Asrar U. H. Sheikh

Investigations into cochannel interference in microcellular mobile radio systems

A microcell interference model termed the Nakagami m/sub x//m/sub y/ model is introduced. The desired signal and the cochannel interferers are assumed to have Nakagami statistics but with different amounts of fading. A special case of this model is obtained when the desired signal has Nakagami statistics while the cochannel interferers are subject to Rayleigh fading. The probability density function of the signal-to-interference ratio in the Nakagami model is derived. This model is also compared with a Rician/Rayleigh microcellular model. Expressions for the outage probabilities in microcell systems are derived. Numerical results show that, compared to medium/large cell systems, the microcellular systems have a lower outage probability. The impact of diversity on the microcellular system is also studied. An improvement of the outage probability due to diversity is observed. >

Outage probability of cellular radio systems using maximal ratio combining in the presence of multiple interferers

In mobile radio systems, antenna diversity is used to combat fading and reduce the impact of cochannel interference. We derived a new expression for probability density functions of the signal-to-interference-plus-noise ratio and apply it to analyze the outage probability (OTP) for a maximal ratio combining diversity system when multiple cochannel interferers are present. Numerical results showing the impact of the number of antenna elements, the number of cochannel interferers, and signal-to-noise ratio on the OTP are presented. Simulation results validating the analytical results are also presented.

Adaptive equalization and diversity combining for mobile radio using interpolated channel estimates

The authors demonstrate the feasibility of a digital cellular radio (DCR) system which employs a jointly adaptive decision-feedback equalizer and diversity combiner. In particular, the authors utilize the current estimates of the channel impulse response (CIR) at each diversity branch to compute the receiver parameters periodically. The authors propose a novel block-adaptive strategy which computes the time-varying CIR by interpolating a set of CIR estimates obtained through periodic training. Although incurring some inherent processing delay and throughput reduction, this interpolation strategy has the advantage of immunity to decision errors which would quite likely occur during a deep fade. It is shown that the system performance is limited, in the form of an irreducible bit error rate at high signal-to-noise ratios (SNRs), by the CIR estimation of the rapidly fading channel. >

The ALOHA systems in shadowed mobile radio channels with slow or fast fading

The influence of receiver capture on the performance of the ALOHA protocol in the presence of shadowing is investigated. The combined effect of Rayleigh fading, log-normal shadowing, and spatial distribution of mobile users is also studied. It is shown that shadowing is similar to fading and near/far phenomena, in that it makes the capture effect possible and provides ALOHA systems with substantial improvements in throughput. It is also confirmed that the superimposed Rayleigh fading, log-normal shadowing, and spatial distribution can further enhance the capture effect, resulting in a ALOHA system with higher throughput. The fast fading effect in ALOHA systems is explored. A very fast fading case is examined which may be considered to be equivalent to interleaving in slow fading. It is found that an ALOHA system under fast fading conditions also benefits from the capture results in higher throughput. >

Indoor mobile radio channel at 946 MHz: Measurements and modeling

The results of propagation measurements carried out in an office complex at 946 MHz are described. The objectives are to investigate the impact of desk population on the average propagation loss, to model average path loss, and to fit a statistical description to the measured results. The measurements are conducted on two identically structured floors, i.e., one fully occupied and the other completely empty. The loss in the fully occupied floor is found to be consistently higher, but the propagation loss factor is more uniform in different directions from the base. Fade margin statistics are obtained for mean signal power levels which are between 50 dB and 60 dB less than the signal power at the base. It is found that the Nakagami fading model fits the results better than the generally accepted model in Rayleigh fading.

Adaptive equalization for co-channel interference in a multipath fading environment

The paper concerns the feasibility and achievable performance of adaptive filtering in an interference-limited multipath fading environment as encountered in indoor wireless communications. In a typical cellular radio application, the performance-limiting impairment is interference due to synchronous data streams from other co-channel and adjacent channel users (CCI and ACI). The receiver under consideration employs an adaptive fractionally spaced decision feedback equalizer (DFE) Which exploits the correlation of the cyclostationary interference to achieve superior performance relative to the worst case when the interference is stationary noise. The paper presents ideal calculations which confirm that significant DFE performance gains are potentially achievable by explicitly accounting for the cyclostationary CCI. Two adaptive DFE strategies are considered. One approach is to adapt the DFE directly using iterative algorithms such as least mean square (LMS) or recursive least squares (RLS). Another approach is to compute the minimum mean square error DFE using an RLS channel impulse response (CIR) estimate and a sample estimate of the CCI autocorrelation obtained from the CIR estimation error during training. The best approach for adaptive equalization, in terms of adaptation speed and system performance, is to employ an RLS DFE which does not explicitly estimate the CIR or the CCI autocorrelation. >

A comparative study of spectrum awareness techniques for cognitive radio oriented wireless networks

Abstract Spectrum scarcity is impeding practical implementations of emerging wireless multimedia applications requiring significantly more frequency spectrum. Cognitive radio (CR) has emerged as a promising solution to the current spectral congestion problem by imparting intelligence to the conventional software defined radio that allows spectrum sharing through opportunistic spectrum access. The principal objective of CR is to optimize the use of under-utilized spectrum through robust and efficient spectrum sensing (SS). This paper introduces cognitive functionality and provides an in-depth comparative survey of various spectrum awareness techniques in terms of their sensing accuracy and computational complexities along with their merits and demerits. Specifically, key challenges in SS are highlighted and possible solutions are discussed. A classification of SS is presented to address the sensing method selection criterion. Both non-cooperative and cooperative sensing schemes are reviewed and open research problems are highlighted to identify future research directions.

Performance and stability analysis of buffered slotted ALOHA protocols using tagged user approach

This paper presents a new approximation approach to analyze slotted ALOHA (S-ALOHA) systems with finite user population having either finite or infinite user buffer capacity. By assuming a symmetric channel, the performance analysis of the overall system is determined by the performance of an arbitrarily selected user, called the tagged user. The service time distribution for the tagged user buffer is found using a state flow graph. This distribution is then applied to the queueing analysis of the tagged user using available classical queueing theory results. The proposed approach can be applied to analysis of systems with a very large user population and user buffer capacity. The distributions and mean values of the important performance indices such as waiting time, queue size, and interdeparture time are obtained. The stability of the system with infinite buffer capacity is also studied. The region of transmission probability p in which the system is always stable and has best performance is obtained. Though the system with finite buffer capacity is considered to be always stable, a comprehensive analysis of the equilibrium points in the system is presented. The analysis presented will allow a proper choice of transmission probability so that the system always operates at the desired equilibrium point.

Unveiling the Hidden Assumptions of Energy Detector Based Spectrum Sensing for Cognitive Radios

Cognitive radio is a promising solution to current problem of spectrum scarcity. It relies on efficient spectrum sensing. Energy detection is the most dominantly used spectrum sensing approach owing to its low computational complexity and ability to identify spectrum holes without requiring a priori knowledge of primary transmission characteristics. This paper offers a comprehensive tutorial on energy detection based spectrum sensing and presents an in depth analysis of the test statistic for energy detector. General structure of the test statistic and corresponding threshold are presented to address existing ambiguities in the literature. The derivation of exact distribution of the test statistic, reported in the literature, is revisited and hidden assumptions on the primary user signal model are unveiled. In addition, the scope of detection probability results is discussed for identifying various classes of random primary transmissions. Gaussian approximations of the test statistic are investigated. Specifically, the roles of signal to noise ratio and performance constraint in terms of probability of detection or false alarm are highlighted when Normal approximations are used in place of exact expressions.

An investigation of block-adaptive decision feedback equalization for frequency selective fading channels

Block adaptation techniques are investigated by computer simulation for decision feedback equalization on a rapidly fading dispersive channel. Tradeoffs between training sequence length and block length are found. Block adaptation, supplemented by linear interpolation of coefficients, is found to be an attractive alternative to more complex continuously adapting recursive least squares adaptation algorithms.<<ETX>>Block adaptation techniques are investigated by computer simulation for decision feedback equalization on a rapidly fading dispersive channel. Trade-offs between training sequence length and block length are found. Block adaptation supplemented by linear interpolation of coefficients is found to be an attractive alternative to more complex continuously-adapting recursive least squares adaption algorithms.