Distributed multisensor parameter estimation in dependent noise

The problem of distributed estimation of a weak nonrandom location parameter &thetas; in additive stationary dependent noise is addressed. Multisensor configurations with and without a coordinator are considered. Dependence in the sensor observations is described by m -dependent, φ-mixing, and p-mixing models. Two cases of interest are addressed: one in which sensor observations are dependent across time but independent across sensors and one in which sensor observations are dependent across both time and sensors. Numerical results on the performance evaluation of the various estimation schemes derived are presented and the relative performances of the various schemes are compared     

Performance of Noncoherent Direct-Sequence Spread-Spectrum Communications Over Specular Multipath Fading Channels

The performance of noncoherent reception in direct-sequence spread-spectrum communications over specular multipath fading channels is investigated. Analytical and numerical results on the average probability of error are presented for direct-sequence spread-spectrum systems employing binary orM-ary frequency-shift-keying modulation with noncoherent demodulation and differential phase-shift-keying modulation with differentially coherent demodulation.     

CelluLAN: A Microcellular Network of Spread-Spectrum Wireless LANs with Efficient Spectral Reuse

The wireless network introduced here, called CelluLAN, can be used as both wireless local area network (LAN) and as a microcellular network. This is achieved by providing the capability of efficient spectral reuse. This capability became possible by spread-spectrum methods that allow the rejection of interference from adjacent CelluLANs. Spread-spectrum also enhances the multiple access throughput by utilizing its delay capture properties. In particular, we have presented the spread-spectrum random access (SSRA) and the spread-spectrum digital sense multiple access (SS-DSMA), which are based on the media access control (MAC) layer protocols of slotted Aloha and DSMA, respectively. These protocols are then enhanced with the capability of joint delay and power capture which is provided at the physical layer. Performance analysis of the system has been carried out to provide the throughput and the uncoded bit error rate of the SSRA and SS-DSMA protocols. The performance analysis is based on a detailed evaluation of the interference power between adjacent CelluLANs or frequency bands in a fully loaded network and with different types of pulse shaping filter. Performance results indicate that the CelluLAN network has satisfactory throughput performance with SS-DSMA protocol and when frequency reuse is one. As shown, the throughput performance is improved significantly when we use the joint delay-power capture technique. We also show that when the spreading factor increases above the point at which the frequency bands overlap, the access throughput begins to decrease.Diakoumis Gerakoulis received his Ph.D. degree from the City University of New York in 1984, his M.S. degree from Polytechnic Institute of New York in 1978, and his B.S. degree from New York Institute of Technology in 1976; all in electrical engineering. From 1984 to 1987 he was assistant professor in the Electrical Engineering Department at Pratt Institute, Brooklyn, New York, and from 1987 to 1989 an associate professor at the Center of Excellence in Information Systems at Tennessee State University. In 1989 he joined AT & T Bell Laboratories as a member of technical staff, where he worked on common channel signaling and radio access technologies for personal communications. In 1996 he joined AT & T laboratories where he was involved in the system design, analysis and performance of common air interfaces for PCS. In 1998 he joined AT & T Labs-Research as a principal member of technical staff where he was involved in wideband access technologies for wireless and digital subscriber lines. In 2004 he joined General Dynamics – Advanced Information Systems where he is currently a senior lead engineer in systems where he is involved in ad hoc and sensor networks.Dr. Gerakoulis holds eight USA patents and he is co-author of the book CDMA: Access and Switching John Wiley, Feb 2001. Dr. Gerakoulis has also published many papers in journals and conference proceedings in the areas of satellite switching and multiple access, spread-spectrum access and synchronization and multi-carrier CDMA for wireless communications.Evaggelos Geraniotis (SM 88) received the Diploma (with highest honors) in Electrical Engineering from the National Technical University of Athens, Athens, Greece, and the M.S. and Ph.D. degrees in EE from the University of Illinois at Urbana-Champaign.From September 1985 to December 2001 he was with the University of Maryland, College Park, where from 1992 to 2001 he was Professor of Electrical Engineering and a member of the Institute for Systems Research and the Center for Satellite and Hybrid Communication Networks. Since January 2002, Dr. Geraniotis has been president of EG Wireless Systems Inc., a consulting company dedicated to research and development in commercial and military wireless communications.Dr. Geraniotiss research has been in communication theory, information theory and their applications with emphasis on wireless communications. His recent work focuses on data modulation, error control coding, multi-user detection and interference cancellation, array processing for receive and transmit diversity, retransmission techniques and multi-access protocols for wireless spread-spectrum and anti-jam communications. The algorithms are applied to cellular, mobile, PCS, fixed wireless, satellite but also to optical, copper-loop and cable networks. He has also worked on multi-media and mixed-media integration and switching for radio and optical neworks as well as on interception, feature-detection, and classification of signals, radar detection and multi-sensor data fusion.He is co-author of the book CDMA: Access and Switching for Terrestrial and Satellite Networks, Joh Wiley, New York, Feb 2001 and and over 300 technical papers in journals and conference proceedings. He serves regularly as a consultant in the above areas for governmental and industrial clients. Dr. Geraniotis has served as Editor for Spread-Spectrum of the IEEE Transactions on Communications from 1989 to 1992.Hany ElGamal was born in Egypt. He obtained his BS in Electrical Engineering from the University of Kairo, Egypt. From September 1999 to December 2002 he was a graduate assistant at the Dept of Electrical Engineering and the Institute of Systems Research of the University of Maryland, College Park, MD. His research interests are in information theory and communication theosry with applications to spread-spectrum systems, interference cancellation, coding and trasmit diversity.     

Optimal strategies for admitting voice and data traffic in networks of LEO satellites using CDMA

Efficient policies are derived for admitting voice and data traffic into networks of low-earth-orbit (LEO) satellites using code-division multiple-access (CDMA) with direct-sequence spread-spectrum (DS/SS) signaling. The satellites act as bent-pipes; no on-board processing or intersatellite links are present. Dual satellite diversity is used to mitigate the effects of shadowing. The policies assume a movable boundary form, allocate optimally the CDMA capacity (PN codes) to voice and data users, and can increase significantly the number of users served while satisfying their bit error rate (BER) requirements. In contrast to direct admission policies that do not take into consideration the statistical features of the traffic, the new policies do account for the different nature of voice and data traffic and the history of prior transmissions/admissions. Two priority schemes are considered: voice users have higher priority than data users, or voice and data users have the same priority. A modified version of our policies can handle two classes of data users: one with high priority which requires real-time delivery and another with low priority that can be queued; the BER requirements of the two data types may differ. Optimal policies have lower voice blocking rates and data packet error rates than direct admission policies.     

Performance of Coherent Direct-Sequence Spread-Spectrum Communications Over Specular Multipath Fading Channels

The performance of coherent direct-sequence spread-spectrum communications over specular multipath fading channels is investigated. The average probability of error of the correlation receiver is derived for an arbitrary number of paths with deterministic or random gain coefficients. The gain coefficients, delays, and phase angles of any two distinct paths are modeled as mutually independent random variables. Numerical results for several values of the system and channel parameters are presented.     

Multiuser demodulation and iterative decoding for frequency-hoppednetworks

Demodulation and decoding for frequency-hopped spread-spectrum multiple-access (FH/SSMA) systems have been traditionally conducted by conventional single-user (noncollaborative) demodulation and error- and erasure-correcting decoding techniques. In this paper, we study the demodulation and decoding aspects of collaborative multiuser reception for FH/SSMA and propose methods which increase the number of users the system can support. In particular, we propose and analyze the optimum maximum a priori probability demodulation of multiple symbols or type, and the use of iterative multiuser decoding after the demodulation. Since hits from one or two other users are the most likely hit events in FH/SSMA, the joint demodulation of two or of three users is performed based on likelihood ratio tests. M-ary frequency-shift keying modulation with noncoherent demodulation and Reed-Solomon codes with hard-decision minimum distance decoding are used in the FH/SSMA system. Results are derived for both synchronous and asynchronous frequency-hop systems. The performance of the proposed multiuser detector in additive white Gaussian noise and flat Rayleigh fading channels is evaluated. Scenarios when all simultaneous users or only a subset of them are collaboratively demodulated and decoded are simulated     

Joint Iterative Multiuser and Narrowband Interference Suppression in Coded Asynchronous DS-CDMA Channels

Multiuser interference suppression in coded direct sequence code division multiple access (DS-CDMA) uplink channels is significantly impacted by the application of the turbo processing concept. This paradigm essentially involves the iterative exchange of soft information between a multiuser demodulator and a bank of single-user decoders, to their mutual benefit. The present work proposes a joint iterative minimum mean square error (MMSE) multiuser and narrowband interference suppressor for coded asynchronous DS-CDMA channels. Since the parameters of the narrowband interference are unknown a priori, the first iteration in this scheme is effectively just MMSE multiuser interference suppression. The outputs of all users' soft decoders (available at the end of each iteration) are fed back to subtract their (estimated) cumulative contribution from the received signal vector. The residue comprises the narrowband interference embedded in wideband noise, so that the former can be piecewise interpolated over chip intervals of appropriate duration, and then subtracted from the received signal, to provide a narrowband-interference-free input signal for the next iteration. The soft-decoded feedback estimates of the interferers' signals are used in each iteration also to perform soft MMSE multiuser interference suppression, in conjunction with subtractive interference cancellation. This scheme performs well at both low and high received signal powers, and displays the successive cancellation property across iterations, exhibiting good near-far resistance. Introducing multisensor arrays at the receiver relaxes the limitations imposed by the inherent suboptimality of MMSE multiuser demodulation, insofar as it enhances interuser separation, now in the spatial sense. Simulations indicate that the performance of the proposed technique surpasses that of all existing suboptimal algorithms in this context.     

Multiple-access capability of frequency-hopped spread-spectrumrevisited: an analysis of the effect of unequal power levels

A method for the evaluation of the probability of error of uncoded asynchronous frequency-hopped spread-spectrum multiple-access communications is presented. For systems with binary FSK modulation this method provides an accurate approximation and a tight upper bound to the bit error probability; for systems with M-ary FSK modulation, it provides tight upper bounds to the symbol error probability. The method enables the computationally efficient averaging of the error probability with respect to the delays, phase angles, and data streams of the different users. It relies on the integration of the product of the characteristic function of the envelope of the branch of the BFSK demodulator, which carries the desired signal, and of the derivative of the characteristic function of the envelope of the other branch. For sufficient frequency separation between the BFSK tones, the method can achieve any desirable accuracy. Moreover, the computational effort required for its evaluation grows linearly with the number of interfering users. In the M-ary case, tight upper bounds based on the union bound and the results of the binary case are derived. The method allows the effect of unequal power levels on other-user interference in FH/SSMA systems to be quantified accurately for the first time. The results indicate that the FH/SSMA systems suffer from the near-far problem, although less seriously than direct-sequence SSMA systems     

Noncoherent Hybrid DS-SFH Spread-Spectrum Multiple-Access Communications

The performance of noncoherent reception in synchronous and asynchronous hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access communication systems operating through additive white Gaussian noise channels is investigated. Systems with binary andM-ary frequency-shift-keying modulation and noncoherent demodulation, as well as systems with differential-phase-shift keying modulation and differentially coherent demodulation, are examined and their probability of error is evaluated for random frequencyhopping patterns and signature sequences. The multiple-access capability of noncoherent hybrid spread spectrum is shown to be superior to that of noncoherent purely frequency-hopped spread spectrum and inferior to that of noncoherent purely directsequence spread spectrum for systems with the same bandwidth expansion. Comparison of hybrid systems with coherent and noncoherent demodulation shows a considerable loss in the performance of the noncoherent systems.     

Presence detection of binary-phase-shift-keyed and direct-sequencespread-spectrum signals using a prefilter-delay-and-multiply device

The specific problem of detecting the presence of either binary-phase-shift-keyed (BPSK) signals or BPSK direct-sequence spread-spectrum (DS/SS) signals with a prefilter-delay-and-multiply (PFDM) device is considered. Using stationary process theory and Fourier analysis, the optimum PFDM structures for signal presence detection of BPSK signals with known bit rates and carrier frequencies and BPSK DS/SS signals with known chip rates and carriers in additive colored Gaussian noise are derived. The structures are optimum in the sense that they maximize the spectral signal-to-noise ratio (SNR) of an output periodic waveform which has fundamental frequency equal to the bit or chip rate of the signal. Two of the optimum structures that are derived and analyzed are the optimal prefilter-square device and the optimal PFDM with delay set to one half of the signal's bit or chip duration