Abstract
In this paper, we introduce an adaptive cooperative system that selects the best relay among the available relay nodes to cooperate with a source node for transmitting the information signal to a destination node if the spectral efficiency can be maximized. This maximization is achieved as long as the average bit error rate is below a target level. The channels are modeled as independent and identically distributed Rayleigh fading channels over the relay nodes, and not identically distributed over the relay hops. Closed-form expressions for the average spectral efficiency and the average bit error rate are derived when adaptive discrete rate M-QAM is used as a practical implementation of the variable-rate transmission. Simulation results show that the proposed scheme, compared with the opportunistic amplify-and-forward fixed relaying and the conventional direct transmission, can provide the maximum average spectral efficiency and perfectly maintain the average bit error rate.