redundancy Version DL 1 |0 | |Reserved |0 | |Modulation Scheme Index DL |24 |

RRC Connection Reconfiguration Message Modulation Scheme Index DL 24


How much bits are transferred in this

1ms transport block size?

It depends on: The MCS (modulation and coding scheme)

The number of resource blocks assigned to the UE

Normal Cyclic Prefix

7 symbols = 0.5 ms

Freq

uen

cy D

om

ain

12

su

bca

rrie

rs =

18

0 k

Hz

Time Domain

Extended Cyclic Prefix

6 symbols = 0.5 ms

12

su

bca

rrie

rs =

18

0 k

Hz

Time Domain

Resource Element 2 bits 4 bits 6 bits


Transport Block Size Tables Look-up table is referenced by the TBS Index and the number of

allocated Resource Blocks

RRC Connection Reconfiguration Message Modulation Scheme Index DL 24


POLL eNB assigns MCS index 12 and 2 resource blocks (RBs). What is the transport block size? 1. 56 2. 144 3. 616 4. 376 5. 440


POLL eNB assigns MCS index 12 and 2 resource blocks (RBs). What is the transport block size? 1. 56 2. 144 3. 616 4. 376 5. 440


Table 7.1.7.2.1-1 Look-up table is referenced by the TBS Index and the number of

allocated Resource Blocks


What affects LTE Cell throughput


Coding Rate


Coding rate

L1

MAC overhead

overhead L1

MAC overhead

overhead

MAC layer selects the modulation and coding scheme configures the physical layer

Code rate: The code rate is defined as the ratio between the transport block size and the total number of physical layer bits per subframe that are available for transmission of that transport block. The code rate is an indication for the redundancy that has been added due to the channel coding process


CQI

Modulation Efficiency Actual coding rate

Required SINR

1 QPSK 0.1523 0.07618 -4.46

2 QPSK 0.2344 0.11719 -3.75

3 QPSK 0.3770 0.18848 -2.55

4 QPSK 0.6016 308/1024 -1.15

5 QPSK 0.8770 449/1024 1.75

6 QPSK 1.1758 602/1024 3.65

7 16QAM 1.4766 378/1024 5.2

8 16QAM 1.9141 490/1024 6.1

9 16QAM 2.4063 616/1024 7.55

10 64QAM 2.7305 466/1024 10.85

11 64QAM 3.3223 567/1024 11.55

12 64QAM 3.9023 666/1024 12.75

13 64QAM 4.5234 772/1024 14.55

14 64QAM 5.1152 873/1024 18.15

15 64QAM 5.5547 948/1024 19.25

The coding rate indicates

how many real data bits

are present out of 1024

while the efficiency

provides the number of

information bits per

modulation symbol.

602/1024 = 0.5879

QPSK = 2bits

Efficiency=

2x0.5879=1.1758 data

bits per symbol

Coding Rate


Coding Rate

602/1024 = 0.5879

QPSK = 2bits

Efficiency=

2x0.5879=1.1758 data

bits per symbol

DL BEARER – 64QAM, Efficiency 5.5

SINR +19,25 High cell throughput

DL BEARER – QPSK Efficiency 0.1523

SINR -4.46 Low cell throughput


Coding Rate


CQI

Modulation Efficiency Actual

coding rate Required

SINR

1 QPSK 0.1523 0.07618 -4.46

2 QPSK 0.2344 0.11719 -3.75

3 QPSK 0.3770 0.18848 -2.55

4 QPSK 0.6016 308/1024 -1.15

5 QPSK 0.8770 449/1024 1.75

6 QPSK 1.1758 602/1024 3.65

7 16QAM 1.4766 378/1024 5.2

8 16QAM 1.9141 490/1024 6.1

9 16QAM 2.4063 616/1024 7.55

10 64QAM 2.7305 466/1024 10.85

11 64QAM 3.3223 567/1024 11.55

12 64QAM 3.9023 666/1024 12.75

13 64QAM 4.5234 772/1024 14.55

14 64QAM 5.1152 873/1024 18.15

15 64QAM 5.5547 948/1024 19.25

Coding Rate

CQI = 15

Terminal Density

High throughput


Code word

L1

MAC overhead

overhead

TRANSPORT BLOCK

• 24 bit checksum (CRC) to the transport block This CRC is used to determine whether the transmission was successful or not, and triggers Hybrid ARQ to send an ACK or NACK

codeword

Transmitter

Transport Block

Transport Block CRC

Compute CRC

Modulation

Receiver

Error detection

Demodulation

Transport Block CRC

NACK

Transport Block CRC

NACK

L1 converts the transport block into a code-word

Re-transmissions will reduce throughput


Adaptive re-transmission If the base station receives the data with errors Two ways for it to respond

1. The base station can trigger a non adaptive re-transmission by sending the mobile a negative acknowledgement on the PHICH. The mobile then re-transmits the data with the same parameters that it used first time around.

2. Alternatively, the base station can trigger an adaptive re-transmission by explicitly sending the mobile another scheduling grant. It can do this to change the parameters that the mobile uses for the re-transmission, such as the resource block allocation or the modulation scheme.

Scheduling grant maximum number of re-transmissions without receiving a positive response

Change parameters like uplink modulation scheme

QPSK for noisy channels


Code word If the transport block is too small, it is padded up to 40 bits If the Transport Block is too big, it is divided into smaller pieces, each of which gets an additional 24 bit CRC A codeword, then, is essentially a transport block with error protection. Note that a UE may be configured to receive one or two transport blocks (and hence one or two codewords) in a single transmission interval Maximum of 2 codewords used to limit signalling requirement (CQI reporting, HARQ acknowledgements, resource allocations)

L1

MAC

TRANSPORT BLOCK

codeword

L1

MAC

TRANSPORT BLOCK

codeword


Codeword

• Maximum of 2 codewords used to limit signalling requirement (CQI reporting, HARQ acknowledgements, resource allocations)

• Transmit diversity provides the fallback when only a codeword is transferred

The number of layers is always less than or equal to the number of antenna ports (transmit antennas).

Layer 1

Layer 2 Codeword 1


Transmit Diversity

Transmit diversity requires multiple antenna elements at the transmitter, and one or more antenna elements at the receiver

3GPP has specified transmit diversity schemes based upon using either 2 or 4 antenna elements at the transmitter

Transmit diversity transfers a single code word during each 1 ms subframe

Modulated

Codeword

Layer 1

Layer 2

Layer mapping for 2 layers Modulated

Codeword Layer 3

Layer 4

Layer mapping for 4 layers

Layer 1

Layer 2


4 Layers

Codewords Layers Mapping

2 4 The first codeword is split (odd/even) between the first two layers , the second codeword is split between the second two layers. Each codeword same length

Layer 1

Layer 2

Layer 3

Layer 4

4 layers – 2 codewords

Codeword 1

Codeword 2

Note that the number of layers is always less than or equal to the number of antenna ports (transmit antennas). The number of layers used in any particular transmission depends (at least in part) on the Rank Indication (RI) feedback from the UE


MIMO

MIMO can transfer either 1 or 2 code words during each 1 ms sub-frame

CQI reporting, link adaptation and HARQ run independently for each code word

Resource Allocation Type (0 or 1)

Resource Block Assignment

TPC Command for PUCCH

HARQ Process Number

Modulation and Coding Scheme

New Data Indicator

Redundancy Version

Modulation and Coding Scheme

New Data Indicator

Redundancy Version

Precoding Information

Transport Block 1 information

Transport Block 2 information

DCI Format 2

The scheduling commands for downlink transmissions are more complicated, and are handled in Release 8 by DCI formats 1 to 1D and 2 to 2A


Cell throughput

CQI = 1

CQI = 15

10Mhz

Maximizing the data rate and spectral efficiency are the main targets in LTE cellular systems.

CQI

Modulation Efficiency Actual

coding rate Required

SINR

1 QPSK 0.1523 0.07618 -4.46

2 QPSK 0.2344 0.11719 -3.75

3 QPSK 0.3770 0.18848 -2.55

4 QPSK 0.6016 308/1024 -1.15

5 QPSK 0.8770 449/1024 1.75

6 QPSK 1.1758 602/1024 3.65

7 16QAM 1.4766 378/1024 5.2

8 16QAM 1.9141 490/1024 6.1

9 16QAM 2.4063 616/1024 7.55

10 64QAM 2.7305 466/1024 10.85

11 64QAM 3.3223 567/1024 11.55

12 64QAM 3.9023 666/1024 12.75

13 64QAM 4.5234 772/1024 14.55

14 64QAM 5.1152 873/1024 18.15

15 64QAM 5.5547 948/1024 19.25


Spectral efficiency

Evolved

Node B

(eNB)

modulation and coding scheme

64QAM 6bits/Hz

64QAM 6bits/Hz

64QAM 6bits/Hz

64QAM 6bits/Hz

A 64 QAM the spectral efficiency cannot exceed N = 6 (bit/s)/Hz If a forward error correction (FEC) code with code rate 1/2 is added, meaning that the encoder input bit rate is one half the encoder output rate, the spectral efficiency is 50% of the modulation efficiency

Different Coding Rates (Bit/s)/Hz per cell It is a measure of the quantity of users or services that can be simultaneously supported by a limited radio frequency bandwidth

Efficiency 5.5547

Efficiency 5.1152

Efficiency 4.5234

Efficiency 3.9023


Maximum data rate for CQI bearer 1 Assumptions: 10 Mz Bandwidth Normal Prefix Coding rate 0.07618 MIMO 1x1

Bandwidth (MHz)

1.4 3 5 10 15 20

# of RBs

6 15 25 50 75 100

Subcarriers 72 180 300 600 900 1200 Normal Cyclic Prefix

7 symbols = 0.5 ms

Freq

uen

cy D

om

ain

12

su

bca

rrie

rs =

18

0 k

Hz

Time Domain

CQI bearer 1

All 50 PRB

MIMO 1x1


0 1 2 3 19

One Sub-frame = 1 mS

10 ms

Number of Traffic symbols bits in a TTI = (4 x12) + (7x12)-6 =126

If QPSK bearer =126 x 2 =252 bits in 1ms

Maximum data rate for CQI bearer 1

4 x12 7x12


7 symbols = 0.5 ms

Freq

uen

cy D

om

ain

12

su

bca

rrie

rs =

18

0 k

Hz

Time Domain


0 1 2 3 19


10 ms

Number of Traffic symbols bits in a TTI = (4 x12) + (7x12)-6 =126


In 1

0 M

hz

you

hav

e 5

0 P

RB

in 1

mS


In one TTI (1mS)you have 50 x 252 bits = 12600 bits per 1mS


0 1 2 3 19


10 ms Number of Traffic symbols bits in a TTI = (4 x12) + (7x12)-6 =126


In 1

0 M

hz

you

hav

e 5

0 P

RB

in 1

mS


In one TTI (1mS)you have 50 x 252 bits = 12600 bits per 1mS

Coding Rate 12600 bits x 0.07618=959.104 bits in 1ms

Bits per second =959.104 x 1000= 959104 kb/s =0.975 Mb/s in 10Mhz


What have we not taken into account?


Each Bearer has a maximum data rate

Antenna 1

1 ms

12 s

ub-c

arr

iers

Bits per second =959.104 x 1000= 959104 kb/s =0.975 Mb/s in 10Mhz

With

ou

t M

IMO

CQI 15

CQI 1

Low throughput

High throughput

With

ou

t M

IMO


Bearers W

ith

ou

t M

IMO


Physical Overhead W

ith

ou

t M

IMO

Antenna 1 Antenna 2


Coverage/Capacity

CQI 1

CQI 15

CQI 14

CQI 13

CQI 12

CQI 11

CQI 10

CQI 9

CQI 8

CQI 7

CQI 6

CQI 5

CQI 4

CQI 1 CQI 3

CQI 2


Summary Cell throughput is dependant on: • Modulation and coding scheme (MCS) (0…31)

and Transport block size • Bandwidth • Normal / Extended Prefix • Transmission modes TX diversity, Su-MIMO etc. • LTE UE categories

CQI

(MCS) (0…31)


7 symbols = 0.5 ms

Freq

uen

cy D

om

ain

12

su

bca

rrie

rs =

18

0 k

Hz

Time Domain


Next Topic

Comparison between GSM, UMTS & LTE


In Closing

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What affects LTE Cell throughput