Contents I. Introduction
THE design of cooperative diversity communication schemes and the analysis of their performance have attracted considerable attention recently. In relevant literature, most works are devoted to cooperative systems employing non-regenerative relays, i.e., using an amplify-and-forward (AaF) strategy. This is mainly due to the fact that AaF schemes offer full diversity gains for large values of signal-to-noise ratio (SNR) [3], [4]. Nevertheless, their practical implementation requires expensive RF chains [5], as opposed to detect-and-forward (DaF) transmission schemes, in which the relays regenerate the received signals with only simple additional digital processing. In [6] a DaF cooperative method is described based on a distributed version of the well known Alamouti code and a suboptimum, as compared to the maximum likelihood (ML), receiver is proposed. In [7] and [5] two different maximal ratio combining (MRC) based cooperative schemes are developed and suboptimum, low complexity detection criteria are proposed. For all the above schemes, derivation of analytical closed-form expressions for the bit error rate (BER) is rather complicated. Recently in [1], a detailed BER analysis of a single relay DaF network has been presented for BPSK signaling, MRC at the destination-receiver, and threshold-based retransmission at the relay. Four different scenaria have been considered in [1], depending on whether instantaneous or average SNR information of the various channels, is available at the relay. For all four scenaria, the optimum thresholds have been derived, that result in the minimum end-to-end BER performance.
