The demand for performance and capacity in cellular systems has generated a great deal of interest in the development 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 detection using multiple transmit and receive antennas for code-division multiple-access (CDMA) communications. We compare, via analytical bit-error-probability calculations, user capacity, and complexity, two linear receiver structures for different antenna configurations. Motivated by its appearance in a number of thirdgenaeration (3G) wideband CDMA standards, we use the Alamouti space-time block code for two-transmit-antenna configurations. We also develop blind adaptive implementations for the two transmit/two receive antenna case for synchronous CDMA in flatfading channels and for asynchronous CDMA in fading multipath channels. Finally, we present simulation results for the blind adaptive implementations. Index Terms-Blind adaptive receiver, multiple antennas, multiuser detection, space-time block code, subspace tracking, wireless communications. I. INTRODUCTION T HE EVER-increasing demand for performance and capacity in cellular wireless systems has prompted the development of myriad advanced signal processing techniques in an effort to utilize these resources more efficiently. The multipleaccess 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

This paper investigates the impact of interuser non-orthogonality and asynchronous communication on the information-outage probability performance of multiuser decode-and-forward (DF) cooperative diversity in a code-division multiple-access (CDMA) uplink. Each user in the proposed system transmits its own data towards the base station and also serves as a relay for other users. We assume full-duplex communication so that each user can transmit and receive simultaneously at the same frequency. Each user attempts to decode the messages of a plurality of other users and forwards the superposition of multiple re-encoded and re-spread messages. Our cooperative scheme employs a sub-optimum decorrelating receiver to suppress the multiuser interference at both the base station and the relay-side. We evaluate the informationoutage probability performance of the proposed scheme in an underloaded, fully-loaded and overloaded CDMA uplink. We consider combining schemes at the base station where the source information is code combined with the relayed information, while the information from multiple relays is either code combined or diversity combined. Under the system parameters contemplated in this paper, diversity combining of the relayed information is nearly as good as code combining because of the associated probabilities of decoding at the relays. We then examine the effect of using practical modulation formats on the information-outage probability performance of the proposed DF multiuser sharing scheme under diversity combining. We see that the performance loss due to modulation constraints and the use of diversity combining instead of code combining is relatively small.

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Recently, Wang and Høst-Madsen developed group-blind multiuser detectors for use in code-division multiple-access (CDMA) uplink environments in which the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. Yu and Høst-Madsen later developed an adaptive version of this detector for synchronous CDMA channels. In this letter, we develop a new low-complexity, high-performance subspace tracking algorithm and apply it to adaptive group-blind multiuser detection in asynchronous multipath CDMA channels. The detector can track changes in the number of users and their composite signature waveforms. We present steady-state performance as well as the ability of the receiver to track changes in the signal subspace. We also address the performance gain of the group-blind detector over its blind counterpart for this application.Index Terms-Adaptive multiuser detection, code-division multiple access (CDMA) uplink, group-blind multiuser detection, subspace tracking.