arXiv subjects

Tamer Khattab

Publications and source records attributed to Tamer Khattab.

62 records · Page 4Linked to original sources

arXiv2014

On the Design of Relay--Assisted Primary--Secondary Networks

The use of $N$ cognitive relays to assist primary and secondary transmissions in a time-slotted cognitive setting with one primary user (PU) and one secondary user (SU) is investigated. An overlapped spectrum sensing strategy is proposed for channel sensing, where the SU senses the channel for $τ$ seconds from the beginning of the time slot and the cognitive relays sense the channel for $2 τ$ seconds from the beginning of the time slot, thus providing the SU with an intrinsic priority over the relays. The relays sense the channel over the interval $[0,τ]$ to detect primary activity and over the interval $[τ,2τ]$ to detect secondary activity. The relays help both the PU and SU to deliver their undelivered packets and transmit when both are idle. Two optimization-based formulations with quality of service constraints involving queueing delay are studied. Both cases of perfect and imperfect spectrum sensing are investigated. These results show the benefits of relaying and its ability to enhance both primary and secondary performance, especially in the case of no direct link between the PU and the SU transmitters and their respective receivers. Three packet decoding strategies at the relays are also investigated and their performance is compared.

cs.IT↗

arXiv2014

Maximum Throughput of a Secondary User Cooperating with an Energy-Aware Primary User

This paper proposes a cooperation protocol between a secondary user (SU) and a primary user (PU) which dedicates a free frequency subband for the SU if cooperation results in energy saving. Time is slotted and users are equipped with buffers. Under the proposed protocol, the PU releases portion of its bandwidth for secondary transmission. Moreover, it assigns a portion of the time slot duration for the SU to relay primary packets and achieve a higher successful packet reception probability at the primary receiver. We assume that the PU has three states: idle, forward, and retransmission states. At each of these states, the SU accesses the channel with adaptive transmission parameters. The PU cooperates with the SU if and only if the achievable average number of transmitted primary packets per joule is higher than the number of transmitted packets per joule when it operates alone. The numerical results show the beneficial gains of the proposed cooperative cognitive protocol.

cs.NI↗

arXiv2014

A DoF-Optimal Scheme for the two-user X-channel with Synergistic Alternating CSIT

In this paper, the degrees of freedom (DoF) of the two-user single input single output (SISO) X-channel are investigated. Three cases are considered for the availability of channel state information at the transmitters (CSIT); perfect, delayed, and no-CSIT. A new achievable scheme is proposed to elucidate the potency of interference creation-resurrection (IRC) when the available CSIT alternates between these three cases. For some patterns of alternating CSIT, the proposed scheme achieves $4/3$ DoF, and hence, coincides with the information theoretic upper bound on the DoF of the two-user X-channel with perfect and instantaneous CSIT. The CSIT alternation patterns are investigated where the patterns that provide extraordinary synergistic gain and dissociative ones are identified.

cs.IT↗

arXiv2014

Protocol Design and Stability Analysis of Cooperative Cognitive Radio Users

A single cognitive radio transmitter--receiver pair shares the spectrum with two primary users communicating with their respective receivers. Each primary user has a local traffic queue, whereas the cognitive user has three queues; one storing its own traffic while the other two are relaying queues used to store primary relayed packets admitted from the two primary users. A new cooperative cognitive medium access control protocol for the described network is proposed, where the cognitive user exploits the idle periods of the primary spectrum bands. Traffic arrival to each relaying queue is controlled using a tuneable admittance factor, while relaying queues service scheduling is controlled via channel access probabilities assigned to each queue based on the band of operation. The stability region of the proposed protocol is characterized shedding light on its maximum expected throughput. Numerical results demonstrate the performance gains of the proposed cooperative cognitive protocol.

cs.NI↗

arXiv2013

Band Allocation for Cognitive Radios with Buffered Primary and Secondary Users

In this paper, we study band allocation of $\mathcal{M}_s$ buffered secondary users (SUs) to $\mathcal{M}_p$ orthogonal primary licensed bands, where each primary band is assigned to one primary user (PU). Each SU is assigned to one of the available primary bands with a certain probability designed to satisfy some specified quality of service (QoS) requirements for the SUs. In the proposed system, only one SU is assigned to a particular band. The optimization problem used to obtain the stability region's envelope (closure) is shown to be a linear program. We compare the stability region of the proposed system with that of a system where each SU chooses a band randomly with some assignment probability. We also compare with a fixed (deterministic) assignment system, where only one SU is assigned to one of the primary bands all the time. We prove the advantage of the proposed system over the other systems.

cs.NI↗

arXiv2013

Multi-user Diversity in Spectrum Sharing Systems over Fading Channels with Average Power Constraints

The multi-user diversity in spectrum sharing cognitive radio systems with average power constraints over fading channels is investigated. Average power constraints are imposed for both the transmit power at the secondary transmitter and the interference power received at the primary receiver in order to provide optimal power allocation for capacity maximization at the secondary system and protection at the primary system respectively. Multiple secondary and primary receivers are considered and the corresponding fading distributions for the Rayleigh and Nakagami-m fading channels are derived. Based on the derived formulation of the fading distributions, the average achievable channel capacity and the outage probability experienced at the secondary system are obtained, revealing the impact of the average power constraints on optimal power allocation in multi-user diversity technique in fading environments with multiple secondary and primary receivers that share the same channel. The obtained results highlight the advantage of having on one hand more secondary receivers and on the other hand fewer primary receivers manifested as an increase in the achievable capacity.

cs.IT↗

arXiv2012

To Relay or Not To Relay in Cognitive Radio Sensor Networks

Recent works proposed the relaying at the MAC layer in cognitive radio networks whereby the primary packets are forwarded by the secondary node maintaining an extra queue devoted to the relaying function. However, relaying of primary packets may introduce delays on the secondary packets (called secondary delay) and require additional power budget in order to forward the primary packets that is especially crucial when the network is deployed using sensors with limited power resources. To this end, an admission control can be employed in order to manage efficiently the relaying in cognitive radio sensor networks. In this paper, we first analyse and formulate the secondary delay and the required power budget of the secondary sensor node in relation with the acceptance factor that indicates whether the primary packets are allowed to be forwarded or not. Having defined the above, we present the tradeoff between the secondary delay and the required power budget when the acceptance factor is adapted. In the sequel, we formulate an optimization problem to minimize the secondary delay over the admission control parameter subject to a limit on the required power budget plus the constraints related to the stabilities of the individual queues due to their interdependencies observed by the analysis. The solution of this problem is provided using iterative decomposition methods i.e. dual and primal decompositions using Lagrange multipliers that simplifies the original complicated problem resulting in a final equivalent dual problem that includes the initial Karush Kuhn Tucker conditions. Using the derived equivalent dual problem, we obtain the optimal acceptance factor while in addition we highlight the possibilities for extra delay minimization that is provided by relaxing the initial constraints through changing the values of the Lagrange multipliers.

cs.NI↗

arXiv2012

Stationarity of Stochastic Processes In The Fractional Fourier Domains

In this paper, we investigate the stationarity of stochastic processes in the fractional Fourier domains. We study the stationarity of a stochastic process after performing fractional Fourier transform (FRFT), and discrete fractional Fourier transform (DFRT) on both continuous and discrete stochastic processes, respectively. Also we investigate the stationarity of the fractional Fourier series (FRFS) coefficients of a continuous time stochastic process, and the stationarity of the discrete time fractional Fourier transform (DTFRFT) of a discrete time stochastic process. Closed formulas of the input process autocorrelation function and pseudo-autocorrelation function after performing the fractional Fourier transform are derived given that the input is a stationary stochastic process. We derive a formula for the output autocorrelation as a function of the $a^{th}$ power spectral density of the input stochastic process, also we derived a formula for the input fractional power spectral density as a function of the fractional Fourier transform of the output process autocorrelation function. We proved that, the input stochastic process must be zero mean to satisfy a necessary but not a sufficient condition of stationarity in the fractional domains. Closed formulas of the resultant statistics are also shown. It is shown that, in case of real input process, the output process is stationary if and only if the input process is white. On the other hand, if the input process is a complex process, it should be proper white process to obtain a stationary output process.

math.CV↗