Transient behavior of the minimum mean-squared error receiver for direct-sequence code-division multiple-access systems

This paper studies the transient behavior of an adaptive near-far resistant receiver for direct-sequence (DS) code-division multiple-access (CDMA) known as the minimum mean-squared error (MMSE) receiver. This receiver structure is known to be near-far resistant and yet does not require the large amounts of side information that are typically required for other near-far resistant receivers. In fact this receiver only requires code timing on the one desired signal. The MMSE receiver uses an adaptive filter which is operated in a manner similar to adaptive equalizers. Initially there is a training period where the filter locks onto the signal that is sending a known training sequence. After training the system can then switch to a decision directed mode and send actual data. This work examines the length of the training period needed as a function of the number of interfering users and the severity of the near far problem. A standard LMS algorithm is used to adapt the filter and so the trade off between convergence and excess mean squared error is studied. It is found that in almost all cases a step size near 1.0/(total input power) gives the best compromise between speed of convergence and excess mean squared error. Also, it is shown that the MMSE receiver can tolerate about 30-40 dB worth of near-far problem before the convergence time becomes unacceptable