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 training@aircominternational.com
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© 2013 AIRCOM International Ltd
About AIRCOM
AIRCOM is the leading provider of mobile network planning,
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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
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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
© 2013 AIRCOM International Ltd
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
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WHAT IS A TRANSPORT BLOCK
RLC
MAC
TCP
IP
/UDP
RLC
HEADER
RLC
RLC
HEADER
MAC HEADER
MAC
TRANSPORT BLOCK
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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
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© 2013 AIRCOM International Ltd
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
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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
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© 2013 AIRCOM International Ltd
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
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Extended Cyclic Prefix
12 subcarriers = 180 kHz
12 subcarriers = 180 kHz
Frequency Domain
Normal Cyclic Prefix
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
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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
© 2013 AIRCOM International Ltd
POLL
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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
© 2013 AIRCOM International Ltd
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
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Coding Rate
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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
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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
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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
© 2013 AIRCOM International Ltd
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
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© 2013 AIRCOM International Ltd
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
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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
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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.
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© 2013 AIRCOM International Ltd
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
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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)
© 2013 AIRCOM International Ltd
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
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Modulated
Codeword
Layer 4
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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
© 2013 AIRCOM International Ltd
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
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Precoding Information
© 2013 AIRCOM International Ltd
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
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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
12 subcarriers = 180 kHz
Normal Cyclic Prefix
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
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© 2013 AIRCOM International Ltd
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
12 subcarriers = 180 kHz
4 x12
Frequency Domain
Normal Cyclic Prefix
7 symbols = 0.5 ms
Time Domain
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© 2013 AIRCOM International Ltd
Maximum data rate for CQI bearer 1
10 ms
0
1
2
3
19
In 10 Mhz you have 50 PRB in 1mS
One Sub-frame = 1 mS
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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
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
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One Sub-frame = 1 mS
In 10 Mhz you have 50 PRB in 1mS
In one TTI (1mS)you have
50 x 252 bits = 12600 bits per 1mS
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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?
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Each Bearer has a maximum data rate
Antenna 1
High throughput
1 ms
CQI 15
Low throughput
CQI 1
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Without MIMO
12 sub-carriers
Without MIMO
Bits per second
=959.104 x 1000= 959104 kb/s
=0.975 Mb/s in 10Mhz
© 2013 AIRCOM International Ltd
Without MIMO
Bearers
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Without MIMO
Physical Overhead
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Antenna 1
Antenna 2
© 2013 AIRCOM International Ltd
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
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© 2013 AIRCOM International Ltd
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
12 subcarriers = 180 kHz
Frequency Domain
Normal Cyclic Prefix
7 symbols = 0.5 ms
Time Domain
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© 2013 AIRCOM International Ltd
Next Topic
Comparison between GSM, UMTS & LTE
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In Closing
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