Abstract-In this paper, we propose a new cooperative communication protocol, which achieves higher bandwidth efficiency while guaranteeing the same diversity order as that of the conventional cooperative schemes. The proposed scheme considers relay selection via the available partial channel state information (CSI) at the source and the relays. In particular, we discuss the multi-node decode-and-forward cooperative scenarios, where arbitrary N relays are available. The source determines when it needs to cooperate with one relay only, and which relay to cooperate with in case of cooperation, i.e., "When to cooperate?" and "Whom to cooperate with?". An optimal relay is the one which has the maximum instantaneous scaled harmonic mean function of its source-relay and relay-destination channel gains. For the symmetric scenario, we derive an approximate expression of the bandwidth efficiency and obtain an upper bound on the symbol error rate (SER) performance. We show that full diversity is guaranteed and that a significant increase of the bandwidth efficiency is achieved. Moreover, we present the tradeoff between the achievable bandwidth efficiency and the corresponding SER. Finally, the obtained analytical results are verified through computer simulations.Index Terms-Cooperative diversity, decode-and-forward cooperative protocol, multi-node wireless relay networks, optimal relay selection.

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Abstract-In this paper we propose a new cooperative protocol, which takes into consideration the partial channel state information (CSI) available at the source. With such protocol a significant improvement in the transmission rate can be achieved in decode-and-forward cooperative transmission, while guaranteeing full diversity order. We derive closed-form expressions for the transmission rate and the symbol error rate (SER) for the M-PSK and the M-QAM signalling. Moreover, we consider two optimization metrics in the protocol design to enhance the system performance; the first is based on minimizing the SER only, while the second is based on minimizing a joint function of both the SER and the transmission rate. Finally, the obtained analytical results are verified through computer simulations.

Abstract-In wireless networks, reducing aggregate transmit power and in many cases, having even power distribution increase the network lifetime. Conventional direct transmission (DTX) scheme results in high aggregate transmit power and uneven power distribution. In this paper, we consider location-aware cooperation-based schemes namely immediate-neighbor cooperation (INC), maximal cooperation (MAX), and wireless network cocast (WNC) that achieve spatial diversity to reduce aggregate transmit power and even power distribution. INC utilizes twouser cooperative communication, resulting in good reduction of aggregate transmit power and low transmission delay; however, power distribution is still uneven. MAX utilizes multi-node cooperative communication, providing incremental diversity to achieve even power distribution and substantial reduction in aggregate transmit power. Transmission delay in MAX, however, grows quadratically with network sizes. As a result, the novel WNC is proposed to achieve incremental diversity as in MAX and low transmission delay as in INC. In WNC, mobile units acting as relays form unique linearly-coded signals from overheard signals and transmit them to the destination, where a multiuser detector jointly detects the symbols from all received signals. Performance evaluation in uniformly distributed networks shows that INC, MAX, and WNC substantially reduce aggregate transmit power while MAX and WNC also provide even power distribution.

Channel estimation error and co-channel interference (CCI) problems are 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 and CCI. Two main performance criteria, namely, the traditional outage probability and the proposed signal-tonoise 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 or CCI. Taking into consideration the channel estimation error, we show that the outage probability is reduced by utilizing cooperative transmission protocols. We also show that cooperative transmission scenarios, in which each cooperating relay forwards its signal over an orthogonal channel, result in lower SNR gap ratio compared to that of the direct transmission. Thus, cooperative transmission schemes are less susceptible to the effect of channel estimation error compared to direct transmission. Moreover, increasing the number of cooperating relays reduces the effect of the channel estimation error more. Timing synchronization error arises in distributed space-time cooperative schemes, in which the cooperating relays are simultaneously transmitting their signals over the same channel. Unlike the channel estimation error, the effect of the timing synchronization error gets worse as the the number of cooperating relays increases. In this work we also study the tradeoff between the timing synchronization error and the channel estimation error, and show their net impact on the system performance. Finally, we illustrate that CCI can be modeled in a similar fashion to the channel estimation error, and hence the cooperative transmission schemes are also less susceptible to the effect of CCI.