CDMA downlink chip-level MMSE equalization and finger placement

In CDMA downlink, MAI arises due to the loss of orthogonality of the spreading codes for each user in frequency-selective channels. We analyze the system when a linear equalizer is used for chip estimation. The analysis is conducted for a general model when arbitrary finger placement with or without multiple receiver antennas is used, of which many CDMA downlink linear receivers are special cases. We show that the equalizer designed to minimize the chip MSE differs only by a scaling factor from the equalizer that maximizes output symbol SNR or minimizes symbol MSE. The performance of the equalizer depends heavily on the finger placement scheme. We propose two low complexity strategies for suboptimal finger placement. One is based on a recursive sequential search and the other is based on heuristic arguments of interference cancellation. Simulation shows that both resulting schemes can achieve similar performance as uniformly spaced tapped-delay-line equalizer but with a much smaller number of fingers.