Optimal power allocation for parallel regenerative two-relayed wireless transmission

Relayed transmission is a way to attain broader coverage by splitting the communication link from the source to the destination into shorter hops at the cost of a reduction of the system spectral efficiency. This communication technique has the advantage of distributing the power between the hops, which leads to longer battery life and lower interference introduced to the rest of the network. In this context, this paper investigates the advantage of using two parallel regenerative relays to cooperate in the transmission of packet data from the source to the destination, and studies the optimal power allocation between these two relays over Rayleigh fading hops. In this purpose, we derive the average Bit Error Rate (BER) expression at the destination. Then, the optimization is done in order to decrease the BER subject to a given power budget used by the two relays. Numerical results shows that the use of the two relays largely enhances the system BER compared to the direct transmission when the power budget allocated to the relays is under a certain threshold. Then, comparing the optimal with the uniform power allocation, we noted that the former is more efficient especially when the relayed links are highly unbalanced, i.e. the link from the source to the relay is either stronger or weaker than the link from the relay to the destination. Furthermore, the relayed transmission outperforms the direct one when the two relays are near the source. Then, investigating the best locations for the relays, we note that it depends on the difference between the source transmit power and the relays power budget.