Local-Maximum-Likelihood Likelihood-Ascent-Search Multiuser Detectors for Emerging Wireless CDMA Networks

Third-generation mobile radio networks have been under intense research. Code division multiple access (CDMA) has emerged as the mainstream air interface solution for the third-generation networks. Multiuser detection is a necessary means for CDMA systems to reduce effect of multiple access interference and hence increase system capacity and alleviate near-far problem. The global maximum likelihood (GML) detector achieves the minimum error probability of joint user detection as well as optimum near-far resistance. However, the complexity of the GML detector grows exponentially with the number of users and is infeasible for practical systems. Although there have been a number of suboptimum multiuser receivers, none of them is claimed to achieve a local maximum likelihood (LML) solution. In this proposed project, we will develop a family of likelihood ascent search (LAS) detectors and a family of local-maximum-likelihood likelihood-ascent-search (LMLAS) detectors which are local maximum likelihood detectors with all neighborhood sizes. These two new families of multiuser detectors can provide tradeoff between error performance spanning from that comparable to the conventional detector, the decorrelating detector, etc., up to that of the GML detector and corresponding computational complexity spanning from linear, quadratic, etc., up to exponential in the number of users. The impracticability of the GML detector is due to the impracticability of carrying out a test for all hypotheses. We will first study the optimum solutions for various tests if only a subset of hypotheses is allowed to be tested due to constraint of complexity. The family of LAS detectors and the family of LMLAS detectors will be then developed in non-blind, semi-blind, group-blind, blind and/or adaptive cases. The characteristics of local maximum likelihood points and local maximum likelihood regions with all neighborhood sizes will be studied. Various criteria and strategies for determination of bit demodulation order will be studied to detour local minimums for improved performance. Performance analysis will include the dynamical stability, monotonic likelihood ascent characterization, expected computational complexity, error probability, asymptotic multiuser efficiency, conditions for equivalence of a local maximum likelihood solution to the global maximum likelihood solution, near-far resistance, optimum power control, and spectral efficiency. The family of random LAS detectors and the family of random LMLAS detectors will also be studied. Studied will also be the application of these two families of detectors to other multiple-in/multiple-out channels (e.g., OFDM), single-user channels, global positioning systems, and image restoration, where the optimum (GML) solution similarly suffers from very heavy computational complexity. The results of this project will contribute to theories of extensive fields of hypothesis testing, multiuser detection, communications, signal and image processing, integer optimization, and neural networks, and to application to broad areas of wireless communications, high-speed data transmission, satellite communication, digital television, magnetic recording, global positioning systems, and image processing. The applicant’s education plans for career development are designed to interest and engage undergraduate and graduate students in modern communication research, especially in the context of wireless communications. Specifically, established will be a wireless communication laboratory comparable to the prototype at the Polytechnic University established by 1999 NSF Undergraduate Faculty Enhancement Program – Wireless Communication Lab, which the applicant attended. This new laboratory will serve for new laboratory courses and research projects of wireless communications open to senior undergraduate students and graduate students. The family of LAS detectors and the family of LMLAS detectors developed in the research of this project will be simulated in the experiments of laboratory course as a special. Other plans include re-designing communication and multiuser detection courses and leading seminars in current wireless and multiuser communication research. Additional important objectives include active development of the applicant’s teaching, mentoring, and motivational skills. These research and education activities will broaden the participation of underrepresented groups in learning of modern communication techniques since the City College of New York is a minority college whose minority students amount to 77% enrollment.