Dual-hop wireless transmission is a technique by which the channel from the source to the destination is divided into two shorter links using relays. By using this transmission, the communications reliability is improved whereas the throughput might go down since the dual-hop transmission is performed by two times. In this letter, we propose a scheme that uses hierarchical modulation at the source and adaptive modulation based on cyclic redundancy check (CRC) code at the relays. The proposed scheme is evaluated in terms of the bit error rate (BER) and throughput performances.

Among spatial diversity schemes, orthogonal space-time block code (OSTBC) and cyclic delay diversity (CDD) have been widely studied for the cooperative wireless relaying system. However, conventional OSTBC and CDD cannot cope with change in the number of relays owing to low throughput or error performance. In this letter, we propose the space-time cyclic delay diversity (STCDD) scheme which provides good error performance and full rate. Simulation results show that bit error rate (BER) performance of the proposed STCDD is superior to that of OSTBC and CDD when sufficient quality of source-relay channels are guaranteed.

In multi-hop wireless networks, the transmitted signal is generally forwarded over several relay terminals. So, the quality of communications is degraded due to these re-transmissions over fading channels in relay terminals. Basically, the conventional cooperative relaying scheme based on cyclic delay diversity (CDD) reduces this degradation because this scheme could have numerous relay terminals which simultaneously transmit the signals. However, we cannot obtain the maximum diversity gain because of the re-transmission without considering relay channel environments. In this letter, to overcome the decreased performance, we propose an adaptive decode-and-forward (DF) relaying scheme based on CDD which uses cyclic redundancy check (CRC) code. Simulation results show that the proposed scheme provides more improvement in error performance than the conventional schemes in multi-hop networks.