Nearest neighbor relay selection with adaptive modulation for improved throughput and scalability of cooperative wireless networks

Cooperative communication is establishing itself as 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. Since mobility and easy handling require terminal miniaturization, cooperative communication is required to fill the gap for MIMO. Conventional cooperative protocols such as amplify-and-forward (AF) and decode-and-forward (DF) attain diversity gain at the cost of reduced spectral efficiency which as a result brings about reduced throughput. In addition, relay selection mechanisms play important role in attaining the required performance. In this paper, adaptive modulation is applied to improve the throughput performance of AF wireless cooperative strategy. Furthermore, a relay selection mechanism known as Nearest Neighbor Selection (NNS) is suggested for scalable implementation of wireless cooperation among users. The source and relay respond to the channel state information (CSI) obtained from the destination as a feedback to change the M-ary Quadrature Amplitude Modulation (M-QAM) types. Operating regions for each modulation type are determined by bit error rate (BER) constraint set by the destination which makes use of maximal ratio combining (MRC) of the signals received from the source and relay. Simulation results show that modulation adaptive cooperation performs better than the non adaptive counterpart in overall throughput and NNS is a promising approach for user scalability of adaptive cooperation.