We study exponential-server timing channels, in which the transmitter encodes a message using a sequence of packet inter-arrival times and the receiver observes the corresponding inter-departure times from an exponential-server queue. We show that if the service rate of one such queue is faster than that of another, then the latter channel is stochastically degraded with respect to the former. This combined with Burke's theorem implies that the slower queue is more entropy increasing than the first. These facts put together provide a converse for the secrecy capacity of the wiretapped version of the channel, matching an earlier achievability result in the literature.

In this paper, we introduce the Inverse Gaussian (IG) fading distribution to model the envelope and the power of the received signal in radio propagation fading channels and free-space optical (FSO) channels. The joint distribution of the IG process and its derivative and the marginal distribution of the derivative are developed. The obtained formulas are then used to derive the second-order statistics of the received signal envelope and the channel capacity under IG and Nakagami-IG (NIG) fading distributions. Numerical results sustaining our analysis are provided, and the impacts of various parameters on the system performance are investigated.

Optical and near-optical band communication systems are known to be intrinsically more secure than comparable RF channels, due to their narrow beamwidths and, in some cases, their high atmospheric absorption. The use of coding against wiretapping for such channels is investigated. For the degraded Poisson wiretap channel model, the secrecy capacity is determined exactly. Moreover, a complete characterization of the rate-equivocation region is presented. For achievability, an optimal code is constructed explicitly by using a code designed by Wyner for the Poisson channel. The converse is proved in two ways: the first method leverages the low-SNR nature of the channel and relies only on simple properties of conditional expectation and classical information inequalities. The second method uses a link recently established between minimum mean square error estimation and mutual information over Poisson channels.

The queueing performance of a (secondary) cognitive user is investigated for a hierarchical network where there are N independent and identical primary users. Each primary user employs a slotted transmission protocol, and its channel usage forms a two-state (busy, idle) discrete-time Markov chain. The cognitive user employs the optimal policy to select which channel to sense (and use if found idle) at each slot. In the framework of effective bandwidths, the stationary queue tail distribution of the cognitive user is estimated using a large deviation approach for which closed-form expressions are obtained when N = 2. Upper and lower bounds are obtained for the general N primary user network. For positively correlated primary transmissions, the bounds are shown to be asymptotically tight. Monte Carlo simulations using importance sampling techniques are used to validate the obtained large deviation estimates.

Providing security guarantees for the information “traveling” through the air is of critical importance for today's applications. Previous research concentrated on studying this problem when the medium used for communication is radio frequency (RF) channels. However, providing security guarantees on RF channels is a complicated task as RF channels' conditions are prone to rapid variations due small scale fading and at a given moment it is difficult to predict which of the main or wiretapper's channel has a better quality. Wireless optical communication is inherently more secure than regular RF communication. Despite this fact, optical communication is not completely immune against wiretapping and signal interception. The purpose of this paper is to study the secrecy capacity of a direct detection optical communication system. For the degraded Poisson wiretap channel, we give a complete characterization of the secrecy capacity in terms of the system parameters.

In this paper a communication system operating over a Gilbert-Elliot channel is studied. The goal of the transmitter is to maximize the number of successfully transmitted bits. This is achieved by choosing among three possible actions: (i) betting aggressively by using a weak code that allows transmission with a high data rate but provides no protection against a bad channel, ii) betting conservatively by using a strong code that perfectly protects the communication against a bad channel but does not allow a high data rate, iii) betting opportunistically by sensing the channel for a fixed duration and then deciding which code to use. The problem is formulated and solved using the theory of Markov decision processes (MDPs). It is shown that the optimal strategy has a simple threshold structure. Closed form expressions and simplified procedures for the computation of the threshold policies in terms of the system parameters are provided.

A novel distribution referred to as N*generalized K, constructed as the product of N statistically independent, but not necessarily identically distributed, generalized K random variables (RVs) is introduced. Statistical characterization of the proposed distribution is carried out via the derivation of closed-form expressions for its moment generating, probability density, cumulative distribution and moment functions. Using the obtained formulas, we present new closed-form expressions for the outage probability, the bit error probability and the average channel capacity of a digital communication system operating over the N cascaded generalized K distributed channels.

This paper presents an outage analysis of wireless systems operating in gamma-shadowed Nakagami-faded environments where the desired signal also suffers from co-channel interference. The interfering signals are also subject to fading and shadowing. Based on the obtained signal to interference ratio (SIR) probability density function (pdf), closed- form expressions for the outage probability are obtained in both cases of statistically identical interferers and multiple interferers with different parameters. The effects on the aforementioned performance metric of the reuse distance and of the combined fading, shadowing and co-channel interference are analyzed subsequently. The newly derived closed-form expressions for the outage probability allow as to assess the effects of the different channel and interference parameters easily.

Composite multipath fading/shadowing environments are frequently encountered in different realistic scenarios. These channels are generally modeled as a mixture of Nakagami-m multipath fading and log-normal shadowing. The resulting composite probability density function (pdf) is not available in closed form, thereby making the performance evaluation of communication links in these channels cumbersome. In this paper, we propose to model composite channels by the G-distribution. This pdf arises when the log-normal shadowing is substituted by the inverse-Gaussian one. This substitution will prove to be very accurate for several shadowing conditions. In this paper we conduct a performance evaluation of single-user communication systems operating in a composite channel. Our study starts by deriving an analytical expression for the outage probability. Then, we derive the moment generating function of the G-distribution, hence facilitating the calculation of average bit error probabilities. We also derive analytical expressions for the channel capacity for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate.

In this letter we provide an analytical expression for the moments of the capacity for the log-normal fading channel. Since the developed expression involves infinite series, we show that the error that results from the truncation of these series is insignificant. We also analyze in more details the ergodic capacity by giving a simpler expression for the remainder of the truncated series. Relying on the fact that the sum of log-normal random variables (RV) is well approximated by another lognormal RV, we further utilize the obtained results to approximate the capacity of diversity combining techniques in correlated lognormal fading channels. The results that we provide in this letter are an important tool for measuring the performance of communication links in a log-normal environment.

This paper presents an analytical framework for performance evaluation of mobile radio systems operating in composite fading/shadowing channels in the presence of colocated co-channel interference. The desired user and the interferers are subject to Nakagami fading superimposed on gamma shadowing. The paper starts by presenting generic closed-form expressions for the signal-to-interference ratio (SIR) probability density function (pdf). From this pdf, closed-form expressions for the outage probability, the average bit error rate and the channel capacity are obtained in both cases of statistically identical interferers and multiple interferers with different parameters. The newly derived closed-form expressions of the aforementioned metrics allow us to easily assess the effects of the different channel and interference parameters. It turns out that the system performance metrics are predominantly affected by the fading parameters of the desired user, rather than by the fading parameters of the interferers.

In this paper, we consider composite multipath/shadowing fading channels characterized by Nakagami- m fading with inverse Gaussian shadowing, and Rice fading with inverse Gaussian shadowing. These channel models are very convenient for land mobile satellite systems and communications with low mobility or stationary users. From an analytical standpoint, one major advantage of these models is that they lead to closed-from expressions for the envelope probability density function (pdf) and the cumulative density function (CDF). Moreover, in this paper, we derive mathematically tractable new expressions for fundamental channel statistics such as the level crossing rate (LCR) and the average fade duration (AFD). Also, we validate our analytical results by providing several numerical examples showing the LCR and the AFD as a function of the Nakagami-m factor, Rice factor, and shadowing spread.

Composite multipath/shadowing fading environments are frequently encountered in different realistic scenarios. These channels are generally modeled as a mixture of Nakagami- m multipath fading and log-normal shadowing. The resulting composite probability density function (pdf) is unfortunately not in closed-form, thereby making the performance evaluation of communication links in these channels cumbersome. In this paper, we propose to model composite channels by the very general G-distribution. This pdf arises when the log-normal shadowing is substituted by the Inverse-Gaussian one. This substitution will prove to be very accurate for several shadowing conditions. In this paper we conduct an exhaustive performance evaluation of communication systems operating in these channels. Our study starts by deriving a closed-form expression for the outage probability. Then, we derive the moment generating function of the G-distribution, hence facilitating the calculation of average bit error probabilities. We also derive closed-form expressions for the channel capacity for three adaptive transmission techniques, namely, (i) optimal rate adaptation with constant power, (ii) optimal power and rate adaptation, and (iii) channel inversion with fixed rate. The different expressions that will be provided are of great importance in assessing the performance of communication systems in composite channels.

This letter investigates the capacity of generalized-k fading channels. This very general model describes accurately composite multipath/shaodwing fading channels which are widely encountered in real-world environments. We derive closed-form expressions for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate. The analytical expressions obtained match perfectly the results obtained by computer simulations. These expressions provide a good tool to assess the spectral efficiency of the aforementioned adaptive transmission techniques over composite channels.

This paper investigates the capacity of generalized- K fading channels. This very general model describes accurately composite multipath/shadowing fading channels which are widely encountered in real-world environments. We derive closed-form expressions for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate. The analytical expressions obtained match perfectly the results obtained by computer simulations. These expressions provide a good tool to assess the spectral efficiency of the aforementioned adaptive transmission techniques over composite channels.

This paper investigates the capacity of log-normal fading channels with receiver channel state information. We provide a closed-form expression for the ergodic capacity of the log-normal fading channel. Since the developed expression involves an infinite series, we show that the error that results from the truncation of this series is insignificant. Relying on the fact that the sum of log-normal Random Variables (RV) is well approximated by another log-normal RV, we further apply the obtained results to find the capacity of correlated log-normal fading channels with Maximum Ratio Combining and Equal Gain Combining. The analytical expressions obtained match perfectly the capacity given by simulations.

This letter proposes a new data-aided carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) communications suitable for frequency-selective channels. The proposed method is based on the transmission of a specially designed synchronization symbol that generates a particular signal structure between the received observation samples at the receiver. This structure is exploited to derive a closed-form expression of the CFO. The proposed solution offers a wide acquisition range with reduced computational load. Simulations over frequency-selective channels confirm the superiority of the proposed method compared to recent data-aided synchronization algorithms

The performance of an OFDM system is highly sensitive to both carrier and sampling frequency offsets. This paper proposes a new joint carrier and sampling frequency offset estimation scheme for OFDM communications suitable for a frequency-selective fading channel. The joint carrier frequency offset (CFO) and sampling frequency offset (SFO) estimation in OFDM is reformulated and resolved as a harmonic retrieval problem. Accurate estimation can be reached even when only one OFDM symbol is used, hence making this method appropriate for offsets with a high level of variability in time. Extension to block processing is presented. Moreover the presented method proved to be robust against large CFO-SFO values, a claim that can not be made for other joint estimation techniques.