### Recent Submissions

• #### 375-nm ultraviolet-laser based non-line-of-sight underwater optical communication

(The Optical Society, 2018-05-04)
For circumventing the alignment requirement of line-of-sight (LOS) underwater wireless optical communication (UWOC), we demonstrated a non-line-of-sight (NLOS) UWOC link adequately enhanced using ultraviolet (UV) 375-nm laser. Path loss was chosen as a figure-of-merit for link performance in this investigation, which considers the effects of geometries, water turbidity, and transmission wavelength. The experiments suggest that path loss decreases with smaller azimuth angles, higher water turbidity, and shorter wavelength due in part to enhanced scattering utilizing 375-nm radiation. We highlighted that it is feasible to extend the current findings for long distance NLOS UWOC link in turbid water, such as harbor water.
• #### Visible light communication using DC-biased optical filter bank multi-carrier modulation

(IEEE, 2018-03-19)
Filter bank multicarrier (FBMC) has become a promising candidate to replace conventional orthogonal frequency-division multiplexing (OFDM) scheme in 5G technology due to its better spectral confinement which results in a reduced inter-channel interference (ICI). However, the viability of using FBMC in visible light communication has not been verified. In this work we present the first experimental validation of the DC-biased optical filter bank multicarrier (DCO-FBMC) modulation scheme over a free-space optical channel. Under different receiving powers, up to three times bit error rate performance improvement has been achieved using DCO-FBMC with different overlapping factors compared to that of conventional DCO-OFDM.
• #### Free-space optical channel characterization and experimental validation in a coastal environment

(The Optical Society, 2018-03-05)
Over the years, free-space optical (FSO) communication has attracted considerable research interest owing to its high transmission rates via the unbounded and unlicensed bandwidths. Nevertheless, various weather conditions lead to significant deterioration of the FSO link capabilities. In this context, we report on the modelling of the channel attenuation coefficient (β) for a coastal environment and related ambient, considering the effect of coastal air temperature (T), relative humidity (RH) and dew point (TD) by employing a mobile FSO communication system capable of achieving a transmission rate of 1 Gbps at an outdoor distance of 70 m for optical beam wavelengths of 1310 nm and 1550 nm. For further validation of the proposed models, an indoor measurement over a 1.5 m distance utilizing 1310 nm, 1550 nm, and 1064 nm lasers was also performed. The first model provides a general link between T and β, while the second model provides a relation between β, RH as well as TD. By validating our attenuation coefficient model with actual outdoor and indoor experiments, we obtained a scaling parameter x and decaying parameter c values of 19.94, 40.02, 45.82 and 0.03015, 0.04096, 0.0428 for wavelengths of 1550, 1310, 1064 nm, respectively. The proposed models are well validated over the large variation of temperature and humidity over the FSO link in a coastal region and emulated indoor environment.
• #### Delivery Time Minimization in Edge Caching: Synergistic Benefits of Subspace Alignment and Zero Forcing

(arXiv, 2017-10-29)
An emerging trend of next generation communication systems is to provide network edges with additional capabilities such as additional storage resources in the form of caches to reduce file delivery latency. To investigate this aspect, we study the fundamental limits of a cache-aided wireless network consisting of one central base station, $M$ transceivers and $K$ receivers from a latency-centric perspective. We use the normalized delivery time (NDT) to capture the per-bit latency for the worst-case file request pattern at high signal-to-noise ratios (SNR), normalized with respect to a reference interference-free system with unlimited transceiver cache capabilities. For various special cases with $M=\{1,2\}$ and $K=\{1,2,3\}$ that satisfy $M+K\leq 4$, we establish the optimal tradeoff between cache storage and latency. This is facilitated through establishing a novel converse (for arbitrary $M$ and $K$) and an achievability scheme on the NDT. Our achievability scheme is a synergistic combination of multicasting, zero-forcing beamforming and interference alignment.
• #### 71-Mbit/s ultraviolet-B LED communication link based on 8-QAM-OFDM modulation

(The Optical Society, 2017-09-14)
A demonstration of ultraviolet-B (UVB) communication link is implemented utilizing quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM). The demonstration is based on a 294-nm UVB-light-emitting-diode (UVB-LED) with a full-width at half-maximum (FWHM) of 9 nm and light output power of 190 μW, at 7 V, with a special silica gel lens on top of it. A −3-dB bandwidth of 29 MHz was measured and a high-speed near-solar-blind communication link with a data rate of 71 Mbit/s was achieved using 8-QAM-OFDM at perfect alignment. 23.6 Mbit/s using 2-QAM-OFDM when the angle subtended by the pointing directions of the UVB-LED and photodetector (PD) is 12 degrees, thus establishing a diffuse-line-of-sight (LOS) link. The measured bit-error rate (BER) of 2.8 × 10−4 and 2.4 × 10−4, respectively, are well below the forward error correction (FEC) criterion of 3.8 × 10−3. The demonstrated high data-rate OFDM-based UVB communication link paves the way for realizing high-speed non-line-of-sight free-space optical communications.
• #### Design and Deployment of Mobile FSO Communication System

(OSA, 2017-05-08)
As a potential solution to many applications, we developed a mobile free-space optical (FSO) system that achieves 1Gbps with transmission distance of 70 m. This system needs minimal preparation to be deployed within an hour.
• #### Performance Evaluation of Underwater Wireless Optical Communications Links in the Presence of Different Air Bubble Populations

(Institute of Electrical and Electronics Engineers (IEEE), 2017-03-16)
We experimentally evaluate the performance of underwater wireless optical communication (UWOC) links in the presence of different air bubbles. Air bubbles of different sizes and densities are generated by using an air pipe in conjunction with a submersible water pump of variable flow rate that help break up large bubbles into smaller bubbles. Received signal intensity measurements show that bubbles significantly degrade the performance of UWOC links. Large bubbles completely obstruct the optical beam and cause a deep fade. However, as the bubble size decreases, the level of deep fade also decreases because the optical beam is less susceptible to complete obstruction and more light reaches the detector. We also show that beam expansion could help mitigate the performance degradation due to the deep fade caused by air bubbles scatters in the channel.
• #### 20-meter underwater wireless optical communication link with 15 Gbps data rate

(The Optical Society, 2016-10-24)
The video streaming, data transmission, and remote control in underwater call for high speed (Gbps) communication link with a long channel length (âˆ¼10 meters). We present a compact and low power consumption underwater wireless optical communication (UWOC) system utilizing a 450-nm laser diode (LD) and a Si avalanche photodetector. With the LD operating at a driving current of 80 mA with an optical power of 51.3 mW, we demonstrated a high-speed UWOC link offering a data rate up to 2 Gbps over a 12-meter-long, and 1.5 Gbps over a record 20-meter-long underwater channel. The measured bit-error rate (BER) are 2.8 Ã— 10-5, and 3.0 Ã— 10-3, respectively, which pass well the forward error correction (FEC) criterion. Â© 2016 Optical Society of America.
• #### Green Cooperative Spectrum Sensing and Scheduling in Heterogeneous Cognitive Radio Networks

(Institute of Electrical and Electronics Engineers (IEEE), 2016-09-12)
In this paper, we consider heterogeneous cognitive radio networks (CRNs) comprising primary channels (PCs) with heterogeneous characteristics and secondary users (SUs) with various sensing and reporting qualities for different PCs. We first define the opportunity as the achievable total data rate and its cost as the energy consumption caused from sensing, reporting, and channel switching operations and formulate a joint spectrum discovery and energy efficiency objective to minimize the energy spent per unit of data rate. Then, a mixed integer nonlinear programming problem is formulated to determine: 1) the optimal subset of PCs to be scheduled for sensing; 2) the SU assignment set for each scheduled PC; and 3) sensing durations and detection thresholds of each SU on PCs it is assigned to sense. Thereafter, an equivalent convex framework is developed for specific instances of the above combinatorial problem. For comparison, optimal detection and sensing thresholds are also derived analytically under the homogeneity assumption. Based on these, a prioritized ordering heuristic is developed to order channels under the spectrum, energy, and spectrum-energy limited regimes. After that, a scheduling and assignment heuristic is proposed and is shown to perform very close to the exhaustive optimal solution. Finally, the behavior of the CRN is numerically analyzed under these regimes with respect to different numbers of SUs, PCs, and sensing qualities.
• #### On the Performance Analysis of Dual-Hop FSO Fixed Gain Transmission Systems

(2015-12-12)
Novel exact closed-form results for the end-to-end performance analysis of dual-hop free-space optical (FSO) fixed-gain relaying systems under heterodyne detection as well as intensity modulation with direct detection techniques in the presence of atmospheric turbulence as well as pointing errors are presented. By using dual-hop FSO relaying, we demonstrate a better system performance relative to the single FSO link. Numerical and Monte-Carlo simulation results are provided to verify the accuracy of the newly proposed results, and a perfect agreement is observed.
• #### 2.3 Gbit/s underwater wireless optical communications using directly modulated 520 nm laser diode

(The Optical Society, 2015-07-30)
We experimentally demonstrate a record high-speed underwater wireless optical communication (UWOC) over 7 m distance using on-off keying non-return-to-zero (OOK-NRZ) modulation scheme. The communication link uses a commercial TO-9 packaged pigtailed 520 nm laser diode (LD) with 1.2 GHz bandwidth as the optical transmitter and an avalanche photodiode (APD) module as the receiver. At 2.3 Gbit/s transmission, the measured bit error rate of the received data is 2.23×10−4, well below the forward error correction (FEC) threshold of 2×10−3 required for error-free operation. The high bandwidth of the LD coupled with high sensitivity APD and optimized operating conditions is the key enabling factor in obtaining high bit rate transmission in our proposed system. To the best of our knowledge, this result presents the highest data rate ever achieved in UWOC systems thus far.
• #### Beaconless Georouting Under The Spotlight: Practical Link Models and Application Scenarios

(Institute of Electrical and Electronics Engineers (IEEE), 2015-06-18)
Beaconless georouting has emerged as a viable packetforwarding technique in distributed wireless networks, particularly for applications requiring portability and scalability. In this paper, we focus on fine-tuning and developing the analytical tools associated with the study of beaconless georouting protocols. For instance, they have been traditionally analyzed and simulated from the perspective of a single hop only. However, end-to-end performance analysis is instrumental when considering practical application scenarios. Furthermore, beaconless georouting protocols have been studied in literature assuming equal communication ranges for the data and control packets. In reality, this is not true since the communication range is actually a function of the packet length (among other factors). Control packets are typically much shorter than data packets. As a consequence, a substantial discrepancy exists in practice between their respective communication ranges, causing many data packet drops. Accordingly, we introduce two simple strategies for bridging the gap between the control and data packet communication ranges. Our primary objective in this paper is to construct a realistic analysis describing the end-to-end performance of beaconless georouting protocols. Two flagship protocols are selected in this paper for further investigation. For a better perspective, the two protocols are actually compared to a hypothetical limit case, one which offers optimal energy and latency performance. Finally, we present four different application scenarios. For each scenario, we highlight the georouting protocol which performs the best and discuss the reasons behind it. © 2007-2012 IEEE.
• #### Statistics of α-μ Random Variables and Their Applications inWireless Multihop Relaying and Multiple Scattering Channels

(Institute of Electrical and Electronics Engineers (IEEE), 2015-06-01)
Exact results for the probability density function (PDF) and cumulative distribution function (CDF) of the sum of ratios of products (SRP) and the sum of products (SP) of independent α-μ random variables (RVs) are derived. They are in the form of 1-D integral based on the existing works on the products and ratios of α-μ RVs. In the derivation, generalized Gamma (GG) ratio approximation (GGRA) is proposed to approximate SRP. Gamma ratio approximation (GRA) is proposed to approximate SRP and the ratio of sums of products (RSP). GG approximation (GGA) and Gamma approximation (GA) are used to approximate SP. The proposed results of the SRP can be used to calculate the outage probability (OP) for wireless multihop relaying systems or multiple scattering channels with interference. The proposed results of the SP can be used to calculate the OP for these systems without interference. In addition, the proposed approximate result of the RSP can be used to calculate the OP of the signal-To-interference ratio (SIR) in a multiple scattering system with interference. © 1967-2012 IEEE.
• #### Resource allocation in heterogeneous cloud radio access networks: advances and challenges

(Institute of Electrical and Electronics Engineers (IEEE), 2015-06)
Base station densification is increasingly used by network operators to provide better throughput and coverage performance to mobile subscribers in dense data traffic areas. Such densification is progressively diffusing the move from traditional macrocell base stations toward heterogeneous networks with diverse cell sizes (e.g., microcell, picocell, femotcell) and diverse radio access technologies (e.g., GSM, CDMA), and LTE). The coexistence of the different network entities brings an additional set of challenges, particularly in terms of the provisioning of high-speed communications and the management of wireless interference. Resource sharing between different entities, largely incompatible in conventional systems due to the lack of interconnections, becomes a necessity. By connecting all the base stations from different tiers to a central processor (referred to as the cloud) through wire/wireline backhaul links, the heterogeneous cloud radio access network, H-CRAN, provides an open, simple, controllable, and flexible paradigm for resource allocation. This article discusses challenges and recent developments in H-CRAN design. It proposes promising resource allocation schemes in H-CRAN: coordinated scheduling, hybrid backhauling, and multicloud association. Simulations results show how the proposed strategies provide appreciable performance improvement compared to methods from recent literature. © 2015 IEEE.
• #### Receive antenna selection for underlay cognitive radio with instantaneous interference constraint

(Institute of Electrical and Electronics Engineers (IEEE), 2015-06)
Receive antenna selection is a low complexity scheme to reap diversity benefits.We analyze the performance of a receive antenna selection scheme in spectrum sharing systems where the antenna that results in highest signal-to-interference plus noise ratio at the secondary receiver is selected to improve the performance of secondary transmission. Exact and asymptotic behaviours of the received SINR are derived for both general and interference limited scenarios over general fading environment. These results are then applied to the outage and average bit error rate analysis when the secondary transmitter changes the transmit power in finite discrete levels to satisfy the instantaneous interference constraint at the primary receiver.
• #### Primary user localisation and uplink resource allocation in orthogonal frequency division multiple access cognitive radio systems

(Institution of Engineering and Technology (IET), 2015-05-21)
In cognitive radio networks, secondary users (SUs) can share spectrum with primary users (PUs) under the condition that no interference is caused to the PUs. To evaluate the interference imposed to the PUs, the cognitive systems discussed in the literature usually assume that the channel state information (CSI) of the link from a secondary transmitter to a primary receiver (interference link) is known at the secondary transmitter. However, this assumption may often be impractical in cognitive radio systems, since the PUs need to be oblivious to the presence of the SUs. The authors first discuss PU localisation and then introduce an uplink resource allocation algorithm for orthogonal frequency division multiple access-based cognitive radio systems, where relative location information between primary and SUs is used instead of CSI of the interference link to estimate the interference. Numerical and simulation results show that it is indeed effective to use location information as a part of resource allocation and thus a near-optimal capacity is achieved. © The Institution of Engineering and Technology 2015.
• #### Performance of BICM-based QAM-SIM OWC over gamma-gamma turbulence channels

(Institute of Electrical and Electronics Engineers (IEEE), 2015-05)
We derive a series expression for the pair-wise error probability (PEP) of bit interleaved coded modulation (BICM)-based subcarrier intensity modulation (SIM) optical wireless communication (OWC) system employing M^{2}-ary quadrature amplitude modulation ( M^{2} -QAM) over the Gamma-Gamma turbulence channels. Using this expression, we develop an upper bound (UB) to predict the bit-error rate performance of such system. Simulation results are presented to verify the analytical results. We also develop an asymptotic UB which reveals that the diversity order depends on the smaller channel parameter and the free distance of the convolutional code. © 1997-2012 IEEE.
• #### Simultaneous Wireless Information and Power Transfer for Decode-and-Forward MIMO Relay Communication Systems

(IEEE, 2015-05)
In this paper, we investigate the simultaneous wireless information and power transfer (SWIPT) for a decode-and-forward (DF) multiple-input multiple-output (MIMO) relay system where the relay is an energy harvesting node. We consider the ideal scenario where both the energy harvesting (EH) receiver and information decoding (ID) receiver at the relay have access to the whole received signal and its energy. The relay harvests the energy while receiving the signal from the source and uses the harvested power to forward the signal to the destination. We obtain the optimal precoders at the source and the relay to maximize the achievable throughput rate of the overall link. In the numerical results, the effect of the transmit power at the source and the position of the relay between the source and the destination on the maximum achievable rate are investigated. © 2015 IEEE.
• #### Asymptotic Ergodic Capacity Analysis of Composite Lognormal Shadowed Channels

(Institute of Electrical and Electronics Engineers (IEEE), 2015-05)
Capacity analysis of composite lognormal (LN) shadowed links, such as Rician-LN, Gamma-LN, and Weibull-LN, is addressed in this work. More specifically, an exact closed-form expression for the moments of the end-to-end signal-to-noise ratio (SNR) of a single composite link transmission system is presented in terms of well- known elementary functions. Capitalizing on these new moments expressions, we present asymptotically tight lower bounds for the ergodic capacity at high SNR. All the presented results are verified via computer-based Monte-Carlo simulations. © 2015 IEEE.
• #### Performance Analysis of FSO Links over Unified Gamma-Gamma Turbulence Channels

(Institute of Electrical and Electronics Engineers (IEEE), 2015-05)
In this work, we present a unified performance analysis of a free-space optical (FSO) link that accounts for pointing errors and both types of detection techniques (i.e. intensity modulation/direct detection as well as heterodyne detection). We, then, present unified exact closed-form expressions for various statistical characteristics of the end-to-end signal-to-noise ratio (SNR) of a single link FSO transmission system. We capitalize on these unified results to offer unified exact closed-form expressions for various performance metrics. Additionally, we derive the asymptotic results for all the expressions derived earlier in the high SNR regime in terms of simple elementary functions. We also derive new asymptotic expressions for the ergodic capacity in the low as well as high SNR regimes in terms of simple elementary functions via utilizing moments. All the presented results are verified via computer-based Monte-Carlo simulations. © 2015 IEEE.