Chip-level MMSE equalization in the forward link of UMTS-FDD: a low complexity approach

Equalization in the forward link of code division multiple access (CDMA) channels has been proposed as an alternative to the RAKE receiver. If the spreading codes are orthogonal (as in UMTS-FDD) and the channel is frequency-flat, the RAKE receiver is optimal. However, frequency-selective fading channels destroy the orthogonality of the spreading codes and chip level equalization is needed in order to restore the orthogonality. We show that the permanent code-multiplexed pilot channel of UMTS-FDD can be used to provide a consistent estimate of the MMSE equalizer. We discuss a low complexity adaptive algorithm that could be used for equalization, namely the stochastic gradient multi-stage Wiener filter (SG-MSWF), which is a sample-by-sample version of the MSWF. Simulations are carried out in a time-varying frequency-selective fading environment and show that the proposed algorithm can achieve performances that are close to those of the optimal with a substantial gain in complexity.