Mahsa Derakhshani

Efficient Cooperative Cyclostationary Spectrum Sensing in Cognitive Radios at Low SNR Regimes

This paper proposes efficient cooperative cyclostationary spectrum sensing schemes in which each secondary-user (SU) performs single-cycle (SC) cyclostationary detection for fast and simple implementation, while collaboration between SUs in final decision on the presence or absence of the primary-user (PU) is explored to improve its performance. As the SUs simultaneously measure the spectral correlation functions at different cycle-frequencies (CF) and exchange their information regarding the measured results, a sufficient number of CFs are effectively examined with parallel searching, which makes the proposed cooperative spectrum sensing more reliable. This paper presents another look at performance evaluation of cyclostationary detectors in terms of deflection coefficients. Outage probability of deflection coefficient is defined as a measure to compare the performance of different cyclostationary detectors in a fading channel. Furthermore, performance of the proposed schemes in terms of false-alarm and detection probabilities is evaluated by analysis and simulation in AWGN and fading channels. Illustrative and analytical results show that the proposed schemes outperform both SC and multi-cycle (MC) cyclostationary detectors, especially in fading channels.

Aggregate Interference and Capacity-Outage Analysis in a Cognitive Radio Network

This paper presents a study on the interference caused by secondary users (SUs) due to misdetection and its effects on the capacity-outage performance of the primary user (PU) in a cognitive radio (CR) network assuming Rayleigh and Nakagami fading channels. The effect of beacon transmitter placement on aggregate interference distribution and capacity-outage performance is studied, considering two scenarios of beacon transmitter placement: a beacon transmitter located 1) at a PU transmitter or 2) at a PU receiver. Based on the developed statistical models for the interference distribution, closed-form expressions for the capacity-outage probability of the PU are derived to examine the effects of various system parameters on the performance of the PU in the presence of interference from SUs. It is shown that the beacon transmitter at the PU receiver imposes less interference and, hence, better capacity-outage probability to the PU than the beacon transmitter at the PU transmitter. Furthermore, the model is extended to investigate the cooperative sensing effect on aggregate interference statistical model and capacity-outage performance considering or (i.e., logical or operation) and maximum likelihood cooperative detection techniques. Simulation results are also provided to verify the developed analytical models.

Cooperative Cyclostationary Spectrum Sensing in Cognitive Radios at Low SNR Regimes

The paper proposes efficient cooperative cyclostationary spectrum sensing schemes in which each secondary user (SU) performs single-cycle cyclostationary detection for fast and simple implementation, while collaboration between SUs in final decision on the presence or absence of the primary user (PU) is explored to improve its performance. As the SUs simultaneously measure the spectral correlation functions at different cycle frequencies (CF) and exchange their information regarding the measured results, a sufficient number of CFs are effectively examined in a short period of time because of parallel searching, which makes the proposed cooperative spectrum sensing more reliable and faster. Performance of the proposed schemes in terms of false-alarm and detection probabilities and deflection coefficients is evaluated by analysis and simulation in AWGN, fading and shadowing channels. Illustrative results show that the proposed schemes outperform both single-cycle (SC) and multi-cycle (MC) cyclostationary detectors, especially in fading and shadowing channels.

Joint resource provisioning and admission control in wireless virtualized networks

This paper studies joint resource provisioning and admission control in wireless virtualized networks (WVN), where one base station of an OFDMA-based wireless network is virtualized into two types of slices with resource-based and rate-based reservations. Aiming to maximize the total rate of WVN, first, the resource provisioning optimization problems are formulated by guaranteeing a minimum requirement for each slice. Via constraint relaxation and variable transformations, an iterative algorithm is developed for power and sub-carrier allocation. Due to the channel variations, WVN suffers from non-zero outage probability, i.e., slice requirements cannot always be met. To prevent this issue, we present an admission control algorithm in which slice requirements are dynamically adjusted based on channel state information. The simulation results demonstrate the effectiveness of our proposed algorithms.

Joint User-Association and Resource-Allocation in Virtualized Wireless Networks

In this paper, we consider the down-link dynamic resource allocation in multi-cell virtualized wireless networks (VWNs) to support the users of different service providers (slices) within a specific region by a set of base stations (BSs) through orthogonal frequency division multiple access (OFDMA). In particular, we develop a joint BS assignment, sub-carrier, and power allocation algorithm to maximize the network sum rate, while satisfying the minimum required rate of each slice. Under the assumption that each user at each transmission instance can connect to no more than one BS, we introduce the user-association factor to represent the joint sub-carrier and BS assignment as the optimization variable vector in the problem formulation. Sub-carrier reuse is allowed in different cells, but not within one cell. As the proposed optimization problem is inherently non-convex and NP-hard, by applying the successive convex approximation (SCA) and complementary geometric programming (CGP), we develop an efficient two-step iterative approach with low computational complexity to solve the proposed problem. For a given problem, Step 1 derives the optimum user-association and subsequently, and for an obtained user-association, Step 2 finds the optimum power allocation. Simulation results demonstrate that the proposed iterative algorithm outperforms the traditional approach in which each user is assigned to the BS with the largest average value of signal strength, and then, joint sub-carrier and power allocation is obtained for the assigned users of each cell. Simulation results reveal a coverage improvement, offered by the proposed approach, of 57% and 71% for uniform and non-uniform users distribution, respectively, leading to higher spectrum efficiency for VWN.

Resource Provisioning in Wireless Virtualized Networks via Massive-MIMO

This letter proposes a dynamic resource provisioning scheme for an OFDMA wireless virtualized network (WVN), where one base-station equipped with a large number of antennas serves users belonging to a number of service providers via different slices. In particular, joint power, sub-carrier, and antenna allocation problems are presented for both perfect and imperfect channel knowledge cases, aiming to maximize a sum-utility while maintaining a minimum rate per slice. Subsequently, relaxation and variable transformation are applied to develop the efficient algorithm to solve the formulated non-convex, combinational optimization problem. Simulation results reveal the benefits of applying a large number of antennas in this setup and evaluate the network performance for different system conditions.

Self-organizing channel assignment for high density 802.11 WLANs

In a dense WLAN deployment, the interfering wireless access points (APs) need to efficiently share the spectrum, and hence, avoid low-performance experience of users due to high collision rates and long backoff overheads. In this paper, we aim to propose a channel assignment scheme in which APs can self-configure their channel choices to mitigate interference and thereby maximize network throughput. Aiming to minimize interference sum utility, a channel assignment problem is formulated as a non-convex optimization problem and solved using difference-of-convex-functions (DC) programming. Subsequently, two distributed algorithms are developed in which each AP independently adapts its channel selection to the optimal values over time by measuring the received interference in different channels. The convergence, complexity, and efficiency of the developed algorithms are studied by simulation results.

Interference and outage analysis in a cognitive radio network with beacon

This paper presents a study on the interference caused by Secondary Users (SUs) due to miss-detection and its effects on the capacity-outage performance of the Primary User (PU) in a cognitive network with beacon. Investigation by simulation indicates that a Gamma distribution can be used to characterize the total interference power from the SUs, and tight upper-bounds on its mean and variance are derived. Based on these results, a closed-form expression of the capacity-outage probability of the PU is developed to examine the effects of various system parameters on the PU performance in the presence of interference from SUs. Simulation results confirm the validity of the developed analytical models.

Adaptive Hopping Transmission Strategy for Opportunistic Spectrum Access

This paper presents an adaptive hopping transmission strategy for secondary users (SUs) to access temporarily idle frequency-slots of a licensed frequency band in consideration of the random return of primary users (PUs), aiming to maximize the overall SU throughput. A SU dynamically hops over multiple idle frequency-slots, each with an adaptive activity factor so that possible PU return in a frequency-slot may destroy only a small fraction of the SU transmission that can be recovered by erasure-correction coding. SU activity factor optimization problems are formulated to develop adaptive SU access algorithms for both centralized and distributed structures. Numerical results confirm the effectiveness and demonstrate performance gains of the proposed approach as compared to existing schemes.

Cognitive MAC Designs: Background

This chapter first presents an overview on the MAC mechanisms currently deployed in IEEE 802.11 WLANs. The basic coexistence capabilities and recent enhancements of 802.11 MAC are discussed as enablers for realizing full cognitive MAC designs. Then, the second part of this chapter reviews various state-of-the-art cognitive MAC designs in OSA networks. We discuss and categorize the MAC design approaches in OSA networks, considering the need for network-wide coordination, the network structure of secondary users, and the transmission model of primary users.