Asymptotic performance of subspace methods for synchronous multirate CDMA systems
Two direct sequence (DS) code division multiple access (CDMA) multirate access methods with a fixed chip rate can be employed to support multirate services: multicode (MC) access and multiple processing gain (MPG) access. In either an MC or an MPG multirate CDMA system, severe intersymbol interference (ISI) may exist due to large channel delay spread relative to the symbol interval. We generalize the blind subspace technique to such a multirate CDMA system in order to estimate possibly long channel impulse response of a desired user. Then, we build a blind minimum mean-square-error (MMSE) detector to detect the user's information. The detection performance can be significantly improved by suppressing ISI, which becomes feasible after the user's channel parameters are estimated. The asymptotic performance of the channel estimator and the detector is analyzed. In particular, for typical distributions of the inputs and channel noise, closed-form expressions for the channel estimation error and the output signal-to-interference-plus-noise ratio (SINR) of the detector are derived as functions of the number of received data samples and system parameters. Simulation results are provided to verify our analysis.