Problem In Wireless Systems Research Articles

Reducing feedback load for OFDM-based wireless system

In this paper, the feedback load reduction problem in wireless systems based on orthogonal frequency division multiplexing (OFDM) is investigated and an opportunistic feedback scheme (OFS) is proposed. The key idea behind OFS is that only the key channel gains which can significantly affect the system throughput are fed back to the BS. Firstly, the key channel gains are proved to belong to a channel gain interval. Secondly, a statistical method is proposed to estimate the channel gain interval. Thirdly, the opportunistic feedback scheme is formulated and the feedback load of OFS is analyzed. The advantage of OFS is threefold: the first is OFS can work in both OFDM-based multicast system and OFDM-based unicast system. The second is the channel fading type of the BS-user link is not required, which is more realistic. The third is OFS can get better feedback load performance compared with other schemes, while achieving almost the same throughput performance compared with that of full feedback scheme.

Optimal scheduling in multi-server queues with random connectivity and retransmissions

We investigate an optimal scheduling problem in a discrete-time, multiserver system of parallel queues. The servers are connected to the queues in a random fashion and scheduled packets complete service successfully with a given probability. This model is suitable for the study of dynamic packet scheduling problems in wireless systems. We study first a two-server system of two infinite-capacity queues with homogeneous arrival, service and connectivity assumptions. We use coupling arguments to prove that a “Most Balancing” scheduling policy is optimal, in a stochastic ordering sense, for this system. We then consider a finite-capacity, non-homogeneous, multi-server and multi-queue system. We develop a dynamic programming model to determine numerically the scheduling policy that optimizes a range of cost functions, including average total queue sizes. We also compare the performance of the optimal policy to that of a few other policies via simulations.

Characterization of Convex and Concave Resource Allocation Problems in Interference Coupled Wireless Systems

This paper investigates the possibility of having convex or concave formulations of optimization problems for interference coupled wireless systems. An axiomatic framework for interference functions proposed by Yates in 1995 is used to model interference coupling in our paper. The paper shows that under certain natural assumptions, the exponential transformation is the unique transformation (up to a positive constant) for “convexification” of resource allocation problems for linear interference functions. Furthermore, it is shown that under certain intuitive assumptions, it is sufficient to check for the joint concavity (convexity) of sum of weighted functions of SINR (inverse SINR) with respect to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">es</i> , where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> is the power vector of the users), if we would like the resulting resource allocation problem to be concave (convex). This paper characterizes the largest class of utility functions and the largest class of interference functions (respectively), which allow a convex and concave formulation of a problem for interference coupled wireless systems. It extends previous literature on log-convex interference functions and provides boundaries on the class of problems in wireless systems, which can be algorithmically tackled by convex optimization techniques.

On demand radio frequency identification based vehicle tracking system

Over the years, many systems have addressed the problem of location sensing. In the past, geographic positioning systems (GPS) have been widely used to track moving objects located outside environments. These systems have several problems such as operational, environmental and high cost. The fixed GPS infrastructure causes several problems in wireless systems. Thus, GPS is considered not a suitable solution for the fixed environment. Due to this, there is a need for the system that can be replaced with less effort to meet future needs. Thus, the purpose of this paper is to discuss the available wireless technologies like radio frequency identification (RFID) techniques and mobile ad hoc sensor network. In doing so, the application of these technologies for remote objects information acquisition and tracking of moving objects is discussed. Further, the authors attempt to develop a communications setup for highways of Pakistan by using RFID and wireless sensor networks techniques to build a network that can be used for object tracking and information acquisition of moving vehicles on highways. This system may be used for variety of purposes especially for security enhancements at highways.

Utility-Based Power Allocation for Multiclass Wireless Systems

We study power-allocation problems in wireless systems through a network utility maximization framework. In this framework, the type of utility function represents the characteristics of each service. Hence, to deal with multiclass services in the system, it is important to accommodate various types of utility functions. In this paper, we consider four types of utility functions that can represent most of the services in wireless networks. We first develop a simple algorithm that solves a general optimization problem with those four types of utility function. We show that, even though the proposed algorithm might not provide the optimal solution, it provides an asymptotically optimal solution, which could be a good approximation to the optimal solution. The proposed algorithm can be applied to various resource-allocation problems in wireless systems. Among them, in this paper, we show that the proposed algorithm can be applied to both uplink and downlink power-allocation problems.

A tutorial on cross-layer optimization in wireless networks

This tutorial paper overviews recent developments in optimization-based approaches for resource allocation problems in wireless systems. We begin by overviewing important results in the area of opportunistic (channel-aware) scheduling for cellular (single-hop) networks, where easily implementable myopic policies are shown to optimize system performance. We then describe key lessons learned and the main obstacles in extending the work to general resource allocation problems for multihop wireless networks. Towards this end, we show that a clean-slate optimization-based approach to the multihop resource allocation problem naturally results in a "loosely coupled" cross-layer solution. That is, the algorithms obtained map to different layers [transport, network, and medium access control/physical (MAC/PHY)] of the protocol stack, and are coupled through a limited amount of information being passed back and forth. It turns out that the optimal scheduling component at the MAC layer is very complex, and thus needs simpler (potentially imperfect) distributed solutions. We demonstrate how to use imperfect scheduling in the cross-layer framework and describe recently developed distributed algorithms along these lines. We conclude by describing a set of open research problems.

Band-Limitation Effects on Microwave Oven Noise in the 2.4-GHz Band

Electromagnetic noise radiated from microwave ovens may cause serious interference problems in wireless systems using the 2.4-GHz band. Since oven noise waveforms show strong dependence on the frequency selectivity of the receiver filters, the effect of band limitation on the interfering oven noise is an important issue for evaluating or comparing the performance degradation of wireless systems subject to interference. To understand these effects, theoretical and experimental investigations of the waveform, peak amplitude, and pulse width of band-limited oven noise are carried out. It is found that the peak amplitude of the received noise changes with the bandwidth in a way very similar to the case of a simple RF tone-burst input. The pulse width of the received noise also changes with the receiver bandwidth but takes a minimum value at a certain receiver bandwidth, which is an essential feature of band-limited microwave oven noise. In addition, an appropriate resolution bandwidth is determined for using a spectrum analyzer to obtain accurate oven noise parameters.