Ramin Hekmat

Connectivity in wireless ad-hoc networks with a log-normal radio model

In this paper we study connectivity in wireless ad-hoc networks by modeling the network as an undirected geometric random graph. The novel aspect in our study is that for finding the link probability between nodes we use a radio model that takes into account statistical variations of the radio signal power around its mean value. We show that these variations, that are unavoidably caused by the obstructions and irregularities in the surroundings of the transmitting and the receiving antennas, have two distinct effects on the network. Firstly, they reduce the amount of correlation between links causing the geometric random graph tend to behave like a random graph with uncorrelated links. Secondly, these variations increase the probability of long links, which enhances the probability of connectivity for the network.Another new result in our paper is an equation found for the calculation of the giant component size in wireless ad-hoc networks, that takes into account the level of radio signal power variations. With simulations we show that for the planning and design of wireless ad-hoc networks or sensor networks the giant component size is a good measure for “connectivity”.

Degree distribution and hopcount in wireless ad-hoc networks

This article is a contribution to mathematical modeling and better understanding of fundamental properties of wireless ad-hoc networks. Our focus in this article is on the degree distribution and hopcount in these networks. The results presented here are useful in the study of connectivity and estimation of the capacity in ad-hoc networks. We model a wireless ad-hoc network as an undirected geometric random graph. For the calculation of the link probability between nodes we have suggested to use a realistic radio model; the socalled log-normal shadowing model. Through a combination of mathematical modeling and simulations we have shown that the degree distribution in wireless ad-hoc networks is binomial for low values of the mean degree. Further, we have investigated the hopcount and have shown that the hopcount in wireless adhoc networks can vary between the expected values for lattice networks and random graphs, depending on radio propagation conditions.

Beam switching support to resolve link-blockage problem in 60 GHz WPANs

In this paper, we propose a solution to resolve link blockage problem in 60 GHz WPANs. Line-of-Sight (LOS) link is easily blocked by a moving person, which is concerned as one of the severe problems in 60 GHz systems. Beamforming is a feasible technique to resolve link blockage by switching the beam path from LOS link to a Non-LOS (NLOS) link. We propose and evaluate two kinds of Beam Switching (BS) mechanisms: instant decision based BS and environment learning based BS. We examine these mechanisms in a typical indoor WPAN scenario. Extensive simulations have been carried out, and our results reveal that combining angle-of-arrival with the received signal to noise ratio could make better decision for beam switching. Our work provides valuable observations for beam switching during point-to-point communication using 60 GHz radio.

Directional MAC Protocol for Millimeter Wave based Wireless Personal Area Networks

Recently, up to 7 GHz license-free spectrum around 60 GHz has been allocated worldwide for high data rate wireless communications. This enables the deployment of WPANs at 60 GHz for short-range multimedia applications up to gigabits per second. In this paper we propose a new scheme to increase the efficiency of MAC layer protocol for WPANs at 60 GHz when directional antennas are used. Our scheme is based on an adaptation of the current IEEE 15.3 standard MAC protocol for WPANs on two aspects. Firstly, we propose a rate-adaptation based scheme to coordinate the directional and omni-directional transmissions in WPANs. Secondly, we propose a novel channel time allocation algorithm, which enables spatial reuse TDMA. The analytical results reveal that our algorithm significantly increases the system capacity.

Performance of Transport Control Protocol Over Dynamic Spectrum Access Links

Dynamic Spectrum Access (DSA) radio devices look for temporarily unoccupied frequency bands and attempt to communicate in them. It is envisioned that DSA can substantially increase the capacity of wireless networks by broadening the utilization of radio resources. Given the ubiquitous use of Internets Transport Control Protocol (TCP), it can be expected that TCP will be used in DSA networks in the future. Whether TCP can efficiently provide stable end-to-end transmissions over DSA links, given their dynamic and unpredictable nature, remained unclear. Therefore, we have studied by simulation the ability of various TCP flavors to efficiently utilize DSA links. We have performed simulations using the TCP stack from the Linux operating system. Our simulations show that modern TCPs can efficiently make use of the dynamic capacity of DSA links for bulk data transmission, under a wide range of conditions, but only if certain requirements are met. We also analytically determine the effect of Primary User (PU) detection errors on TCP performance and conclude that the dominating component responsible for TCP throughput reduction in a DSA environment is the observation time, not, as one might expect, PU detection errors.

Near-far effects in land mobile random access networks with narrow-band Rayleigh fading channels

The near-far effect of random access protocols in mobile radio channels with receiver capture is investigated. To this end, the probability of successful reception of a packet from a terminal at a known distance from the central receiver is obtained taking into account Rayleigh fading, UHF propagation attenuation, and the statistics of contending packet traffic in radio nets employing slotted ALOHA, carrier sense multiple access (CSMA) or inhibit sense multiple access (ISMA) protocols. Various models of receiver capture are compared, namely packet error rates for synchronous detection in slow- and fast-fading channels, and the probability that the signal-to-interference ratio is above a required threshold. >

Quality of service assessment of opportunistic spectrum access: a medium access control approach

Opportunistic spectrum access (OSA) is a promising new spectrum management approach that will allow coexistence of both licensed and opportunistic users in each spectrum band, potentially decreasing the spectrum licensing costs for both classes of users. However, this has significant implications on the QoS experienced by the licensed and opportunistic spectrum users. In this article we investigate how tolerant to secondary user activity a licensed user should be so as to provide dependable communication with sufficient QoS to an opportunistic user. We also look at key multichannel MAC features for such OSA networks proposed in the literature, and discuss how the design of control channel management affects the QoS of opportunistic users as a function of the tolerance of licensed users. We quantify the trade-off between dependability of the OSA network and the dependability of licensed users. The main conclusion is that opportunistic users can indeed achieve good QoS, as long as the licensed users are not highly active. For example, in one of the scenarios we studied, opportunistic users can achieve a delay below 100 ms if licensed user activity stays below 30 percent.

State of the Art in Opportunistic Spectrum Access Medium Access Control Design

Opportunistic spectrum access (OSA) allows unlicensed secondary networks to share licensed spectrum in space and time, but only when licensed users are not using the spectrum. Thus this novel spectrum management technique started drawing the attention of researchers recently. Although many interesting approaches have been proposed, most techniques are early proposals that often only cover a subset of the problems related to OSA. In this paper, we give an extensive overview of medium access control design challenges specific to OSA, while discussing the main approaches proposed so far in the literature. We give an extensive survey of protocols proposed and discuss which features are not explored yet and which one need to be looked at more carefully.

Self-Adaptive Neighbor Discovery in Ad Hoc Networks with Directional Antennas

In this paper we present a novel neighbor discovery protocol for networks with directional antennas. The novelty of this protocol lies in two aspects. Firstly, in order to cope with mobility issues, we adjust the frequency of neighbor discovery attempts according to the dynamics of the network Secondly, to improve power efficiency and reduce overhead, the protocol has the ability to limit neighbor discovery attempts in directions where no new neighbors are likely to be found. The superior performance of our protocol is shown through simulations. Furthermore, we provide an analytical model to analyze directional neighbor discovery protocols in general. This model reveals the impacts of using directional antennas in neighbor discovery process. The accuracy of this analytical model is validated through simulations.

Interference power sum with log-normal components in ad-hoc and sensor networks

The log-normal shadowing radio model has frequently been used to model radio propagation conditions. There exist accurate calculation methods for estimation of interference power sum statistics in fixed-topology wireless networks based on this radio model. Here we publish essential additions to these estimation methods to expand their use to sensor networks and ad-hoc networks with changing topology. To our best knowledge this has not been done before. Taking into account radio propagation conditions, density of nodes, size of the network, traffic load per node and MAC protocol characteristics, we present a calculation method for the estimation of interference power sum statistics in wireless ad-hoc and sensor networks. The accuracy of the calculation method is verified by simulations. We highlight the influence of MAC protocols on interference and show that an increase in network size or in node density does not necessarily lead into higher interference values. Our results can be deployed to estimate the network capacity.