Successive Interference Cancellation in DS-CDMA Systems
Doctoral thesis, 1998
There is every indication that direct-sequence code division multiple access (DS-CDMA) will be one of the multiple access schemes for the third generation wireless mobile systems. CDMA is a flexible multiple access method suitable for supporting many new services (speech, data, video, etc), which are becoming increasingly important for mobile communications. Besides supporting multiple data rates, future systems also have to meet enhanced performance and capacity demands. A CDMA system employing conventional receiver techniques is limited in capacity, since its performance can be significantly degraded by multiple access interference and the near-far effect. These problems can, however, be combated with multiuser detection. The multiuser detector studied in this thesis is the successive interference canceller (SIC), chosen because of its relatively low complexity and suitability for hardware implementation. Additionally, this thesis analyses interference cancellation (IC) in combination with variable data rate systems.
The initial study analyses the single- and multistage SIC for asynchronous, multirate DS-CDMA systems. Two multirate schemes are considered, which are based on mixed (or multiple) modulation formats and multicode. These schemes are compared for the additive white Gaussian noise (AWGN) and the Rayleigh fading channels. A conventional linear SIC is used for the single-path channel, but the corresponding SIC adapted to the multipath channel includes both linear and non-linear SIC together with RAKE combining. The results indicate that it is possible to achieve a performance close to the single-user bound for both multirate schemes in systems with fairly high load.
Furthermore, the multi-stage SIC is analysed using a matrix-algebraic approach, leading to the description of the SIC as a linear matrix filtering. Using this representation, an analytical expression for the bit error rate is derived, and the convergence behaviour of the SIC is studied. A group-wise successive interference canceller (GSIC) is also analysed and a general analytical expression for the performance of the GSIC scheme with any linear group detector is given. Four structures are considered for the group detector: the matched filter, the decorrelator, the parallel interference canceller and the MMSE detector. Convergence characteristics, the effect of the grouping and the performance of systems with and without weighted cancellation are analysed. Finally, the matrix-algebraic approach is applied to multirate systems employing the GSIC based on the matched filter group detector, and the multirate systems based on variable processing gain and multicode schemes are compared.
direct-sequence code division multiple access (DS-CDMA)
successive interference cancellation