Bit and Power Loading for OFDM-Based Three-Node Relaying Communications

Bit and power loading (BPL) techniques have been intensively investigated for the single-link communications. In this paper, we propose a margin-adaptive BPL approach for orthogonal frequency-division multiplexing (OFDM) systems assisted by a single cooperative relay. This orthogonal half-duplex relay operates either in the selection detection-and-forward (SDF) mode or in the amplify-and-forward (AF) mode. Maximum-ratio combining is employed at the destination to attain the achievable distributed spatial diversity-gain. Assuming perfect channel knowledge is available at all nodes, the proposed approach is to minimize the transmit-power consumption at the target throughput (average number of bits/symbol) and the target link performance. With respect to various power-constraint conditions, we investigate two distributed resource-allocation strategies, namely flexible power ratio (FLPR) and fixed power ratio (FIPR). The FLPR strategy is proposed for scenarios without individual local power constraint. The source power and relay power have a flexible ratio for each subcarrier. The FIPR strategy is proposed for scenarios with individual local power constraint. The source power and relay power have a fixed ratio for each subcarrier. Computer simulations are carried out to evaluate the proposed approach with respect to the relay location. Significant performance improvement is observed in terms of both the symbol-error-rate and the transmit-power efficiency.