Title :
A Modified Decorrelated Delay Lock Loop for Synchronous DS-CDMA Systems
Author :
Wong, W.K. ; Wu, Y.T. ; Leung, S.H. ; Zhu, Y.S.
Author_Institution :
Dept. of Electron. Eng., City Univ. of Hong Kong, Hong Kong
Abstract :
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.
Keywords :
code division multiple access; delay lock loops; S-curve; computer simulations; delay lock loop; gold code sequences; low-signal-to-noise-ratio; multiple-access interference; synchronous DS-CDMA systems; synchronous direct-sequence code-division multiple-access systems; Delay Lock Loop; Delay lock loop; direct-sequence code division multiple access (DS-CDMA); direct-sequence code-division multiple access (DS-CDMA); multiple access interference (MAI); multiple-access interference (MAI); synchronization;
Journal_Title :
Vehicular Technology, IEEE Transactions on
DOI :
10.1109/TVT.2008.915516