A Modified Decorrelated Delay Lock Loop for Synchronous DS-CDMA Systems

• Published in: IEEE transactions on vehicular technology 2008-09, Vol.57 (5), p.3293-3299
• Main Authors: Wong, W.K., Wu, Y.T., Leung, S.H., Zhu, Y.S.
• Format: Article
• Language: English
• Subjects: Applied sciences; Arm; Bias; Circuit properties; Circuits of signal characteristics conditioning (including delay circuits); Coding, codes; Decorrelation; Delay; Delay lock loop; Detection, estimation, filtering, equalization, prediction; Detectors; direct-sequence code-division multiple access (DS-CDMA); Electric, optical and optoelectronic circuits; Electronic circuits; Electronics; Exact sciences and technology; Information, signal and communications theory; Interference suppression; Locks; Multiaccess communication; Multiple access interference; multiple-access interference (MAI); Multiuser systems; Noise reduction; Offsets; Signal and communications theory; Signal to noise ratio; Signal, noise; Studies; synchronization; Synchronous; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Time measurements; Timing; Tracking; Tracking loops; Transmission and modulation (techniques and equipments); Weighting
• ISSN: 0018-9545; 1939-9359
• DOI: 10.1109/TVT.2008.915516

This paper presents a modified decorrelated delay lock loop (MD-DLL) for synchronous direct-sequence code-division multiple-access (DS-CDMA) systems under multiuser environments. The structure of the MD-DLL is similar to that of a traditional delay lock loop (T-DLL), aside from the fact that the despreading signals in T-DLL are replaced by a linear combination of all the users' code sequences with various weightings and timing offsets. The basic principles of MD-DLL are given as follows: 1) to use the decorrelating technique of a multiuser detector to remove the multiple-access interference at the on-time code position instant and 2) to choose a proper timing offset for the code sequences to reduce the weighting imbalance between the two arms of the tracking loop and suppress noise enhancement. The proposed MD-DLL scheme solves the tracking bias problems in a decorrelated delay lock loop under a low-signal-to-noise-ratio (SNR) condition. Computer simulations based on Gold code sequences are used to verify the findings. It is shown that MD-DLL achieves a better and stable S-curve with negligible tracking bias, better rms tracking error and mean-time-to-lose-lock performance under high-SNR conditions, and stronger near-far resistance than T-DLL.