Spectrum Efifciency Based On Optimization Of C-harq Protocol

Currently, as multimedia, mobile broadband Internet data transmission services increased rapidly, the high data rate wireless access demands the same rate of that. In the mobile environments, there are two severe challenges. First, the radio spectrum resources for the mobile communication system are limited, the issue of how to improve the spectral efficiency has been placed in front of us. Secondly, the channel fading and interference encountered in the high-speed mobile environments are more serious. How to overcome the impact of fading and enhance the quality of the received signals, is also a problem.In recent years, one of the latest progresses in wireless communication researches is multiple-input multiple-output(MIMO), which need not take additional frequency or time resources for excellent network performance and quality of service. However, it is difficult to install multiple antennas at a terminal because of the size of mobile terminals and other constraints. Therefore, cooperative diversity was proposed to achieve spatial diversity in such circumstances. In the cooperative diversity networks, collaboration terminals repeat the partner’s information bits, which does not make use of the spectrum effectively. It has aroused general interests in how to improve the spectral efficiency at the same time of obtaining cooperative diversity gain in a cooperative diversity system.In this paper, various of cooperative strategies are first described. Secondly, the impact of system frequency resources utilization caused by the cooperation is analyzed, and the Shannon capacity of the amplify-and-forward(AF) cooperative system involved two users is presented. Aiming at optimizing the spectral efficiency in the cooperative diversity systems, an improved approach based on cross-layer to jointly design adaptive modulation and coding (AMC) at the physical layer and cooperative hybird automatic repeat request (HARQ) protocol at the data link layer is prosed in the main part of this paper. The numerical results and computer simulation indicate that the proposed scheme can not only improve the practicability of itself and lower the system complexity relatively but also noticeably enhance the spectral efficiency at medium and low signal-to-noise-ratio (SNR) compared with the system that employs AMC alone at the physical layer. This gain is achieved by taking advantage of the cooperative channel state information (CSI), which takes up more system resources. On the other hand, the improvement also depends on that the collaborative users can receive the source’s information packets correctly, so we should select the nodes in superior conditions to collaborate.