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UBC Theses and Dissertations

Blind multiuser detection for time-frequency spread multicarrier CDMA Tam, Wilson


Multicarrier transmission technology has shown tremendous potential in realizing high data rates for next generation broadband wireless communication systems. Multicarrier modulation schemes are robust to frequency selective fading which are inherent in broadband wireless channels. This thesis considers blind multiuser detection for the recently proposed time frequency spread multicarrier CDMA (TF-MC-CDMA) system in which the system uses both time and frequency domain spreading to achieve diversity in both time and frequency domains. The challenge for TF-MC-CDMA multiuser detection is to mitigate multiple access interference (MAI) in both the time and the frequency domain. The objective of this thesis is to develop and analyze the performance of blind multiuser detection algorithms for TF-MC-CDMA for three types of channel models: AWGN with MAI, slowly fading downlink Rayleigh multipath channels, and the slowly fading downlink multipath channels with Doppler shift induced intercarrier interference (ICI). A new suboptimal decoupling technique for blind multiuser detection for TF-MCCDMA in AWGN with MAI is proposed. It is found that the original TF-MC-CDMA blind multiuser detection problem can be suboptimally decoupled into two blind multiuser DS-CDMA problems. These two problems can be solved separately using blind DS-CDMA multiuser techniques. Our effort focused on using blind linear multiuser detectors in which we investigated into four types of blind detection methods: blind direct matrix inversion (DMI) method, blind CMOE RLS method, blind subspace multiuser detection method using SVD and PASTd adaptive subspace tracking. The suboptimal decoupling technique for blind multiuser detection for TF-MC-CDMA is extended to slowly fading downlink Rayleigh multipath channels known as type I detectors. Computer simulation results show that type I detectors do not work well in slowly fading multipath channels even though such a scheme provides very good performance in AWGN with MAI. In slowly fading channels, the orthogonality of the time domain signature sequences is preserved. We propose a type II detector which uses a cascade implementation with the time domain detection output acts as the input of the frequency domain detection. Computer simulations show that type II detectors provide much better performance than type I detectors. Blind multiuser detection for TF-MC-CDMA is further extended for slowly fading Rayleigh multipath channels with Doppler shift induced ICI. In mobile channels, Doppler shifts combined with multipath effects create random subcarrier frequency shifts which in turn cause subcarrier frequency mismatch. Such mismatch leads to the loss of orthogonality among subcarriers thus creating ICI. Our analysis shows that Doppler shifts induced ICI has the effect of destroying the common-channel property in downlink channels. The downlink TF-MC-CDMA signal becomes a quasi-uplink signal because of user dependent subchannel gains. The type II detector for slowly fading Rayleigh multipath channels is further extended to apply in such channels with Doppler shift induced ICI. It is shown through analysis and computer simulations that the proposed type II detectors implemented using CMOE RLS algorithm, blind subspace SVD algorithm, and the blind subspace PASTd adaptive subspace tracking algorithm, without modifications, provide robust performance in multipath channels with Doppler shift induced ICI.

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