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Graduate Paper Kate Gleason College of Engineering
Rochester Institute of Technology Dubai Silicon Oasis
Dubai, U.A.E.
Copyright © 2013 Rochester Institute of Technology
Hybrid Automatic Repeat Request in LTE
Chowdhury Mizan Mahmood Taher Dr. Muhieddin Amer
Student of Electrical Engineering Professor of Electrical Engineering
ABSTRACT
Wireless communication systems are
continuing to grow and evolve amidst high
performance, expectations and requirements.
Therefore the 3GPP has considered LTE to ensure
future competitions. Obtaining accurate channel
quality is not feasible due to various errors. These
errors tend to degrade the system throughput. The
Hybrid ARQ (HARQ) comes into use in such
situations to provide fast re-transmission and
lowering the error rate in the physical link. A HARQ
mechanism allows one to mend the errors using
techniques such as Incremental Redundancy and
Chase Combining along with certain protocols
depending on the timing and modulation selection. In
this paper, the various HARQ schemes will be
discussed. To save the radio resources, the number of
HARQ retransmission in the downlink may be
limited. Therefore adequate result using an algorithm
proposed in an earlier research work has been
provided in this paper to make more efficient
utilization of the radio link. This paper also focuses
on the performance of HARQ schemes in the
OFDMA downlink operation. The throughput and
PER of these schemes are compared based on the
buffering memory available.
NOMENCLATURE
ARQ – Automatic Repeat Request
ACK – Acknowledgement
BLER – Block Error Rate
CC – Chase Combining
CRC – Cyclic Redundancy Check
DL - Downlink
eNB – e Node Base Station
HARQ – Hybrid Automatic Repeat Request
IR – Incremental Redundancy
MCS – Modulation and Coding Scheme
NACK – Negative Acknowledgement
OFDMA – Orthogonal Frequency Division
Multiple Access
PER – Packet Error Rate
RTT – Round Trip Time
SAW – Stop and Wait
SINR – Signal to Interface Noise Ratio
SNR – Signal to Noise Ratio
TTI – Transmission Time Interval
UE – User Equipment
I. INTRODUCTION
Data transmissions in wireless channels are
subject to errors because of variations in the signal
quality received. Link Adaptation can handle such
errors to some degrees. However, counteractions
cannot be done to receiver noise and interference that
are unpredictable. Therefore a Forward Error
Correction is used in all wireless systems. The main
principle beyond forward error-correction coding is
to introduce redundancy in the transmitted signal. In
this, the parity bits are added to the information bits
prior to transmission. These parity checks are
computed from the information bits using a method
given by the coding structure used.
The other approach to handle transmission errors
is to use Automatic Repeat Request. In this approach,
the receiver employs an error detection code to detect
the received packet contains error or not. A positive
Acknowledgement (ACK) is sent by the receiver to
the transmitter when no error is detected in the
packet. In case of an error occurring, the receiver
discards the received error packet and transmits a
negative Acknowledgement (NACK) to the
transmitter. Thus the transmitter re-transmits the
information after receiving the NACK.
2
Most of the modern wireless systems, including
LTE deploy Hybrid ARQ (HARQ), which uses a
combination of Forward Error Coding (FEC) and
ARQ scheme in which unsuccessful attempts are
used in FEC decoding instead of being discarded.
The received packets are discarded and the receiver
requests retransmissions of corrupted packets.
The first proposal of Hybrid ARQ was in [14]
and since then, numerous publications have appeared
(see literature [13] and references therein). In
principle, any error-detection and error correction
code can be used. But most of the practical hybrid
ARQ schemes rely on cyclic redundancy check code
for error detection and convolution or turbo codes for
error correction.
A. ARQ Protocols:
The ARQ can be divided into three categories:
Stop-And-Wait (SAW)
Go-Back-N and
Selective repeat protocols.
SAW Stop-and-wait ARQ is the simplest type of
Scheme. A transmitter sends one packet at a time.
After sending a packet, the transmitter waits for
acknowledgment (ACK) or negative
acknowledgment (NACK) and does not send any new
packets until it receives either an ACK or a NACK as
shown in Figure 1. In case of successful transmission,
the receiver sends an ACK and the transmitter
transmits the next information. On the other hand if
decoding of a packet is failed, a NACK signal is sent
by the receiver.
Figure 1 – SAW
Thus, the transmitter retransmits the missing
packet. It can be seen that the stop-and-wait protocol
requires the receiver to buffer at most one packet that
is currently being decoded. High transmission delays
are a major disadvantage for SAW protocol. This is
because the transmitter has to wait for ACK/NACK
feedback before proceeding to the next transmissions.
Therefore, the waiting times can be quite long due to
a combination of transmission delays as well as
receiver processing times. In the LTE system, the
packet transmission time is only one subframe (1
ms). But it requires seven subframe (7 ms) waiting
time before the packet in error can be retransmitted
using Hybrid ARQ.
Go-Back-N
Waiting time problem associated with the
SAW protocol is solved using the Go-Back-N. In this
case, the transmitter does not wait for the ACK to be
received from the receiver and keeps sending a
number of packets specified by a window size as
shown in Figure 2.
Figure 2 – Go-Back-N
When a negative acknowledgment (NACK)
for a missing packet is received, the transmitter starts
retransmitting packets starting from the missing
packet. Here also buffering is done at most one
packet at the receiver. Due to the small memory
3
available for buffering, it can be noted that after
sending a NACK, the receiver ignores all subsequent
packets until it receives retransmission for the
missing packet. The main drawback of the Go-Back-
N approach is duplicate transmissions as the
transmitter retransmits some packets that are already
successfully decoded at the receiver.
Selective Repeat ARQ:
Duplicate transmission and waiting time is
eventually solved by Selective Repeat. In selective
repeat ARQ protocol, the sending process continues
to send a number of packets specified by a window
size even after loss of packet as shown in Figure 3.
Figure 3 – Selective Repeat Request
In this system, the receiving process will
continue to accept and acknowledge packets sent
after an error occuring. Therefore buffering takes
place in both transmitter and receiver. We note that
the selective repeat ARQ protocol retransmits only
the missing packets and therefore avoids the
duplicate transmissions problem of Go-Back-N. In
addition, as the packets can be sent continuously,
unlike stop-and-wait, there is no waiting time
problem.
B. HARQ transmission based on timing and
Adaptation
Figure 4 – HARQ classification
Figure 4 lists the various types of HARQ
transmission based on the timing and adaptation. The
4 types namely, Synchronous Non-Adaptive,
Synchronous Adaptive, Asynchronous Non-Adaptive
and Asynchronous Adaptive, have been described in
the following lines.
Synchronous Non-Adaptive
As the name suggests, in synchronous
HARQ protocol, the retransmissions happen at fixed
time intervals. With N = 8, if the first subblock (SB)
is transmitted in time subframe number 0, the first
retransmission attempt can only take place in
subframe number 8 and similarly the second
retransmission in subframe number 16 as shown in
figure 5.
Figure 5 – Synchronous Non-Adaptive [4]
The main benefit of this type of HARQ is
that the control information needs to be transmitted
along with the first subblock only. However, a major
drawback is that the retransmitted subblocks cannot
be scheduled at time-frequency resources
experiencing good channel conditions at the time of
retransmissions. Moreover, the MCS and resource
4
format cannot be adapted at the time of
retransmission according to the prevailing channel
conditions.
Synchronous Adaptive
Figure 6 shows an example of Synchronous
Adaptive HARQ. It allows one to change the
resource allocation and MCS information for
retransmissions while maintaining the time
synchronicity.
Figure 6 – Synchronous Adaptive [4]
In this case, the control information is sent
along with the retransmissions as the resource
allocation, MCS and MIMO precoding can change
for retransmissions. The synchronous adaptive
hybridARQ scheme thus allows scheduling
retransmissions at frequency resources experiencing
good channel conditions at the time of the
retransmissions and hence recuperates some
frequency-selective scheduling gains.
Asynchronous Non-Adaptive
An asynchronous non-adaptive hybrid ARQ
protocol allows one to schedule retransmissions in
time as shown in Figure 7. In such HARQ, resource
allocation, MCS and MIMO formats kept the same as
the initial transmission. Only the control information
carrying UE ID, hybrid ARQ process and redundancy
version is carried with every retransmission. When
the prevailing channel quality is good, the time-
domain channel sensitive scheduling can be
performed for retransmissions.
Figure 7 – Asynchronous Non-Adaptive [4]
The main drawback of this scheme is limited
flexibility. The retransmissions resource allocation,
MCS and MIMO formats cannot be adapted.
Asynchronous Adaptive
An asynchronous adaptive hybrid ARQ
scheme [6] provides full flexibility for
retransmissions. Retransmissions are done the same
way as the original transmissions as shown in Figure
8. Both the Adaptation and Timing are adjusted
according to the channel quality. Therefore,
retransmission timing, resource allocation, MCS and
MIMO formats are all adapted according to the
prevailing channel and resource conditions at the
time of retransmission.
Figure 8 –Asynchronous Adaptive
One major drawback of this HARQ is that the
full control information needs to be sent with
retransmissions. Even if the timing, resource
allocation, MCS and MIMO formats of the
5
retransmissions are unchanged relative to the first
transmission, the control information in
asynchronous adaptive HARQ needs to be
transmitted. The receiver decodes a packet when it
receives the control information, which indicates the
presence of a transmission.
C. Flexibility v/s Overhead
Figure 9 – Flexibility v/s Overhead [4]
Depicted in Figure 9 is flexibility versus
overhead trade off for various hybrid ARQ schemes.
The synchronous non-adaptive scheme occupies the
lowest overhead as well as provides the lowest
flexibility. On the other hand, the asynchronous
adaptive scheme provides the best flexibility at the
expense of the largest overhead. The synchronous
adaptive scheme and the asynchronous non-adaptive
scheme provide some flexibility such as avoiding
resource conflicts with persistent allocations with
intermediate overhead. The overhead for the
synchronous adaptive scheme is expected to be larger
than the asynchronous adaptive scheme as resource
allocation generally contributes the most to the total
overhead.
D. Scheduling grant control message
Figure 10 – Scheduling grant control message
contents [4]
As mentioned earlier the Hybrid ARQ sends
control information along with the data. The Figure
10 shows all the fields that the control message
contains. UE ID indicates the User Equipment (UE)
for which the data transmission is intended. The new
data indicator (NDI) is used to indicate if a subblock
is a new packet transmission or to retransmission for
a previous packet. The resource assignment indicates
which time frequency resource units are allocated to
the UE. The modulation indicates one of the
supported modulations. The payload size or transport
block size gives the data information block size. The
hybrid ARQ information consists of hybrid ARQ
process number, redundancy version and new data
indicator. The MIMO control information includes
information on transmission rank and precoding, etc.
E. Hybrid ARQ with Soft Combining
The main objective of soft combining is to store
received packet in a buffer memory and combining
with the data retransmission to obtain a single packet
that is much more reliable than its original
constituent. The simplest form of such scheme was
proposed by Chase [5] in 1985. Therefore it was
named Chase Combining. This scheme involves the
retransmission by the transmitter of the same coded
data packet. The receiver decodes these packets and
combines these multiple copies of the transmitted
packet.
Figure 11 – Chase Combining [16]
Chase combining can be seen as additional
repetition coding as each retransmission is an
identical copy of the original transmission. Therefore
the accumulation of received Eb/N0 for every
retransmission occurs, due to the fact that no new
redundancy is being transmitted.
Incremental redundancy is another type of
soft combined Hybrid ARQ. In this scheme,
progressive parity packets are sent in each
transmission of the packet, instead of sending simple
repeats of the coded data packet. The decoder later
combines all the transmissions and decodes the
packet at a lower code rate.
6
Figure 12 – Incremental Redundancy [16]
It is proved that incremental redundancy can
give superior performance due to coding gain at
retransmissions. However, this gain comes at the
expense of additional UE complexity because the
buffering required in the case of incremental
redundancy is higher than in the case of Chase
combining.
The LTE system supports the operation of Chase
combining as well as incremental redundancy.
II. Analysis of HARQ in 3GPP LTE
Previously we have seen the working of 2
main Hybrid ARQ techniques, Chase Combing and
Incremental Redundancy. The effectiveness of these
2 techniques has been studied and analyzed
extensively. This section will look into the general
performance characteristics of each of these schemes.
The overall performance of both the schemes, in
terms of BLER for respective SINR will be studied
and analyzed thoroughly. The delay in system and the
effect of HARQ on the system delay are also looked
into.
In the following subsections of this section,
a suitable system model will be implemented. Before
that we shall see how the Hybrid ARQ works in LTE
system. After that the HARQ model for this
simulation will be considered. Finally the results of
the simulations are presented.
A. Hybrid ARQ in LTE system
In LTE system incremental redundancy (IR)
based hybrid ARQ with Chase combining as a special
case of IR is employed. In terms of timing and
adaptivity, asynchronous adaptive (AA) hybrid ARQ
is used in the downlink while synchronous adaptive
hybrid ARQ is employed in the uplink. The new data
indicator (NDI) field in the uplink scheduling grant is
used to indicate if the grant is for a retransmission of
a previous transmission or grant for a new transport
block transmission.
If the control message is received with the NDI
bit toggled, this means that eNB is scheduling a new
uplink transmission. On the other hand, if NDI is not
toggled, this means a retransmission of the previous
transmission attempt. Moreover, if no uplink
scheduling assignment is received while an ACK is
received on the Physical Hybrid Automatic Repeat
Request Indicator Channel (PHICH), this indicates
successful transmission of the uplink transport block.
B. Number of Hybrid ARQ processes
The N-channel SAW deployed in LTE
consists of number of channels or number of HARQ
processes depending on the buffering and delays.
This is defined as the number of HARQ processes
that can be initiated at a given time. This Number is
given by:
[( )
] (1)
In this equation:
= Propagation time between the eNB and UE
= Subblock transmission time
= Processing time of UE
= Transmission time of Ack/Nack
= Processing time of eNB
Figure 13 shows the relationship between all
the above parameters represented in a Round Trip
Time (RTT)
Figure 13 – RTT [4]
7
It can be seen that the propagation time for
the cell sizes between eNB and UE is much smaller
than the subblock transmission and the time required
for processing. Therefore this propagation time can
be neglected. Compromising between latency and
signal overhead, the is chosen to be one subframe
(1ms). This was because a smaller subblock
transmission time will only accommodate smaller
information on the transport block. Thus it requires a
larger overhead.
, the UE processing time, is selected
based on the compromise of latency and complexity.
This is due to the UE complexity increases for a
smaller value of . The eNB processing time
accounts for the decoding of the ACK/NACK by the
eNB and also the scheduling for the new transport
block. and in LTE are selected equal to 1
subframe (3ms).
is the time required for the transmission
of ACK/NACK. The value of this is considered 1
subframe (1ms) in the LTE system.
Parameter Symbol Value
Propagation
Time Negligible
Subblock
Transmission
Time
1ms
UE Processing
Time 3ms
Ack
Transmission
Time
1m
eNB Processing
Time 3ms
Table 1 – RTT parameters for LTE [4]
Table 1 summarizes all the values used in
the Round Trip Time (RTT). Using all the values
from the table in the equation for , the
following numerical value can be obtained.
[( )
]
Therefore 8 HARQ processes have been
obtained from this analysis. Furthermore, it can be
seen that an ACK/NACK response for a given
subframe is transmitted in n+4 subframe. This is
illustrated in figure 14.
Figure 14 – HARQ in LTE [4]
C. Channel Quality Indicator
Channel Quality Indicator (CQI) is a quantity of 4-
bits which indicate the maximum rate of data that can
be handled by the mobile. This quantity depends on
the signal to interface plus noise ration on the
receiving end, as a high data rate is achieved at high
SINR. But advanced receivers can be exceptional by
providing high data rate at low SINR as this factor
also depends on the implementation of the receiver.
Table 2 – CQI table [19]
Table 2 shows the CQI table in terms of downlink
modulation scheme and the coding rate. The base
station uses this received CQI in order to calculate
the modulation and coding scheme. And it uses only
1 modulation and coding scheme per transport block
in the downlink transmission.
D. System Model
Given the SINR and MCS, the BLER is
derived based on early models put forward by
Chawla and Goldsmith [7, 8]. The following equation
is used in deriving the BLER
(2)
8
= SINR, valid for
M = Modulation coding scheme used
Where ,
The efficiency of modulation code rate is
given by [9]
(3)
Can be written as
(4)
= Signal received power
= Noise density
= Bandwidth
The implementation was carried out in a
single cell with users in varying number. It has been
assumed that 25RBs are used in a cell. One eNodeB
is needed to control the cell. Users report the SINR to
the eNodeB.
The HARQ process used is asynchronous
HARQ process in the downlink and a synchronous
HARQ process in the uplink. Both these processes
have been discussed earlier. Asynchronous will allow
eNodeB to transmit whenever it has packet available
to be scheduled. While a synchronous HARQ can
only transmit in a fixed time slot.
A Channel Quality Indicator (CQI) concept
is used here which maps the SINR to the transmitted
data bits and the modulation coding scheme. Based
on some previous work, a simplified version of the
CQI table is used here in the implementation. This
CQI is used by eNodeB to determine the modulation
and coding scheme.
E. Algorithm and HARQ model
HARQ helps in delivering service levels.
Priority to packets needing transmission can be
provided by tying a scheduler closely with HARQ.
Figure 15 – Flowchart of ACK/NACK process
A flowchart of the ACK/NACK process is
depicted in the figure 15. It can be seen that if the
random assigned number is below a threshold BLER,
the packet seemed to be corrupted or contain error.
Therefore a NACK is being sent in this case.
HARQ uses both Chase Combining and
Incremental Redundancy. Therefore models of these
2 methodologies are looked into on the LTE type
environment.
Frame K received
from transmitter
Random assigned
number checked
K>effective
BLER
Send ACK Send NACK
Return to main
Yes No
9
Figure 16 – Flowchart of Incremental
Redundancy
Figure 17 – Model of Chase Combining
The working of chase combining as well as
incremental redundancy was previously discussed.
These 2 flowcharts explain the operations that take
place in these methodologies.
The CQI is mapped using some simplified
values for this model. SINR is included in here. The
eNodeB uses the CQI to calculate the MCS. Table 1
depicts the downlink SINR to data rate mapping.
Table 3 – Downlink SINR to Data Rate
mapping [17]
F. Simulation and System Parameters
C++ programming language has been used
here for the coding and compiling of the simulation.
Therefore a faster execution of the program was
possible.
LTE standards were emulated without
introducing complexity to the system. But some extra
requirements had to be added. Firstly, the eNodeB
needs to wait for 8ms before it can send a
retransmission of the packet [3]. The time taken to
send an Ack/Nack is 1ms by the UE. The eNodeB
and UE takes 3ms each in processing the frame.
Therefore a total of 8ms is required for the data
transmission to the receiving of the Ack/Nack by
eNodeB.
Secondly, parallel running of the HARQ
process was made ensured. It has been proved in part
B of section II that the number of HARQ processes
used in LTE is 8. Therefore 8 parallel HARQ
processes are kept running. Different time slots are
allocated for the transmitter to run several HARQ
processes. So the transmitter is able to send frames to
the same corresponding user but with different
HARQ processes.
In the system parameter, we consider a
single cell environment consisting of 1-40 users. At a
particular instance, not every user has data to send.
NACK received
Retransmit
<=4
Yes
Drop code
rate to next
level down
Send packet
with parity bits
to scheduler
Finish
Discard the
packet and
notify
scheduler
No
NACK received
Retransmit
<=4 Re-queue
packet for
transmission
Send packet to
scheduler for
retransmission
Discard the
packet and
notify scheduler Finish
Yes
No
10
Data frames can be lost en-route or corrupted due to
random fluctuations created in the channel. The
probability of ACK being interpreted with NACK is
[10]. Therefore it has been excluded from the
simulation.
As seen earlier in the flowchart, in case of a
NACK, a packet will go through the process of being
re-scheduled and finally getting discarded if the
threshold time is exceeded.
G. Results
Results were obtained after running several
batches of files, with each file being repeated in the
same simulation 20 times.
A contrast between Chase Combining and
Incremental Redundancy is obtained first. This
simulation was done for 64 QAM.
Figure 18 – Chase Combining v/s Incremental
Redundancy [1]
The figure 18 clearly shows Incremental
Redundancy outperforming Chase Combining.
Previous works on this issue was studied earlier in
literature [11].
System delay is an important aspect of data
transmission. We create a situation where no HARQ
is used, i.e. the transmitter doesn’t wait for the
ACK/NACK and no re-transmission of packet takes
place. This situation has been simulated against using
Incremental Redundancy and Chase Combining.
Figure 19 – System Delay performance [1]
It can be seen from figure 19 that
introducing HARQ to the system has a huge impact
on the system delay. A difference of roughly 50ms is
spotted from the graph. Thus the usage of HARQ
increases the delay in the system.
Now the question arises whether using
HARQ will be beneficial or not? The answer can be
found by studying the simulated figure 20 of
throughput versus the BLER between HARQ and no
HARQ.
Figure 20 – System Throughput versus BLER [1]
Figure 20 clearly shows that using HARQ
has better throughput when BLER is around or
higher. Thus HARQ provides higher throughput at
the expense of system delay.
11
III. Limiting HARQ retransmission in
the downlink for poor radio condition
HARQ is implemented by the MAC module
called the HARQ entity in LTE. The techniques by
which HARQ helps to recover data have been
discussed in the previous sections. But it does so with
the expense of consuming radio resources. In this
section, a proposal based on a previous work is
analyzed to limit the maximum HARQ
retransmission in order to save radio resources for
UEs in the state of poor radio link conditions.
The Physical Downlink Control Channel
(PDCCH) is used by eNodeB to signal about the
allocation of resources to be shared by UEs. The
resources allocated to the PDCCH can be varied. But
in case of amount of resources being too small, the
UL and DL data schedulers will not be able to
schedule all the UEs needed to be served. Whereas if
the amount is too large, then the resources that could
have been used for data transmission will be wasted.
These allocations of resources to the PDCCH are
addressed in the literature [12].
A. Proposal and Analysis
In order to recover data correctly, the HARQ
retransmissions consume radio resources. However,
the maximum number of retransmissions in downlink
is proposed to be limited and made less than 3 to save
radio resources with poor radio conditions for the
UEs.
The following points will justify this proposal.
At a poor radio link condition, an UE does
not have the ability to make good use of the
resources. The uses of resources are done
less efficiently and thus leading to lower cell
throughput. Therefore the resources that can
be saved from reduced HARQ
retransmissions can be utilized properly by
the UE during the good radio link and thus
improving cell throughput.
The eNodeB has the ability to easily
implement reduction of the number of
HARQ retransmissions in downlink. The UE
sends the CQI report to the eNodeB
indicating the quality of the downlink radio
link. Whenever the CQI value is too low, the
eNodeB can simply reduce the maximum
number of HARQ retransmissions. The
eNodeB may use a mapping of maximum
number of HARQ retransmissions with CQI
value. It is for eNodeB to decide of how
much limitation of retransmission should be
applied with what value of CQI.
As seen earlier, the HARQ retransmission
uses the asynchronous adaptive in the
downlink. So the full control information
has to be sent with each transmission
leading to significant signaling overhead on
PDCCH. Thus, additional resources can be
saved by avoiding overhead signaling when
the numbers of retransmissions are reduced.
Even after all the 3 retransmission, more
likely chances exist for the receiver to fail in
decoding the data packet in a poor radio link
condition. Thus, the radio resources for the
attempt are wasted in the retransmissions.
But this scenario can be minimized if lesser
retransmissions are used leading to lesser
wastage during the poor channel link
B. Results
The simulation was done using a LTE Link
Level Simulator with poor radio link conditions.
HARQ retransmissions have been performed from 0
to 3 in the poorest radio link condition of CQI values
1 and 2. The following values in tables are taken
from the simulated outputs of SINR versus BLER
plots [2].
Analysis for CQI 1
BLER
SNR 0 tx 1 re tx 2 re tx 3re tx
-9.5 0.83 0.72 0.61 0.6
-9 0.8 0.67 0.57 0.58
-8 0.7 0.53 0.5 0.5
-7.2 0.6 0.43 0.42 0.42
-6 0.42 0.4 0.33 0.33
-5 0.35 0.3 0.27 0.27
-4 0.25 0.2 0.2 0.2
-3 0.15 0.15 0.15 0.15
-2 0.1 0.1 0.1 0.1
-1 0.06 0.06 0.06 0.06
0 0.04 0.04 0.04 0.04
1 0.02 0.02 0.02 0.02
2 0 0 0 0
3 0 0 0 0
4 0 0 0 0 Table 4 – SINR versus BLER in CQI 1
12
Figure 21 – Plot of the table 4
Analysis for CQI 2
BLER
SNR 0 tx 1 re tx 2 re tx 3re tx
-9.5 0.95 0.9 0.82 0.74
-9 0.93 0.85 0.7 0.7
-8 0.9 0.8 0.65 0.6
-7.2 0.85 0.68 0.6 0.58
-6 0.7 0.57 0.5 0.52
-5 0.6 0.43 0.4 0.4
-4 0.45 0.33 0.33 0.33
-3 0.38 0.25 0.25 0.25
-2 0.23 0.2 0.2 0.2
-1 0.19 0.16 0.16 0.16
0 0.1 0.1 0.1 0.1
1 0.05 0.05 0.05 0.05
2 0.02 0.02 0.02 0.02
3 0.01 0.01 0.01 0.01
4 0 0 0 0 Table 5 – SINR versus BLER in CQI 2
Fig 22 – Plot for table 5
CQI of 1 and 2 occupy the lowest values in
terms of radio conditions. Therefore CQI of 1 is the
poorest radio link. The table 4 and table 5 shows that
the Block Error Rate improves with higher
retransmissions. Same scenario can be seen in the
graph plots of figure 21 and figure 22. But it’s not
very substantial. Therefore, a sacrifice of this
improvement can be justified allowing saving of
resources as mentioned previously.
In short, the simulation result reiterates the
proposal that has suggested earlier in [2] by this
simulated model that HARQ retransmission can be
minimized to 1 or at best 2 with poor radio link
especially when the network is overloaded. It can
also lead to a better scheduling methodology for DL
resource allocation in LTE.
IV. Performance Evaluation of various
HARQ schemes
To evaluate the performance of HARQ, we
have classified them into 4 types of schemes. The
Type–I is the simplest where a retransmission request
is sent to the transmitter through the use of CRC
(Cyclic Redundancy Check) and the corrupted packet
is discarded. After this, the transmitter will engage in
retransmitting the same packet until it is successfully
decoded or until the maximum limit of retransmission
is reached.
Next is the Type-I with Chase Combining
where the packets with errors are stored in a buffer
and the values are combined according to the weights
of signal to noise ratio. Type-II is the Full
Incremental Redundancy which operation has been
discussed earlier in section I. Type –III is Partial IR.
13
In Partial IR the retransmitted packets are chase
combined with previous packets.
A. Channel Modeling
The Orthogonal Frequency Division
Multiple Access (OFDMA) is implemented in the
downlink of LTE. This is a multiple access scheme
which is based on OFDM where data transmission
takes place on different subcarriers to different users.
More detailed parameters for the downlink
transmission can be found in literature [15]. Figure
23 depicts the proposed model of LTE OFDMA
system employing Hybrid ARQ. As can be seen, a
Cyclic Redundancy Check (CRC) is used in the
receiving end to enable error detection. And the
parameters used for this simulation is given in the
table 6.
Figure 23 – LTE OFDMA system model
[18]
Table 6 – LTE OFDMA downlink parameters [18]
The simulation involved here uses the
Spatial Channel Model Extension for 3GPP. Such
type of channel model is used by the European
WINNER project. This Model Extension utilizes 3
environments namely Suburban Macro, Urban Macro
and Urban Micro. For this simulation Urban Macro
has been used.
The maximum number of retransmission is 3
as discussed earlier. In, LTE 3 data modulations are
supported namely QPSK, 16QAM and 64QAM. For
this simulation, 3 MCS are considered as shown in
the table 7.
MCS Modulation Coding Rate
1 QPSK 0.5
2 16QAM 0.75
3 64QAM 0.75 Table 7 – Modulation and Coding schemes (Edited
from [18])
As can be seen from the table 7, the lowest MCS
carries the lowest modulation as well as the lowest
coding rate. As the MCS increases, the Modulation
and Coding rate increases. For this particular
simulation only 3 MCS values have been considered
B. Enhancement of HARQ schemes:
The M-QAM constellation comprises of 2
components. These are the in-phase (I) and the
quadrature (Q). Using the Gray encoding, these
components are mapped to a complex symbol.
For 16-QAM, every four bits (I1Q1I2Q2)
represent a symbol of constellation which is made to
transmit over the communication channel. The
Channel
CRC
Check
FFT
Demodulation
Remove
CP
Turbo
Decoder
De puncture
Bit- De
interleaving
Ack/Nack
CRC
Encoder
IFFT
Modulation
Add CP
Turbo
Encoder
Puncturing
Bit- interleaving
Inpu
t data
Deco
ded
data
14
unequal error protection exists among the four bits of
constellation which make up the 16-QAM symbol.
Among these constellation bits, there is high error
probability in least significant bit (LSB) compared
with the most significant bit (MSB).
Taking an example, it takes 3 times more
perturbation in the real or imaginary part than the 3rd
bit of the 16 QAM constellation symbol for the first
bit of the same symbol to be demodulated
erroneously. Cutting the story short, the MSBs are
three times more reliable than LSBs in inducing bit
error. Therefore, bits are rearranged so that lower
protected bits are given high protection during the
retransmission to compensate for the above issue.
Thus in the table 8 the rearrangement of constellation
for 16-QAM is given
Table 8 – Constellation Rearrangement for
16QAM [18]
Such a similar rearrangement can be applied
to 64-QAM for achieving the maximum benefit.
OFDM subcarriers in a frequency selective
channel are prone to distortion. This leads to different
received signal quality. Fading occurs in pairs of
subcarriers which are uncorrelated given that they are
separated wider than coherence bandwidth in the
frequency domain. Therefore code bits are assigned
to subcarriers in retransmission to achieve frequency
diversity. The method is implemented by shifting the
code bits with an appropriate step which is larger
than the channel coherent bandwidth.
A sub-carrier rearrangement scheme is
deployed in this experiment as shown in the figure
24.
Figure 24 - Subcarrier Rearrangement [18]
It can be seen how the data are shifted in
each re-transmission. Main merit being that there is
no hassle of using a buffer and only shifting
operation is done.
C. Simulated Metrics:
For the evaluation of performance of the
different schemes, 2 methods of metrics are
considered, Packet Error Rate (PER) and the
Throughput.
PER is defined as the residual packet error
rate after a maximum number of retransmission.
Throughput is defined as the number of
correct bits per channel that are received. The
measuring unit is bits per second. It can be written in
the form of equation as:
( )
( )
Here R is the transmitted bit rate and N is
the average number of retransmissions.
D. Simulated results:
Referring to the table of modulation and
coding schemes, various results have been simulated.
These simulated results were summarized in the form
of a table for comparing the PER and Throughput
between the various techniques. The values were
obtained from simulated outputs [18]. The first table
shows the gradual change in PER at a specific SNR
for all the MCS values.
PER Analysis
ARQ MCS 1 MCS 4 MCS 6
Types PER SNR PER SNR PER SNR
(dB) (dB) (dB)
Type 1 - Simple ARQ 0.4 5 0.8 10 1 10
Type 1 - with CC 0.05 5 0.2 10 0.6 10
Type 2 - Full IR 0.02 5 0.009 10 0.006 10
Type 3 - Partial IR 0.02 5 0.02 10 0.01 10
Table 9 – PER Summary
15
From Table 9, Observation can be made that
Type III and type II outperforms the rest of the type
of schemes by having the lowest PER in terms of
SNR for all the MCS. In MCS 1, both the IR
techniques are well above CC as they occupy the
highest decoding gain obtained over retransmission.
This occurs at a lower code rate as this MCS has a
code rate of ½. For coding rate ½, Full IR no longer
gains over Partial IR as both methods are successful
in reducing the coding rate of mother code to 1/3
with just one retransmission. Type I with CC is still
better than the type I with simple ARQ.
The values under MCS 4 show a clear view
about the behavior of the scheme as the MCS is
increased. Type I with CC is more prominently better
than the Type I with Simple ARQ. But Again the
Type II and Type III out performs both of these
schemes. It can also be observed that Type II, being
full IR, has a slight advantage over the Type III in
this case.
As the MCS is increased to the maximum
according to our table, we can see that the result
becomes more prominent. Full IR has an edge and
outperforms the other HARQ schemes in terms of
reliability. This is clearly seen under the MCS 6
which is the highest MCS used.
At a high coding rate (3/4), it takes 6
retransmissions by the Partial IR to reduce the coding
rate down to the mother code by transmitting all
required redundancy bits. But Full IR reduces the
coding rapidly because more redundancy bits are sent
in its transmission.
The PER values in all 3 simulated MCS
prove that Full IR has to offer the maximum coding
gain at the expense of highest buffer requirement.
Chase Combining is easier to implement compared to
Incremental redundancy with low memory
requirement. The type I with simple ARQ offers the
worst performance but with least complexity and no
memory requirement.
While considering the throughput
performance of all these schemes, we observe a
similar pattern in the conclusion compared to the
PER
Throughput Analysis
ARQ Types
MCS 1 MCS 4 MCS 6
(MBPS) SNR (MBPS) SNR (MB PS) SNR
(dB) (dB) (dB)
Type 1 - Simple ARQ 3.6 5 2.5 10 7 15
Type 1 - with CC 5.5 5 8 10 16 15
Type 2 - Full IR 5.8 5 17 10 25 15
Type 3 - Partial IR 5.8 5 15 10 23 15
Table 10 - Throughput Summary
For a lower value of MCS, it can be seen
that again the Type II and Type III overcome the rest
of the schemes in terms of throughput this time. This
is shown as a summary in table 10. Type I with CC is
also far ahead compared to simple ARQ.
From the values under MCS 4 and MCS 6,
Full IR occupies the highest throughput performance.
Partial IR comes next in throughput performance
with less memory requirements. Chase Combining on
the other hand has relatively poor throughput
performance, still better than the simple ARQ. But in
the next section of the simulated results, it can be
seen that the CC scheme benefits more from the
frequency and constellation diversity.
E. Enhanced HARQ performance:
Previously in the section of Enhanced
HARQ Schemes, the rearrangement of constellation
and the sub carrier rearrangement schemes have been
defined. Based on these 2, the performance of Chase
Combining is investigated.
16
PER and Throughput for Chase Combining
CC Types SNR(dB) PER Throughput
(MBPS)
CC 10 0.18 8
CC - Subcarrier Rearrangement 10 0.04 12
CC – Constellation Rearrangement 10 0.07 11
CC - Combined 10 0.01 13
Table 11 – PER and Throughput for Chase
Combining
The table 11 shows the comparison of
various enhanced schemes in terms of Chase
Combining for MCS 4. Clear observations can be
seen that the inclusion of constellation rearrangement
and subcarrier rearrangement improves the
performance of the system. For the same SNR, the
PER for the combination of CC techniques
outperforms the conventional CC by a good margin.
Even the individual performances of these techniques
are better than the CC alone. Similar variation can be
spotted in the throughput of the system.
Comparatively the subcarrier rearrangement has a
slightly upper hand to the constellation
rearrangement.
Figure 25 - PER for MCS 6 for Enhanced HARQ
[18]
Figure 25 is the last simulated result of this
model. This graph shows the PER for Enhanced
HARQ for all the schemes in MCS 6. It can be
interestingly seen that by the addition of enhanced
parameters, the performance gap between the Full IR,
Partial IR and CC has been reduced to approximately
2dB.
V. CONCLUSION AND FUTURE
WORK
The variation in received signal quality depends
on reflection, refraction, scattering, and diffraction.
Thus the RF signals are prone to errors. The various
suitable techniques that can be used as an error
detecting scheme are demonstrated in this paper with
the desired results obtained.
LTE systems employ incremental redundancy
(IR) based hybrid ARQ with chase combining as a
special case of IR. Asynchronous Adaptive (AA)
hybrid ARQ in the downlink and Synchronous
Adaptive hybrid ARQ in the uplink is used, in regard
to timing and adaptivity. The Round Trip Time
equals 8ms with 3ms processing time for each UE
and eNB.
The introduction of Hybrid ARQ into the system
puts a huge impact on the system delay. It increases
the delay in the system by roughly 50ms. But it offers
gain in the throughput performance leading to
improvisation in the quality of service for cell-edge
users. Thus, it provides better throughput in the
expense of system delay.
At poor channel conditions, UE is unable to
make the best use of the radio resources. Thus the
proposal of limiting the HARQ retransmission in
poor radio conditions was stated in this paper. Results
showed that there is improvement in the BLER with
higher retransmission but is not very much
substantial. Therefore considering the points stated in
section III, it is feasible to employ retransmissions as
low as 1 or 2 during poor conditions allowing saving
of resources and leading to better scheduling
methodologies for Downlink resource allocation.
Results have shown that Incremental
Redundancy is better in terms of throughput
compared to the Chase Combining. Full IR achieves
the best performance in terms of PER and
Throughput but with the cost of high buffer
requirement. If we consider the cost of memory, CC
will be favorable as it has a comparable performance
at a lower cost for lower MCS.
17
The Partial IR gives a good tradeoff between the
cost of memory and performance. Thus we can
conclude the justification of LTE using this scheme.
The use of frequency diversity and constellation
diversity along with the Hybrid ARQ schemes, offer
better throughput performance for various MCS. The
subcarrier rearrangement technique when coupled
with the constellation rearrangement is able to give a
significant improvement for existing Hybrid ARQ
schemes. This factor can be looked into as a future
work for enhancing the performance and stability of
error handling in radio links.
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