A Comparison of chip rate MMSE and symbol rate MMSE in multiuser uplink long code DS-CDMA

We investigate a chip rate implementation of the MMSE multiuser receiver in the case of uplink UMTS context where data are scrambled by long codes (Gold sequences). The main problem is the resolution of a high dimension linear system involving a band-diagonal auto-correlation matrix. Using a Cholesky (or LDU) decomposition and taking advantage of the multiuser correlation expression, we show that the complexity of this chip rate MMSE is similar to that of the more classical symbol rate MMSE receiver. Then, a theoretical equivalence between these two receivers is demonstrated. Finally, simulation results are presented which show the performance gains of the MMSE receiver when compared to the RAKE receiver.