Mitigating channel estimation error via cooperative communications

Abstract

Abstract-Channel estimation error problem is among the main causes of performance degradation in wireless networks. In this paper, we investigate the impact of cooperative communications on mitigating the effect of channel estimation error. Two main performance criteria, namely, the traditional outage probability and the proposed signal-to-noise ratio (SNR) gap ratio, are utilized to characterize such impact. The SNR gap ratio measures the reduction in the SNR due to channel estimation error. Taking into consideration the channel estimation error, we show that the outage probability is reduced by utilizing cooperative transmission. We also show that cooperative transmission results in lower SNR gap ratio compared to that of the direct transmission. Thus, cooperative transmission is less susceptible to the effect of channel estimation error compared to direct transmission. Finally, we illustrate that increasing the number of cooperating relays reduces the effect of the channel estimation error more. I. INTRODUCTIONRecently, cooperative communications for wireless networks have gained much interest due to its ability to mitigate fading in wireless networks through achieving spatial diversity, while resolving the difficulties of installing multiple antennas on small communication terminals. Cooperative communication protocols make use of the broadcast nature of wireless channels, where a number of relay nodes are assigned to help a source in forwarding its information to its destination, hence forming a virtual antenna array. Various cooperative diversity protocols were proposed and analyzed in [4]. It was shown that the conventional cooperative communication scenario, in which each of the available N relays forwards the source's information over an orthogonal channel (e.g. time or frequency slot), achieves full diversity order equal to N +1. However, the bandwidth efficiency drops to 1/(N + 1) symbols per channel use (SPCU). In [5], [6] various relay selection cooperative schemes, in which only one relay forwards the source's information, have been proposed. It was illustrated that relay-selection cooperative schemes achieve high bandwidth efficiency while guaranteeing full diversity order. Distributed space-time cooperative schemes, in which the helping relays simultaneously forward modified version of the source's information, have been proposed and analyzed in [7], [8]. It was shown that distributed cooperative schemes, under certain conditions, can achieve full diversity order.Channel estimation error, caused possibly by Doppler shift or noise on the pilot signals, can cause dramatic performance degradation in wireless networks. In [9], it was examined that

Impact of Channel Estimation Errors in Cooperative Transmission over Nakagami-m Fading Channels

“…In this section, we adopt a concept, called the SNR gap ratio, proposed in [12], to our cooperative transmission scenarios to give a scale to quantify the SNR loss caused by the channel estimation errors. The SNR gap ratio in [12] is defined as follows…”

“…A DF cooperative protocol proposed in [12] is adopted to our system, where the availability of a relay node assisting the source-to-destination communication depending on the quality of the received signal. In this protocol, a transmission of one symbol is implemented in N + 1 phases.…”

“…Imperfect channel estimation problem in DF systems consists of individual estimation of source-to-relay and relay-to-destination channels. In [12], the outage probability of DF systems with channel estimation errors was well studied, where performance analysis could be explained by a similar approach as point-to-point networks. On the other hand, in AF systems, a cascaded channel consisting of source-to-relay and relayto-destination links needs to be estimated and the effect of channel estimation errors in the two links should be jointly considered.…”

“…Therefore, investigating the performance of cooperative systems with imperfect channel estimation will be significatively important to practice. There have been increasing studies on this topic recently [12]- [17]. Imperfect channel estimation problem in DF systems consists of individual estimation of source-to-relay and relay-to-destination channels.…”

“…• The average SNR gap ratio is employed by our system to quantify the impact of channel estimation errors, which was proposed in [12] and shows a new method to quantify the reduction in the SNR due to the channel estimation errors. We derive the exact expressions of the average SNR gap ratio for both DF and AF systems and present the comparison between them to investigate different degrees of the impact.…”