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This paper deals with the power allocation problem in a wireless relay network employing differential modulation technique and Alamouti distributed space-time code, where signals from both the direct and relay paths are taken into account in the detection process. We propose a sub-optimal power allocation scheme based on the maximization of an approximate receive covariance expression. The proposed scheme jointly allocates the transmit power of both the source and relay nodes under a total transmit power and per-node transmit power constraints. We can increase the effectiveness of the proposed power allocation algorithm by grouping the relays with respect to their individual covariances. Numerical experiments show that the proposed covariance maximization algorithm combined with the relay grouping algorithm offers a respectable BER performance improvement, compared to the conventional schemes. Finally, we demonstrate that the proposed scheme requires only a small number of iterations to achieve convergence, making it attractive to practical systems.