Department Publications

This paper studies a cellular network in which base station (BS) locations are modeled using a homogenous spatial Poisson point process, and user locations are modeled using an arbitrary stationary homogenous point process. The objective is to maximize the Coverage-Intensity Product (CIP), which represents the average number of users covered per unit area, under constraints on transmit power, BS intensity and transmit power density (per unit area). Expressions for the CIP are obtained considering two user coverage models that differ from each other depending on the BS-user association. Using these expressions, solutions for the optimization problem are provided. All these results are presented for general bounded path loss models that satisfy some mild conditions. The results are also illustrated using a specific bounded path loss model to provide further insights.

The outage probability of a multiuser two-hop amplify-and-forward relaying system employing opportunistic scheduling is investigated. A practically important case where there are cochannel interference signals present at the network is considered for the analysis. Exact expressions and closed-form lower bounds are derived for the outage probability. Exact closed-form expressions are derived for the system outage probability when interference signals are present at the relay and at the destination separately. A closed-form lower bound is derived for the outage probability when the relay and the destination nodes are affected by interference simultaneously. The effects of channel state information feedback delay are investigated for special cases. In addition, asymptotic outage probability results are derived to obtain useful insights on the effects of interference and feedback delay. The novel expressions can be …

This paper studies the structure of downlink sum-rate maximizing selective decentralized feedback policies for opportunistic beamforming under finite feedback constraints on the average number of mobile users feeding back. First, it is shown that any sum-rate maximizing selective decentralized feedback policy must be a threshold feedback policy. This result holds for all fading channel models with continuous distribution functions. Second, the resulting optimum threshold selection problem is analyzed in detail. This is a nonconvex optimization problem over finite-dimensional Euclidean spaces. By utilizing the theory of majorization, an underlying Schur-concave structure in the sum-rate function is identified, and the sufficient conditions for the optimality of homogenous threshold feedback policies are obtained. Applications of these results are illustrated for well-known fading channel models such as Rayleigh …

Opportunistic beamforming is a reduced feedback communication strategy for vector broadcast channels which requires partial channel state information (CSI) at the base station for its operation. Although reducing feedback, this strategy in its plain implementations displays a linear growth in the feedback load with the total number of users in the system n, which is an onerous requirement for large systems. This paper focuses on a more stringent but realistic O(1) feedback constraint on the feedback load. Starting with a set of statistically identical users, we obtain the tradeoff curve tracing the Pareto optimal boundary between feasible and infeasible feedback-capacity pairs for opportunistic beamforming. Any point on this tradeoff curve can be obtained by means of homogeneous decentralized threshold feedback policies, which are rate-wise optimal, in which a user feeds back only if the received signal quality is …

The performance of a random vector quantization based transmit precoding distributed antenna system is investigated in terms of the achievable ergodic rate. An approximate expression is derived for the ergodic rate of the system considering the effects of path loss, Rayleigh fading and out-of-cell interference. A moment matching technique is used to approximate the desired signal power and the total interference power distributions of the system using a Gamma distribution. The proposed approximation estimates the performance of the system accurately and saves computational time in simulations.

The outage probability of a recently proposed relay network with multiple antennas at the relay is analyzed. The relay uses maximal ratio combining to combine the received signals, while it uses maximal ratio transmission for the transmission. It is assumed that the reception at the relay and the destination are corrupted by multiple co-channel interfering signals. Closed-form outage probability expressions are derived when the relay is operated in the amplify-and-forward mode with fixed gain. Both cases of equal power and unequal power interferers are considered.

The outage probability of a multiuser amplify-and-forward relaying protocol is investigated. The practical case where there are co-channel interference signals present at the network is considered for the analysis. Exact expressions and closed-form lower bounds are derived for the outage probability. Three network configurations are analyzed based on the presence of co-channel interference in the system.

Opportunistic beamforming (OBF) is a well-known communication technique that utilizes partial channel state information (CSI) to obtain multiuser diversity gains in the downlink. We focus on the structure of the optimal homogenous threshold feedback policy that maximizes the ergodic downlink sum-rate for OBF under a peak feedback load constraint, which we model by using a multi-packet reception model for the uplink. We solve the resulting quasi-convex optimization problem by obtaining a formula for the sum-rate maximizing feedback probability. While providing insights on the implications of our results in practical systems, we also illustrate the tradeoff between feedback and rate by obtaining the Pareto optimal boundary between feasible and infeasible feedback-rate pairs.

The paper presents a video transmission technique for surveillance systems deployed over wireless networks. In order to be integrated into modern wireless and IP based networks, video surveillance systems should possess the ability to deal with variable network quality. The developed technique allows the system to adaptively compensate for unpredictable changes in the network. A statistical method is used to predict parameters related to network quality, and accordingly vary the frame rate of the video stream transmitted.

Real Time Location Estimation systems (RTLS) developed for outdoor environments have shown significant errors due to changes in atmospheric conditions, multipath propagation and inherent signal strength variations. In this paper, we present an improved Wi-Fi based RTLS solution for the outdoors that reduces the above impacts. In addition this paper presents the development of a portable device for implementing the above RTLS solution. The research makes use of Euclidean, standard deviation based and probabilistic algorithms. Data smoothening is done to reduce the multipath propagation issues. The different algorithm combinations are compared via their error cumulative distribution functions.

A novel approach is introduced for the analysis of energy detection of unknown deterministic signals, where the energy detector uses dual diversity reception. The energy detector is assumed to employ a low complexity diversity technique, namely square-law combining. A general case where the two diversity branches are correlated is treated by the analysis. Detection and false alarm probabilities for the energy detector are derived for correlated Rayleigh and Rician fading channels.

Novel representations for the multivariate probability density function and cumulative distribution function of the non-central chi-square distribution are derived. The solutions are expressed in terms of well known functions which are available in common mathematical software. When the number of degrees of freedom, m, is even, the new representation has the advantage that only a single-integral computation is needed to compute a N-dimensional distribution. The new representations are used to numerically evaluate the outage probability of multiple input multiple output systems with receiver antenna selection, operating in correlated Rician fading. The well known equal correlation model, which is considered to be a useful model for a closely placed set of antennas, is used for the analysis.

Optimum capacity scaling in the downlink of a single-cell multiple-input multiple-output communication system can be achieved by a communication strategy called opportunistic beamforming in which information carrying beams are randomly formed and users are opportunistically scheduled based on their partial channel state information. Even though opportunistic beamforming reduces the amount of feedback required to achieve optimum capacity scaling laws, the number of users feeding back in its plain implementations still grows linearly with the total number of users in the system, which is an onerous requirement on the feedback channel. In this paper, we focus on a more stringent but realistic O(1) feedback constraint on the feedback channel, and obtain the tradeoff curve tracing the Pareto optimal boundary between feasible and infeasible feedback-rate pairs. We show that any point on this tradeoff curve …

This paper introduces and solves the sum rate maximization problem at the multiple-input multiple-output (MIMO) downlink as a function optimization problem subject to feedback constraints at the uplink. It is first shown that this optimization problem can be reduced to a finite dimensional non-convex optimization problem. Then, the resulting problem can be solved by investigating Schur-concavity of the aggregate communication rate across multiple traffic flows. Necessary and sufficient conditions for the rate optimality of homogenous threshold feedback policies are established. With some surprise, it is shown that homogenous thresholding is not always rate-wise optimal even if mobile users experience the same channel conditions statistically. Applications of these results are illustrated for Rayleigh fading channels.

An analytical framework to evaluate the exact performance of multihop amplify-and-forward relaying in the presence of co-channel interference is presented. The new results are formulated based on the recently introduced generalized transformed characteristic function method. The new framework is used to evaluate the exact performance measures such as average symbol error probability, outage probability and ergodic capacity, when the multihop relaying system is operating in identically distributed fading channels with equal power interferers at the relays and the destination.

Novel representations for the multivariate probability density function (PDF) and cumulative distribution function (CDF) of the equi-correlated non-central chi-square (χ 2 ) distribution are derived. The new representations are given as single integral solutions in terms of well known mathematical functions which are available in common mathematical software packages. The advantage of the new representations is that only a single integral computation is needed to evaluate the PDF and CDF for an arbitrary number of dimensions. The new representations are used to numerically evaluate the outage probability of a single user multiple input multiple output system with receiver antenna selection diversity, operating in correlated Rician fading channels. The well known constant (equal) correlation model, which is considered to be a useful model for a closely placed set of antennas, is used for the analysis in this paper. A …

The Disaster and Emergency Warning Network (DEWN) is a last-mile disaster early warning system based on a suite of mobile technologies. It alerts citizens of impending disasters through SMS/Cell Broadcast activated remote-alarm devices, cell broadcasting to handsets and multi-lingual SMS.

The paper presents the development and the experimental verification of a technique for the identification of obstructions in the line-of-sight path between two or more radio transceivers in a multipath radio environment. The objective of this technique is to use RSS measurements from a mesh of simple sensor nodes to identify intrusions.

Threshold feedback policies are well known and provably rate-wise optimal selective feedback techniques for communication systems requiring partial channel state information (CSI). However, optimal selection of thresholds at mobile users to maximize information theoretic data rates subject to feedback constraints is an open problem. In this paper, we focus on the optimal threshold selection problem, and provide a solution for this problem for finite feedback systems. Rather surprisingly, we show that using the same threshold values at all mobile users is not always a rate-wise optimal feedback strategy, even for a system with identical users experiencing statistically the same channel conditions. By utilizing the theory of majorization, we identify an underlying Schur-concave structure in the rate function and obtain sufficient conditions for a homogenous threshold feedback policy to be optimal. Our results hold for …

Beamforming techniques utilizing only partial channel state information (CSI) has gained popularity over other communication strategies requiring perfect CSI thanks to their lower feedback requirements. The amount of feedback in beamforming based communication systems can be further reduced through selective feedback techniques in which only the users with channels good enough are allowed to feed back by means of a decentralized feedback policy. In this paper, we prove that thresholding at the receiver is the rate-wise optimal decentralized feedback policy for feedback limited systems with prescribed feedback constraints. This result is highly adaptable due to its distribution independent nature, provides an analytical justification for the use of threshold feedback policies in practical systems, and reinforces previous work analyzing threshold feedback policies as a selective feedback technique without …

In this paper, a new algorithm for indoor localization using WLAN signal strengths is proposed. The objective of the work was to design a simple and efficient indoor positioning algorithm with good accuracy. The proposed algorithm uses the fingerprinting technique as a pre-processing step. The second step is the application of trilateration within the nearest neighbours found during the first step. The paper describes the algorithm, the environment used to test the algorithm and results obtained. The results show that the proposed algorithm has 44% better accuracy compared to basic fingerprinting technique and the accuracy is 73% better compared to basic trilateration technique. The algorithm has been demonstrated to be independent of the propagation environment and works with no knowledge of the positions and the transmit powers of the WLAN access points in the vicinity. Thus, it is a simple and effective …

Simple expressions for the symbol error rate of some coherent and noncoherent modulation schemes with dual maximal ratio combining operating in identically distributed correlated Nakagami-q fading are derived. The new expressions are derived based on a decorrelation transformation applied to the correlated branches. New expressions are given as both closed-form and finite range single integrals of elementary mathematical functions, which can be easily evaluated using mathematical software.