Hazim Tawfik

A new channel assignment algorithm for dual-mode cellular system

In a cellular system, the primary concern of a system designer is to use the available spectrum as efficiently as possible. Meanwhile, the performance must be maintained above an acceptable level. In the American system which supports both analog and dual-mode subscribers, one could allow an overflow from a certain type of traffic to the other to reduce blocking for both types of subscribers. This method may result in a possible scenario where analog subscribers use dual-mode radios or dual-mode subscribers use analog radios or both. In this paper, a new algorithm is proposed to dynamically reassign calls to their native radios in an attempt to increase the total carried traffic in the system. Simulation results are presented to quantify this increase in traffic.

Spectrum sensing via reconfigurable antennas: fundamental limits and potential gains

We propose a novel spectrum sensing technique in cognitive radio networks that provides diversity and capacity benefits using a single antenna at the Secondary User (SU) receiver. The proposed scheme is based on a reconfigurable antenna: an antenna that is capable of altering its radiation characteristics by changing its geometric configuration. Each configuration is designated as an antenna mode or state and corresponds to a distinct channel realization. Based on an abstract model for the reconfigurable antenna, we tackle two different settings for the cognitive radio problem and present fundamental limits on the achievable diversity and throughput gains. First, we explore the “to cooperate or not to cooperate” tradeoff between the diversity and coding gains in conventional cooperative and non-cooperative spectrum sensing schemes, showing that cooperation is not always beneficial. Based on this analysis, we propose two sensing schemes based on reconfigurable antennas that we term as state switching and state selection. It is shown that each of these schemes outperform both cooperative and non-cooperative spectrum sensing under a global energy constraint. Next, we study the “sensing-throughput” trade-off, and demonstrate that using reconfigurable antennas, the optimal sensing time is reduced allowing for a longer transmission time, and consequently better throughput. Moreover, state selection can be applied to boost the capacity of SU transmission.

Random Aerial Beamforming for Underlay Cognitive Radio With Exposed Secondary Users

In this paper, we introduce the exposed secondary-user (SU) problem in underlay cognitive radio systems, where both the secondary-to-primary and primary-to-secondary channels have a line-of-sight (LoS) component. Based on a Rician model for the LoS channels, we show, both analytically and numerically, that LoS interference hinders the achievable SU capacity when interference constraints are imposed at the primary user (PU) receiver. This is caused by the poor dynamic range of interference channel fluctuations when a dominant LoS component exists. To improve the capacity of such a system, we propose the use of an electronically steerable parasitic array radiator (ESPAR) antenna at the secondary terminals. An ESPAR antenna involves a single radio frequency (RF) chain and has a reconfigurable radiation pattern that is controlled by assigning arbitrary weights to

Performance of the dual-hop decode and forward relaying systems over Nakagami-m fading channels

M

A Spectrum Sensing Scheme for Partially Polarized Waves over α-μ Generalized Gamma Fading Channels

orthonormal basis radiation patterns via altering a set of reactive loads. By viewing the orthonormal patterns as multiple virtual dumb antennas, we randomly vary their weights over time, creating artificial channel fluctuations that can perfectly eliminate the undesired impact of LoS interference. This scheme is termed as random aerial beamforming (RAB) and is well suited for compact and low-cost mobile terminals as it uses a single RF chain. Moreover, we investigate the exposed-SU problem in a multiuser setting, showing that LoS interference hinders multiuser interference diversity and affects the growth rate of the SU capacity as a function of the number of users. Using RAB, we show that LoS interference can, in fact, be exploited to improve multiuser diversity by boosting the effective number of users.

Opportunistic spectrum sharing using dumb basis patterns: The Line-of-Sight interference scenario

A closed-form expression is derived for the probability density function (PDF) of the instantaneous received end-to-end signal-to-noise ratio (SNR) of dual-hop decode-and-forward (DF) relaying systems over Nakagami-m fading channel. The derived expression is used to evaluate the average symbol error probability for different coherent modulation techniques over this fading channel. Moreover, an expression for the outage probability is derived for this channel model. All the derived expressions are in closed forms and reduce to the well-known fading channel distributions. The effects of fading parameters on the system performance were studied. It is shown that an increase in the Nakagami-m parameter gives improvement in the system performance.

Spectrum sensing via reconfigurable antennas: Is cooperation of secondary users indispensable?

A spectrum holes sensing approach is developed for Cognitive Radio (CR) systems based on the estimation of the Stokes parameters of partially polarized electromagnetic waves. The performance of the proposed approach is compared with the energy detection method. An expression for the probability of detection based on the proposed method is derived using α - μ generalized gamma fading model.

An Innovative Cooperative Spectrum Sensing Algorithm with Non-ideal Feedback Channels and Delay Considerations

We investigate a spectrum-sharing system with non-severely faded mutual interference links, where both the secondary-to-primary and primary-to-secondary channels have a Line-of-Sight (LoS) component. Based on a Rician model for the LoS channels, we show, analytically and numerically, that LoS interference hinders the achievable secondary user capacity. This is caused by the poor dynamic range of the interference channels fluctuations when a dominant LoS component exists. In order to improve the capacity of such system, we propose the usage of an Electronically Steerable Parasitic Array Radiator (ESPAR) antenna at the secondary terminals. By viewing the orthonormal patterns of an ESPAR antenna as multiple virtual dumb antennas, we randomly vary their weights over time creating artificial channel fluctuations that can perfectly eliminate the undesired impact of LoS interference. Because the proposed scheme uses a single RF chain, it is well suited for compact and low cost mobile terminals.

Novel Teletraffic Analysis of GSM/EDGE Orthogonal Sub Channel (OSC) and Application to Spectrum Re-farming

This work presents an analytical framework for characterizing the performance of cooperative and non-cooperative spectrum sensing schemes by figuring out the tradeoff between the achieved diversity and coding gains in each scheme. Based on this analysis, we try to answer the fundamental question: can we dispense with SUs cooperation and still achieve an arbitrary diversity gain? It is shown that this is indeed possible via a novel technique that can offer diversity gain for a single SU using a single antenna. The technique is based on the usage of a reconfigurable antenna that changes its propagation characteristics over time, thus creating an artificial temporal diversity. It is shown that the usage of reconfigurable antennas outperforms cooperative as well as non-cooperative schemes at low and high Signal-to-Noise Ratios (SNRs). Moreover, if the channel state information is available at the SU, an additional SNR gain can also be achieved.

A spectrum sensing scheme for partially polarized waves

Cognitive radio (CR) is used to overcome the spectrum scarcity problem, which results from fixed allocation of wireless bands. CR allows the unlicensed secondary users to exploit the idle spectrum, which is not occupied by any licensed primary user (PU), thus increasing the overall spectrum utilization. In this paper, we first propose a simple cooperative sensing algorithm, which combines the local decision at each CR along with a group decision received from a fusion center to produce a collective decision on the existence of the PU. The performance of the algorithm is investigated over ideal and non-ideal reporting channels, from the fusion center to the CR devices, both analytically and via simulations. Furthermore, the effect of cooperation delay, which causes the decisions received by the CR device from the fusion center to be outdated, is extensively studied, both analytically and via simulations. To overcome the significant performance degradation due to the effect of delay, an extra local sensing cycle is performed at the CR side upon reception of the group decision. Results show that the proposed algorithm outperforms the conventional hard decisions technique and exhibits a comparable performance to the soft decisions approach at a considerably lower complexity. Moreover, the algorithm is shown to enjoy more robustness against reporting channel errors than the conventional hard decisions-based algorithm. Finally, the extra sensing cycle is shown to dramatically improve the performance for different delay scenarios.