The performance of robust adaptive modulation over wireless channels with non reciprocal interference

Adapting a modem's modulation scheme can significantly enhance the resulting performance over a wireless communications link. Non-adaptive modems are generally designed conservatively for worst-case channel conditions. Adaptive modems are able to exploit the higher channel capacity when good conditions are encountered. A number of different metrics can be used to drive the adaptation process (i.e. Automatic Repeat reQuest (ARQ) or Segment Error Rate (SER)). Not surprisingly, certain metrics are far more successful than others. This paper presents an investigation of adaptive modulation in the presence of interference. In particular, the case of non-reciprocal interference is studied (i.e., cases where the level of interference is different on the forward and reverse links). Research results have shown that if adaptation is performed assuming a reciprocal channel, then poor performance will result for the link experiencing a higher level of interference. In this study, an algorithm is used to calculate a measure of the link quality. This value is then fed back to the other end of the link to adapt subsequent transmissions. Results indicate that the throughput of such a system can be considerably improved when transmitting adaptively using any one of DBPSK, DQPSK or 8DPSK modulation schemes. It is proposed that this simple approach be adopted for short range, ad hoc, Wireless Personal Area Networks (WPAN).