Jawad Salehi - Profile on Academia.edu (original) (raw)

Papers by Jawad Salehi

On Fundamental Limits of Energy Efficient Signaling in Visible Light Communication Systems

IEEE Wireless Communications Letters, 2023

arXiv (Cornell University), Apr 4, 2017

arXiv (Cornell University), Apr 25, 2016

In this paper, we present a multi-user resource allocation framework using fragmented-spectrum sy... more In this paper, we present a multi-user resource allocation framework using fragmented-spectrum synchronous OFDM-CDMA modulation over a frequency-selective fading channel. In particular, given pre-existing communications in the spectrum where the system is operating, a channel sensing and estimation method is used to obtain information of subcarrier availability. Given this information, some real-valued multi-level orthogonal codes, which are orthogonal codes with values of {±1, ±2, ±3, ±4, ...}, are provided for emerging new users, i.e., cognitive radio users. Additionally, we have obtained a closed form expression for bit error rate of cognitive radio receivers in terms of detection probability of primary users, CR users' sensing time and CR users' signal to noise ratio. Moreover, simulation results obtained in this paper indicate the precision with which the analytical results have been obtained in modeling the aforementioned system.

arXiv (Cornell University), Aug 29, 2015

In this paper we analyze and investigate the bit error rate (BER) performance of multiple-input m... more In this paper we analyze and investigate the bit error rate (BER) performance of multiple-input multiple-output underwater wireless optical communication (MIMO-UWOC) systems. In addition to exact BER expressions, we also obtain an upper bound on the system BER. To effectively estimate the BER expressions, we use Gauss-Hermite quadrature formula as well as approximation to the sum of log-normal random variables. We confirm the accuracy of our analytical expressions by evaluating the BER through photon-counting approach. Our simulation results show that MIMO technique can mitigate the channel turbulence-induced fading and consequently, can partially extend the viable communication range, especially for channels with stronger turbulence.

arXiv (Cornell University), Aug 17, 2015

In this paper, we analytically study the performance of multiple-input multiple-output underwater... more In this paper, we analytically study the performance of multiple-input multiple-output underwater wireless optical communication (MIMO UWOC) systems with on-off keying (OOK) modulation. To mitigate turbulence-induced fading, which is amongst the major degrading effects of underwater channels on the propagating optical signal, we use spatial diversity over UWOC links. Furthermore, the effects of absorption and scattering are considered in our analysis. We analytically obtain the exact and an upper bound bit error rate (BER) expressions for both optimal and equal gain combining. In order to more effectively calculate the system BER, we apply Gauss-Hermite quadrature formula as well as approximation to the sum of lognormal random variables. We also apply photon-counting method to evaluate the system BER in the presence of shot noise. Our numerical results indicate an excellent match between the exact and upper bound BER curves. Also a good match between the analytical results and numerical simulations confirms the accuracy of our derived expressions. Moreover, our results show that spatial diversity can considerably improve the system performance, especially for channels with higher turbulence, e.g., a 3 × 1 MISO transmission in a 25 m coastal water link with log-amplitude variance of 0.16 can introduce 8 dB performance improvement at the BER of 10 -9 .

arXiv (Cornell University), Mar 9, 2016

In this paper, we evaluate the end-to-end bit error rate (BER) of point-to-point underwater wirel... more In this paper, we evaluate the end-to-end bit error rate (BER) of point-to-point underwater wireless optical communication (UWOC) systems with multi-hop transmission. To do so, we analytically derive the BER expression of a single-hop UWOC link as the building block for end-to-end BER evaluation. We also apply photon-counting method to evaluate the system BER in the presence of shot noise. Moreover, we use Gauss-Hermite quadrature formula to obtain the closed-form solutions for the system BER in the case of log-normal underwater fading channels. Our analytical treatment involves all the impairing effects of the underwater optical channel, namely absorption, scattering and fading. Numerical results demonstrate that multihop transmission by alleviating the aforementioned impairing effects of the channel, can significantly improve the system performance and extend the viable end-to-end communication distance. For example, dual-hop transmission in 22.5 m and 45 m coastal water links can provide 17.5 dB and 39 dB performance enhancement at the BER of 10 -6 , respectively.

Optimal Placement of Access Points in Cellular Visible Light Communication Networks: An Adaptive Gradient Projection Method

IEEE Transactions on Wireless Communications, Oct 1, 2020

In this paper, a new approach toward the optimization of Access Point (AP) placement in cellular ... more In this paper, a new approach toward the optimization of Access Point (AP) placement in cellular Visible light Communication (VLC) networks is proposed based on the projected gradient algorithm. The objective of the optimal placement problem is to maximize the average throughput of the network subject to constraints on the minimum illumination level and the minimum rate of the users. This optimization framework gives the enhanced AP deployment in case of users with static, nomadic, or completely mobile behavior. Taking the distribution of users, the receiver’s field of view, reflection from walls and interference from neighboring APs into account makes the deployment problem complicated. To solve the arising intricate optimization problem, we derive analytical expressions for gradients of the objective and use them in the gradient ascent algorithm. The proposed adaptive gradient projection method then realizes the constraints. This method relaxes the projection onto a high dimensional space to planar projections, which are implemented using efficient tools from computational geometry. The proposed method eliminates the need for an intractable exhaustive search to find the optimal placement of VLC APs in cellular VLC networks, while it gives either exact optimum or very close approximations to the optimal placement. It is shown, with the aid of properties of convolution of parametric concave functions, that in some practical cases the objective function is unimodal and has no local optimum. Simulation results show a significant improvement in the throughput, SINR and outage probability of the system when the access points are deployed according to their optimized placement, determined by the algorithm. On the other hand, proper constraints on the minimum achievable rate and minimum illumination level improve the worst-case performance of the network to the desired extent.

An Analytical Framework for the Performance Evaluation of Node- and Network-Wise Operation Scenarios in Elastic Optical Networks

IEEE Transactions on Communications, May 1, 2014

ABSTRACT

arXiv (Cornell University), Dec 5, 2016

In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre... more In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre channel. Laguerre behavior is used to model vorious types of optical systems and networks such as optical amplifiers, short distance visible light communication systems with direct detection and coherent code devision multiple access (CDMA) networks. Bounds are derived for short distance visible light communication systems and coherent CDMA networks. These bounds are separated in three main cases: when both average and peak power constrants are imposed, when peak power constraint is inactive and when only peak power constraint is active. Optical intensity modulation with direct detection (IM/DD) is one of the most prevalent methods to communicate through optical channels and networks due to its simplicity in design and implementation. In these channels, information is modulated onto intensity domain and thus, all symbols have non-negative values. To find the capacity of such channels, first one should obtain the statistical expression of the channel. There has already been presented several channel statistics model for IM/DD channels such as Poisson and Gaussian intensity channels and also the capacity of these channels are investigated. In [1]-[5], upper and lower bound for discrete-time Poisson channel are proposed under different conditions. Upper and lower bounds for the capacity of the Guassian optical intensity channels are also evaluated in [6]-[10] using various methods such as sphere packing, duality approach and maxentropic method. Moser presents the capacity results of optical intensity channels with input-dependent gaussian noise under peak and average power conditions .

arXiv (Cornell University), Jan 23, 2018

Optical signal propagation through underwater channels is affected by three main degrading phenom... more Optical signal propagation through underwater channels is affected by three main degrading phenomena, namely absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path, and then examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the channel coherence time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beamcollimator at the transmitter side and aperture averaging lens at the receiver side are experimentally investigated. We show that the use of a transmitter beam-collimator and/or a receiver aperture averaging lens suits single-lobe distributions such that the generalized Gamma and exponentiated Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the channel coherence time is on the order of 10 -3 seconds and larger which implies to the slow fading turbulent channels.

arXiv (Cornell University), Dec 3, 2017

Inspired by the ever-increasing demand for higher data transmission rates and the tremendous atte... more Inspired by the ever-increasing demand for higher data transmission rates and the tremendous attention toward all-optical signal processing based on miniaturized nanophotonics, in this paper, for the first time, we investigate the integrable design of coherent ultrashort light pulse code-division multiple-access (CDMA) technique, also known as femtosecond CDMA, using all-dielectric metasurfaces (MSs). In this technique, the data bits are firstly modulated using ultrashort femtosecond optical pulses generated by mode-locked lasers, and then by employing a unique phase metamask for each data stream, in order to provide the multiple access capability, the optical signals are spectrally encoded. This procedure spreads the optical signal in the temporal domain and generates low-intensity pseudo-noise bursts through random phase coding leading to minimized multiple access interference. This paper comprehensively presents the principles and design approach to realize fundamental components of a typical femtosecond CDMA encoder, including the grating, lens, and phase mask, by employing high-contrast CMOS-compatible MSs. By controlling the interference between the provided Mie and Fabry-Perot resonance modes, we tailor the spectral and spatial responses of the impinging light locally and independently. Accordingly, we design a MS-based grating with the highest possible refracted angle and, in the meantime, the maximized efficiency which results in a reasonable diameter for the subsequent lens. Moreover, to design our MS-based lens commensurate with the spot size and distance requirements of the pursuant phase mask, we leverage a new optimization method which splits the lens structure into central and peripheral parts, and then design the peripheral part using a collection of gratings converging the impinging at the subsequent phase mask. This work can be regarded as a pioneering attempt to make a bridge between the recent tremendous advancement in the nonophotonics technology and that of optical communication networks to extend the boundaries and facilitate their effective miniaturized deployment.

In this paper, we investigate PN-sequences with ideal autocorrelation property and the consequenc... more In this paper, we investigate PN-sequences with ideal autocorrelation property and the consequences of this property on the number of +1s and -1s and run structure of sequences. We begin by discussing and surveying about the length of PNsequences with ideal autocorrelation property. From our discussion and survey we introduce circulant matrix representation of PN-sequence. Through circulant matrix representation we obtain system of non-linear equations that lead to ideal autocorrelation property. Rewriting PN-sequence and its autocorrelation property in {0, 1} leads to a definition based on Hamming weight and Hamming distance and hence we can easily prove some results on the PN-sequences with ideal autocorrelation property.

arXiv (Cornell University), Aug 17, 2015

In this paper, we characterize the performance of relay-assisted underwater wireless optical code... more In this paper, we characterize the performance of relay-assisted underwater wireless optical code division multiple access (OCDMA) networks over turbulent channels. In addition to scattering and absorption effects of underwater channels, we also consider optical turbulence as a log-normal fading coefficient in our analysis. To simultaneously and asynchronously share medium among many users, we assign a unique optical orthogonal code (OOC) to each user in order to actualize OCDMAbased underwater network. The most significant challenge in underwater optical communication is in the ability to extend the short range of its coverage. In order to expand the viable communication range, we consider multi-hop transmission to the destination. Moreover, we evaluate the performance of a relayassisted point-to-point UWOC system as a special case of the proposed relay-assisted OCDMA network. Our numerical results indicate significant performance improvement by employing intermediate relays, e.g., one can achieve 32 dB improvement in the bit error rate (BER) of 10 -6 using only a dual-hop transmission in a 90 m point-to-point clear ocean link.

Iet Optoelectronics, Dec 1, 2015

Wireless sensor networks enable a variety of application scenarios due to their flexibility, the ... more Wireless sensor networks enable a variety of application scenarios due to their flexibility, the wide range of hardware solutions, as well as their simplicity. However, new challenges arise that make applications viable, such as power consumption, robust communication, limited storage and processing. One of the main elements among communication and power consumption optimization aspects is related to the protocol stack and especially the medium access layer, due to the nature of the wireless environment. Thus, this article proposes a multi-channel mechanism for asynchronous receiver-initiated MAC for low-power WSN. The proposed protocol reduces energy consumption due to message contention and idle listening through an initial recognition mechanism and an efficient multichannel mechanism based on knowledge of the best channel in each node of the network. For validation of the protocol, evaluations were made for consumption, latency and rate of delivery of the network in a web application, comparing the multichannel asynchronous protocol to the synchronous protocol. The results show that the synchronous protocol presents slight superiority in communication performance, whereas the asynchronous protocol presents lower consumption.

arXiv (Cornell University), Dec 5, 2016

In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre... more In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre channel. Laguerre behavior is used to model various types of optical systems and networks such as optical amplifiers, short distance visible light communication systems with direct detection and coherent code division multiple access (CDMA) networks. Bounds are derived for short distance visible light communication systems and coherent CDMA networks. These bounds are separated in three main cases: when both average and peak power constraints are imposed, when peak power constraint is inactive and when only peak power constraint is active. Optical intensity modulation with direct detection (IM/DD) is one of the most prevalent methods to communicate through optical channels and networks due to its simplicity in design and implementation. In these channels, information is modulated onto intensity domain and thus, all symbols have non-negative values. To find the capacity of such channels, first one should obtain the statistical expression of the channel. There has already been presented several channel statistics model for IM/DD channels such as Poisson and Gaussian intensity channels and also the capacity of these channels are investigated. In [1]-[5], upper and lower bound for discrete-time Poisson channel are proposed under different conditions. Upper and lower bounds for the capacity of the Guassian optical intensity channels are also evaluated in [6]-[10] using various methods such as sphere packing, duality approach and maxentropic method. Moser presents the capacity results of optical intensity channels with input-dependent gaussian noise under peak and average power conditions .

arXiv: Information Theory, 2016

In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre... more In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre channel. Laguerre behavior is used to model various types of optical systems and networks such as optical amplifiers, short distance visible light communication systems with direct detection and coherent code division multiple access (CDMA) networks. Bounds are derived for short distance visible light communication systems and coherent CDMA networks. These bounds are separated in three main cases: when both average and peak power constraints are imposed, when peak power constraint is inactive and when only peak power constraint is active.

Resource Allocation in Space Division Multiplexed Elastic Optical Networks Secured With Quantum Key Distribution

IEEE Journal on Selected Areas in Communications, 2021

Elastic Optical Network (EON) is a promising solution to address the high capacity, low latency, ... more Elastic Optical Network (EON) is a promising solution to address the high capacity, low latency, and flexibility requirements of the upcoming 5th-generation (5G) networks. Furthermore, Multi-Core Fibers (MCFs) and Space Division Multiplexing (SDM) technique can be utilized to overcome the capacity limitation of the conventional Single Mode Fibers (SMFs). On the other hand, Quantum Key Distribution (QKD) is an effective solution to address the security issues in 5G transport networks. In this paper, we investigate the performance of QKD over elastic optical networks with multi-core fibers and address the resource allocation problem for quantum and classical channels of QKD (QChs and CChs) and conventional data channels (DChs). To do so, we calculate the background noise caused by different noise sources and accordingly calculate the Secret Key Rate (SKR) in quantum channels. Then, we propose an Integer Linear Programming formulation and a heuristic algorithm to allocate network resources (spectrum, core, and links) to QChs, CChs, and DChs, with the objective of maximizing the secret key rate and minimizing the number of utilized frequency slots (FSs). Finally, we evaluate the proposed ILP and heuristic algorithm in terms of SKR and the number of utilized FSs. In our simulations, we consider core and metro topologies, fixed and distance adaptive launch power for classical signals, different fiber specifications, and different assumptions regarding the relative locations of quantum and classical channels in a multi-core fiber.

Anticipatory Approaches for Resource Allocation in LiFi Networks

2019 2nd West Asian Colloquium on Optical Wireless Communications (WACOWC), 2019

As a promising solution for future indoor access networks, resource allocation in Visible light c... more As a promising solution for future indoor access networks, resource allocation in Visible light communication or LiFi networks is subject to lots of researches. An interesting approach for network algorithm design is to use some knowledge about the future of the network. With this regards, the anticipatory design may improve the performance of the system in terms of delay and throughput. This paper reviews the state-of-the-art anticipatory algorithms proposed in the literature, given different prediction capabilities. The key element that all of these algorithms share is to find an event that correlates the current actions to the performance of the network in the future. Apart from the amount of possible prediction, two main approaches have been proposed for problem formulation. The optimal anticipatory resource allocation problem can be formulated in terms of a T-step look-ahead optimization problem. This approach requires more accurate information about the future of the network and yields higher performance improvements. The objective function can be adjusted to achieve high fairness and throughput or low latency in the system. On the other hand, the resource allocation algorithm can be expressed as a matching problem on a bipartite graph. The partial knowledge of the future can be added to the problem using proper heuristic preference functions.

2015 IEEE Wireless Communications and Networking Conference Workshops (WCNCW), 2015

In Cognitive Radio (CR), in order to avoid interference in the Primary Users (PU), some subcarrie... more In Cognitive Radio (CR), in order to avoid interference in the Primary Users (PU), some subcarriers need to be deactivated. In systems based on Multicarrier Code Division Multiple Access (MC-CDMA), this causes losing orthogonality among different spreading codes, leading to poor Bit Error Rate (BER) performance. The performance of such system can be improved by using Generalized Hadamard Codes (GHC) instead of conventional Hadamard codes which avoids the loss of orthogonality. This is due to the fact that unlike conventional Hadamard codes, GHC are conjectured to exist for any arbitrary length. In this paper, we propose using a novel spreading code, namely, GHC for synchronous MC-CDMA in CR networks. The new spreading codes help the CR systems maintaining the data rate while improving the performance by eliminating the orthogonality loss. Finally, the performance of the MC-CDMA system using GHC is evaluated and it is shown that the proposed system outperform the system with conventional Hadamard codes.

Scientific Reports, Feb 22, 2018

Optimal wavelength assignment in dense-wavelength-division-multiplexing (DWDM) systems that integ... more Optimal wavelength assignment in dense-wavelength-division-multiplexing (DWDM) systems that integrate both quantum and classical channels is studied. In such systems, weak quantum key distribution (QKD) signals travel alongside intense classical signals on the same fiber, where the former can be masked by the background noise induced by the latter. Here, we investigate how optimal wavelength assignment can mitigate this problem. We consider different DWDM structures and various sources of crosstalk and propose several near-optimal wavelength assignment methods that maximize the total secret key rate of the QKD channels. Our numerical results show that the optimum wavelength assignment pattern is commonly consisted of several interspersed quantum and classical bands. Using our proposed techniques, the total secret key rate of quantum channels can substantially be improved, as compared to conventional assignment methods, in the noise dominated regimes. Alternatively, we can maximize the number of QKD users supported under certain key rate constraints.

On Fundamental Limits of Energy Efficient Signaling in Visible Light Communication Systems

IEEE Wireless Communications Letters, 2023

arXiv (Cornell University), Apr 4, 2017

arXiv (Cornell University), Apr 25, 2016

In this paper, we present a multi-user resource allocation framework using fragmented-spectrum sy... more In this paper, we present a multi-user resource allocation framework using fragmented-spectrum synchronous OFDM-CDMA modulation over a frequency-selective fading channel. In particular, given pre-existing communications in the spectrum where the system is operating, a channel sensing and estimation method is used to obtain information of subcarrier availability. Given this information, some real-valued multi-level orthogonal codes, which are orthogonal codes with values of {±1, ±2, ±3, ±4, ...}, are provided for emerging new users, i.e., cognitive radio users. Additionally, we have obtained a closed form expression for bit error rate of cognitive radio receivers in terms of detection probability of primary users, CR users' sensing time and CR users' signal to noise ratio. Moreover, simulation results obtained in this paper indicate the precision with which the analytical results have been obtained in modeling the aforementioned system.

arXiv (Cornell University), Aug 29, 2015

In this paper we analyze and investigate the bit error rate (BER) performance of multiple-input m... more In this paper we analyze and investigate the bit error rate (BER) performance of multiple-input multiple-output underwater wireless optical communication (MIMO-UWOC) systems. In addition to exact BER expressions, we also obtain an upper bound on the system BER. To effectively estimate the BER expressions, we use Gauss-Hermite quadrature formula as well as approximation to the sum of log-normal random variables. We confirm the accuracy of our analytical expressions by evaluating the BER through photon-counting approach. Our simulation results show that MIMO technique can mitigate the channel turbulence-induced fading and consequently, can partially extend the viable communication range, especially for channels with stronger turbulence.

arXiv (Cornell University), Aug 17, 2015

In this paper, we analytically study the performance of multiple-input multiple-output underwater... more In this paper, we analytically study the performance of multiple-input multiple-output underwater wireless optical communication (MIMO UWOC) systems with on-off keying (OOK) modulation. To mitigate turbulence-induced fading, which is amongst the major degrading effects of underwater channels on the propagating optical signal, we use spatial diversity over UWOC links. Furthermore, the effects of absorption and scattering are considered in our analysis. We analytically obtain the exact and an upper bound bit error rate (BER) expressions for both optimal and equal gain combining. In order to more effectively calculate the system BER, we apply Gauss-Hermite quadrature formula as well as approximation to the sum of lognormal random variables. We also apply photon-counting method to evaluate the system BER in the presence of shot noise. Our numerical results indicate an excellent match between the exact and upper bound BER curves. Also a good match between the analytical results and numerical simulations confirms the accuracy of our derived expressions. Moreover, our results show that spatial diversity can considerably improve the system performance, especially for channels with higher turbulence, e.g., a 3 × 1 MISO transmission in a 25 m coastal water link with log-amplitude variance of 0.16 can introduce 8 dB performance improvement at the BER of 10 -9 .

arXiv (Cornell University), Mar 9, 2016

In this paper, we evaluate the end-to-end bit error rate (BER) of point-to-point underwater wirel... more In this paper, we evaluate the end-to-end bit error rate (BER) of point-to-point underwater wireless optical communication (UWOC) systems with multi-hop transmission. To do so, we analytically derive the BER expression of a single-hop UWOC link as the building block for end-to-end BER evaluation. We also apply photon-counting method to evaluate the system BER in the presence of shot noise. Moreover, we use Gauss-Hermite quadrature formula to obtain the closed-form solutions for the system BER in the case of log-normal underwater fading channels. Our analytical treatment involves all the impairing effects of the underwater optical channel, namely absorption, scattering and fading. Numerical results demonstrate that multihop transmission by alleviating the aforementioned impairing effects of the channel, can significantly improve the system performance and extend the viable end-to-end communication distance. For example, dual-hop transmission in 22.5 m and 45 m coastal water links can provide 17.5 dB and 39 dB performance enhancement at the BER of 10 -6 , respectively.

Optimal Placement of Access Points in Cellular Visible Light Communication Networks: An Adaptive Gradient Projection Method

IEEE Transactions on Wireless Communications, Oct 1, 2020

In this paper, a new approach toward the optimization of Access Point (AP) placement in cellular ... more In this paper, a new approach toward the optimization of Access Point (AP) placement in cellular Visible light Communication (VLC) networks is proposed based on the projected gradient algorithm. The objective of the optimal placement problem is to maximize the average throughput of the network subject to constraints on the minimum illumination level and the minimum rate of the users. This optimization framework gives the enhanced AP deployment in case of users with static, nomadic, or completely mobile behavior. Taking the distribution of users, the receiver’s field of view, reflection from walls and interference from neighboring APs into account makes the deployment problem complicated. To solve the arising intricate optimization problem, we derive analytical expressions for gradients of the objective and use them in the gradient ascent algorithm. The proposed adaptive gradient projection method then realizes the constraints. This method relaxes the projection onto a high dimensional space to planar projections, which are implemented using efficient tools from computational geometry. The proposed method eliminates the need for an intractable exhaustive search to find the optimal placement of VLC APs in cellular VLC networks, while it gives either exact optimum or very close approximations to the optimal placement. It is shown, with the aid of properties of convolution of parametric concave functions, that in some practical cases the objective function is unimodal and has no local optimum. Simulation results show a significant improvement in the throughput, SINR and outage probability of the system when the access points are deployed according to their optimized placement, determined by the algorithm. On the other hand, proper constraints on the minimum achievable rate and minimum illumination level improve the worst-case performance of the network to the desired extent.

An Analytical Framework for the Performance Evaluation of Node- and Network-Wise Operation Scenarios in Elastic Optical Networks

IEEE Transactions on Communications, May 1, 2014

ABSTRACT

arXiv (Cornell University), Dec 5, 2016

In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre... more In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre channel. Laguerre behavior is used to model vorious types of optical systems and networks such as optical amplifiers, short distance visible light communication systems with direct detection and coherent code devision multiple access (CDMA) networks. Bounds are derived for short distance visible light communication systems and coherent CDMA networks. These bounds are separated in three main cases: when both average and peak power constrants are imposed, when peak power constraint is inactive and when only peak power constraint is active. Optical intensity modulation with direct detection (IM/DD) is one of the most prevalent methods to communicate through optical channels and networks due to its simplicity in design and implementation. In these channels, information is modulated onto intensity domain and thus, all symbols have non-negative values. To find the capacity of such channels, first one should obtain the statistical expression of the channel. There has already been presented several channel statistics model for IM/DD channels such as Poisson and Gaussian intensity channels and also the capacity of these channels are investigated. In [1]-[5], upper and lower bound for discrete-time Poisson channel are proposed under different conditions. Upper and lower bounds for the capacity of the Guassian optical intensity channels are also evaluated in [6]-[10] using various methods such as sphere packing, duality approach and maxentropic method. Moser presents the capacity results of optical intensity channels with input-dependent gaussian noise under peak and average power conditions .

arXiv (Cornell University), Jan 23, 2018

Optical signal propagation through underwater channels is affected by three main degrading phenom... more Optical signal propagation through underwater channels is affected by three main degrading phenomena, namely absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path, and then examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the channel coherence time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beamcollimator at the transmitter side and aperture averaging lens at the receiver side are experimentally investigated. We show that the use of a transmitter beam-collimator and/or a receiver aperture averaging lens suits single-lobe distributions such that the generalized Gamma and exponentiated Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the channel coherence time is on the order of 10 -3 seconds and larger which implies to the slow fading turbulent channels.

arXiv (Cornell University), Dec 3, 2017

Inspired by the ever-increasing demand for higher data transmission rates and the tremendous atte... more Inspired by the ever-increasing demand for higher data transmission rates and the tremendous attention toward all-optical signal processing based on miniaturized nanophotonics, in this paper, for the first time, we investigate the integrable design of coherent ultrashort light pulse code-division multiple-access (CDMA) technique, also known as femtosecond CDMA, using all-dielectric metasurfaces (MSs). In this technique, the data bits are firstly modulated using ultrashort femtosecond optical pulses generated by mode-locked lasers, and then by employing a unique phase metamask for each data stream, in order to provide the multiple access capability, the optical signals are spectrally encoded. This procedure spreads the optical signal in the temporal domain and generates low-intensity pseudo-noise bursts through random phase coding leading to minimized multiple access interference. This paper comprehensively presents the principles and design approach to realize fundamental components of a typical femtosecond CDMA encoder, including the grating, lens, and phase mask, by employing high-contrast CMOS-compatible MSs. By controlling the interference between the provided Mie and Fabry-Perot resonance modes, we tailor the spectral and spatial responses of the impinging light locally and independently. Accordingly, we design a MS-based grating with the highest possible refracted angle and, in the meantime, the maximized efficiency which results in a reasonable diameter for the subsequent lens. Moreover, to design our MS-based lens commensurate with the spot size and distance requirements of the pursuant phase mask, we leverage a new optimization method which splits the lens structure into central and peripheral parts, and then design the peripheral part using a collection of gratings converging the impinging at the subsequent phase mask. This work can be regarded as a pioneering attempt to make a bridge between the recent tremendous advancement in the nonophotonics technology and that of optical communication networks to extend the boundaries and facilitate their effective miniaturized deployment.

In this paper, we investigate PN-sequences with ideal autocorrelation property and the consequenc... more In this paper, we investigate PN-sequences with ideal autocorrelation property and the consequences of this property on the number of +1s and -1s and run structure of sequences. We begin by discussing and surveying about the length of PNsequences with ideal autocorrelation property. From our discussion and survey we introduce circulant matrix representation of PN-sequence. Through circulant matrix representation we obtain system of non-linear equations that lead to ideal autocorrelation property. Rewriting PN-sequence and its autocorrelation property in {0, 1} leads to a definition based on Hamming weight and Hamming distance and hence we can easily prove some results on the PN-sequences with ideal autocorrelation property.

arXiv (Cornell University), Aug 17, 2015

In this paper, we characterize the performance of relay-assisted underwater wireless optical code... more In this paper, we characterize the performance of relay-assisted underwater wireless optical code division multiple access (OCDMA) networks over turbulent channels. In addition to scattering and absorption effects of underwater channels, we also consider optical turbulence as a log-normal fading coefficient in our analysis. To simultaneously and asynchronously share medium among many users, we assign a unique optical orthogonal code (OOC) to each user in order to actualize OCDMAbased underwater network. The most significant challenge in underwater optical communication is in the ability to extend the short range of its coverage. In order to expand the viable communication range, we consider multi-hop transmission to the destination. Moreover, we evaluate the performance of a relayassisted point-to-point UWOC system as a special case of the proposed relay-assisted OCDMA network. Our numerical results indicate significant performance improvement by employing intermediate relays, e.g., one can achieve 32 dB improvement in the bit error rate (BER) of 10 -6 using only a dual-hop transmission in a 90 m point-to-point clear ocean link.

Iet Optoelectronics, Dec 1, 2015

Wireless sensor networks enable a variety of application scenarios due to their flexibility, the ... more Wireless sensor networks enable a variety of application scenarios due to their flexibility, the wide range of hardware solutions, as well as their simplicity. However, new challenges arise that make applications viable, such as power consumption, robust communication, limited storage and processing. One of the main elements among communication and power consumption optimization aspects is related to the protocol stack and especially the medium access layer, due to the nature of the wireless environment. Thus, this article proposes a multi-channel mechanism for asynchronous receiver-initiated MAC for low-power WSN. The proposed protocol reduces energy consumption due to message contention and idle listening through an initial recognition mechanism and an efficient multichannel mechanism based on knowledge of the best channel in each node of the network. For validation of the protocol, evaluations were made for consumption, latency and rate of delivery of the network in a web application, comparing the multichannel asynchronous protocol to the synchronous protocol. The results show that the synchronous protocol presents slight superiority in communication performance, whereas the asynchronous protocol presents lower consumption.

arXiv (Cornell University), Dec 5, 2016

In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre... more In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre channel. Laguerre behavior is used to model various types of optical systems and networks such as optical amplifiers, short distance visible light communication systems with direct detection and coherent code division multiple access (CDMA) networks. Bounds are derived for short distance visible light communication systems and coherent CDMA networks. These bounds are separated in three main cases: when both average and peak power constraints are imposed, when peak power constraint is inactive and when only peak power constraint is active. Optical intensity modulation with direct detection (IM/DD) is one of the most prevalent methods to communicate through optical channels and networks due to its simplicity in design and implementation. In these channels, information is modulated onto intensity domain and thus, all symbols have non-negative values. To find the capacity of such channels, first one should obtain the statistical expression of the channel. There has already been presented several channel statistics model for IM/DD channels such as Poisson and Gaussian intensity channels and also the capacity of these channels are investigated. In [1]-[5], upper and lower bound for discrete-time Poisson channel are proposed under different conditions. Upper and lower bounds for the capacity of the Guassian optical intensity channels are also evaluated in [6]-[10] using various methods such as sphere packing, duality approach and maxentropic method. Moser presents the capacity results of optical intensity channels with input-dependent gaussian noise under peak and average power conditions .

arXiv: Information Theory, 2016

In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre... more In this paper, new upper and lower bounds are proposed for the capacity of discrete-time Laguerre channel. Laguerre behavior is used to model various types of optical systems and networks such as optical amplifiers, short distance visible light communication systems with direct detection and coherent code division multiple access (CDMA) networks. Bounds are derived for short distance visible light communication systems and coherent CDMA networks. These bounds are separated in three main cases: when both average and peak power constraints are imposed, when peak power constraint is inactive and when only peak power constraint is active.

Resource Allocation in Space Division Multiplexed Elastic Optical Networks Secured With Quantum Key Distribution

IEEE Journal on Selected Areas in Communications, 2021

Elastic Optical Network (EON) is a promising solution to address the high capacity, low latency, ... more Elastic Optical Network (EON) is a promising solution to address the high capacity, low latency, and flexibility requirements of the upcoming 5th-generation (5G) networks. Furthermore, Multi-Core Fibers (MCFs) and Space Division Multiplexing (SDM) technique can be utilized to overcome the capacity limitation of the conventional Single Mode Fibers (SMFs). On the other hand, Quantum Key Distribution (QKD) is an effective solution to address the security issues in 5G transport networks. In this paper, we investigate the performance of QKD over elastic optical networks with multi-core fibers and address the resource allocation problem for quantum and classical channels of QKD (QChs and CChs) and conventional data channels (DChs). To do so, we calculate the background noise caused by different noise sources and accordingly calculate the Secret Key Rate (SKR) in quantum channels. Then, we propose an Integer Linear Programming formulation and a heuristic algorithm to allocate network resources (spectrum, core, and links) to QChs, CChs, and DChs, with the objective of maximizing the secret key rate and minimizing the number of utilized frequency slots (FSs). Finally, we evaluate the proposed ILP and heuristic algorithm in terms of SKR and the number of utilized FSs. In our simulations, we consider core and metro topologies, fixed and distance adaptive launch power for classical signals, different fiber specifications, and different assumptions regarding the relative locations of quantum and classical channels in a multi-core fiber.

Anticipatory Approaches for Resource Allocation in LiFi Networks

2019 2nd West Asian Colloquium on Optical Wireless Communications (WACOWC), 2019

As a promising solution for future indoor access networks, resource allocation in Visible light c... more As a promising solution for future indoor access networks, resource allocation in Visible light communication or LiFi networks is subject to lots of researches. An interesting approach for network algorithm design is to use some knowledge about the future of the network. With this regards, the anticipatory design may improve the performance of the system in terms of delay and throughput. This paper reviews the state-of-the-art anticipatory algorithms proposed in the literature, given different prediction capabilities. The key element that all of these algorithms share is to find an event that correlates the current actions to the performance of the network in the future. Apart from the amount of possible prediction, two main approaches have been proposed for problem formulation. The optimal anticipatory resource allocation problem can be formulated in terms of a T-step look-ahead optimization problem. This approach requires more accurate information about the future of the network and yields higher performance improvements. The objective function can be adjusted to achieve high fairness and throughput or low latency in the system. On the other hand, the resource allocation algorithm can be expressed as a matching problem on a bipartite graph. The partial knowledge of the future can be added to the problem using proper heuristic preference functions.

2015 IEEE Wireless Communications and Networking Conference Workshops (WCNCW), 2015

In Cognitive Radio (CR), in order to avoid interference in the Primary Users (PU), some subcarrie... more In Cognitive Radio (CR), in order to avoid interference in the Primary Users (PU), some subcarriers need to be deactivated. In systems based on Multicarrier Code Division Multiple Access (MC-CDMA), this causes losing orthogonality among different spreading codes, leading to poor Bit Error Rate (BER) performance. The performance of such system can be improved by using Generalized Hadamard Codes (GHC) instead of conventional Hadamard codes which avoids the loss of orthogonality. This is due to the fact that unlike conventional Hadamard codes, GHC are conjectured to exist for any arbitrary length. In this paper, we propose using a novel spreading code, namely, GHC for synchronous MC-CDMA in CR networks. The new spreading codes help the CR systems maintaining the data rate while improving the performance by eliminating the orthogonality loss. Finally, the performance of the MC-CDMA system using GHC is evaluated and it is shown that the proposed system outperform the system with conventional Hadamard codes.

Scientific Reports, Feb 22, 2018

Optimal wavelength assignment in dense-wavelength-division-multiplexing (DWDM) systems that integ... more Optimal wavelength assignment in dense-wavelength-division-multiplexing (DWDM) systems that integrate both quantum and classical channels is studied. In such systems, weak quantum key distribution (QKD) signals travel alongside intense classical signals on the same fiber, where the former can be masked by the background noise induced by the latter. Here, we investigate how optimal wavelength assignment can mitigate this problem. We consider different DWDM structures and various sources of crosstalk and propose several near-optimal wavelength assignment methods that maximize the total secret key rate of the QKD channels. Our numerical results show that the optimum wavelength assignment pattern is commonly consisted of several interspersed quantum and classical bands. Using our proposed techniques, the total secret key rate of quantum channels can substantially be improved, as compared to conventional assignment methods, in the noise dominated regimes. Alternatively, we can maximize the number of QKD users supported under certain key rate constraints.