Investigation On Several Issues Of Spreading Sequences, Adaptive Modulation And Coding In CDMA Systems

In the code division multiple access (CDMA) system, the multiple access interference (MAI) mitigation capability as well as system capacity are determined, to a great extent, by the correlation properties of spreading sequences; while adaptive modulation and coding (AMC) with high spectral efficiency is the key technique to obtain high speed and low symbol error rate (SER) performance in mobile data transmission. Concerning with the direct sequence (DS) CDMA system, this thesis focuses on several issues related to orthogonal spreading sequences and AMC, including correlation properties of orthogonal variable-length spreading sequences and their effects on DS CDMA system performance, performance of multicarrier (MC) DS CDMA system with generalized orthogonal (GO) sequences, adaptive modulation with throughput maximization, performance of adaptive Turbo-QAM coded modulation in generalized fading channel and performance of AMC in WCDMA high speed downlink packet access (HSDPA).First of all, correlation properties of orthogonal variable-length spreading sequences are analyzed and new closed-form expressions of MAI of the CDMA systems are derived in terms of aperiodic partial correlation of variable-length sequences. Then, the self-interference (SI), MAI and bit-error-rate (BER) performance of the desired user in a multi-rate CDMA system are analyzed. It is shown that the OVSF sequence length of the interference user has significant influence on the BER performance of the desired user, while the effect of sequence length of the desired user is negligible. With the downlink simulation model constructed, performance of wideband CDMA under different radio propagation environment and vehicular speed is simulated. Based on system model of MC DS CDMA under flat fading channel with relative shifts of spreading sequences within 1 chip and frequency selective fading channel with arbitrary relative chip shifts, performance of multicarrier CDMA system with GO sequences under different radio environment is investigated. Results obtained demonstrate that, due to the existence of zero-correlation-zone (ZCZ) of the GO sequences, near ideal performance can still be achieved even when high speedradio environment is encountered. In addition, obvious interference mitigation as well as significant performance improvement can still be observed when the relative shifts of spreading sequences exceed ZCZ.Next, based on the throughput performance of fixed modulation schemes in packet transmission, adaptive modulation with throughput maximization under fading channel is investigated and significant throughput improvement is obtained. At the same time, analysis of effects of Gaussian distributed and uniform distributed feedback error of the channel fading behavior on throughput performance of adaptive MQAM reveals that the obvious throughput loss shall be observed as far as the error exceeds certain limit. Also, new expression of SER of MQAM in frequency selective fading channel with RAKE combining is obtained, and throughput performance adaptive MQAM with RAKE receiver in multipath Rayleigh fading channel is analyzed. Moreover, new close-form BER expressions of MQAM and MPSK in Nakagami-/?? fading channel are derived, which include the known analytical results as a special case. Furthermore, analytical model for adaptive coded modulation is proposed, through which channel SNR switching thresholds with the aim of throughput maximization of adaptive coded modulation are obtained with the method of Lagrange, and then the average throughput performance of the adaptive system is obtained.Finally, performance of adaptive modulation and coding in 11SDPA system is investigated. Multi-code channel structure of physical layer, coded modulation schemes and adaptive transmission parameter selection are analyzed. Simulation implementation of fading compensation in flat fading channel and maximum ratio combining (MRC) in frequency selective fading channel is investigated. It is shown in our simulation results that throughput maximization has...