Performance Study Of Distributed Space-Time-Code-OFDM System

Broadband is the development direction of wireless communication. OFDM technology has obvious advantages in the progress of broadband. MIMO technique can enhance the system capacity and performance effectively. But it can’t be applied among single-antenna mobile terminals without the use of cooperate communication technology. Space-time coding is one of the most important techniques of MIMO system. Among all the space-time coding techniques, linear dispersion codes (LDC) can make a better trade-off between diversity gain and multiplexing gain. In this paper, DLDC-OFDM system is constituted by the combination of linear dispersion codes, cooperative communication technology and OFDM. The performance of the system is studied. Firstly, the relay schemes are studied. The BER performance of amplify-forwardand decode-forward are analyzed in theory, and the simulation results show that the amplify-forward can achieve a full diversity gain under high SNR. Secondly, the coding guidelines of linear dispersion codes are also studied and theperformance of DLDC is studied in theory. Combined with OFDM technique, the space-frequency coding scheme of DLDC-OFDM system is provided using two-input and two-output model, the influence of different coding schemes and modulation modes on BER performance of the DLDC-OFDM system is simulated under perfect channel estimation. Simulation results show that bigger dispersion metrics often have a better BER performance at the same code structure. According to channel condition number, a new adaptive coding and decoding scheme is proposed for DLDC-OFDM system. Each relay node selects dispersion matrix and modulation type adaptively according to channel state and different detection methods are used according to the threshold of channel condition number adaptively. Simulation results show that the proposed adaptive coding and decoding scheme can control the BER performance and decoding complexity effectively.Finally, the performance of DLDC-OFDM system and the proposed adaptive coding and decoding scheme is studied under imperfect channel estimation. The BER performance degradation of bigger dispersion metrics is also less than that of small dispersion metrics while the modulation order is increased in the presence of channel estimation error. But for the same modulation order, the degradation of BER performance caused by the channel estimation error of small dispersion metrics is less than that of bigger dispersion metrics. And the proposed adaptive scheme is still valid under channel estimation error.