Joint Hierarchical Modulation And Network Coding In Asymmetric Multiple-User Multiple-Relay Wireless Networks

There exists a problem of asymmetry in multiple-user multiple-relay wireless networks. The distance between a user node and a base station (BS)is much longer than the distance between a user node and a relay, so the channel of User-BS is worse than that of User-Relay because of path loss in general. If the user nodes use low order modulation, then the good channel of User-Relay will be wasted. However if the user nodes use high order modulation, the bad channel of User-BS will degrade the system using network coding.Network coding has changed the traditional concept that the relay can only forward the message it received and it improves the throughput of the network by combining the messages linearly and non-linearly. Hierarchical modulation can provide protection to the more important information by allocating more energy to it. It can provide different data rates to receivers of different channel conditions.In order to overcome the problem of asymmetry in multiple-user multiple-relay wireless networks, this paper proposes a new strategy named joint hierarchical modulation(HM) and network coding(NC) by combining HM and NC to improve the performance of the system. The approximate end-to-end bit error ratios (BER) of two-user two-relay networks under AWGN channel and Raleigh channel are got to prove the correctness of simulations. The paper first makes simulations of the proposed strategy using4/16-QAM hierarchical modulation in AWGN and Raleigh channel. By comparison with the conventional NC scheme, it is proved that the joint hierarchical modulation and network coding strategy performs better under AWGN channel and low signal-to-noise ratio(SNR) under Rayleigh channel. Then simulations of different priority parameters are made to clarify that the optimal scheme can be found in different channel conditions. After that simulations of the new scheme using4/64-QAM and4/8-QAM are made in AWGN and Rayleigh channel. It proves that the new strategy performs better using high order hierarchical modulation. In addition, we successfully apply the new strategy to m-user n-relay wireless networks by using random network coding to improve the performance of the system.