Near maximum likelihood multiuser receivers for direct sequence code division multiple access

Wideband wireless access based on direct-sequence code-division multiple access (DS-CDMA) has been adopted for third-generation mobile communications systems. Hence, DS-CDMA downlink communications systems form the platform for the work in this thesis. The principles of the spread spectrum concept and DS-CDMA technology are first outlined, including a description of the system model and the conventional receiver. The two classes of codes used in this system, namely spreading codes and forward error correction codes (including Turbo codes), are discussed. Due to the fact that practical communications channels are non-ideal, the performance of an individual user is interference limited. As a result, the capacity of the system is greatly restricted. Fortunately, multiuser detection is a scheme that can effectively counteract this multiple access interference. However, the optimum multiuser detection scheme is far too computationally intensive for practical use. Hence, the fundamental interest here is to retain the advantages of multiuser detection and simplify its implementation. The objective of the thesis is to investigate the optimum multiuser receiver, regarded on a chip level sampling basis. The aim is to reduce the complexity of the optimum receiver to a practical and implementable level while retaining its good performance. The thesis first reviews various existing multiuser receivers. The chip-based maximum likelihood sequence estimation (CBMLSE) detector is formulated and implemented. However, the number of states in the state-transition trellis is still exponential in the number of users. Complexity cannot be reduced substantially without changing the structure of the trellis. A new detector is proposed which folds up the original state-transition trellis such that the number of states involved is greatly reduced. The performance is close to that of the CBMLSE. The folded trellis detector (FTD) can also be used as a preselection stage for the CBMLSE. The FTD selects with high accuracy the few symbol vectors that are more likely to be transmitted. The CBMLSE is then used to determine the most likely symbol vector out of the small subset of vectors. The performance of this scheme is as good as the CBMLSE. The FTD is also applied in an iterative multiuser receiver that exploits the powerful iterative algorithm of Turbo codes

Spatial diversity in MIMO communication systems with distributed or co-located antennas

The use of multiple antennas in wireless communication systems has gained much attention during the last decade. It was shown that such multiple-input multiple-output (MIMO) systems offer huge advantages over single-antenna systems. Typically, quite restrictive assumptions are made concerning the spacing of the individual antenna elements. On the one hand, it is typically assumed that the antenna elements at transmitter and receiver are co-located, i.e., they belong to some sort of antenna array. On the other hand, it is often assumed that the antenna spacings are sufficiently large, so as to justify the assumption of independent fading. In this thesis, the above assumptions are relaxed. In the first part, it is shown that MIMO systems with distributed antennas and MIMO systems with co-located antennas can be treated in a single, unifying framework. In the second part this fact is utilized, in order to develop appropriate transmit power allocation strategies for co-located and distributed MIMO systems. Finally, the third part focuses on specific synchronization problems that are of interest for distributed MIMO systems