First generation wireless communications systems in the form of analog cellular mobile radio systems have little changed since they have been launched to the market 15 years ago. As the demand for wireless access of fixed networks grows, the weakness of the first generation technology became increasingly apparent. Specifically, they are not designed to support different transmission services. Cellular systems which use digital modulation techniques (called digital cellular) offer large improvements in capacity and system performance. In particular, they enable the transmission of different kinds of services, such as voice and data transmissions. Among those digital cellular systems such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Direct Sequenc...[ Read more ]

First generation wireless communications systems in the form of analog cellular mobile radio systems have little changed since they have been launched to the market 15 years ago. As the demand for wireless access of fixed networks grows, the weakness of the first generation technology became increasingly apparent. Specifically, they are not designed to support different transmission services. Cellular systems which use digital modulation techniques (called digital cellular) offer large improvements in capacity and system performance. In particular, they enable the transmission of different kinds of services, such as voice and data transmissions. Among those digital cellular systems such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Direct Sequence Code Division Multiple Access (DS/CDMA), there does not appear to be a single multiple accessing technique that is superior to others in all circumstances. In wireless communication, the problem of multipath fading and interference from other users in the cellular reuse environment limit the performance of data transmission. Since DS/CDMA systems are capable of rejecting multipath and interference, it is expected that DS/CDMA systems can be the strong candidate for the technology of the future wireless communications systems. However, this technique still suffers from various problems. For example, in conventional DS/CDMA receivers, the data can be detected by correlating the received signal with the signature sequence of the desired user. This receiver actually demodulates each user as if the other users were not present. This leads to a receiver that is not optimum against the non-Gaussian multi-access interference. In particular, as the number of users increases it can become Multiple Access Interference (MAI) limited. Another major disadvantage is the near-far problem which refers to the phenomenon of high power interferers completely destroying communications from lower power transmitters. Therefore, this technique suffers from a severe reduction in the system performance.

Recently, there has been a lot of interest in the multiuser detection problem of spread-spectrum multi-access systems in which these questions are addressed. This interest was motivated by the work of Verdli where it has been shown that the shortcomings of DS/CDMA systems are not inherent to the system itself, but are rather a consequence of the conventional detection techniques. In particular, the optimum multiuser detector of DS/CDMA communications has been shown to be near-far and MA1 resistant and that significant performance can be gained by optimum, as opposed to conventional, demodulation. Unfortunately, the optimum multiuser detector requires very complex centralized implementation and knowledge of the received energies of the various users. Specifically, its complexity grows exponentially as the number of users increases. To reduce the complexity of the optimum multiuser detector, various suboptimum multiuser detectors have been developed in the past several years. These detectors vary in complexity and have been mainly studied over the AWGN channel.

In this thesis, we propose two multiuser detection methods for an asynchronous DS/CDMA communications network that is operating over multipath Rayleigh fading links. Specifically, we first propose and analyze a multiuser receiver that is based on Cascade Cochannel Interference (CI) cancellations. The second proposed method is again a multiuser detector strategy which involves the addition of Forward Error Correction (FEC) to the Cascade CI cancellation method. Numerical results will show that the proposed multistage detectors, which alleviate the detrimental effects of the near-far problem and MAI, significantly improve the system performance.