Yehuda Ben-Shimol

Two-dimensional mapping for wireless OFDMA systems

The recent Orthogonal Frequency Division Multiple Access (OFDMA) transmission technique is gaining popularity as a preferred technology in the Broadband Wireless Access (BWA) emerging standards. In standards 802.16-2004 and 802.16e, the basic allocation units are comprised of sub-channels and OFDMA time symbols; each sub-channel is a group of sub-carriers, so that all the sub-channels are considered equally adequate to all users. We study the naturally arising new approach of two-dimensional mapping of incoming requests into the matrix that represents the system resources, where each allocation is of an arbitrary multi-rectangular shape (to the best of our knowledge, this approach has not been discussed elsewhere). We define a cost model and constraints related to practical OFDMA systems, which depend on the spatial shape of the two-dimensional allocation; the main objective function is the spatial efficiency. We show that the arising problem, even in its simplest form, is NP-hard . We present run-time efficient heuristic solutions for various mapping problems, taking into account the above QoS and OFDMA related constraints. In particular, a novel solution for two-dimensional mapping under priority constraints is suggested. Extensive simulations with parameters of real systems were used to investigate the performance of the proposed solutions in terms of throughput, delay and system load. The results show that high throughput can be achieved with relatively simple mapping algorithms. We believe that the proposed two-dimensional mapping approach is prospective, due to its fitness to modern standards

Automated antenna positioning algorithms for wireless fixed-access networks

Abstract This article addresses a real-life problem - obtaining communication links between multiple base station sites, by positioning a minimal set of fixed-access relay antenna sites on a given terrain. Reducing the number of relay antenna sites is considered critical due to substantial installation and maintenance costs. Despite the significant cost saved by eliminating even a single antenna site, an inefficient manual approach is employed due to the computational complexity of the problem. From the theoretical point of view we show that this problem is not only NP hard, but also does not have a constant approximation. In this paper we suggest several alternative automated heuristics, relying on terrain preprocessing to find educated potential points for positioning relay stations. A large-scale computer-based experiment consisting of approximately 7,000 different scenarios was conducted. The quality of alternative solutions was compared by isolating and displaying factors that were found to affect the standard deviation of the solutions supplied by the tested heuristics. The results of the simulation based experiments show that the saving potential increases when more base stations are needed to be interconnected. The designs of a human expert were compared to the automatically generated solutions for a small subset of the experiment scenarios. Our studies indicate that for small networks (e.g., connecting up to ten base stations), the results obtained by human experts are adequate although they rarely exceed the quality of automated alternatives. However, the process of obtaining these results in comparison to automated heuristics is longer. In addition, when more base station sites need to be interconnected, the human approach is easily outperformed by our heuristics, both in terms of better results (fewer antennas) and in significant shorter calculation times.

Efficient mapping of voice calls in wireless OFDMA systems

We address the problem of efficient resource allocation and its description for constant bitrate traffic (e.g., voice calls) in wireless OFDMA systems. In such systems resources are allocated to users in a two-dimensional map. The representations of the allocated resources have to be broadcast, thus generating a substantial amount of overhead transmission. Here we present an efficient mapping and allocation representation algorithm that increase the total resource utilization by reducing the mapping overhead of the uplink frame for constant bitrate voice sessions. We use the IEEE 802.16 standard for a system model and investigate by simulation the performance of the proposed solution. The results show that the required overhead can be reduced greatly

SPLAST: a novel approach for multicasting in mobile wireless ad hoc networks

Trees of special properties are required to provide efficient network management of group communications in mobile ad hoc networks. Usually, such trees try to balance between the requirements to minimize the total tree cost and the requirement to minimize the maximal shortest path. This work presents a novel solution for efficient multicast trees that fulfill both requirements called SPLAST. The following discussion covers the development process, starting from centralized static solution, through distributed implementation to a complete distributed algorithm that cope with various scenarios that are relevant to wireless ad hoc networks by efficient management and maintenance of the underlying components of the algorithm. Simulation inquiry shows that the average performance of SPLAST is attractive as well.

Prediction of frequency dependence of path loss and link-budget design for various terrestrial communication links

In this work, we continue the analysis of a unified probabilistic approach and the corresponding stochastic multiparametric model of wave propagation above the terrain, including microcell, rural forested, mixed residential and urban, environments. Taking into account diffraction effects caused by the Earths curvature, we extend this model to the radio links with ranges of more than 10 km covering rural macrocell communication and describe effects of interference caused by terrain curvature. We concentrate on path loss dependence in the frequency domain within the UHF/X-band propagation channels both for urban and rural areas and on link-budget design in such types of communication links. It is shown that the frequency dependence of loss and fading characteristics in terrestrial communication links depend significantly on terrains topography, all features of obstructions, natural and man-made, as well as on the terminal antennas elevation with respect to obstruction surrounding. Then, based on the same stochastic approach, we give practical recommendations on how to design the link budget based on the concept of fast fading, shadowing effects, and average path loss within various terrestrial communication links and compare them with existing approximate prediction methods.

Contribution to the problem of near‐zone inverse Doppler effect

The existence of an inverse Doppler effect in free space is again scrutinized, following some papers predicting the existence of such phenomena in the near zone of a moving oscillating three-dimensional dipole. In the present paper the wave scattered from a perfectly conducting thin cylinder moving in the presence of a plane electromagnetic wave is analyzed. The response of such a cylinder may be considered as due to either a two-dimensional monopole or a two-dimensional dipole in accordance with the polarization of the incident wave. An intensive numerical spectral estimation based on the fundamental definitions of the terms “frequency,” “spectrum,” and “uncertainty” is performed on the scattered wave at various ranges. An inverse Doppler effect was not found for the two-dimensional case. The same analysis was applied to the case of a moving three-dimensional radiating dipole, reconfirming previous results which showed the presence of an inverse Doppler effect in the near zone of the three-dimensional dipole.

Wave propagation in moving chiral media: Fizeau's experiment revisited

The Fizeau experiment is discussed as a concrete example for investigating wave propagation in nonsimple moving media. Exact special relativistic formalism is used throughout, and first-order approximations are developed from the exact forms. No Doppler frequency shifts occur to an observer in the laboratory frame of reference, because in Fizeaus experiment the moving fluid is contained within stationary boundaries. Consequently, only phase shifts are measurable. The results show that in order to measure the velocity effects from the interference fringes, one has to adequately modify the construction of the original Fizeau experiment. A relativistically exact model for first-order in velocity was developed for the chiral medium giving a relatively simple formalism and enabling an easy solution to propagation and scattering of electromagnetic waves in the presence of moving chiral media.

Efficient mapping of multiple VoIP vocoders in WiMAX systems

This paper addresses the problem of efficient mapping and allocation of voice over IP (VoIP) traffic with multiple VoIP vocoders in WiMAX systems. We show that by taking advantage of the periodicity of VoIP frames, generated by multiple vocoder types, and by applying semi-fixed allocations in the existing resource allocation process, the mapping overhead can be reduced substantially. We present a system model that allows the evaluation of the proposed solutions for the problem at hand. Using this model, we show that the problem is NP hard and present a set of observations on the resource allocation process leading to good mapping decisions from the perspective of mapping overhead. We present a new mapping algorithm that implement the decisions and show by extensive simulations that the proposed mapping algorithm can reduce the overhead down to 25% of the overhead experienced in orthogonal frequency division multiple access (OFDMA)-based systems such as WiMAX and IEEE802.16, while retaining a low computational complexity. Copyright

Interference-aware cooperative routing in wireless networks

We study interference-aware, many-to-many cooperative routing algorithms in wireless networks. Network performance is measured in terms of effective rate achieved when multiple frames are transmitted sequentially over a single cooperative path. Motivated by the overall network performance when multiple concurrent sessions are taken into consideration, we develop heuristic algorithms that reduce the interference caused by a session to a small geographic neighborhood. This allows the following frames and possibly the rest of the network to operate efficiently. We introduce an interference-aware performance metric based on the effective rate and evaluate the performance of several cooperative routing algorithms. Our heuristic algorithms show an improvement of 10 – 20% in the effective rate over traditional cooperative routing algorithms and an improvement of 10% over simple routing without cooperative transmission. We show that cooperation has little if any performance gain over simple relaying in terms of effective rate when inter-frame interference is not taken into consideration.

Efficient mapping of VoIP traffic in wireless OFDMA systems using semi-fixed allocations

This paper addresses the problem of efficient broadcasting of resource allocations descriptors for CBR applications (e.g., VoIP) in OFDMA-based wireless systems. In such systems mapping is the process of allocating resources and representing them is the mapping overhead. Existing mapping algorithms require an allocation descriptor to be broadcast in each frame for each allocated resource for every served subscriber. Information concerning the allocations is crucial for proper system operation, therefore it is broadcast on the wireless down link with low modulation and high repetition rate. The mapping overhead which is a function of the number of served subscribers is usually large and consumes a substantial part of down link resources. We present the notion of semi fixed allocations that allows a substantial reduction of the mapping overhead. We show that by taking advantage of the periodicity of VoIP frames and by applying semi fixed allocations in the existing resource allocation process, the mapping overhead can be reduced substantially. We present an innovative system model for the problem on hand and for evaluating the solutions. Using this model we show that the problem is in NP hard and present a set of observations on the resource allocation process leading to good mapping decisions from the perspective of mapping overhead. We present new mapping algorithms implementing those mapping decisions and show that by extensive simulations with these mapping algorithms the overhead can be reduced to 10% of the overhead experienced in OFDMA-based systems such as WiMAX and IEEE802.16, while retaining a low computational complexity.