Cooperative Diversity And Relay Selection Schemes For Wireless Networks

The wireless channel suffers from fading, which can seriously degrade detection performance. Spatial diversity can effectively combat the deleterious effects of fading, which is generated by transmitting signals from different locations, allowing independently faded versions of the signal at the receiver. To realize spatial diversity, the nodes in wireless networks need to support more than one antenna (e.g., multiple-input multiple-output (MIMO) system). However, due to size, cost, or hardware limitations, a wireless node may not be able to support multiple transmit antennas.Cooperative communication in wireless systems has recently emerged to overcome this problem. The basic idea is that single-antenna nodes in a wireless networks share their antennas and transmit cooperatively as a virtual MIMO system, thus realizing spatial diversity to mitigate fading without relying on multiple antennas. Based on the level of signal processing at the relay, cooperative relaying schemes can be classified as amplify-and-forward (AF) relaying and decode-and-forward (DF) relaying. In DF relaying, the relay detects and then retransmits the detected signal, whereas the relay just amplifies and retransmits the noisy signal in AF relaying.Many issues in cooperative communications still need to be addressed. An important question is cooperative diversity scheme, i.e., when and how to cooperate. Another important issue is relay selection method in multi-relay networks. In other words, who to cooperate if there more than one relays? Traditionally, all available nodes are requested to relay signals between the source and destination, and the destination will combine all received signal with maximum ratio combining (MRC) technique. It has recently been proposed, however, that for each source-destination transmission, only a single best node should be selected to act as a relay. The resulting scheme outperforms MRC schemes and can be implemented in a distributed fashion with limited feedback. The results showed that this scheme has the same diversity order in terms of the outage probability as the regular cooperative networks, where all the relays participate in sending the source signal to the destination.Based on the aforementioned issues of cooperative networks, we concentrate our efforts on the research of cooperative diversity and relay selection schemes for wireless networks. The main contribution and innovation can be summarized as follows:(1) A novel relaying scheme in conjunction with incremental DF relaying and selective DF relaying strategies, termed incremental-selective DF relaying, is proposed and analyzed. Closed-form expressions for bit error rate (BER) of both the incremental-selective DF relaying scheme and the incremental DF relaying scheme are derived. In addition, the effect of signal-to-noise ratio (SNR) threshold on the BER is discussed.(2) We analyze the performance of SNR-based hybrid decode-amplify-forward (HDAF) relaying cooperative diversity networks over independent non-identical flat Rayleigh fading channels with MRC technique. Closed-form expression for the BER of the HDAF relaying scheme is derived. Computer simulations are carried out to illustrate and validate the correctness of analytical results. Besides, the impacts of the SNR threshold and relay location on the performances of BER are investigated.(3) We analyze the symbol error rate (SER) performance of cooperative diversity networks with opportunistic DF relaying. Assuming that channels suffer from independent nonidentical Rayleigh fading and symbols are M-PSK modulated, the approximate closed-form SER expressions are derived for opportunistic relaying with adaptive DF (ADF) and fixed DF (FDF) protocols, respectively. Simulations are carried out to validate the theoretical analysis.(4) The performances of selection cooperation (SC) are investigated in a scenario based on DF relaying and where equal gain combining (EGC) technique is adopted at the destination. Assuming that channels suffer from independent non-identical Rayleigh fading, we first derive the cumulative distribution function (CDF) and probability density function (PDF) for the total instantaneous SNR at the destination after EGC. Then, these statistical functions are used to derive closed-form expressions for outage probability and average SER of selection cooperation. Besides, an improved selection cooperation (ISC) scheme for DF relaying is proposed based on selection cooperation (SC) strategy. Assuming that channels suffer from independent non-identical Rayleigh fading, we derive closed-form expression for the exact SER of ISC scheme.Finally, problems to be solved in this field and future research topics are summarized.