Mitigating interference in wireless local loop DS-CDMA systems

In wireless local loop (WLL) systems employing direct sequence spread-spectrum (DS/SS) code-division multiple-access (CDMA), the use of closed-loop timing and power control, short orthogonal spreading codes, and directional antennas at the subscribers result in a vastly different interference scenario compared to mobile cellular systems in which asynchronous subscribers use omnidirectional antennas. In the WLL reverse link, residual timing offsets and multipath propagation give rise to intrasector interference, and subscribers in other sectors give rise to intersector interference. We address the improvements afforded by replacing the conventional despreader by an adaptive linear minimum-mean-square-error (MMSE) detector that exploits the cyclostationary nature of the above interference. We compare the conventional and MMSE detectors for varying degrees of intrasector interference by varying the timing error, and for varying degrees of intersector interference by considering a system with two carrier frequencies per cell and a system with universal frequency reuse (and hence twice the capacity). Our results indicate that, compared to the despreader, the MMSE detector effectively mitigates the interference to enable a WLL system with higher capacity and less stringent timing control.