Georges El-Howayek

Efficient Dynamic Spectrum Sharing in Cognitive Radio Networks: Centralized Dynamic Spectrum Leasing (C-DSL)

In this paper, we propose the concept of centralized dynamic spectrum leasing (C-DSL), in which multiple primary users belonging to the same primary system participate in the spectrum leasing process with secondary users (potential bidder for spectrum) under centralized control. We develop a new game-theoretic user interaction model suitable for C-DSL in a cognitive radio network. Dynamic Spectrum Leasing (DSL), proposed in allows active participation of both primary and secondary users in the spectrum sharing process. Motivated by network spectrum utilization considerations, we propose generalizations to the primary system utility function defined in and a new utility function for the secondary users. We also generalize the proposed non-cooperative C-DSL game to allow for linear multiuser detectors at the secondary base stations. We formulate the conditions on the primary system and the secondary user utility functions so that the proposed C-DSL game has desired equilibrium properties. We prove that the proposed C-DSL game has unique Nash equilibria (NE) under both matched filter (MF) and linear minimum mean-squared error (LMMSE) receivers. Equilibrium performance of the system and robustness of the proposed game theoretic adaptive implementation are investigated through simulations.

Distributed Dynamic Spectrum Leasing (D-DSL) for Spectrum Sharing over Multiple Primary Channels

We propose a new architecture for dynamic spectrum sharing called the distributed dynamic spectrum leasing (D-DSL) and a game-theoretic framework for its implementation on a cognitive radio network. In D-DSL, each available frequency channel is assigned to a primary user who may independently lease the channel to secondary users. Secondary users are allowed to transmit in multiple channels simultaneously. We establish conditions for the system to reach an equilibrium and analyze the robustness of the proposed game theoretic D-DSL in time-varying channels. For the same amount of available primary spectrum, D-DSL leads to better overall performance compared to previously proposed Centralized-DSL.

Plasmonic field confinement for separate absorption-multiplication in InGaAs nanopillar avalanche photodiodes

Avalanche photodiodes (APDs) are essential components in quantum key distribution systems and active imaging systems requiring both ultrafast response time to measure photon time of flight and high gain to detect low photon flux. The internal gain of an APD can improve system signal-to-noise ratio (SNR). Excess noise is typically kept low through the selection of material with intrinsically low excess noise, using separate-absorption-multiplication (SAM) heterostructures, or taking advantage of the dead-space effect using thin multiplication regions. In this work we demonstrate the first measurement of excess noise and gain-bandwidth product in III–V nanopillars exhibiting substantially lower excess noise factors compared to bulk and gain-bandwidth products greater than 200 GHz. The nanopillar optical antenna avalanche detector (NOAAD) architecture is utilized for spatially separating the absorption region from the avalanche region via the NOA resulting in single carrier injection without the use of a traditional SAM heterostructure.

Dynamic Spectrum Leasing (DSL) in Dynamic Channels

Dynamic Spectrum Leasing (DSL) was recently proposed in [1]-[4] as a new way to achieve dynamic spectrum sharing (DSS). Unlike previously considered dynamic spectrum access (DSA) proposals, DSL allows for the spectrum owner, called the primary user, to dynamically adjust the amount of interference it is willing to tolerate from secondary users. In response, the secondary users update their transmit powers to maximize a suitably chosen reward function. In previous work, it has been shown that the best response adaptations will converge to a unique Nash equilibrium under the assumptions of a quasi-static channel conditions and for a fixed number of secondary users. In this paper, we investigate the convergence and equilibrium performance of the proposed DSL-game in the presence of slow time-varying fading and time-varying secondary system size. Our results show that while the DSL best response adaptation algorithm is reasonably robust against these dynamics, there is a trade-off between performance and the CSI update rate.

On the Use of Gaussian Approximation in Analyzing the Performance of Optical Receivers

The analytical calculation of the bit error rate (BER) of digital optical receivers that employ avalanche photodiodes (APDs) is challenging due to 1) the stochastic nature of the avalanche photodiodes impulse-response function and 2) the presence of intersymbol interference (ISI). At ultrafast transmission rates, ISI becomes a dominant component of the BER, and its effect on the BER should be carefully addressed. One solution to this problem, termed the bit-pattern-dependent (PD) approach, is to first calculate the conditional BER given a specific bit pattern and then average over all possible bit patterns. Alternatively, a simplifying method, termed the bit-pattern-independent (PI) approach, has been commonly used whereby the average bit stream is used to calculate the distribution of the receiver output, which, in turn, is used to calculate the BER. However, when ISI is dominant, the PI approximation is inaccurate. Here, the two approaches are analytically compared by analyzing their asymptotic behavior and their bounds. Conditions are found to determine when the PI method overestimates the BER. The BER found using the PD method exponentially decays with the received optical power, whereas for the PI approach, the BER converges to a constant, which is unrealistic. For an InP-based APD receiver with a 100-nm multiplication layer, the PI method is found to be inaccurate for transmission rates beyond 20 Gb/s.

Error Probabilities for Optical Receivers That Employ Dynamically Biased Avalanche Photodiodes

A novel theory was recently reported for the avalanche multiplication process in avalanche photodiodes (APDs) under dynamic reverse-biasing conditions. It has been shown theoretically that the bit-synchronized, periodic modulation of the electric field in the multiplication region can offer improvements in the gain-bandwidth product by reducing intersymbol interference in optical receivers. This paper reports a rigorous formulation of the sensitivity of optical receivers that employ dynamically biased APDs. To enable the sensitivity analysis, a recurrence theory is developed to calculate the joint probability distribution function (PDF) of the stochastic gain and avalanche buildup time in APDs that are operated under dynamic biasing. It is shown that in an ideal buildup-time limited scenario, a minimum receiver sensitivity of -20 dBm is predicted at an optimal gain of approximately 47 for a 60 Gb/s communication system, compared to a minimum of 0 dBm in the static-bias case. The receiver sensitivity analysis also reveals that, as the peak-to-peak voltage of the dynamic reverse bias increases, the device optimal gain increases while maintaining a short avalanche buildup time and reduced ISI. Of course, a point of diminishing return exists in practice when the tunneling current in the multiplication region becomes dominant.

Ad-hoc wireless sensor network based underwater diffusive source localization

In this paper, we develop a statistical method for estimating the location of a diffusive source using an ad-hoc network of wireless sensors capable of sensing a diffusive field/environment. In particular, we focus on an underwater oil spill scenario along with statistical algorithms for analysis of sensor data leading to efficient estimation of the parameters of interest. We derive a maximum likelihood (ML) based parametric estimation method for localizing the diffusive source. The Cramer-Rao lower bound (CRLB) for the ML estimator performance is also derived as a measure of the theoretical performance bound. The effectiveness of the proposed method is also shown through numerical simulations.

Breaking the buildup-time limit of sensitivity in avalanche photodiodes by dynamic biasing.

Avalanche photodiodes (APDs) are the preferred photodetectors for direct-detection, high data-rate long-haul optical telecommunications. APDs can detect low-level optical signals due to their internal amplification of the photon-generated electrical current, which is attributable to the avalanche of electron and hole impact ionizations. Despite recent advances in APDs aimed at reducing the average avalanche-buildup time, which causes intersymbol interference and compromises receiver sensitivity at high data rates, operable speeds of commercially available APDs have been limited to 10Gbps. We report the first demonstration of a dynamically biased APD that breaks the traditional sensitivity-versus-speed limit by employing a data-synchronous sinusoidal reverse-bias that drastically suppresses the average avalanche-buildup time. Compared with traditional DC biasing, the sensitivity of germanium APDs at 3Gbps is improved by 4.3 dB, which is equivalent to a 3,500-fold reduction in the bit-error rate. The method is APD-type agnostic and it promises to enable operation at rates of 25Gbps and beyond.

Efficient spectrum sharing with autonomous primary users: Distributed dynamic spectrum leasing (D-DSL)

In this paper we introduce a new architecture for dynamic spectrum sharing called the distributed dynamic spectrum leasing (D-DSL) and a game theoretic framework for its implementation on a cognitive radio network. In D-DSL, it is assumed that each channel is assigned to a primary user who may lease the channel to secondary users. The spectrum owners can thus dynamically adjust the amount of interference they are willing to tolerate from the secondary users. The secondary users compete with each other in order to achieve maximum possible Quality of Service without violating the interference cap imposed by the primary users on each channel. The secondary users are allowed to transmit in more than one channel simultaneously. The utility of each primary user is in general an increasing function of the amount of allowed secondary interference. Under these conditions in addition to the competition among secondary users, the primary users also compete with each other to let more secondary users to access their channels. We establish conditions for the system to reach an equilibrium and analyze the performance. It is shown that the performance of the secondary system improves by increasing the availability of primary channels.