Chip-level LMMSE equalization for downlink MIMO CDMA in fast fading environments

In this paper, we consider linear MMSE equalization for wireless downlink transmission with multiple transmit and receive antennas in fast fading environments. We propose a new algorithm based on the conjugate-gradient algorithm with enhanced channel estimation. In order to be robust to the channel variations, the channel coefficients are estimated by using a weighted sliding window. Two methods to determine optimal weights with respect to the Doppler frequency are proposed. The algorithm has been simulated in a fast fading environment (vehicular A with a velocity for the mobile station of 120 km/h). We show by simulations that good performance is obtained in a correlated fast fading environment with reasonable complexity. Moreover, this approach outperforms methods based on basic sliding window or forgetting factor and the LMS algorithm.