Diversity and Interference Aspects in the Downlink of Code Division Multiple Access Systems

Doctoral thesis, 2004

This thesis consists of six included papers and technical reports, plus an introduction. The topic studied is interference and diversity aspects in code division multiple access (CDMA) systems for mobile cellular communications. The focus in the papers is on the downlink - the communication link from the base station to the mobile terminal. A performance limiting factor in the downlink of single-carrier CDMA systems is multiple access interference due to lost orthogonality on the time-dispersive radio channel. On such channels, orthogonal frequency division multiplexing (OFDM) is a very powerful technique and, recently, several proposals of CDMA for OFDM systems have been made. Although some results for single-carrier systems are provided (paper A), it is to the area of OFDM-CDMA that the main contributions of this thesis are made (papers B-F). In the included papers, low-complexity single-user detectors are assumed. In agreement with existing results, it is concluded that direct-sequence spreading in OFDM systems is beneficial when the number of active users is low compared to the length of the spreading codes used, i.e., for low system loads. However, assuming a single-cell scenario and a channel code of a sufficiently large free distance, it is at high system loads concluded that coded orthogonal frequency division multiplexing access (OFDMA) outperforms all the considered direct-sequence schemes. This is most clearly the case in a near-far scenario - a situation that naturally will occur in the downlink due to the requirement for different transmit powers for different users. It is also concluded that coded OFDMA achieves essentially the same robustness against inter-cell interference as schemes that use direct-sequence spreading. If a low complexity detector is required, the results presented in this thesis indicate that coded OFDMA is a competitive candidate for the downlink of mobile cellular systems.