Optimum power allocation based adaptive modulation for cooperative wireless networks

Cooperative communication is becoming a viable means of performance enhancement for wireless networks. It provides diversity by emulating multi-input and multi-output (MIMO) system for devices that are spatially too small to harbor multiple antennas. Conventional cooperative protocols such as amplify-and-forward (AF) and decode-and-forward (DF) attain diversity gain at a cost of reduced spectral efficiency which becomes a cause for reduced throughput. To fight this reduction in throughput different approaches can be implemented. In this paper, adaptive modulation in conjunction with optimal power allocation is applied both at the source and the relay. In the process, the bit error rate (BER) at the destination, after performing optimal power allocation, is used as a constraint to determine the operating SNR regions of different M-QAM modulation types to be applied by the source and relay. Experimental results on AF strategy show that adaptive modulation with optimal power allocated BER as a constraint has better performance over the equal power allocated counterpart. The spectral efficiency and throughput performance differences become even higher when channel conditions worsen.