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IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 19, NO. 8, AUGUST 2001

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Guest Editorial Multiuser Detection Techniques With Application to Wired and Wireless Communications Systems I

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ULTIUSER detection techniques are essential for achieving near-optimal performance in communication systems where signals conveying the desired information are received in the presence of ambient noise plus multiple-access interference. With the exploding interest from both the research community and industry in wireless code-division multiple-access (CDMA) systems, the application of multiuser detection techniques to wireless systems is becoming increasingly important. The addition of multiuser detection capabilities to the various elements of networks of mobile individual communicators promises to be key for enabling the significant increases in capacity needed for future network services. The tremendous potential of multiuser detection in wired communications systems has also been recognized. For example, such techniques hold the potential for allowing a dramatic increase in the achievable user rates for digital subscriber line (DSL) technologies. In particular, these techniques can be applied to the emerging very-high-speed DSL (VDSL) technology, as crosstalk signals at the input of a VDSL receiver can be viewed as interfering signals that share the same channel as the desired signal. The leitmotiv of developments in multiuser detection is represented by the reduction in complexity of practical receivers with respect to that of optimal receivers, which is known to increase exponentially with the number of active users and with the delay spread of the channel, while achieving near-optimum performance. A further element that is being recognized as essential to reap the full benefits of interference suppression is the joint application of multiuser detection with other techniques such as spatial-temporal processing and iterative decoding. The rapid growth and the significance of the applications of multiuser detection techniques to wired and wireless communications is reflected in a substantial need for in-depth treatment of the various aspects of this topic across traditional research field boundaries. In fact, the topic of multiuser detection is interdisciplinary in nature, as indicated by the overwhelming response we received from researchers in the areas of communications, signal processing, and information theory, who submitted approximately 100 manuscripts for this special issue. Because of this overwhelming response, two issues will be published on this topic, a decision that is justified by the high quality exhibited by many of the submitted papers, which could not be accommodated in a single issue. The twenty papers selected for this first issue place their emphasis on the investigation of challenging practical as well as theoretical

Publisher Item Identifier S 0733-8716(01)07232-8.

aspects of multiuser detection, encompassing low-complexity multiuser detection techniques, channel identification methods, blind equalization of multi-input multi-output channels as well as CDMA channels, information-theoretic limits on the capabilities of multiuser detection, spatial-temporal processing, multicarrier CDMA systems, and achievable performance of coded CDMA systems. Limited implementation complexity for various classes of multiuser detectors is the main issue treated by the first group of papers. The paper by Müller and Verdú tackles the problem of approximating linear multiuser detectors for vector channels with crosstalk by weighted matrix polynomials. The results of this paper help dispel the misconception that increasing spectral efficiency by multiuser detection involves significant additional complexity. Complexity of implementation and algorithms based on maximum-likelihood sequence detection (MLSD), instead of matrix methods, are addressed in “The Application of Semidefinite Programming for the Detection in CDMA,” by Tan and Rasmussen. The so-called interior point methods of optimization theory are used to assist in obtaining polynomial-complexity algorithms to determine ML estimates of all the users’ transmitted signals. “Successive Interference Cancellation with SISO Decoding and EM Channel Estimation,” by Kobayashi et al., addresses the problem of error-propagation in successive interference cancellation (SIC) caused by errors in intermediate symbol decisions and/or channel estimates, through an iterative technique that combines soft-input/soft-output (SISO) channel decoding with expectation-maximization (EM) channel estimation. The proposed technique is seen to achieve near-single-user performance, and thereby to outperform conventional methods significantly. Low-complexity joint detection techniques for third generation mobile radio systems are the subject of the paper by Vollmer et al. The focus is on Fourier techniques that exhibit the smallest computational complexity among the various approaches considered and are well-suited for implementation using parallel hardware architectures. Identification of multiuser channels represents one of the toughest challenges for the realization of effective multiuser detectors. “Channel Estimation and Multiuser Detection in Long-Code DS/CDMA Systems” by Buzzi and Poor addresses the problems of channel identification and multiuser detection for direct-sequence CDMA systems that use long codes. The channel identification procedure introduced has complexity proportional to only the square of the spreading factor, a reduction from the cubic complexity of existing algorithms. The next two papers deal with the identification of crosstalk coupling functions among the twisted pairs in DSL systems. Zeng et al.

0733–8716/01$10.00 © 2001 IEEE

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IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 19, NO. 8, AUGUST 2001

propose the concept of an impartial third-party site in which crosstalk identification is performed and the spectra from different users are monitored. This approach would help solve the problem of spectral compatibility between the signals used for transmission by different operators in the multi-operator environment, which is emerging from the unbundling process that has been undertaken in many parts of the world. Galli et al. also recognize that, from a spectrum management point of view, it is important to be able to identify all crosstalk signals. They propose a nonmodem-based approach where crosstalk sources are identified in the frequency domain by finding the maximum correlation with a “basis set” of representative measured crosstalk coupling functions. Blind multiuser detection is often the only choice left to a designer when it is not possible or economical for the various elements in a communications network to be coordinated such that multiuser channel identification can be performed by assuming that all transmitted and received signals are known. In “Blind Estimation and Equalization of MIMO Channels via Multi-Delay Whitening,” by Tugnait and Huang, blind channel estimation and blind minimum mean-square-error (MMSE) equalization of multiple-input multiple-output (MIMO) systems is considered by assuming primarily that the second-order statistics of the data are known. The basis of the approach is the design of multiple zero-forcing equalizers that whiten the data at multiple delays. Blind equalization and signal separation of CDMA signals is the subject of “Blind Asynchronous Multiuser CDMA Receivers for ISI Channels Using Code-Aided CMA” by Tugnait and Li. This detector requires only the knowledge of the desired user’s spreading code, and uses several versions of the constant modulus algorithm (CMA) to extract the signals of the desired user and other active users in the presence of multiuser interference and multipath-induced intersymbol interference (ISI). The proposed blind detector is shown to outperform several other blind approaches proposed previously. “An Improved Blind Adaptive MMSE Receiver for Fast Fading DS-CDMA Channels,” by Chen and Mitra, addresses the performance penalty suffered by blind adaptive MMSE detection compared with trained adaptive methods. By exploiting the fact that the desired linear receiver depends only on the interference correlation matrix (and not the entire data correlation matrix), an improved blind adaptive method is developed, the analysis of which shows strong performance gains over previously developed blind adaptive methods. A new class of linear multiuser detectors suitable when users employ binary shift keying (BPSK) over fading channels is presented in “Asymptotic Analysis of Improved Linear Receivers for BPSK-CDMA Subject to Fading” by Tulino and Verdú. These linear conjugate detectors exploit the antipodal structure of the BPSK signal and the knowledge of the fading coefficients of active users at the base station receiver. The tools of asymptotic distribution of the spectrum of large random matrices are used to show significant output signal-to-noise ratio (SNR) improvement relative to the classical MMSE receivers. Suboptimal approximations to these receivers are proposed for systems with long spreading codes. In “Multicell Uplink Spectral Efficiency of Coded DS-CDMA With Random Signatures,” Zaidel joins Shamai and Verdú in examining

capacity for a variety of power allocation schemes in CDMA. Single-user results are used and extended to examine the possible spectral efficiencies that could be attained. The joint application of multiuser detection and spatial–temporal processing represents one of the most significant results that have recently emerged in the application of multiuser detection techniques to wireless systems. “Iterative Multiuser Detection, Macrodiversity Combining, and Decoding for the TDMA Cellular Uplink,” by Valenti and Woerner, considers the exploitation of the macrodiversity provided by multiple base stations to enhance the performance of overloaded TDMA systems. The proposed technique iterates between SISO multiuser detection applied at each base station and joint channel decoding applied on soft information from all relevant base stations. It is seen that this technique allows multiple co-channel TDMA users per cell with minimal performance loss. “Overloaded Array Processing With Spatially Reduced Search Joint Detection,” by Hicks et al., examines reduced complexity trellis-based joint detection in an antenna array system, where the number of cochannel signals exceeds the number of antenna elements. The proposed algorithm is based on the delayed decision-feedback sequence estimation (DDFSE) for signals with ISI. It is shown that the proposed method significantly outperforms the optimal linear beam-former and its performance is comparable to that of the joint MLSD at greatly reduced complexity. “Space–Time Iterative and Multistage Receiver Structures for CDMA Mobile Communication Systems,” by Marinkovic et al., proposes a practical partial cancellation method that allows on-line estimation of the weighting coefficients in parallel interference cancellers. The cancellers are then applied jointly with iterative receiver structures and performance is further enhanced by receive adaptive array antennas and space–time processing. Linear MMSE multiuser receivers for asynchronous multicarrier CDMA (MC-CDMA) systems over frequency-selective fading channels are analyzed in “Partial Sampling MMSE Interference Suppression in Asynchronous Multicarrier CDMA System,” by Zong et al. It is shown that these receivers are superior to conventional diversity combining schemes. The timing acquisition requirement of the MMSE receivers is removed using a novel partial sampling (PS-MMSE) receiver structure. Reduced complexity approaches to the proposed PS-MMSE receivers are also described. “A Comparison of Long Versus Short Spreading Sequences in Coded Asynchronous DS-CDMA Systems,” by Tang et al., considers the effects of long versus short spreading codes on the performance of (convolutionally and turbo-)coded direct-sequence CDMA systems, and on the accuracy of several analytical methods for such performance analysis. It is seen that long-code systems have the edge in performance for conventional matched-filter detection due to the averaging effects of the long code, but that the performance edge transfers to the short-code systems when the short code is exploited in linear MMSE multiuser detection. The joint application of multiuser detection and iterative decoding techniques holds the promise to achieve both interference suppression and large coding gains, thus allowing practical systems to operate close to capacity. “Bayesian Monte Carlo Multiuser Receiver for Space–Time Coded Multicarrier CDMA

IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 19, NO. 8, AUGUST 2001

Systems,” by Yang et al., considers the use of Markov chain Monte Carlo (MCMC) methods for the implementation of multiuser detection in space–time block coded multicarrier CDMA systems operating in unknown frequency-selective channels. By exploiting the orthogonality properties of the space–time block code, this SISO technique applies soft interchange between the multiuser detector and the outer decoders to achieve high-performance/low-complexity adaptive multiuser detection and decoding for such systems. “Turbo Greedy Multiuser Detection,” by AlRustamani et al., considers the joint iterative multiuser detection and turbo-decoding in which the multiuser detection stage is implemented with a greedy multiuser detector. This approach significantly lowers the complexity over the use of MAP multiuser detection (from exponential to polynomial in the number of users), while suffering only a mild performance loss (less than 0.5 dB). Finally, “Joint Iterative Decoding of Serially Concatenated Coded CDMA,” by Shi and Schlegel considers iterative joint channel decoding and multiuser detection for CDMA channels in which the channel code itself is a serially concatenated code. Viewing this configuration as a system of three serially concatenated codes (the third of which is the CDMA channel), the authors use a variance transfer function approach to the analysis of the constituent codes to design the

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codes and the decoding schedule. The resulting designs achieve performance very near the Shannon limit on the multiple-access channel.

GIOVANNI CHERUBINI, Guest Editor IBM Zurich Research Lab CH-8803 Ruschlikon, Switzerland JOHN M. CIOFFI, Guest Editor Stanford University Department of Electrical Engineering Stanford, CA 94305–9515 USA ALEXANDRA DUEL-HALLEN, Guest Editor North Carolina State University Department of Electrical and Computer Engineering Raleigh, NC 27695–7914 USA H. VINCENT POOR, Guest Editor Princeton University Department of Electrical Engineering Princeton, NJ 08544–5263 USA WILLIAM H. TRANTNER, J-SAC Board Representative

Giovanni Cherubini (S’80–M’82–SM’94) received the Dr.Ing. degree in electrical engineering (summa cum laude) from the University of Padova, Italy, in 1981, and M.S. and Ph.D. degrees in electrical engineering from the University of California, San Diego, in 1984 and 1986, respectively. In 1984, he received a scholarship from M/A-COM Linkabit, San Diego, for his work on the synchronization of spread-spectrum systems. Since 1987, he has been with the IBM Zurich Research Laboratory, Rüschlikon, Switzerland. His research interests include the study of transceiver architectures for high-speed data transmission and signal processing for bandwidth-efficient communications. In 1995, Dr. Cherubini served as Guest Editor for the Special Issue of the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS on Copper Wire Access Technologies for High-Performance Networks. From 1995 to 1996, he was co-editor of the IEEE Standard 100BASE-T2 for Fast Ethernet transmission over voice-grade cables. Since 1999, he has been Editor for CDMA Systems for the IEEE TRANSACTIONS ON COMMUNICATIONS.

John M. Cioffi (S’77–M’78–SM’90–F’96) received the B.S.E.E. degree from the University of Illinois, Urbana–Champagne, in 1978 and the Ph.D.E.E. degree from Stanford University, Stanford, CA, in 1984. He was with Bell Laboratories, Holmdel, NJ, from 1978 to 1984 and IBM Research, San Jose, CA, from 1984 to 1986. He has been with Stanford University as an electrical engineering Professor from 1986 to present. He founded Amati Communications Corporation, Palo Alto, CA, in 1991 (it was purchased by Texas Instruments in 1997) and was officer/director from 1991 to 1997. He currently is on the boards or advisory boards of BigBand Networks, Coppercom, GoDigital, Ikanos, Ionospan, Ishoni, IteX, Marvell, Kestrel, Charter Ventures, and Portview Ventures, and is a Member of the US National Research Council’s CSTB. His specific interests are in the area of high-performance digital transmission. Dr. Cioffi received the following awards: IEEE Kobayashi Medal in 2001, IEEE Millennium Medal in 2000, IEE J. J. Tomson Medal in 2000, 1999 University of Illinois Outstanding Alumnus, 1991 IEEE COMMUNICATIONS MAGAZINE best paper, 1995 ANSI T1 Outstanding Achievement Award, and NSF Presidential Investigator from 1987 to 1992. He has also received awards from the National Academy of Engineering in 2001. He has published over 200 papers and holds over 40 patents, most of which are widely licensed, including basic patents on DMT, VDSL, and V-OFDM.

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Alexandra Duel-Hallen received the B.S. degree in mathematics from Case Western Reserve University, Cleveland, OH, in 1982, the M.S. degree in computer, information and control engineering from the University of Michigan, Ann Arbor, in 1983, and the Ph.D. degree in electrical engineering from Cornell University, Ithaca, NY in 1987. From 1987 to 1990, she was a Visiting Assistant Professor with the School of Electrical Engineering, Cornell University. From 1990 to 1992, she was with the Mathematical Sciences Research Center, AT&T Bell Laboratories, Murray Hill, NJ. She is an Associate Professor with the Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, which she joined in January 1993. Her current research interests are in equalization, wireless communications, and multiuser detection. From 1990 to 1996, Dr. Duel-Hallen was Editor for Communication Theory for the IEEE TRANSACTIONS ON COMMUNICATIONS.

H. Vincent Poor (S’72–M’77–SM’82–F’87) received the Ph.D. degree in electrical engineering and computer science in 1977 from Princeton University, NJ. He is currently a Professor with the Department of Electrical Engineering, Princeton University. He is also affiliated with Princeton’s Department of Operations Research and Financial Engineering, and with its Program in Applied and Computational Mathematics. From 1977 until 1990, he was a Faculty Member with the University of Illinois, Urbana–Champaign. He has also held visiting and summer appointments at several universities and research organizations in the United States, Britain, and Australia. His research interests are in the area of statistical signal processing and its applications, primarily in wireless multiple-access communication networks. His publications in this area include the book, Wireless Communications: Signal Processing Perspectives. Dr. Poor is a member of the National Academy of Engineering, and is a Fellow of the Acoustical Society of America, the American Association for the Advancement of Science, and the Institute of Mathematical Statistics. He has been involved in a number of IEEE activities, including serving as President of the IEEE Information Theory Society in 1990, and as a member of the IEEE Board of Directors in 1991 and 1992. Among his other honors are the Frederick E. Terman Award of the American Society for Engineering Education (1992), the Distinguished Member Award from the IEEE Control Systems Society (1994), the IEEE Third Millennium Medal (2000), and the IEEE Graduate Teaching Award (2001).