24-12-2012, 02:57 PM
Throughput evaluation of HARQ Schemes with Packet and Code Combining over multipath fading channels for DS-SSS
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Abstract
In this paper, the performance of both ARQ and HARQ I, with Packet Combining (PC), and HARQ II, with
Code Combining (CC), are evaluated in terms of throughput efficiency over multipath fading channels. The analysis is done in
two extreme scenarios where the channel is assumed to be constant and independent during the different transmissions. The
theoretical results are then compared to those obtained by computer simulations and confirm the great advantage of using a PC
strategy.
Introduction
The High Speed Downlink Packet Access (HSDPA) is a
new mechanism proposed by 3GPP [1] to support higher
data transmission rate for mobile users, and to provide
streaming, interactive and background services with a good
quality of service. This is accomplished by using different
techniques such as Hybrid Automatic Repeat reQuest
(HARQ) scheme. HARQ consists in combining ARQ and
FEC schemes [2]. Two types of Hybrid ARQ schemes
[2] have been identified in which error correction followed
by error detection are applied at every received packet.
In HARQ I schemes, the correction of errors is first attempted
at each received packet. If this fails, the entire
packet is discarded and its retransmission is requested.
Each subsequent retransmission is decoded independently
of the prior retransmissions. The main disadvantage of
ARQ and HARQ I schemes is their low throughput at low
signal to noise ratios. In fact, when a packet is detected in
error, it must be retransmitted and, therefore, for low or
mean signal to noise ratios, the number of transmissions
needed before receiving a packet correctly is high and then
the transmission rate is reduced to an unacceptable level.
In HARQ II schemes, the receiver stores the erroneous
packets in a buffer so that they can be reused after subsequent
retransmissions.
Systemmodel
In this section, we describe the channel decoder input for
DS-SSS using a Rake receiver. The Rake receiver is assumed
to have a perfect knowledge of complex path gain
modules {αl}L
l=1, where L is the number of paths. If path
delays are well separated and in the absence of Inter Symbol
Interference and Multi-User Interference, the Rake receiver
output for symbol sn can be written as [9]
ARQ with Packet Combining
In the conventional ARQ scheme, the transmitter sends a
packet consisting of k information bits and np parity bits
for error detection. At the receiver, packets declared in error
are discarded and replaced by another copy. The performance
of conventional ARQ schemes can be improved
if the erroneous packets are not discarded but averaged
with the new packet to reduce the effect of the channel
noise and therefore the average number of transmissions.
Let Tr be the average number of transmission attempts
that must be made before a packet is accepted by the receiver.
Numerical and simulation results
In this section, we compare theoretical and simulation results
in terms of throughput efficiency evolution with respect
to Es/N0 for a two paths Rayleigh fading channel
(L = 2). The code used for HARQ I and HARQ II with
Code Combining is a half rate convolutional encoder of
constraint length m + 1 = 7 and generators polynomials
(133,171). The free distance df and the distance spectra
adf of repetition codes of HARQ II with Code Combining
are listed in table VI of [3]. In order to compare the
performance of the different schemes, the same length of
the transmitted packets must be maintained. In the simulations,
the transmitted packet length was equal to 960
bits. The parameters of the different schemes are listed in
Table 1.
Conclusion
In this paper, we have derived the performance of ARQ
and HARQ I with PC in multipath Rayleigh fading channels.
The analysis is done in two extreme scenarios where
the channel is assumed to be constant and independent
during the different transmissions. We have shown that
the performance improvement brought by PC is greater
when the channel is constant during the different transmissions.