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Volume 3, No. 4, October 2007        ISSN: 1744-2397

A SPECIAL ISSUE on theory and practice of Ad-Hoc Networks, Optical Communications, Signal Processing and Teletraffic

Guest Editors:

Dr. Michael D. Logothetis
University of Patras, Greece

Prof. Anthony C. Boucouvalas
University of Peloponesse, Greece

Title: VOICE CAPACITY ANALYSIS OF IEEE 802.11 WIRELESS LANS SUPPORTING VOIP
Authors: D. S. Amanatiadis, V. Vitsas, A. Manitsaris, I. Mavridis, P. Chatzimisios, A. C. Boucouvalas,.........................................................................................................pp. 132-141
Abstract: There is a tremendous growth in the deployment and usage of voice applications over Wireless Local Area Networks (WLANs) in recent years. The voice capacity of 802.11 WLANs is receiving an increasing attention by researchers. In this paper we use analytical models presented in the literature for saturation throughput, average packet delay and packet drop probability to evaluate voice traffic over WLANs. Delay, jitter and packet loss, the key factors that impact packet voice quality, are studied in the WLAN environment. We propose a methodology that determines the voice capacity of WLANs and we examine the effect of packetization interval and transmission rate on voice capacity using various voice codecs. Analytical results for voice capacity match results presented in the literature using extensive simulations.
Keywords: IEEE 802.11b, Voice Capacity, Average Packet Delay, Drop Probability.

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Title: HIGH BIT RATE OPTICAL WIRELESS SYSTEMS FOR SWARM UNMANNED AERIAL VEHICLES: A FEASIBILITY STUDY
Authors: K. Zettl, S. S. Muhammad, C. Chlestil, E. Leitgeb, A. Friedl, N. P. Schmitt, W. Rehm,....................................................................................................................pp.142-150
Abstract:  Unmanned Aerial Vehicles (UAV) flying in swarm formations carrying a variety of sensors for monitoring and surveillance, are a possible future application for both civil and military use. Different requirements for the guidance and control of UAVs have already been identified, however, the communication facilities between mobile platforms and air-to-ground links are restricted to low bit rate radio based technology. In the near future data rates in the area of 100 Mbit/s to 1 Gbit/s will be needed to handle multiple sensor information in real-time. Thus, UAVs in combination with Free Space Optics (FSO) is a new challenge and will become a necessity for all scenarios where real-time delivery of high date rates is essential. With the current systems (already delivering up to 2.5 Gbit/s) the optical solutions stand a firm ground in the competition against the microwaves. Therefore this paper provides some possible FSO-Link scenarios for UAV swarms and looks into technical aspects, like the optical link budgets, various weather conditions on the link path and some geometrical beam issues.
Keywords: Free Space Optics (FSO), Link Availability, Networking Architectures, Optical Wireless, Reliability, Unmanned Aerial Vehicles (UAV).

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Title: NONLINEAR CONVOLUTIVE BLIND SOURCE SEPARATION OF NON-STATIONARY SIGNALS
Authors: J. Zhang, L. C. Khor, W. L. Woo, S. S. Dlay,......................................pp. 151-159
Abstract: A new algorithm is proposed in this paper to solve blind source separation of post-nonlinear convolutive mixtures of non-stationary sources. Both convolutive mixing and post-nonlinear distortion are included in the proposed model. Based on the generalized Expectation-Maximization (EM) algorithm, the Maximum Likelihood (ML) approach is utilized to estimate the parameters in the model. A set of polynomials is used to estimate the post-nonlinear distortion. In the E-step, the sufficient statistics associated with the source signals are estimated while in the M-step, the parameters are optimized by using these statistics. Generally, the nonlinear distortion renders the statistics intractable and difficult to be formulated in a closed form. However, the use of Extended Kalman Smoother (EKS) around a linearized point facilitates the E-step tractable and the statistics needed can be solved by linear equations. With these statistics available, the post-nonlinear distortion can then be estimated by a set of self-updated polynomials.
Keywords: Nonlinear blind source separation, convolutive mixture, non-stationary sources, Extended Kalman Smoother.

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Title: APPLICATION OF A BATCH MARKOVIAN ARRIVAL PROCESS TO IP TRAFFIC
Authors: S. Nishimura, N. Yatomi,......................................................................pp.160-170
Abstract: It has been widely recognized that traffic including Internet has a self-similar nature on time scales. Some stochastic processes are proposed in order to capture the behavior of real IP traffic. We consider a batch Markovian arrival process (BMAP) introduced by Neuts. It is quite flexible and its queueing models are analytically tractable. We study the calculation of the stationary probability of the number of customers in a BMAP/G/1 queue, by using the inverse discrete Fast Fourier Transform (FFT) of the vector generating function. And we consider the estimation of rate matrices of a BMAP, using the EM algorithm from real traffic. In order to evaluate the validity of the estimated BMAP, two kinds of characteristics are given. First, by comparing graphs of bytes per time units for the traffic data and the estimated BMAP, we evaluate the corresponding self-similarity. Second, in order to check the fitness of the estimated BMAP, we compare the numbers of customers for a single server queueing model derived from the above spectral method and the simulation result of the original traffic data.
Keywords: Queue, BMAP/G/1, Stationary Probability, IP Traffic, EM Algorithm.

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Title: ENHANCED SOA STATIC AND DYNAMIC PERFORMANCES OVER A RELATIVE WIDE BAND OF SIGNAL WAVELENGTH BY OPTICAL INJECTION AT TRANSPARENCY
Authors: M. Amaya, A. Sharaiha, J. Le Bihan, F. Ginovart,................................pp. 171-180
Abstract: We experimentally demonstrate the influence of a high power continuous wave (CW) injection, as a holding beam (HB), into an SOA at the gain transparency wavelength, on the device’s static and dynamic performances over a wide range of signal wavelength. Our experiments show that the holding beam injection into the SOA increases its 3-dB saturation output power and decreases its dynamic gain recovery time over a broadband of optical signal wavelength, without sacrificing its non-saturated gain neither changing its noise figure (NF). In addition, we demonstrate that the holding beam injection is a helpful technique for reducing the inter-channel crosstalk induced between signals due to the cross-gain modulation (XGM) effects in the SOA. By bit error rate (BER) measurements, we show that the crosstalk penalties imposed between two optical signals, modulated at 2.5 Gb/s and traveling through the SOA, can be considerably reduced by a holding beam power (of 19 dBm) injection at the gain transparency wavelength (1480 nm) inside the SOA cavity in counter-propagating direction.
Keywords: Semiconductor Optical Amplifier (SOA), Holding Beam (HB), Saturation Output Power, Dynamic Gain Recovery Time, Noise Figure (NF), Inter-Channel Crosstalk Penalty, Bit Error Rate (BER).

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