The Research On Adaptive Modulation And Coding Based On Orthogonal Frequency Division Multiplexing Systems

OFDM is a multi-carrier transmission technique for wireless communication system. Due to its high data rate transmission capacity with high spectral efficiency, flexible modulation modes and its robustness to multi-path delay, OFDM has been the core physical technique in the fourth generation communication systems. With variable code, variable power and variable rate, adaptive modulation and coding can provide high throughput under the frame error rate constraint. In the next generation communication systems, there are plenty of IP packet and high-rate data services. To guarantee the transmission quality and enhance the transmission rate, it is an inevitable tendency to apply adaptive modulation and coding to OFDM systems.Policies of adaptive modulation and coding in OFDM systems are studied in this paper. The main researches and contributions are as follows:1. In this paper, the principles of OFDM technique and adaptation of modulation in OFDM system are first introduced. Typical algorithms for adaptive modulation are summarized and compared.2. With Matlab software, a simulation model of a baseband OFDM system is established. The BER performance under different conditions are simulated and compared. The simulation results prove the validity of OFDM technique and can provide a valuable theoretic reference to practical application.3. An adaptive modulation and coding scheme combining adaptive frame length is proposed. With uniform power allocation, the complexity the proposed scheme is low. The average channel bit error rate is the key parameter to combine adaptive coding and adaptive modulation. In different noise environments, different parameters are set for different coding schemes given the target frame error rate. Using these parameters, we complete rate allocation according to the instantaneous channel state information. And the coding scheme with the highest spectral efficiency will be selected. Simulation results show that the proposed scheme has a satisfying FER. Compared to the widely used policy with fixed threshold, the proposed scheme improves the transmission rate effectively.