An adaptive receiver for dynamic traffic DS/CDMA packet transmission

The adaptive minimum mean squared error (MMSE) based detector is one of the most attractive CDMA receivers because of its relatively simple structure and good performance. However, most of the previous research on such receivers was based on the assumption that the system traffic is fixed during the receiver processing. We propose the use of the MMSE filter in a dynamic traffic system, such as in packet transmission systems. With decision directed (DD) update of the filter coefficients, we can adapt to the dynamic traffic. Under some extreme cases when the user traffic increases significantly, the pure DD mode may fail or diverge and the condition under which it fails is also derived. To assure convergence, the blind PASTd (Projection Approximation Subspace Tracking) method is first used before switching to the DD mode. It is shown that the combined PASTd and DD based MMSE filter is a robust receiver and can achieve almost the same performance as a receiver with training sequences for every packet, both in the uplink and the downlink. We also show that the receiver behavior is quite different when the traffic increases or decreases