Chapter 7 Multiuser Detection

We have discussed a simple method of MAI suppression in Chapter 6. The idea of MAI suppression stems form the single-user detection philosophy, in which we treat signals from other users as interference. In this chapter, we will introduce another way, namely multiuser detection, to tackle the near-far problem. In the paradigm of multiuser detection, we jointly detect data signals from all the users. There is not MAI since signals form all users are treated as the desired signals. However, one should note that this does not mean that we do not need to design spreading sequences with “good” correlation properties for the users. For example, if two users use exactly the same sequence spreading sequence and their transmissions are synchronous, there is not way we can reliably resolve the data signals of the two users from the received signal even if we detect them jointly. On the other hand, with properly designed sequences, the near-far problem is solved implicitly by performing multiuser detection. Theoretically, receivers based on multiuser detection usually outperform, but are usually more complex than, receivers based on single-user detection. The applicability of multiuser receivers depends on system design issues, such as the security of joint detection, the implementation complexity, and the availability of information required to perform multiuser detection. For example, let us consider a typical wireless cellular system. It would be difficult to employ multiuser receivers at the mobile units for forward-link transmission because of the limitation on the implementation complexity and the availability of information about other users. However, multiuser detection could be a viable choice in the base-station for reverse-link transmission.