Research On The Key Techniques Of Multicarrier Systems In Multipath Fading Channel

The next generation wireless communication systems shall support multiuser access and high data rate transmission. One of the research goals in physical layer is to find effective way to combat multipath interference and multiple access interference. Combining OFDM and CDMA techniques is a promising solution for next generation wireless communication systems, which can support multiuser access and is robust against multipath interference and multiple access interference.This thesis presents researches on the key techniques of multiuser wireless broadband communication against multipath fading, including OFDM modulation to combate multipath interference by parrell transmission, Orthogonal Complementary Code possessing perfect correlation property and channel-shortening equalization tO shorten channel impulse response. The conclusions of research provide references to system design and parameters optimization. The main contents of this work conclude:With deep insight into present multicarrier CDMA systems, the baseband system model of MC-OCC-CDMA based on OFDM and OCC spreading is derived. The basic modules of baseband signal processing are realized and verified in ADI DSP integration development environment. In the MC-OCC-CDMA systems, multipath interference is eliminated in uplink, both multipath interference and multiple access interference are eliminated in downlink.Because of using OFDM modulation, MC-OCC-CDMA is sensitive to the carrier frequency offsets. Effects of timing errors and carrier frequency offsets on the MC-OCC-CDMA system performance are analyzed. The sensitivities of MC-OCC-CDMA, MC-CDMA and MC-DS-CDMA system performances to frequency offsets are compared. With the same number of subcarriers and equal bandwidth, three fully loaded systems have the same performace degradtion. The analytical results are instructive for the study of system synchronization.To improve system bandwidth efficiency and power efficiency in time dispersive channel conditions, the model, algorithm and computational complexity of MMSE channel shortening equalization are investigated. A new optimal delay estimation method is proposed, which reduces the computational complexity greatly at slight performance loss. The MMSE channel shortening equalization algorithm for OFDM system with water-filling power allocation is studied. The expressions of equalizer coefficients and signal correlation matrix are derived. Then the relation between the performance and equalizer parameters is analysed through simulations. Simulation results of system symbol error rate validate that the channel shortening equalization can improve the system performance, while keeping high bandwidth efficiency and power efficiency.