Achieving full diversity order by means of a cooperative- ARQ protocol

Cooperative protocols are a new and promising research field that can lead to a better understanding of wireless ad-hoc networking. In this paper we focus on cooperative hopby-hop ARQ (automatic repeat request) protocols. The main objective of a hop-by-hop ARQ is to prevent, by means of frame retransmission, the loss of frames due to transmission errors. Classically, when the receiver node detects errors in the received frame, it asks for a frame retransmission to the transmitter node. The key idea of cooperative ARQ is that nodes others than transmitter and receiver can play a role in the protocol operation, exploiting some of the characteristics of wireless media such as natural broadcast of wireless transmission, and leading to significant improvements in protocol performance. Examples of cooperative hop-by-hop ARQ protocol are [2], [3] , [4] and [6]. In all cases, authors report significant improvements in terms of transmission power, transmission range or throughput. Cooperative ARQ protocols exploit receiver diversity [1], a well-known wireless transmission technique. The classical optimal way of exploiting receiver diversity is Maximal Ratio Combining (MRC). In MRC, nodes use an array of antennas with a minimal separation of around half wavelength. Signals received by each antenna are coherently combined before detection, using an estimate of the received SNR as weight for each branch. For Rayleigh channels, for instance, it can be shown that the Bit Error Rate (BER) versus ?, the average Signal to Noise Ratio (SNR), is given by: