Ariton E. Xhafa

Propagation model for the HVAC duct as a communication channel

Heating, ventilation, and air conditioning (HVAC) ducts in buildings are typically hollow metal pipes which can be used as waveguides to carry signals and provide network access to offices. Knowledge of channel properties is crucial to designing such a communication system. The paper presents a propagation model for a straight HVAC duct terminated at both ends. At high frequencies, this duct behaves as a multimode waveguide with a transmitting antenna coupling in and a receiving antenna coupling out. We derive a simple analytical expression for the frequency response of this channel using conventional techniques. Experimental data taken on real circular ducts excited by monopole probe antennas confirm the theoretical results. This model represents an initial step toward the development of a tool for planning a wireless distribution system using building HVAC ducts.

High-speed Internet access via HVAC ducts: a new approach

We report a novel technique for inexpensive high-speed Internet access in buildings. Our work shows that, one can use heating, ventilation, and air conditioning (HVAC) ducts for indoor wireless transmission systems and networks. Measurements and system calculations show that coverage distances in excess of 100 meters from the base station and data rates of up to 100 Mbps should be possible, when HVAC system is used in conjunction with OFDM technology.

Dynamic priority queueing of handover calls in wireless networks: an analytical framework

In this paper, we present an analytical framework for dynamic priority queueing of handover calls in wireless networks. The framework employs a queueing discipline with two classes of priority for handover calls. Two queues, first priority and second priority, are employed for the two priority classes of handover calls. The priority of queued handover calls is not based only on the received signal strength, but also on the remaining time in the overlap region between two cells. We also incorporate a priority transition between handover calls in the queue; specifically, a second-priority handover call in the second-priority queue, based on certain criteria, can become a first-priority handover call and join the first-priority handover queue. In addition, the event that a handover call could finish its call while waiting in the queue is taken into account in the analysis. This event was not taken into consideration in previous related studies and, as a result, these previous studies overestimate handover failure probability. Our results also show that the predictions of the analytical framework developed in this paper are in very good agreement with simulation results. The developed analytical framework is comprehensive and can also cope with several priority schemes proposed by other researchers in the literature. For example, it is shown that, under certain conditions, the proposed framework converges to first-in-first-out queueing of handover calls. One can easily modify the proposed framework to incorporate priority schemes that use guard channels for handover calls. It is also shown that one could potentially use the framework developed in this paper in integrated voice/data networks, as well as for handover between different network types. The proposed analytical framework is anticipated to be a very useful tool in evaluating performance of present and future wireless networks employing dynamic priority queueing for handovers and in designing more efficient handover algorithms.

Handover Performance of Priority Schemes in Cellular Networks

In this paper, we study the performance of three handover priority schemes - the pure guard channel method (GCM), GCM with first-in-first-out (GCM-FIFO), and dynamic priority queuing (DPQ) - and, for the first time, compare them for different scenarios and provide guidelines as to which scheme to use under which scenario. In the first scenario, the channel holding time is assumed to follow an exponential distribution. For this scenario, we derive new accurate formulas to evaluate the performance of GCM and GCM-FIFO schemes. Our results show that existing analytical frameworks for GCM and GCM-FIFO overestimate the handover failure probability. Recent field measurements have shown that the best fits to measured data for channel holding time are lognormal and mixed lognormal distributions rather than an exponential distribution. We quantify and compare the handover performance of GCM, GCM-FIFO, and DPQ for ideal (i.e., exponential channel holding time distribution) and for practical (i.e., lognormal and mixed lognormal channel holding time distributions) scenarios. Our results show that, depending on the cell type, the number of channels per cell, and the traffic load, the operator should use either DPQ or GCM. For example, for high handover traffic and high traffic load, the operator should use GCM-FIFO; for light-to-moderate traffic load and a high number of channels per cell (40 or more), DPQ seems to be the best option. To make the handover scheme adaptive, a switching mechanism can be used such that, depending on the time of the day, traffic load, number of channels per cell, and cell type, the handover scheme that provides the best performance is selected. The guidelines provided in our paper are, in most cases, valid for both exponential and pure/mixed lognormal distributions.

MAC performance of IEEE 802.16e

I n this paper, for the first time ,w e evaluate the impact of different parameters, such as MAC frame size, MAC protocol data unit (PDU) size, and number of connections on MAC performance of IEEE 802.16e. Our results show that contrary to the common belief, frame packing; i.e., aggregating more than one MAC PDU in a single frame, yields minimal improvements for aggregated frame sizes greater than 1500 bytes. Our results also show that if the number of connections increases from 1 to 50 in a non-sectorized cell, MAC efficiency drops by approximately 40%. Since increasing the number of connections implies an increase in the number of information elements (IE); i.e., feedback/control information, one has to find ways to reduce MAC overhead by reducing or combining IEs in an intelligent way.

Practical frequency hopping sequence design for interference avoidance in 802.15.4e TSCH networks

As the popularity of wireless communication increases rapidly, the unlicensed Industrial Scientific and Medical (ISM) band becomes crowded. Interference avoidance thus becomes an important issue when designing wireless communication in the unlicensed ISM band. As shown in related works, channel hopping with blacklisting is a promising mechanism addressing interference in 802.15.4e time slotted channel hopping (TSCH) networks. However, the blacklisting mechanism changes the number of channels used, and thus requiring the frequency hopping sequence (FHS) be regenerated. A time-random channel selection mechanism avoids regeneration overhead; however, it breaks close-to-optimality property of the default FHSs, which in turns leads to high probability of interference. In this paper, we propose a new algorithm that generates FHSs in presence of interference without regeneration overhead, and at the same time maintains the optimality property. With the new proposed mechanism, we can generate FHSs with one designed original sequence. Analysis and simulation results show that optimal/near optimal properties are achieved by these new FHSs.

Impulse response of the HVAC duct as a communication channel

Heating, ventilation, and air conditioning (HVAC) ducts in buildings behave as multimode waveguides when excited at radio frequencies and thus, can be used to distribute radio signals. The channel properties of the ducts are different from the properties of a usual indoor propagation channel. In this paper, we describe physical mechanisms which affect the HVAC channel impulse response and analyze their influence on the delay spread. Those mechanisms include antenna coupling, attenuation, and three types of dispersion: intramodal, intermodal, and multipath. We analyze each type separately and explore the behavior of the delay spread as a function of distance in straight ducts. Experimental channel measurements taken on real ducts confirm the validity of our model.

A new queuing scheme for handoffs in 3G wireless networks

An analytical framework which employs a multiple queuing scheme (MQS) for handoff calls in wireless networks is considered. Results show that, MQS and delay-dependent priority queuing (DDPQ) reported in Xhafa and Tonguz (2001) reduce handoff dropping probability by 40% when compared to the first-in-first-out (FIFO) scheme. It is also shown that, if one compares the MQS and DDPQ schemes, MQS achieves lower handoff dropping probability when the percentage of high-mobility handoff call users is greater than 38%.

A simple path-loss prediction model for HVAC systems

In this paper, we present a simple path-loss prediction model for link budget analysis in indoor wireless local area networks that use heating, ventilation, and air conditioning (HVAC) cylindrical ducts in the 2.4-2.5-GHz industrial, scientific, and medical band. The model we propose predicts the average power loss between a transmitter-receiver pair in an HVAC duct network. This prediction model greatly simplifies the link budget analysis for a complex duct network, making it a convenient and simple tool for system design. The accuracy of our prediction model is verified by an extensive set of experimental measurements.

Does mixed lognormal channel holding time affect the handover performance of guard channel scheme

While most analytical studies on handover performance assume exponential channel holding times, recent field measurements have shown that channel holding time follows a mixed lognormal distribution. In this paper, the impact of mixed lognormal channel holding time on the handover performance of guard channel scheme with first-in first-out (GCM-FIFO) queuing examined. Our results show that the handover performance of GCM-FIFO is almost the same as the one when the channel holding time follows an exponential distribution. Hence, to evaluate the handover performance of PCS networks where the channel holding time follows a mixed lognormal distribution, one could use the analytical framework developed for the case of exponential channel holding time distributions.