Research For Adaptive Modulation Techniques Based On Ofdm Systems

Orthogonal Frequency Division Multiplexing (OFDM) with anti-inter-symbol intereference, high frequency efficiency, simpleness in hardware implementation, and many other advantages, is considered as one of the key technologies in the next generation broadband wireless communication systems. Adaptive modulation techniques can significantly improve system performance and bandwidth efficiency by adaptively choosing modulation scheme according to channel state information. With the increasing of user demand, the next-generation communication networks have to provide a large number of IP packet services and high-speed data services. In order to ensure the transmission quality and increase the transmission rate, OFDM combined with the adaptive modulation technique has become one of the most possible trends in current communication systems. This thesis focuses on the adaptive modulation techniques in OFDM system. The main contribution is listed as follows:1. Summarize the basic principles of OFDM system and adaptive modulation, and build a complete simulation model of adaptive modulation OFDM system in MATLAB simulation platform. Study three optimal adaptive schemes for OFDM systems using the following optimization objections: (1) minimizing the bit error rate (BER); (2) minimizing the transmission power; (3) maximizing the transmission rate.2. According to the principle of reliability, the OFDM adaptive modulation techniques which can minimize the BER are studied. The Fisher adaptive algorithm can minimize the BER of systems while maintaining the same transmission rate and power. This thesis derives the Fisher algorithm by performing Lagrange multiplier. Furthermore, the Fisher algorithm is simplified for practical systems with few modulation types. Simulation results show that the new algorithm can greatly reduce the computational complexity at the expense of small performance loss compared with the Fischer algorithm.3. According to the principle of validity, the OFDM adaptive modulation techniques which can minimize the transmit power and maximize the transmission rate are studied. Several optimal algorithms were presented to reduce the transmission power or increase the transmission rate under the given BER. Since these algorithms suffer from extensive iterative process, two novel adaptive schemes are derived in closed-form expression by optimizing the corresponding objective function. They totally avoid the iterative procedure and significantly reduce the implementation complexity, so they are more suitable for broadband wireless communication system. Simulation results reveal that the proposed algorithms can approach optimal performance.