The Study, Based On Adaptive Technology For Multi-carrier Transmission System

Digital wireless communication systems are now changing from voice communication to multimedia communication. Tremendous growth in demand of the users and limitation of our frequency resource needs more efficient transmission techniques to realize high efficiency of spectrum utilization. From this point, M-ary Quadrature Amplitude Modulation (MQAM), MultiCarrier Modulation (MCM) and adaptive modulation techniques are good candidates in next generation mobile radio system.As a new technique for high data rate transmission, MCM systems have been extensively used in high-speed data transmission applications. Their main characteristic over other modulation schemes is that they transmit data in parallel subcarriers across the channel. As we know, the effects of the Inter Symbol Interference (ISI) are negligible so long as the delay spread is significantly shorter than the duration of one transmitted symbol. This requirement can be met in MCM system by serial to parallel (S/P) conversion and IFFT at the transmitter. With MCM techniques, BER performance can be improved remarkably over frequency selective fading scenarios when high data rate is required.There are many approaches to carry out MCM. Orthogonal Frequency Division Multiplexing (OFDM) and Filtered MultiTone modulation (FMT) are two typical techqniues among them. Due to the different purposes of the two techniques, OFDM and FMT are employed in different communication standards. Because of the immunity to ISI and the robustness to the time dispersion of the channel, OFDM has been used in many communication standards, such as digital video broadcasting (DVB), digital audio broadcasting (DAB), high performance loacal area network (HIPERLAN/2), IEEE 802.11a and etc. For the disadvantage of OFDM in sensitivity to frequency offset, whereas, FMT is an idea choice for communication in the environment with abundant Doppler frequency shift and coarsesynchronization technique. More immunity to Inter Channel Interference (ICI) can be achieved by sacrificing the partly immunity to IS I in FMT. It has been widely used in wire communication standards. In order to utilize the system resources as much as possible, adaptive techniques are widely employed in the commuincation systems, including:1. Adaptive modulation and power control. Adaptive modulation is a typical one of adaptive techniques. It has been extensively used in wireless communication systems in order to enhance the frequency efficiency and the ability of resisting noise. Simulations are made over multicarrier modulation system. Methods advocated here can increase the frequency efficiency remarkably while maintaining the BER performance of the system. In addition, power control method is employed at the same time. Simulation results can prove the efficiency of this method.2. Adaptive bit and power loading. Wireless channel is separated into several sub-channels in MCM systems. One of the advantages is that we can distribute the transmitting power and bits flexibly and reasonably according to the channel scenarios. So we can see that, adaptive bit and power loading is a unique adaptive technique in MCM system. In order to reduce the complexity of the algorithm in MCM system, we modified the greedy algorithm and proposed a method based on sub-band instead of sub-channel. Simulation results in FMT prove the efficiency of this method.3. Adaptive image transmission. Adaptive image transmission can be a representative application of adaptive technique. Here we combined source division and adaptive sub-channel distribution techqnues to accomplish the adaptive image transmission over slow Rayleigh fading channel. We proposed two kinds of methods to carry it out. Simulation results and theory analysis demonstrate that we can improve the subjective quality of the received image remarkably and enhance the ability of resisting noise significantly.Because of the limiation of various impersonal reasons, partly simulation and analysis for adaptive technique in MCM system have been done in this paper. There are many topics, such as the application of adaptive technique inrestraining the Peak to Average Power Ratio (PAPR) in MCM and the application in Coded Modulation (CM), expected for the further exploration in theory. More study on these topics will be done in future work.