On Diversity of Cooperative Relaying Protocols

We focus on the achievable diversity order and probability of error performance in an uncoded cooperative relaying scenario with multiple relays when the destination has no knowledge of the channel gains corresponding to the source to relay links. We consider a hard-decision adaptive decode-and- forward protocol wherein each relay transmits only if its received signal strength is above a threshold. For such a protocol we show that by allowing the threshold to change with the average SNR, the achievable diversity order can be made arbitrarily close to the spatial diversity available. We also analyze a pure amplify- and-forward protocol and show via numerical results that the achievable diversity order is close to 1 as opposed to 3 when there are two participating relays. By choosing a threshold that varies with the average SNR, it is observed that the adaptive decode- and-forward protocol significantly outperforms the amplify-and- forward and decode-and-forward protocols.