The dynamic length equaliser and its application to the DS-CDMA systems

• Main Author: Wei, Xusheng
• Published: University of Edinburgh 2005
• Subjects: 621.382
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.663613

From the introduction of wireless communications in the 1980s, the wireless communication market has grown explosively and produced demand for more capacity. Since the direct sequence code division multiple access (DS-CDMA) technique has more capacity than the current time division multiple access technique, it has been adopted for third generation mobile communications systems and it serves as the platform for the work in this thesis. At the downlink of the DS-CDMA systems, the multiple access interference (MAI) will be introduced due to the non-zero cross correlation between the spreading codes with arbitrary time shifts. Thus, the system's capability is limited by the amount of interference instead of the background noise. To achieve full capacity utilization, it is crucial to suppress the interference in the system. Linear multiuser detection (MUD) and symbol level equalisers are two common methods to suppress the MAI at the downlink. However, these methods have considerable computational complexity and more importantly, they are not suitable for most cases of the downlink where the long scrambling codes are used. Recently, chip level equalisers have been proposed to partially restore the orthogonality of the spreading codes by inverting the channel transfer function prior to the despreading. The MAI can then be suppressed by the conventional correlator after the chip level equalisers. Due to the time varying channel in the downlink, adaptive implementation should be adopted at the mobile terminal. The length of the adaptive filter is an important factor which affects all the aspects of its performance measures such as the convergence rate, computational complexity and MSE (mean square error) performance. However, till now, little research work related to this important parameter has been done. In this thesis, the relation between the MSE performance and the length of the adaptive filters based on several different adaptive algorithms is firstly given. Then the influence of the value of the threshold parameters on the performance of dynamic length algorithm is analyzed. Based on this analysis, a new type of dynamic length algorithms is proposed and its implementation issues both under the static channel environment and the time varying channel environment are also considered. Then, the relationship between the MSE performance and the tracking performance with the length of the chip level equalisers is given for both the chip rate implementation and symbol rate implementation scenario. The dynamic length algorithm proposed is used at the downlink of CDMA systems. Both the chip rate implementation and symbol rate implementation are considered. The performance of this new type of the chip level equalisers outperforms the corresponding fixed length chip level equalisers with a marginal increase in computational complexity.