Performance of Power Allocation Schemes in a Two-Hop AF Relay System with Faded Direct Link

Though power allocation has been studied extensively in the literature, as energy efficiency becomes more important in "green" wireless communication, we revisit to study the differences in SNR vs BER-based power allocation (PA) for relay communication when the direct link is severely faded. The average SNR and average BER expressions are first derived for an amplify-and-forward relaying protocol with Rayleigh fading channels. Based on the derived expressions at the destination through two-hop communication, closed-form expressions for optimum transmit power are obtained at the source and the relay. It is observed that the higher the total transmit power is, the bigger portion of it should be allocated to the relay independent of its location. For a given end-to-end BER, optimally selected relay's location is relatively more closer to the source if BER-based PA is employed than that of the SNR-based PA. It is also noted that the BER-based power allocation scheme can achieve 1dB performance improvement over the SNR-based counterpart when relay is centrally located and is in the higher transmit power regime resulting in energy saving, for a two-hop wireless system with no avail of direct communication between transceivers.