IEEE TRANSACTIONS ON SIGNAL PROCESSING, VOL. 50, NO. 6, JUNE 2002 1261 Blind Adaptive Space–Time Multiuser Detection With Multiple Transmitter and Receiver Antennas Daryl Reynolds, Xiaodong Wang, Member, IEEE, and H. Vincent Poor, Fellow, IEEE Abstract—The demand for performance and capacity in cel- lular systems has generated a great deal of interest in the devel- opment of advanced signal processing techniques to optimize the use of system resources. In particular, much recent work has been done on space–time processing in which multiple transmit/receive antennas are used in conjunction with coding to exploit spatial diversity. In this paper, we consider space–time multiuser detec- tion using multiple transmit and receive antennas for code-division multiple-access (CDMA) communications. We compare, via ana- lytical bit-error-probability calculations, user capacity, and com- plexity, two linear receiver structures for different antenna con- figurations. Motivated by its appearance in a number of third- genaeration (3G) wideband CDMA standards, we use the Alam- outi space–time block code for two-transmit-antenna configura- tions. We also develop blind adaptive implementations for the two transmit/two receive antenna case for synchronous CDMA in flat- fading channels and for asynchronous CDMA in fading multipath channels. Finally, we present simulation results for the blind adap- tive implementations. Index Terms—Blind adaptive receiver, multiple antennas, mul- tiuser detection, space–time block code, subspace tracking, wire- less communications. I. INTRODUCTION T HE EVER-increasing demand for performance and capacity in cellular wireless systems has prompted the de- velopment of myriad advanced signal processing techniques in an effort to utilize these resources more efficiently. The multiple- access technique that has received the most attention, and the one on which many of these signal processing techniques are based, is direct-sequence code-division multiple-access (DS-CDMA or, simply, CDMA). CDMA or wideband CDMA (WCDMA) is one of the more promising candidates for third-generation (3G) cellular services [1]. One of the new technologies that is being considered for 3G and later generation WCDMA standards is space–time processing. Generally speaking, space–time processing involves the exploitation of spatial diversity using Manuscript received March 8, 2001; revised February 22, 2002. D. Reynolds and X. Wang were supported in part by the U.S. National Science Foundation (NSF) under Grants CCR-9875314 and CCR-9980599. H. V. Poor was sup- ported in part by NSF Grant CCR-9980580 and the New Jersey Center for Wire- less Telecommunications. The associate editor coordinating the review of this paper and approving it for publication was Dr. Alex C. Kot. D. Reynolds is with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843 USA (e-mail: reynolds@ee.tamu.edu). X. Wang was with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843 USA. He is now with the Department of Electrical Engineering, Columbia University, New York, NY 10027 USA (e-mail: wangx@ee.columbia.edu). H. V. Poor is with the Department of Electrical Engineering, Princeton Uni- versity, Princeton, NJ 08544 USA (e-mail: poor@ee.princeton.edu). Publisher Item Identifier S 1053-587X(02)04388-X. multiple transmit and/or receive antennas and, perhaps, some form of coding. The initial focus was on systems that use one transmit antenna and multiple receive antennas [2]–[8]. Re- cently, however, much of the work in this area has focused on transmit diversity schemes that use multiple transmit antennas. They include delay schemes [9]–[12] in which copies of the same symbol are transmitted through multiple antennas at different times, the space–time trellis coding algorithm developed by Tarokh et al. in [13], and the simple space–time block coding (STBC) scheme developed by Alamouti [14], which has been adopted in a number of 3G WCDMA standards [15], [16]. A gen- eralization of the Alamouti space–time block coding concept is developed in [17]. It has been shown that these techniques can significantly improve capacity [18], [19]. Recently, some work has been completed on space–time multiuser detection using multiple antennas at both the trans- mitter and receiver. In [20], for example, the authors considered maximum-likelihood space–time multiuser detection in a CDMA system using orthogonal spreading codes. An appli- cation of space–time block coding to CDMA appears in [21]. However, this work assumes a perfectly known channel and does not investigate blind adaptive algorithms or make use of the popular Alamouti space–time code. In the present work, we consider the performance of linear space–time multiuser detec- tion using multiple transmit and receive antennas for CDMA systems using nonorthogonal codes. First, we will compare two different linear receiver structures (linear diversity combining and space–time detection) for various antenna configurations. Motivated by the use of STBC in 3G proposals, we will utilize this block code for two-transmit-antenna configurations. Then, we develop blind adaptive implementations of the two transmit/two receive antenna configuration for synchronous CDMA in flat-fading channels and for asynchronous CDMA in fading multipath channels. The adaptive techniques developed here are blind, in the sense that the only information known to the receiver is the signature sequence of the user of interest. The remainder of this paper is organized as follows. In Section II, we analyze and compare two different linear re- ceiver structures that are appropriate for CDMA with multiple transmit and/or receive antennas. In Section III, we develop blind adaptive implementations of the space–time receiver structure for synchronous CDMA in flat-fading channels. In Section IV, we extend the sequential adaptive implementation to asynchronous CDMA in fading multipath channels. In Section V, we present simulation results, and Section VI concludes the paper. 1053-587X/02$17.00 © 2002 IEEE