[Show abstract][Hide abstract] ABSTRACT: This paper investigates the performance of different precoding schemes for a multiuser MIMO VLC system with channel estimation errors, an assumption that is commonly neglected in the literature. In particular, dirty paper coding, channel inversion, and block diagonalization, are considered for interference mitigation under imperfect channel state information. The impact of the variation of the beam angles of the transmitters and the field of view (FOV) of the receivers on the system performance is also examined. Simulation results reveal that, dirty paper coding provides the best performance under perfect channel state information (CSI). However, under imperfect CSI, suboptimal linear precoding schemes will give better performance. Furthermore, tuning the transmitting angles and the FOVs can significantly improve the system performance.
[Show abstract][Hide abstract] ABSTRACT: Dynamic power allocation and power splitting, in a multicarrier two-hop link with a wireless powered relay, is investigated. We first formulate the corresponding optimization problem, which consists of the joint optimization -in terms of achievable rate- of, 1) the dynamic power allocation among multiple channels and, 2) the selection of the power splitting ratio between information processing and energy harvesting at the relay, when amplify-and-forward is applied. This is a non-convex optimization problem, which is mapped to a convex one and optimally solved using one-dimensional search and dual decomposition, while a suboptimal efficient iterative method is also proposed. Simulations reveal a significant increase in the throughput, when comparing the proposed approach with two alternative power allocation schemes, while they verify the effectiveness of the fast-converging iterative solution.
No preview · Article · Aug 2015 · IEEE Wireless Communication Letters
[Show abstract][Hide abstract] ABSTRACT: This paper presents an adaptive approach to address the two main problems associated with the time varying Doppler shift, the first being the acceleration effects on the cyclic-prefix (CP) correlation and the second, the effect of a sudden change in the velocity direction between packets on the entire orthogonal frequency division multiplexing (OFDM) symbols. In addition, this paper considers the residual Doppler shift or carrier frequency offset (CFO) that was estimated iteratively within a range according to a design based on the sub-carrier spacing using pilots, which are basically utilized for the purpose of channel estimation. Furthermore, the proposed algorithm adopts three estimations of the symbol timing offset. These estimations are centroid-based localization over an anticipated CP window, first order expectation and autocorrelation of the received CP with its replica. Subsequently, a penalization algorithm is applied in order to drop the anomalous parameter among them. Therefore, the consequences of the inflection point that accompanies the abrupt change in the velocity are mitigated and a reliable time varying Doppler shift is obtained. This Doppler shift is fine tuned in an iterative manner. Compared with the block-based Doppler compensation approach, the proposed technique works with variable speed during packet duration. In addition, it exploits the available bandwidth more efficiently by utilizing a single preamble of linear-frequency-modulation (LFM) to detect the start of the packet. The proposed receiver was evaluated through simulations and sea trials conducted over 500 m and 1000 m channel ranges. In simulations, a model was designed to imitate the time varying Doppler shift with two scenarios (expansion/compression) in combination with a various multipath delay spread. The simulation results confirm that the proposed system accommodates an acceleration up to m/s2 during the packet time.
[Show abstract][Hide abstract] ABSTRACT: Novel analytic solutions are derived for integrals that involve the
generalized Marcum Q-function, exponential functions and arbitrary powers.
Simple closed-form expressions are also derived for the specific cases of the
generic integrals. The offered expressions are both convenient and versatile,
which is particularly useful in applications relating to natural sciences and
engineering, including wireless cpmmunications and signal processing. To this
end, they are employed in the derivation of the channel capacity for fixed rate
and channel inversion in the case of correlated multipath fading and switched
Full-text · Article · May 2015 · IEEE Signal Processing Letters
[Show abstract][Hide abstract] ABSTRACT: In this paper, a study is conducted into using an auxiliary ring to enhance the captured (confined) electric field at the gap of dipole nanoantenna for the use in solar rectennas. The system is constructed of main resonator, the dipole antenna, and a booster resonator, the auxiliary ring. It is generally accepted that the dipole antenna captures limited amount of solar radiation. Thus, boosting its performance with an additional resonant element will increase the captured electric field at its gap, and hence, increase the conversion efficiency of the solar rectenna. The designed structure has been simulated using FEM-based software. The results showed that the performance of the rectenna can be improved by adding an additional resonator. This can be achieved by exploiting the mutual coupling between the two elements. Furthermore, the effect of the distance between the ring and the dipole has been studied. Finally, different shapes of the ring have been used and it is found that the circular ring has the best performance.
[Show abstract][Hide abstract] ABSTRACT: In this paper, a new receiver structure is proposed for uplink interleaved division multiple access (IDMA) on multipath fading channels. In a conventional IDMA rake detector, jointly removing inter symbol interference (ISI) and multiple access interference (MAI) is achieved by exchanging extrinsic log-likelihood ratio (LLR) chips between an elementary signal estimator (ESE) and a posteriori probability decoders (DEC)s in a turbo-like manner. The same exchanging principle takes place between a centralized decision feedback equalizer (CDFE) and the DECs in CDFE-IDMA systems. However, the proposed IDMA receiver can be considered a mixture between the two mentioned receivers as it employs an adaptive DFE for each user to remove the ISI effects while MAI is eliminated by a parallel interference canceller (PIC). The PIC utilizes the same ESE principles in removing MAI effects from received symbols which is more efficient than cross-over filters in a CDFE-IDMA system. Moreover, the ISI abstraction in DFE is more active in removing ISI than ESE. Simulation results demonstrate the superiority of DFE-IDMA over the other systems' performances found in the literature for both optimal and adaptive algorithms (AA).
[Show abstract][Hide abstract] ABSTRACT: Energy harvesting has been recognized as a key technique for improving the performance of energy constrained networks. In this paper, the application of wireless information and power transfer, an emerging concept of energy harvesting, to wireless cooperative networks is considered. Particularly the focus is on a cooperative network in which a source communicates with a destination with the help of relays and the relays are randomly deployed according to homogeneous Poisson point process. According to the available channel state information, three different strategies to use the available relays are studied, and their impact on the outage probability and diversity gain is characterized by applying stochastic geometry. Particularly the developed analytical results demonstrate that the use of energy harvesting relays does not degrade the achievable diversity gain of cooperative protocols, but has a deteriorating effect on the outage performance. Simulation results are provided to confirm the accuracy of the developed analytical results and facilitate a better performance comparison.
[Show abstract][Hide abstract] ABSTRACT: This paper studies different secrecy rate optimization problems for a multiple-input–multiple-output (MIMO) secrecy channel. In particular, we consider a scenario where a communication through a MIMO channel is overheard by a multiple-antenna eavesdropper. In this secrecy network, we first investigate two secrecy rate optimization problems: 1) power minimization and 2) secrecy rate maximization. These optimization problems are not convex due to the nonconvex secrecy rate constraint. However, by approximating this secrecy rate constraint based on Taylor series expansion, we propose iterative algorithms to solve these secrecy rate optimization problems. In addition, we provide the convergence analysis for the proposed algorithms. These iterative optimization approaches are developed under the assumption that the transmitter has perfect channel state information. However, there are practical difficulties in having perfect channel state information at the transmitter. Hence, robust secrecy rate optimization techniques based on the worst-case secrecy rate are considered by incorporating channel uncertainties. By exploiting the S-Procedure, we show that these robust optimization problems can be formulated into semidefinite programming at low signal-to-noise ratios (SNRs). Simulation results have been provided to validate the convergence of the proposed algorithms. In addition, numerical results show that the proposed robust optimization techniques outperform the nonrobust schemes in terms of the worst-case secrecy rates and the achieved secrecy rates.
Full-text · Article · May 2014 · IEEE Transactions on Vehicular Technology
[Show abstract][Hide abstract] ABSTRACT: In this paper, the application of wireless information and power transfer to
cooperative networks is investigated, where the relays in the network are
randomly located and based on the decode-forward strategy. For the scenario
with one source-destination pair, three different strategies for using the
available relays are studied, and their impact on the outage probability and
diversity gain is characterized by applying stochastic geometry. By using the
assumptions that the path loss exponent is two and that the relay-destination
distances are much larger than the source-relay distances, closed form
analytical results can be developed to demonstrate that the use of energy
harvesting relays can achieve the same diversity gain as the case with
conventional self-powered relays. For the scenario with multiple sources, the
relays can be viewed as a type of scarce resource, where the sources compete
with each other to get help from the relays. Such a competition is modeled as a
coalition formation game, and two distributed game theoretic algorithms are
developed based on different payoff functions. Simulation results are provided
to confirm the accuracy of the developed analytical results and facilitate a
better performance comparison.
Full-text · Article · Mar 2014 · IEEE Transactions on Wireless Communications
[Show abstract][Hide abstract] ABSTRACT: A new cost efficient automated planning and optimization method is proposed for OFDMA future-generation cellular networks targeting throughput maximization. The mathematical formulation is a non-linear multi-objective optimization problem subject to minimum interference, cost and similar resource constraints at each cell within a defined heterogeneous traffic environment. The fundamental objective is to maximize the individual cell throughput without deteriorating it over other cells, which results in a throughput equilibrium maximization over multiple cells. This implicitly implies traffic and co-channel interference congestion avoidance across the network whilst maintaining both cost efficiency and quality of service (QoS) policies. Optimal solution existence is subject to the network size, traffic and computational complexity constraints which converges to a throughput equilibrium or alternatively to the well known Nash Equilibrium (NE).
Full-text · Article · Mar 2014 · IEEE Transactions on Wireless Communications
[Show abstract][Hide abstract] ABSTRACT: Resource allocation for a downlink orthogonal frequency division multiple access (OFDMA) system ensures that capacity and throughput are maximized. In the literature, either channel coding is not considered or only a fixed code rate is specified. To overcome this problem, an optimized cross-layer resource allocation strategy that distributes the user, bit and power optimally has been proposed for adaptive modulation and coding (AMC) based single-cell downlink OFDMA systems. The proposed strategy, which is referred to as optimized user, bit and power allocation (OUBPA), considers throughput as a new cost function in terms of spectral efficiency and bit-error rate (BER) to allocate the available resources. An optimized approach is presented for solving the throughput maximization problem and simulation results demonstrating that the throughput performance of the OFDMA-AMC system using OUBPA strategy outperforms conventional techniques.
[Show abstract][Hide abstract] ABSTRACT: In this paper, a study is conducted into optimization of quantum and coupling efficiency of solar rectennas. The optical antenna impedance is calculated using Method of Moment, whereas the Metal/Insulator/Metal (MIM) diode characteristics are calculated using Simmons' formula of tunneling current. The diode resistance and the responsivity have been determined based on the I–V characteristics of the MIM diode. Additionally, the quantum and coupling efficiency have been calculated from the diode and antenna resistances as well as the diode responsivity. Furthermore, both efficiencies have been optimized by finding the optimum values of the insulator layer thickness and the metal work function difference.
[Show abstract][Hide abstract] ABSTRACT: The exploitation of already deployed wireless local area networks
(WLAN)s (e.g., WiFi access points (AP)s) has attracted considerable
attention, as an efficient and practical method to improve the
performance of beyond 4G wireless networks. In this paper, we propose a
novel communication paradigm to satisfy the performance demands of
future wireless networks: a hybrid Cellular/WLAN network architecture
with wireless offloading. In contrast to the commonly adopted practice
of WiFi offloading, where the WLAN APs have a wired backhaul (e.g.,
Digital Subscriber Line), we propose a wireless offloading approach,
where the WLAN APs will share their wireless cellular broadband
connection with other users. These users will select their serving node,
i.e., the macro-cell eNodeB or a WLAN AP, based on a certain selection
criterion. Thus a challenging research field is originated, where
interfering effects and wireless resources limitations play a dominant
role. Important performance metrics of the proposed hybrid scheme,
including the bit error probability, the ergodic capacity and the
average signal-to-interference-plus noise ratio, are theoretically
studied and closed form expressions are derived for the single-user case
with multiple interferers, for both identical and non-identical fading
conditions. Also, based on the general multi-cellular hybrid
WLAN-Cellular concept, we first propose a intercell interference
minimization approach. Then we present a novel scheme for achieving
frequency reuse equal to one within a single macro-cell, under specific
performance criteria and constraints, that guarantee the overall cell or
the individual user QoS requirements.
[Show abstract][Hide abstract] ABSTRACT: In this paper, an investigation is presented into nanogap-based bowtie nanoarrays for THz energy detection, with the aim of optimizing their geometrical parameters utilizing finite-element method (FEM) based simulations. The bowtie elements of the array are connected by feeding lines, which are used to transfer the captured electric field away from the antenna's center to a common feeding gap. The performance of the bowtie nanoarray has been compared with a single bowtie element constructed using the same device area. The obtained results demonstrated that the array outperforms the single element bowtie. Subsequently, a parametric study is carried out on important geometrical parameters of the array to optimize its performance. The results demonstrate that the optimum spacing between array elements is 2.9 m, whereas the best line width of the feeding lines is 50 nm. Additionally, it has been found that a 25 nm gap offers the highest electric field at resonance. Furthermore, a bowtie nanoarray is designed based on the optimized parameters of the parametric study. Finally, the effects of the incident angle and curvature of the edges of the array elements and feeding lines have also been studied and their impact on the overall performance is presented in this paper.
Full-text · Article · Sep 2013 · IEEE Transactions on Terahertz Science and Technology
[Show abstract][Hide abstract] ABSTRACT: Throughput maximization is generally the major objective when allocating resources in orthogonal frequency division multiple access (OFDMA)networks. Traditionally, dynamic allocation methods were developed to exploit multi-user diversity in these networks. These techniques achieved significant gain in throughput by adopting relaxed convex models to define system upper bound capacity. Frequency diversity, on the other hand, is only considered to a certain extent in order to meet user service constraints. The vast majority of existing research relies on these techniques. Separately, research considering combining frequency diversity and multi-user diversity in full has been scarce. The results of our research in this paper show that using this dual diversity combining can substantially maximize system capacity and resource efficiency, and minimize outage probability whilst users' quality of service (QoS) demands are maintained.
[Show abstract][Hide abstract] ABSTRACT: In this letter, a Sum-Product algorithm (SPA) utilizing soft distances is shown to be more resilient to impulsive noise than conventional likelihood-based SPAs, when the noise distribution is unknown. An efficient version of the soft distance SPA is also developed but with half the storage requirements and running time.
Full-text · Article · Jun 2013 · IEEE Transactions on Communications
[Show abstract][Hide abstract] ABSTRACT: In this paper, an investigation is presented into infrared nanoantennas for solar energy harvesting at 10-μm wavelength, where considerable solar energy is available. These antennas have been modeled using both the integral equation model and the circuit model. The method of moments has been utilized to solve Hallen's and Pocklington's integral equations to find the current distribution over the antenna surface by considering the conductivity and the dielectric properties of gold at this wavelength, as well as its effect on the antenna performance. For verification of the obtained results, this antenna has been simulated using a finite element method-based electromagnetic simulator and both results were found to be consistent. In addition, the metal/insulator/metal (MIM) diode has been studied and its equivalent circuit is presented. Furthermore, a solar rectenna has been constructed by overlapping the antenna arms over a small area to incorporate the MIM diode. The circuit of the solar rectenna, with mathematical expressions for the elements of the equivalent circuit, is demonstrated. Finally, a parametric study into the effect of the MIM diode on the captured voltage is conducted.
Full-text · Article · May 2013 · IEEE Journal of Selected Topics in Quantum Electronics