146 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 57, NO. 1,JANUARY 2008 A Multipath Detection Scheme for CDMA Systems With Space–Time Spreading Mohamed Abou-Khousa, Student Member, IEEE, Ali Ghrayeb, Senior Member, IEEE, and Mohamed El-Tarhuni, Senior Member, IEEE Abstract—Space–time spreading (STS) is an appealing open- loop transmit diversity scheme, which has recently been included into the cdma2000 standard. It has been shown that the perfor- mance of the STS scheme is highly sensitive to fading coefficient estimation errors, particularly when the channel is highly time dispersive. In practical systems, channel estimation is normally performed after the multipath components are resolved, which suggests that improving multipath detection reduces such esti- mation errors. Motivated by this, we address, in this paper, the problem of multipath detection in STS-based code division mul- tiple access (CDMA) systems. We first extend the conventional energy-based multipath detection scheme (EMDS) to cope with the spatial channel structure. We derive approximate expressions for the probability of detection and probability of false alarm. It is shown that the errors produced by the conventional scheme in detecting the potential multipath components severely impact the performance of the receiver. To improve upon the EMDS, we introduce and analyze an improved multipath detection scheme (IMDS) based on the estimation of the interference power in the individual resolved multipath components. The efficacy of the proposed scheme stems from the fact that the interference in each potential path is estimated and subtracted before that path is detected. We also present a simple and realizable version of the proposed IMDS detection scheme. Our results show that the pro- posed scheme not only improves the bit-error-rate performance significantly but also utilizes the pilot power much more efficiently. Index Terms—Code division multiple access (CDMA) systems, multipath detection, space–time spreading (STS). I. I NTRODUCTION T RANSMIT diversity techniques constitute promising means to combat slow fading in third-generation code division multiple access (CDMA) systems. Among all the tech- niques, open-loop transmit diversity techniques are particularly appealing to the system designer, as they do not reduce the up- link capacity since no channel information feedback is needed. Manuscript received March 20, 2006; revised January 28, 2007, April 16, 2007, and May 2, 2007. The work of M. Abou-Khousa and A. Ghrayeb was supported in part by the Natural Sciences and Engineering Research Council of Canada under Grant N00858. This work was presented in part at the IEEE Vehicular Technology Conference, Stockholm, Sweden, May–June 2005. The review of this paper was coordinated by Prof. T. J. Lim. M. Abou-Khousa was with the Department of Electrical and Computer Engineering, Concordia University, Montreal, QC H3G 1M8, Canada. He is now with the Department of Electrical Engineering, Missouri University of Science and Technology (Missouri S&T), Rolla, MO 65409 USA (e-mail: maamc2@mst.edu). A. Ghrayeb is with the Department of Electrical and Computer Engi- neering, Concordia University, Montreal, QC H3G 1M8, Canada (e-mail: aghrayeb@ece.concordia.ca). M. El-Tarhuni is with the Department of Electrical Engineering, American University of Sharjah, Sharjah, United Arab Emirates (e-mail: mtarhuni@ aus.edu). Digital Object Identifier 10.1109/TVT.2007.905327 Recently, the so-called space–time spreading (STS) scheme has been included in the cdma2000 standard as an open-loop transmit diversity with two transmit antennas and one receive antenna option. The STS scheme was first proposed in [1] and independently in [2]. Due to its potential for enhancing the transmission reliability and the capacity of the system without penalizing the system resources, e.g., bandwidth and spreading codes, it has been subsequently adopted by the cdma2000 standard [3]–[5]. Since its introduction, the STS scheme has been subjected to extensive performance analysis and evaluation (see [3] and references therein). Moreover, various studies have been conducted to compare the performance of the STS scheme with other open-loop transmit diversity schemes, including space–time transmit diversity [6], [7] and orthogonal transmit diversity [3], [8]. In [9] and [10], the performance of a reverse link STS-based CDMA system is analyzed over frequency se- lective fading channels with imperfect estimation of the fading coefficients while assuming that the delays are perfectly known. Therein, it is shown that as the number of multipath components increases, the estimation errors significantly impact the sys- tem performance. More recently, a comprehensive performance evaluation of the STS scheme for the forward link has been discussed in [3]. The majority of the relevant work in the literature suggests that the STS scheme is rather sensitive to the fading coeffi- cient estimation errors, particularly when the channel is highly time dispersive. In realistic systems, channel estimation errors are normally composed of not only the errors in estimating the fading coefficients themselves but also the impairment in detecting the multipath components impinging at the receiver front end [11]. The latter errors have a profound effect on the performance of the system since the estimation of the fading coefficients of a certain multipath component is normally car- ried out after that path is identified. Hence, accurate multipath detection is of great importance for STS-based CDMA sys- tems, as it impacts the overall performance of the receiver. To date, neither the problem of multipath detection for STS-based CDMA systems nor the effect of imperfect multipath detection on the performance of the STS scheme has been investigated. Guenach et al. [12], [13] propose an iterative estimation scheme for the channel-fading coefficients and the path delays, with emphasis on the uplink channel. They also consider single transmit and single receive antenna systems, i.e., no STS. This scheme can also be applied to the downlink channel, but given the associated high computational complexity, it may be prohibitive to implement this scheme in mobile terminals. 0018-9545/$25.00 © 2008 IEEE