Design and performance analysis of resources management and multiuser detection in CDMA systems


Student thesis: Doctoral Thesis

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  • Kwok Kai SOO

Related Research Unit(s)


Awarding Institution
Award date2 Oct 2007


With the growing deployment of third generation mobile systems, it is expected thatcode division multiple access (CDMA) technique will continue to play a dominant rolein mobile communication in the near future to provide high speed mobile dataservices. The capacity of a CDMA mobile system is mainly limited by the multipleaccess interference and the near-far problem because the orthogonality of thespreading codes cannot be maintained at the receiver due to varying channel condition.The success of the CDMA mobile system depends greatly on the measures ofresources management and multiuser detection to counteract the multiple accessinterference. Closed loop power control and admission control are the two major typesof resources management schemes. The closed loop power control is a feedback control scheme to regulate thetransmission power of the mobile based on the power control decision feedback fromthe base station. The closed loop power control consists of two parts – inner loop andouter loop. In the inner loop power control, the base station derives the power controlcommand by comparing the received signal with a prescribed target. This powercontrol command is fed back to the mobile to adjust its transmission power directly. The outer loop power control regulates the control target of the inner loop according tothe mobile’s current radio channel condition. The outer loop adjusts the transmissionpower of the mobile indirectly by means of the inner loop control target. In this thesis, a new inner loop power control scheme is proposed utilizing the iterativesymmetric successive overrelaxation (SSOR) preconditioned technique to adjust thetransmission power of a block of mobiles within a cell. It is able to speed up theconvergence of the inner loop power control with restricted link gain informationexchange for the mobiles within a cell. A higher system capacity is achieved by theproposed inner loop power control scheme. Moreover, a new outer loop power controlscheme is designed to regulate the inner loop control target adaptively. It follows the“waterfilling” strategy such that a higher control target is set for the inner loop whenthe mobile is in good channel conditions and vice versa when in adverse channelconditions. It is shown that the proposed waterfilled outer loop power control schemecan lower the average outage probability and can have a fast response in tracking thevarying channel condition closely. Admission control is critical to the success of a CDMA system in regulating networkaccess to guarantee the fulfillment of quality requirements for different mobiles and to maximize resource utilization while preventing system overload due to the multipleaccess interference. A simple admission control criterion is derived with a dynamicadmission boundary based on the intra-cell and other-cell interference power receivedat the base station. The proposed admission criterion can adapt to the current loadingof the cell in a practical multi-cell multiple services CDMA system with imperfectclosed loop power control and shadowing effect due to channel characteristics. Multiuser detection is a powerful technique to reduce the multiple access interferenceby exploiting the signals of the other mobiles. Nevertheless, the computationalcomplexity of the optimal multiuser detection is too high to be implemented inpractice. Two sub-optimal multiuser detectors are proposed from differentperspectives. The first one is leveraged on a new evolutionary computationaltechnique called particle swarm optimization. The second one is a hybridization of theSSOR preconditioned technique applied in the inner loop power control and a reducedcomplexity recursive scheme. It is shown that they can achieve a significant reductionin computational complexity with competent bit error rate performance.

    Research areas

  • Demodulation (Electronics), Code division multiple access