International Telemetering Conference Proceedings / October 18-21, 2004 / Town & Country Resort, San Diego, California / The decorrelating detector can afford good data estimates because it does not need to know many parameters of the received signal. However, it shows great performance deprivation when the background noise is high. On the other hand, partial parallel interference canceller(PPIC) has the potential to combat the near-far problem and have much lower computation complexity. But its performance depends on the initial data estimate. An improved PPIC scheme is proposed in this paper to combat the near-far problem. It utilizes the advantages of the two detectors by combining them. The focus of this paper is on the BER performance and the near-far resistance capability of the proposed scheme. Computer simulations demonstrate that the proposed detector has good BER performance and near-far resistance capability.

In this thesis, we investigate the CDMA (Code Division Multiple Access) multi- user detection scheme where the DD (Decorrelating Detector) is used to eliminate the multiple access interference. The DD algorithm need to compute the inverse of the matrix involves a great deal of computation, especially when the number of users is large. A recursive method with the LMS (Least-Mean-Square) algorithm, namely the decorrelating transversal filter, to detect users¡¦ signals adaptively can reduce greatly the computational complexity of a CDMA multi-user detector. In this thesis, we focus on the hardware FPGA (Fdield Programmable Gate Array) implementation of the decorrelating transversal filter. The functional system simulation of the decorrelating transversal filter is carried out by using Matlab first. Then this filter is implemented by the Xilinx FPGA and its system performance is also verified.

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The study of the spatial aspects of multipath in wireless communications environments is an increasingly important addition to the study of the temporal aspects in the search for ways to increase the utilization of the available wireless channel capacity. Traditionally, multipath has been viewed as an encumbrance in wireless communications, two of the major impairments being signal fading and intersymbol interference. However, recently the potential advantages of the diversity offered by multipath rich environments in multiuser communications have been recognised. Space time coding, for example, is a recent technique which relies on a rich scattering environment to create many practically uncorrelated signal transmission channels. Most often, statistical models have been used to describe the multipath environments in such applications. This approach has met with reasonable success but is limited when the statistical nature of a field is not easily determined or is not readily described by a known distribution.¶ Our primary aim in this thesis is to probe further into the nature of multipath environments in order to gain a greater understanding of their characteristics and diversity potential. We highlight the shortcomings of beamforming in a multipath multiuser access environment. We show that the ability of a beamformer to resolve two or more signals in angle directly limits its achievable capacity.¶ We test the probity of multipath as a source of spatial diversity, the limiting case of which is co-located users. We introduce the concept of separability to define the fundamental limits of a receiver to extract the signal of a desired user from interfering users’ signals and noise. We consider the separability performances of the minimum mean square error (MMSE), decorrelating (DEC) and matched filter (MF) detectors as we bring the positions of a desired and an interfering user closer together. We show that both the MMSE and DEC detectors are able to achieve acceptable levels of separability with the users as close as λ/10.¶ In seeking a better understanding of the nature of multipath fields themselves, we take two approaches. In the first we take a path oriented approach. The effects on the variation of the field power of the relative values of parameters such as amplitude and propagation direction are considered for a two path field. The results are applied to a theoretical analysis of the behaviour of linear detectors in multipath fields. This approach is insightful for fields with small numbers of multipaths, but quickly becomes mathematically complex.¶ In a more general approach, we take a field oriented view, seeking to quantify the complexity of arbitrary fields. We find that a multipath field has an intrinsic dimensionality of (πe)R/λ≈8.54R/λ, for a field in a two dimensional circular region, increasing only linearly with the radius R of the region. This result implies that there is no such thing as an arbitrarily complicated multipath field. That is, a field generated by any number of nearfield and farfield, specular and diffuse multipath reflections is no more complicated than a field generated by a limited number of plane waves. As such, there are limits on how rich multipath can be. This result has significant implications including means: i) to determine a parsimonious parameterization for arbitrary multipath fields and ii) of synthesizing arbitrary multipath fields with arbitrarily located nearfield or farfield, spatially discrete or continuous sources. The theoretical results are corroborated by examples of multipath field analysis and synthesis.