Coherent Code Tracking for Spatial Transmit Diversity DS-CDMA Systems
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Coherent Code Tracking for Spatial Transmit Diversity DS-CDMA Systems Garrey W. Rice DSP Enabled Communications Group, Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow G1 1XW, Scotland, UK Email: [email protected]
Iain G. Stirling DSP Enabled Communications Group, Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow G1 1XW, Scotland, UK Email: [email protected]
R. W. Stewart DSP Enabled Communications Group, Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow G1 1XW, Scotland, UK Email: [email protected] Received 6 February 2004; Revised 23 February 2005 Spatial transmit diversity schemes are now well integrated into third-generation cellular mobile communication system specifications. When DS-CDMA-based technology is deployed in typical macro- and microcell environments, multipath diversity and spatial diversity may be exploited simultaneously by a 2D RAKE receiver. The work presented in this paper focuses on taking advantage of spatial transmit diversity in synchronising the 2D RAKE structure. We investigate the use of coherent and noncoherent techniques for tracking the timing parameters of each multipath component. It is shown that both noncoherent and coherent techniques benefit from transmit diversity. Additionally the performance gap between these two techniques increases with the number of antennas. Keywords and phrases: DS-CDMA, 2D RAKE, transmit diversity, coherent delay-locked loop.
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INTRODUCTION
For direct-sequence code-division multiple-access (DSCDMA) communications, antenna diversity techniques have been proposed and implemented to mitigate the effects of multipath fading [1, 2]. One economical way of deploying such diversity is to use multiple antennas at the base station, and a single antenna at the mobile station [3]. With such an architecture, uplink reception can be carried out using adaptive beamforming solutions. Downlink reception relies on the transmission of distinct waveforms at the transmit antennas which can then be separated at the single receive antenna. It is well known that CDMA-based systems are able to exploit and recombine signal components of different delays using RAKE-style receiver structures. Recently 2dimensional (2D) RAKE receivers have been proposed which are designed to exploit both delayed signal components (time diversity) and spatial diversity simultaneously. Although time diversity is exploited by the overall operation of a 2D RAKE receiver, it is of no benefit to the delay tracking functions which underpin its operation.
In this paper, we consider techniques for tracking the delay of a received signal component when spatial transmit diversity is employed. Closed loop synchronization techniques are generally used for this purpose. The most popular is the noncoherent delay-locked loop (DLL) [4]. However, the WCDMA (wideband CDMA) specifications provide known pilot symbols in both link directions making the coherent DLL a viable alternativ
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