Multiuser Delay-Tracking CDMA Receiver
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Multiuser Delay-Tracking CDMA Receiver Wei Zha Department of Electrical and Computer Engineering, Faculty of Applied Science, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Email: [email protected]
Steven D. Blostein Department of Electrical and Computer Engineering, Faculty of Applied Science, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Email: [email protected] Received 31 January 2002 and in revised form 7 May 2002 We propose to integrate delay tracking into a successive interference cancellation (SIC) code division multiple access (CDMA) multiuser detector. Delay-robust SIC was initially proposed for CDMA multiuser detection when there exist 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. The amplitude of the error signal can be 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 pulses and band-limited chip pulses are shown. Keywords and phrases: CDMA, multiuser detection, synchronization, delay tracking, successive interference cancellation.
1.
INTRODUCTION
The capacity of a code division multiple access (CDMA) system is limited by the multiple access interference (MAI) from other users. CDMA multiuser detection at the basestation, which utilizes known user spreading codes for detection, is an effective method to suppress MAI and improve receiver performance. The optimal multiuser detector has exponential computational complexity [1]. Several low complexity multiuser detectors including the decorrelating detector, MMSE detector, successive interference cancellation (SIC) [2], and parallel interference cancellation (PIC) receiver have been proposed [3]. These multiuser detectors all need accurate time delay information from all users. If the time delays are estimated at the receiver even within a small error, the performance will degrade dramatically [4]. There is a rich literature on initial delay estimation (acquisition) of either a single user or of all users [5, 6]. However, little research has addressed the problem of tracking the multiuser delays after the initial acquisition. Known multiuser delay-tracking receivers can be categorized into two types: those based on the extended Kalman filter (EKF) [7, 8, 9] and those based on delay-locked loops
(DLLs) combined with interference cancellation [10, 11]. Both types of delay trackers require the initi
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