LTE Radio Capacity & Throughput

AIRCOM LTE Webinar Series:

What affects LTE Cell throughput © 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

Adam Moore – Learning & Development Manager  With AIRCOM since 2006  Member of CIPD

Contact us at [email protected] 2

© 2013 AIRCOM International Ltd

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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 5


What affects Cell throughput DATA

Relay Application DATA

TCP/UDP

DATA

IP

PDCP

GTP-U

RLC

UDP

MAC

IP

L1

L1/L2

PDCP DATA

RLC

DATA

MAC L1

DATA

UE

6

eNode B


User Plane Application Rate

Application

Non Real Time

overhead

Non Real Time

Real Time

TCP

overhead

Application

UDP

overhead

IP

overhead

RLC

TCP

overhead

PDCP

Real Time UDP

IP

PDCP overhead

RLC

RLC layer will concatenate or segment the data coming from PDCP layer into correct block size 7


WHAT IS A TRANSPORT BLOCK RLC

MAC

TCP IP /UDP

RLC HEADER

RLC

RLC HEADER

MAC HEADER

MAC

TRANSPORT BLOCK

8


User Plane Application Rate

Application

Non Real Time

overhead

Non Real Time

Real Time

TCP

overhead

Application

UDP

overhead

IP

PDCP

16QAM 4 bits

64QAM 6bits

RLC

overhead

RLC

overhead

MAC

overhead

MAC

L1 UE

Different coding Rates

UDP

IP

PDCP

overhead

overhead

TCP

overhead

QPSK 2 bits

Real Time

overhead

L1 UE

MAC layer selects the modulation and coding scheme configures the physical layer 9


Normal Cyclic Prefix

12 subcarriers = 180 kHz

Frequency Domain

LTE UE categories

Resource Element 2 bits 4 bits 6 bits

7 symbols = 0.5 ms Time Domain

10


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

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RRC Connection Reconfiguration Message |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 |

Since the size of transport block is not fixed MCS Index

RRC Connection Reconfiguration Message Modulation Scheme Index DL 24 12


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

7 symbols = 0.5 ms Time Domain 13

Extended Cyclic Prefix



Frequency Domain


Resource Element 2 bits 6 symbols = 0.5 ms 4 bits Time Domain 6 bits © 2013 AIRCOM International Ltd

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

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POLL

15

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

16

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

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What affects LTE Cell throughput

18


Coding Rate

19


Coding rate overhead overhead

MAC L1

overhead overhead

MAC L1

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

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Coding Rate

21

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

602/1024 = 0.5879 QPSK = 2bits Efficiency= 2x0.5879=1.1758 data bits per symbol

SINR +19,25

High cell throughput

DL BEARER – 64QAM, Efficiency 5.5

SINR -4.46

Low cell throughput DL BEARER – QPSK Efficiency 0.1523

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Coding Rate

23


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

24

CQI = 15

High throughput

Terminal Density © 2013 AIRCOM International Ltd

Code word overhead

MAC

• 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 Receiver

Transmitter Transport Block

TRANSPORT BLOCK

Error detection

Compute CRC Transport Block

overhead

CRC

Demodulation

Modulation

L1

Re-transmissions will reduce throughput Transport Block

codeword L1 converts the transport block into a code-word 25

CRC

NACK Transport Block

CRC

NACK © 2013 AIRCOM International Ltd

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. Scheduling grant maximum number of re-transmissions without receiving a positive response Change parameters like uplink modulation scheme QPSK for noisy channels 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. 26


Code word MAC

MAC

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

TRANSPORT BLOCK TRANSPORT BLOCK

A codeword, then, is essentially a transport block with error protection. L1

L1

codeword

codeword

27

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)


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 Layer 1

Codeword 1 Layer 2

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

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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

Layer mapping for 4 layers Layer 1

Layer mapping for 2 layers Modulated Codeword

Layer 1

Layer 2 Layer 3

Layer 2 29

Modulated Codeword

Layer 4 © 2013 AIRCOM International Ltd

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

4 layers – 2 codewords Codeword 1

Layer 1 Layer 2

Codeword 2

Layer 3 Layer 4

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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 DCI Format 2 Resource Allocation Type (0 or 1) Resource Block Assignment TPC Command for PUCCH HARQ Process Number

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

Modulation and Coding Scheme New Data Indicator

Transport Block 1 information

Redundancy Version Modulation and Coding Scheme New Data Indicator

Transport Block 2 information

Redundancy Version 31

Precoding Information


Cell throughput 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

Maximizing the data rate and spectral efficiency are the main targets in LTE 10Mhz cellular systems.

CQI = 15

CQI = 1 32


Spectral efficiency Different Coding Rates 64QAM 6bits/Hz

Efficiency 4.5234

64QAM 6bits/Hz

64QAM 6bits/Hz

Efficiency 5.5547

64QAM 6bits/Hz

modulation and coding scheme Efficiency 3.9023

Efficiency 5.1152

Evolved Node B (eNB)

(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

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

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

Frequency Domain



Bandwidth 1.4 (MHz)

3

5

10

15

20

# of RBs

6

15

25

50

75

100

Subcarriers

72

180

300

600

900

1200

All 50 PRB CQI bearer 1 MIMO 1x1 7 symbols = 0.5 ms Time Domain

34


Maximum data rate for CQI bearer 1 10 ms

0

1

2

3

19

One Sub-frame = 1 mS

7x12

Number of Traffic symbols bits in a TTI = (4 x12) + (7x12)-6 =126 If QPSK bearer =126 x 2 =252 bits in 1ms


4 x12

Frequency Domain


7 symbols = 0.5 ms Time Domain

35



0

1

2

3

19

In 10 Mhz you have 50 PRB in 1mS


36

Number of Traffic symbols bits in a TTI = (4 x12) + (7x12)-6 =126 If QPSK bearer =126 x 2 =252 bits in 1ms

In one TTI (1mS)you have 50 x 252 bits = 12600 bits per 1mS © 2013 AIRCOM International Ltd


0

1

2

3

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

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

19


In 10 Mhz you have 50 PRB in 1mS

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

37

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 © 2013 AIRCOM International Ltd

What have we not taken into account?

38


Each Bearer has a maximum data rate

Antenna 1

High throughput 1 ms CQI 15

Low throughput CQI 1

39

Without MIMO

12 sub-carriers

Without MIMO

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


Without MIMO

Bearers

40


Without MIMO

Physical Overhead

41

Antenna 1

Antenna 2


Coverage/Capacity 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 3

CQI 1 CQI 1

CQI 2

42


Summary

(MCS) (0…31)

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


Frequency Domain


7 symbols = 0.5 ms

Time Domain

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Next Topic

Comparison between GSM, UMTS & LTE

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In Closing  Thank you for attending  Webinars webpage – keep up to date and register to receive email alerts on new webinars http://www.aircominternational.com/Web inars.aspx

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LTE Radio Capacity & Throughput

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