On the Statistics of Cognitive Radio Capacity in Shadowing and Fast Fading Environments (Journal Version)

In this paper we consider the capacity of the cognitive radio channel in different fading environments under a low interference regime. First we derive the probability that the low interference regime holds under shadow fading as well as Rayleigh and Rician fast fading conditions. We demonstrate that this is the dominant case, especially in practical cognitive radio deployment scenarios. The capacity of the cognitive radio channel depends critically on a power loss parameter, $\alpha$, which governs how much transmit power the cognitive radio dedicates to relaying the primary message. We derive a simple, accurate approximation to $\alpha$ in Rayleigh and Rician fading environments which gives considerable insight into system capacity. We also investigate the effects of system parameters and propagation environment on $\alpha$ and the cognitive radio capacity. In all cases, the use of the approximation is shown to be extremely accurate.Comment: Submitted to the IEEE Transactions on Wireless Commun. The conference version of this paper appears in Proc. IEEE CrownCom, 200

Level Crossing Rates of Interference in Cognitive Radio Networks

The future deployment of cognitive radios is critically dependent on the fact that the incumbent primary user system must remain as oblivious as possible to their presence. This in turn heavily relies on the fluctuations of the interfering cognitive radio signals. In this letter we compute the level crossing rates of the cumulative interference created by the cognitive radios. We derive analytical formulae for the level crossing rates in Rayleigh and Rician fast fading conditions. We approximate Rayleigh and Rician level crossing rates using fluctuation rates of gamma and scaled noncentral $\chi^2$ processes respectively. The analytical results and the approximations used in their derivations are verified by Monte Carlo simulations and the analysis is applied to a particular CR allocation strategy.Comment: submitted to the IEEE Transactions on Wireless Communication

Interference and Deployment Issues for Cognitive Radio Systems in Shadowing Environments

In this paper we describe a model for calculating the aggregate interference encountered by primary receivers in the presence of randomly placed cognitive radios (CRs). We show that incorporating the impact of distance attenuation and lognormal fading on each constituent interferer in the aggregate, leads to a composite interference that cannot be satisfactorily modeled by a lognormal. Using the interference statistics we determine a number of key parameters needed for the deployment of CRs. Examples of these are the exclusion zone radius, needed to protect the primary receiver under different types of fading environments and acceptable interference levels, and the numbers of CRs that can be deployed. We further show that if the CRs have apriori knowledge of the radio environment map (REM), then a much larger number of CRs can be deployed especially in a high density environment. Given REM information, we also look at the CR numbers achieved by two different types of techniques to process the scheduling information.Comment: to be presented at IEEE ICC 2009. This posting is the same as the original one. Only author's list is updated that was unfortunately not correctly mentioned in first versio

Performance of Cognitive Radio Systems with Imperfect Radio Environment Map Information

In this paper we describe the effect of imperfections in the radio environment map (REM) information on the performance of cognitive radio (CR) systems. Via simulations we explore the relationship between the required precision of the REM and various channel/system properties. For example, the degree of spatial correlation in the shadow fading is a key factor as is the interference constraint employed by the primary user. Based on the CR interferers obtained from the simulations, we characterize the temporal behavior of such systems by computing the level crossing rates (LCRs) of the cumulative interference represented by these CRs. This evaluates the effect of short term fluctuations above acceptable interference levels due to the fast fading. We derive analytical formulae for the LCRs in Rayleigh and Rician fast fading conditions. The analytical results are verified by Monte Carlo simulations.Comment: presented at IEEE AusCTW 2009. Journal versions are under preparation. This posting is the same as the original one. Only author's list is updated that was unfortunately not correctly mentioned in the first versio

A Max-Min Task Offloading Algorithm for Mobile Edge Computing Using Non-Orthogonal Multiple Access

To mitigate computational power gap between the network core and edges, mobile edge computing (MEC) is poised to play a fundamental role in future generations of wireless networks. In this letter, we consider a non-orthogonal multiple access (NOMA) transmission model to maximize the worst task to be offloaded among all users to the network edge server. A provably convergent and efficient algorithm is developed to solve the considered non-convex optimization problem for maximizing the minimum number of offloaded bits in a multi-user NOMAMEC system. Compared to the approach of optimized orthogonal multiple access (OMA), for given MEC delay, power and energy limits, the NOMA-based system considerably outperforms its OMA-based counterpart in MEC settings. Numerical results demonstrate that the proposed algorithm for NOMA-based MEC is particularly useful for delay sensitive applications.Comment: 5 pages, 5 figure

A Minorization-Maximization Method for Optimizing Sum Rate in Non-Orthogonal Multiple Access Systems

Non-orthogonal multiple access (NOMA) systems have the potential to deliver higher system throughput, compared to contemporary orthogonal multiple access techniques. For a linearly precoded multiple-input multiple-output (MISO) system, we study the downlink sum rate maximization problem, when the NOMA principles are applied. Being a non-convex and intractable optimization problem,we resort to approximate it with a minorization-maximization algorithm (MMA), which is a widely used tool in statistics. In each step of the MMA, we solve a second-order cone program, such that the feasibility set in each step contains that of the previous one, and is always guaranteed to be a subset of the feasibility set of the original problem. It should be noted that the algorithm takes a few iterations to converge. Furthermore, we study the conditions under which the achievable rates maximization can be further simplified to a low complexity design problem, and we compute the probability of occurrence of this event. Numerical examples are conducted to show a comparison of the proposed approach against conventional multiple access systems. NOMA is reported to provide better spectral and power efficiency with a polynomial time computational complexity.Comment: Submitted for journal publicatio

Aspects of Design and Analysis of Cognitive Radios and Networks

Recent survey campaigns have shown a tremendous under utilization of the bandwidth allocated to various wireless services. Motivated by this and the ever increasing demand for wireless applications, the concept of cognitive radio (CR) systems has rendered hope to end the so called spectrum scarcity. This thesis presents various different facets related to the design and analysis of CR systems in a unified way. We begin the thesis by presenting an information theoretic study of cognitive systems working in the so called low interference regime of the overlay mode. We show that as long as the coverage area of a CR is less than that of a primary user (PU) device, the probability of the cognitive terminal inflicting small interference at the PU is overwhelmingly high. We have also analyzed the effect of a key parameter governing the amount of power allocated to relaying the PU message in the overlay mode of operation in realistic environments by presenting a simple and accurate approximation. Then, we explore the possibilities of statistical modeling of the cumulative interference due to multiple interfering CRs. We show that although it is possible to obtain a closed form expression for such an interference due a single CR, the problem is particularly difficult when it comes to the total CR interference in lognormally faded environments. In particular, we have demonstrated that fitting a two or three parameter lognormal is not a feasible option for all scenarios. We also explore the second-order characteristics of the cumulative interference by evaluating its level crossing rate (LCR) and average exceedance duration (AED) in Rayleigh and Rician channel conditions. We show that the LCRs in both these cases can be evaluated by modeling the interference process with gamma and noncentral χ2 processes, respectively. By exploiting radio environment map (REM) information, we have presented two CR scheduling schemes and compared their performance with the naive primary exclusion zone (PEZ) technique. The results demonstrate the significance of using an intelligent allocation method to reap the benefits of the tremendous information available to exploit in the REM based methods. At this juncture, we divert our attention to multiple-input multiple-output (MIMO) CR systems operating in the underlay mode. Using an antenna selection philosophy, we solve a convex optimization problem accomplishing the task and show via analysis and simulations that antenna selection can be a viable option for CRs operating in relatively sparse PU environments. Finally, we study the impact of imperfect channel state information (CSI) on the downlink of an underlay multiple antenna CR network designed to achieve signal-to-interference-plus-noise ratio (SINR) fairness among the CR terminals. By employing a newly developed convex iteration technique, we solve the relevant optimization problem exactly without performing any relaxation on the variables involved