CDMA Multiuser Delay-Tracking and Detection

We propose a low complexity CDMA multiuser delay tracking receiver as an alternative to the more complex extended Kalman filter (EKF) based CDMA multiuser delay trackers. It integrates delay-tracking into CDMA multiuser detection by the delay-robust successive interference cancellation (SIC) technique, which was initially proposed for robust CDMA multiuser detection when there exists time delay estimation errors. When the delay error is small, the true user signature vector can be approximately expanded around the estimated time delay as a linear combination of an estimated signature vector and an error vector. This error vector is equivalent to a single-branch realization of the delay-late delay locked loop in a conventional single user CDMA receiver. The error signal is recursively estimated and cancelled in the multistage SIC iterations by using the tentative feedback decisions. Since the relative amplitude of the error signal provides information about the delay error, we propose the application of error signal feedback to improve the delay estimation. In a sliding window implementation, the delay-robust SIC is used to track multiple time varying user delays. For constant or slowly varying channels, a soft-decision function can be used to improve the delay-robust SIC. For fast fading channels, a linear decision function is used. Delay tracking results for both rectangular chip pulse and band-limited chip pulses are shown.