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.

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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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