TABLE OF CONTENTS The superposition of the signals transmitted by the users in a multiple access spread spectrum, also known as Code Division Multiple Access (CDMA), system cause considerable interference to the desired signal if the users' signature waveforms are not orthogonal to each other all the time, a situation which is unlikely to occur in mobile-originated calls.
A lot of research has focused on reducing or cancelling the multiple access interference in order to improve the CDMA receivers' performance. The initial approach was to design an improved single-user detector operating efficiently in multi-user channel by applying advanced adaptive signal processing algorithms. An overview of the work in this area is presented in Section 6.6 of the last chapter. It is worth noting that these detectors are preferred by individual mobile users because knowledge of the parameters (signature waveforms, timing, amplitude and phase) of the interfering users is not desired.
The second approach considers the detection of signals associated with a group of users where spreading codes, timing information and possibly signals amplitude and phase are known and used jointly to better detect each user. These devices are called multi-user detectors and are suitably used at the base station since the parameters of the group are known or can be estimated.
Multi-user detection comes under titles such as group or joint detection and related topics are interference cancellation and co-channel interference suppression. These topics will be treated in more detail in this chapter. The theoretical...
