| In this master's thesis, the issue of signal detection in the downlink of Code Division Multiple Access (CDMA) is addressed. First, we observe the CDMA downlink in a frequency-selective slow fading channel as a linear combination of convolved independent symbol sequences and develop a higher order statistics (HOS) based novel symbol estimation method. We call this method as Blind Multiuser Separation (BMUS), which inherently benefits from quasi-orthogonality of the user spreading codes and the independence among transmitted user, and evaluate its applicability in a "short code" limited knowledge CDMA scenario. Later we develop 3G WCDMA downlink specific chip-level space-time equalizers capable of providing uncoded BER of ≤10-3 in fast time varying frequency-selective Rayleigh faded wireless channels. An adaptive version of linear minimum mean-square error (LMMSE) equalizer based on conjugate gradient algorithm (CGA), which avoids the direct estimation of inverse correlation matrix, is derived in the generalized framework of multiple receive antennas for very high speed mobile receivers (≥120 kmph). The singleuser channel estimate obtained using common code-multiplexed pilot channel (CPICH) is further refined by exploitation of time correlations among samples of any single multi-path via weighted sliding window approach. An adaptive MMSE method based on CGA for handling soft hand-off is also developed. Simulation results suggest that even in the events of very high spectral loading the performance of CGA based adaptive MMSE equalizer with moderate computational load is close to that of the LMMSE equalizer employing matrix inversion and it is a possible solution of much labored problem of multiuser detection in WCDMA downlink with "long codes". |
