This paper presents the optimization and performance enhancement of multiuser detection in Direct Sequence Code Division Multiple Access (DS-CDMA) system by using subspace blind linear Minimum Mean Square Error (MMSE) detector. The proposed scheme uses efficient Bit Error Rate (BER) by use of Gold sequence, the performance of subspace linear MMSE multiuser detection algorithms, and yields better performance and lower computational complexity in contrast to DMI blind linear MMSE detector algorithms. A simulation example is given to compare for various codes such as M-sequence, and Gold-sequence.

In this simulation, several Multiuser Detectors (MUD) are compared with the conventional detector, the partial Minimum Mean Square Error (MMSE) detector, and the full MMSE detector (Wang and Poor, 1998). While the partial MMSE detector can reduce only intra-cell interference, the full MMSE detector can reduce both intra-cell interference and inter-cell interference with the assumption that all spreading codes such as M-sequence or Gold-sequence of both known and unknown users are known. The performances of the blind MMSE MUD were evaluated. For both synchronous systems and asynchronous systems, the blind MMSE MUD using the subspace method outperforms the blind MMSE MUD using the direct method because it gains high resolution from the subspace decomposition. After some symbols, the Signal to Interference-plus-Noise Ratio (SINR) of the blind MMSE detector using the subspace method crosses over the partial MMSE detector and converges to the full MMSE detector. In the case of random code, it crosses over the partial MMSE detector and the conventional detector faster than in the case of gold code because the partial MMSE detector and the conventional detector have worse performance in the case of random code (Shimon, 1996; and Wang and Poor, 1998 and 2004).

Code Division Multiple Access (CDMA) relies on the use of spread spectrum techniques to achieve multiple communication channels. With CDMA (Proakis and Salehi, 2002; Tranter et al., 2004; and Wang and Poor, 2004) each user's narrow band signal is modulated by a high rate special code (pseudo-random binary sequence). This causes the spreading of the bandwidth of the user's signal resulting in a wideband signal. A large number of CDMA users share the same frequency spectrum. If CDMA is viewed either in time or frequency domain, the signals appear to be overlapping; they are separated by their special code. In the receivers, the signals are correlated by the appropriate pseudo-random code which de-spreads the spectrum. The other users' signals whose codes do not match are not despread and therefore only appear as noise and represent a self-interference generated by the system. The Signal-to-Interference Ratio (SIR) for CDMA is determined by the ratio of the desired signal power to the total interference power from all the other users. The capacity of CDMA is therefore only limited by the amount of interference that can be tolerated from other users. This is why the capacity of CDMA is said to be interference limited (unlike Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) capacities which are primarily bandwidth limited) (Proakis and Salehi, 2002; Tranter et al., 2004; and Wang and Poor, 2004).

Telecommunications Journal, Multiuser Detection, Code Division Multiple Access, CDMA, MMSE Detector Algorithms, Synchronous Systems, Multiple Communication Channels, Multiple Access Interference, Direct Matrix Inversion, Blind Detector, Batch Processing Method, MMSE Multiuser Detectors.