Bouchra Senadji

Detecting relay attacks with timing-based protocols

Distance-bounding protocols have been proposed as a means of detecting relay attacks, also known as mafia fraud. In this paper we present the first symmetric key based distance-bounding protocol that is also resistant to so-called terrorist fraud, a variant of mafia fraud. Distance-bounding protocols require a communication channel that can exchange single bits with extremely low latency. This unconventional communication requirement has prompted Hancke and Kuhn to assert in a recent publication that ultra wide band (UWB) radio is necessary to achieve a useful distance-bounding resolution for RF security devices (contactless smart cards, RFID tags and the like). We analyse this assertion and present an alternative, novel communication approach that leverages the phenomena of side channel leakage to deliver a low latency channel. Our proposal is capable of detecting sophisticated relay attacks without resorting to the considerable expense and complexity of UWB radio. We present experimental results to support our arguments.

Mobile unit velocity estimation based on the instantaneous frequency of the received signal

This paper presents a new mobile station velocity estimator based on the first moment of the instantaneous frequency (IF) of the received signal. The effects of shadowing, additive noise, and scattering distribution on the proposed velocity estimator are analyzed. We show that, unlike velocity estimators based on the envelope and quadrature components of the received signal, the proposed estimator is robust to shadowing. We also prove that the performance of the IF-based estimator is only mildly affected by the presence of additive noise. Finally, by using simulations we show that the performance of the proposed IF-based estimator is superior to that of existing velocity estimators.

Ricean K-factor estimation in mobile communication systems

We propose an estimator for the Ricean K-factor which has applications in mobile communication systems. The estimator is based on the statistics of the instantaneous frequency (IF) of the received signal. Using simulations, we show that the proposed estimator is robust to shadowing and its mean-square error (MSE) is significantly lower than that of the envelope-based estimators.

Complex-lag polynomial Wigner-Ville distribution

This paper introduces a fourth order polynomial Wigner-Ville distribution (PWVD) which uses complex time lags to achieve optimal energy concentration for signals with non-linear FM laws. It is shown that the new complex-time fourth order PWVD provides an unbiased estimate of the instantaneous frequency of signals with polynomial phases up to order four.

Spectrum Sensing Optimisation for Dynamic Primary User Signal

We propose a multi-layer spectrum sensing optimisation algorithm to maximise sensing efficiency by computing the optimal sensing and transmission durations for a fast changing, dynamic primary user. Dynamic primary user traffic is modelled as a random process, where the primary user changes states during both the sensing period and transmission period to reflect a more realistic scenario. Furthermore, we formulate joint constraints to correctly reflect interference to the primary user and lost opportunity of the secondary user during the transmission period. Finally, we implement a novel duty cycle based detector that is optimised with respect to PU traffic to accurately detect primary user activity during the sensing period. Simulation results show that unlike currently used detection models, the proposed algorithm can jointly optimise the sensing and transmission durations to simultaneously satisfy the optimisation constraints for the considered primary user traffic.

A handover decision procedure based on the minimization of Bayes criterion

In mobile radio communications, inter-cell handover algorithms represent a critical issue for future pico and micro cellular systems. Two conflicting criteria must be optimized: the number of unnecessary handovers between two adjacent base-stations and the number of lost calls. A handover algorithm is proposed which utilizes a decision criterion such as signal strength. It allows for optimal trade-off between the number of unnecessary handovers and the number of lost calls. It is based on Bayes criterion which is commonly used in detection and estimation techniques. An optimal threshold /spl eta/ is derived using this criterion, which depends on the decision criterion and is dynamically adjusted by taking into account the observations. The decision of handover is made by comparing the distance d between the mobile station and the current base-station to this threshold /spl eta/.<<ETX>>

A new handover decision algorithm for microcellular systems

A new handover decision algorithm for microcellular systems is proposed. A handover is optimal at a cell boundary in line-of-sight situations or a street corner in non-line-of-sight situations. Since the probability of handover is maximum at these locations, it is chosen as a new criterion for processing a handover. The method is based on the real-time computation of the probability of handover from the received signal strengths, and on tracking its global maximum by calculating its gradient. Simulation results of the proposed algorithm are presented and the performance of the method is discussed.

Estimating the Ricean K-factor for mobile communication applications

In this paper, we propose two estimators for the Ricean K-factor, which has applications in mobile communication systems. The estimators are based on the first two moments of the envelope of the received signal. The effects of scattering distribution, additive noise and shadowing on the proposed estimators are analyzed. We show that the proposed estimators are robust to non-isotropic scattering and their performance is only mildly affected by shadowing.

Velocity estimation in cellular systems based on the time-frequency characteristics of the received signal

Estimating the velocity of the mobile units is of great importance in hierarchical cellular systems since the satisfactory handover of mobiles to macro or micro-cells depends on its present velocity. In this contribution, the first moment of the instantaneous frequency (IF) of the received signal, is proposed as a new velocity estimator. The performance of this estimator in the presence of Rayleigh fading, log-normal shadowing, and additive white Gaussian noise is compared with the conventional zero crossing rate (ZCR) estimator. Simulations show that the performance of the proposed estimator is superior to that of the ZCR velocity estimator, in the presence of additive noise. Also it has been shown that the IF velocity estimator is robust in the presence of shadowing.

Estimation of the hysteresis value for handover decision algorithms using Bayes criterion

In mobile radio communications, inter-cell handover is the process whereby a call in progress is maintained while the mobile unit passes through different cells. Current handover decision algorithms compare the difference between the received signal strengths from different base-stations to a hysteresis value. The proposed paper is a contribution towards an optimal estimation of the hysteresis value using Bayes criterion. An expression of the threshold value h is derived in terms of the standard deviation a of the log-normal shadowing affecting the received signals and the ratio /spl eta/ between the a priori probability of handover and probability of no handover. The performance of the handover decision algorithm is evaluated in terms of the probability of false alarm, or probability of unnecessary handover, and probability of detection, or probability of successful handover.