AIRCOM LTE Webinar Series:
What affects LTE Cell throughput
About the Presenters Graham Whyley Lead Lead Technical Trainer AIRCOM Technical Technical Master Trainer Trainer since 2005 2005 Currently responsible for all LTE LTE training course creation and delivery Over 20 years of training experience at companies including British Telecom Telecom and Fujitsu –
Adam Moore Learning Learning & Development Manager With AIRCOM since 2006 Member of CIPD –
Contact us at
[email protected]
About the Presenters Graham Whyley Lead Lead Technical Trainer AIRCOM Technical Technical Master Trainer Trainer since 2005 2005 Currently responsible for all LTE LTE training course creation and delivery Over 20 years of training experience at companies including British Telecom Telecom and Fujitsu –
Adam Moore Learning Learning & Development Manager With AIRCOM since 2006 Member of CIPD –
Contact us at
[email protected]
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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
DATA
Relay Application DATA
TCP/UDP
DATA
IP
PDCP
GTP-U
RLC
UDP
MAC
IP
L1
L1/L2
PDCP DATA DATA
DATA
RLC MAC L1 UE
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
WHAT IS A TRANSPORT BLOCK RLC
MAC
TCP IP /UDP
RLC HEADER
RLC
RLC HEADER
TRANSPORT BLOCK
MAC HEADER
MAC
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
Normal Cyclic Prefix
LTE UE categories n i a m o D y c n e u q e r F
z H k 0 8 1 = s r e i r r a c b u s 2 1
Resource Element 2 bits 4 bits 6 bits
7 symbols = 0.5 ms Time Domain
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 |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
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
n i a m o D y c n e u q e r F
z H k 0 8 1 = s r e i r r a c b u s 2 1
Extended Cyclic Prefix
z H k 0 8 1 = s r e i r r a c b u s 2 1
7 symbols = 0.5 ms Time Domain
Resource Element 2 bits 6 symbols = 0.5 ms 4 bits Time Domain 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 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
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
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
Coding Rate
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
CQI = 15
High throughput
Terminal Density
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
CRC
Demodulation
Modulation
overhead
L1
Re-transmissions will reduce throughput Transport Block
codeword L1 converts the transport block into a code-word
CRC
NACK Transport Block
NACK
CRC
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.
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
codeword
L1
codeword
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).
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
Modulated Codeword
Layer 2 Layer 3
Layer 2
Layer 4
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
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 Redundancy Version
Transport Block 2 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
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
Maximum data rate for CQI bearer 1 Assumptions: 10 Mz Bandwidth Normal Prefix Coding rate 0.07618 MIMO 1x1 Normal Cyclic Prefix
n i a m o D y c n e u q e r F
z H k 0 8 1 = s r e i r r a c b u s 2 1
Bandwidth (MHz)
1.4
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
Maximum data rate for CQI bearer 1 10 ms
0
1
2
3
19
One Sub-frame = 1 mS
Normal Cyclic Prefix
4 x12
7x12
Number of Traffic symbols bits in a TTI = (4 x12) + (7x12)-6 =126 If QPSK bearer =126 x 2 =252 bits in 1ms
n i a m o D y c n e u q e r F
z H k 0 8 1 = s r e i r r a c b u s 2 1
7 symbols = 0.5 ms Time Domain
Maximum data rate for CQI bearer 1 10 ms
0
1
2
3
19
One Sub-frame = 1 mS
S m 1 n i B R P 0 5 e v a h u o y z h M 0 1 n I
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
Maximum data rate for CQI bearer 1 10 ms
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
One Sub-frame = 1 mS
In one TTI (1mS)you have 50 x 252 bits = 12600 bits per 1mS S m 1 n i B R P 0 5 e v a h u o y z h M 0 1 n I
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 Bits per second =959.104 x 1000= 959104 kb/s =0.975 Mb/s in 10Mhz O M I M t u o h t i 2 W 1
s r e i r r a c b u s
Antenna 1
High throughput
1 ms CQI 15
Low throughput CQI 1
O M I M t u o h t i W
Bearers
O M I M t u o h t i W
Physical Overhead
O M I M t u o h t i W
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 2
CQI 1 CQI 1
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
• •
Normal Cyclic Prefix
• •
n i a m o D y c n e u q e r F
z H k 0 8 1 = s r e i r r a c b u s 2 1
7 symbols = 0.5 ms
Time Domain
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