Savo Glisic

Advanced frequency hopping modulation for spread spectrum WLAN

A wireless LAN standard developed by IEEE committee P802.11 operates in the unlicensed 2.4 GHz ISM band. This band is a very hostile environment due to many unpredictable interference sources, such as microwave ovens, utilizing the same frequency bands. At the same time the allowed signal power density that can be used is limited in order to minimize the interference to other users in the same band. In order to avoid these interference and keep a low signal power density the standard supports both direct sequence (DS) and frequency hopping (FH) modulation for these applications. The new test beds for multimedia wireless (WLANs) also use FH modulation. We analyze the performance of a modification of the FH multiple frequency shift keying (MFSK) system that includes a multitone multiamplitude MFSK signal, designated as amMFSK modulation. In this case, in order to meet the transmitted power density limits imposed in the ISM band, the signal energy is split into m separate tones. This makes the system more vulnerable to noise and fading, but still the overall flow of useful information will be increased. The results demonstrate that under the large range of the signal, channel and interference parameters this system offers better performance.

Automatic decision threshold level control (ADTLC) in direct-sequence spread-spectrum systems based on matched filtering

An automatic decision threshold level control scheme in direct sequence spread spectrum systems is described and analyzed. Both parameters of interest, probability of false alarm Pa and signal correct detection probability Pd are controled simultaneously. Numerical results and discussion are presented.

Modeling the Dynamics of Coalition Formation Games for Cooperative Spectrum Sharing in an Interference Channel

Although establishing cooperation in a wireless network is a dynamic process, most game theoretic coalition formation models proposed in the literature are static. We analyze a dynamic coalition formation game based on a Markovian model for the spectrum sharing problem in an interference channel. Our model is dynamic in the sense that distributed transmitter/receiver pairs, with partial channel knowledge, reach stable coalition structures (CSs) through a time-evolving sequence of steps. Depending on an interference environment, we show that the game process either converges to the absorbing state of the grand coalition or to the absorbing state of internal and external stability. We also show that, due to myopic links, it is possible that the core of the game is nonempty, but links cannot form the grand coalition to utilize the core rate allocations. We then formulate a condition for the formation of the stable grand coalition. Using simulation we show that coalition formation yields significant gains in terms of average rates per link for different network sizes. We also show average maximum coalition sizes for different distances between the transmitters and their own receivers. Finally, we analyze the mean and variance of the time for the game to reach the stable coalition structures.

Advanced CDMA for Wireless Communications

Within this paper some selected topics currently being subject of extensive research for applications in future code-division multiple-access (CDMA) networks are reviewed. The presentation focuses on capacity aspects of advanced CDMA systems utilizing multiuser receivers. A comprehensive review of the multiuser demodulation theory and techiques is presented. Special emphasis is on optimal, suboptimal linear equalizer type and interference cancellation multiuser receivers. Future trends in practical system development are also discussed.

Two-dimensional code acquisition in time and angular domains

The extension of conventional delay-domain code acquisition to the angular domain is explored. The uncertainty region is partitioned into a number of delay and angular cells. In single-path channels where the interference is modeled as temporarily and spatially white, there exists an optimum number of angular cells minimizing the mean acquisition time. Mean acquisition times up to three times shorter can be attained with the two-dimensional approach. A rather similar behavior is in general found in Lth-order equal amplitude multipath channels, where paths are contiguous in the delay or angular domains. The strategy employed to search through the uncertainty region may have a considerable impact on acquisition performance. The results reveal that the search should proceed not in the direction of the multipath spread but in the other available domain. Proper selection of the search strategy can reduce the synchronization time by a factor of up to two. Two-dimensional code acquisition in scenarios with spatially nonuniform interference is also investigated. In general, the acquisition performance is degraded by the presence of nonuniform interference in the angular domain.

Automatic decision threshold level control in direct-sequence spread-spectrum systems

An algorithm for automatic decision threshold-level control in direct-sequence spread-spectrum systems is presented and analyzed. Using this algorithm, instantaneous setting of the threshold is possible. An additional loop called a threshold loop is used to improve the system performance. This loop is based on a constant false alarm rate (CFAR) criterion. The purpose of the algorithm is to set a decision threshold in the system which will provide a small probability of false alarm (P/sub 3/ to 0) and a large probability of signal detection (P/sub d/ to 1) at the same time. The analysis has shown that the threshold probability distribution function (PDF) is narrower (equaling better performance) in the case of the CFAR algorithm, but this comes at the cost of a considerably larger integration time for the system (the system time response is slower). As a compromise, the instantaneous algorithm is modified so that k successive samples of the signal are used to determine the value of the threshold. This permits better control of the shape and position of the threshold PDF with respect to the P/sub 3/ and P/sub d/ curves. At the same time, the time response of the system is good and can be easily controlled. >

1-persistent carrier sense multiple access in radio channels with imperfect carrier sensing

A comprehensive analysis of a 1-persistent carrier-sense multi-access (CSMA) system using a radio channel with imperfect carrier sensing is presented and discussed. It is shown that a careful optimization of the channel state detector parameters must be performed in order to get good system performance. If the threshold of the detector is too high, the system will tend to behave like an unslotted ALOHA; if the threshold is too low, the system throughput will be zero. It is also shown that for the larger average packet rate G the system throughput is decreased. This is because for the same probability of correct channel sensing, the probability of incorrect transmissions from the waiting mode is increased. >

Code division multiple access communications

1: Introduction. The Evolution of Spread Spectrum Multiple-Access Communications R.A. Scholtz. 2: Information Theory & Spread Spectrum. Towards an Information Theory of Spread Spectrum Systems J.L. Massey. Performance Limits of Error Correcting Coding in Multi-Cellular CDMA Systems With and Without Interference Cancellation A.J. Viterbi. Optimum PN Sequences for CDMA Systems D.V. Sarwate. Block Demodulation -- an Overview E. Biglieri, E. Bogani, M. Visintin. 3: Interference Suppression. Adaptive Multiuser Detection S. Verdu. Spatial and Temporal Filtering for Co-Channel Interference in CDMA R. Kohno. Interference Suppression for CDMA Overlays of Narrowband Waveforms L.B. Milstein, J. Wang. DS/CDMA Successive Interference Cancellation J.M. Holtzman. 4: Performance Analysis. Rake Reception for a CDMA Mobile Communicaton System with Multipath Fading D.L. Noneaker, M.B. Pursley. Frequency Hopped Systems for PCS D.E. Borth, P.D. Rasky, G.M. Chiasson, J.F. Kepler. Spread Spectrum System Synchronization A. Polydoros, S. Glisic. 5: CDMA Applications. Design Aspects of a CDMA Cellular Radio Network W.C.Y. Lee. Consumer Communications Based on Spread Spectrum Techniques M. Nakagawa. Optimal Policies for Multi-Media Integration in CDMA Networks E. Geraniotis, Y-W. Chang, W-B. Yang. CDMA for Mobile LEO Satellite Communciations R. Pickholtz, B.R. Vojcic. Standardization in a Wireless Environment D. Schilling, J. Taylor, J. Garodnik.

Design study for a CDMA-based LEO satellite network: downlink system level parameters

The performance analysis of a new concept of a code-division multiple-access (CDMA) based low Earth orbit (LEO) satellite network for mobile satellite communications is presented and discussed. The starting point was to analyze the feasibility of implementing multisatellite and multipath diversity reception in a CDMA network for LEO satellites. The results are used to specify the design parameters for a system experimental test bed. Due to the extremely high Doppler, which is characteristic of LEO satellites, code acquisition is significantly simplified by using a continuous wave (CW) pilot carrier for Doppler estimation and compensation. The basic elements for the analysis presented are: the channel model, the pilot carrier frequency estimation for Doppler compensation, and multipath and multisatellite diversity combining.

Efficient Solutions for Weighted Sum Rate Maximization in Multicellular Networks With Channel Uncertainties

The important problem of weighted sum rate maximization (WSRM) in a multicellular environment is intrinsically sensitive to channel estimation errors. In this paper, we study ways to maximize the weighted sum rate in a linearly precoded multicellular downlink system where the receivers are equipped with a single antenna. With perfect channel information available at the base stations, we first present a novel fast converging algorithm that solves the WSRM problem. Then, the assumption is relaxed to the case where the error vectors in the channel estimates are assumed to lie in an uncertainty set formed by the intersection of finite ellipsoids. As our main contributions, we present two procedures to solve the intractable nonconvex robust designs based on the worst case principle. The proposed iterative algorithms solve semidefinite programs in each of their steps and provably converge to a locally optimal solution of the robust WSRM problem. The proposed solutions are numerically compared against each other and known approaches in the literature to ascertain their robustness towards channel estimation imperfections. The results clearly indicate the performance gain compared to the case when channel uncertainties are ignored in the design process. For certain scenarios, we also quantify the gap between the proposed approximations and exact solutions.