In this work, we address a novel framework for next-generation distributed radio access. In contrast with existing studies, where all remote radio heads (RRHs) in a distributed network are directly connected to a central unit (CU), an alternative architecture for advanced flexibility is proposed. In our setup, only one of the RRHs, namely the$primary$RRH, communicates directly with the CU, while the connectivity between the rest RRHs, namely$secondary$RRHs, and the CU is achieved through the primary RRH via free-space optical links. Assuming that users exploit non-orthogonal multiple access (NOMA) for their transmissions, we introduce two successive interference cancellation (SIC) cooperation schemes, depending on the one-directional or bidirectional communication between the RRHs, as well as, a four-step centralized algorithm for efficient user-RRH association and decoding order operations is proposed. The feasibility of the suggested schemes is adequately demonstrated by deriving analytical expressions for users' outage probabilities and providing valuable insights into the high signal-to-noise ratio regime. Furthermore, the performance of the proposed system under various weather conditions is investigated via simulation and analytical results. The comparison with a benchmark scheme, where all RRHs are directly connected to the CU and cooperate with each other via ideal links, is provided and it is revealed that although the performance of the proposed system model is weather dependent, in most of the practical cases it achieves similar performance with the ideal benchmark.

In this paper, we introduce a theoretical framework for analyzing the performance of multi-reconfigurable intelligence surface (RIS) empowered terahertz (THz) wireless systems subject to turbulence and stochastic beam misalignment. In more detail, we extract a closed-form expression for the outage probability that quantifies the joint impact of turbulence and misalignment as well as the effect of transceivers’ hardware imperfections. Our results highlight the importance of accurately modeling both turbulence and misalignment when assessing the performance of multi-RIS-empowered THz wireless systems.

The ambitious goal of light-fidelity (LiFi) technology to provide seamless user mobility in indoor environments requires strong link connections to confront any stochasticity that possibly comes out. In this context, a robust LiFi network should effectively address the spatial random terminal locations inside its coverage volume. In contrast with the previously published works dealing mainly with randomness in two dimensions, the present study aims to provide a more realistic framework by treating the problem in the three-dimensional space. In view of the foregoing, a LiFi configuration is adopted where a terminal lies at a random position inside its truncated conic coverage volume at a distance from the transmitter. Some key statistics for a distance-related metric and the channel gain, including the probability density function, are derived in the first place. The investigation is then headed on the performance analysis of the LiFi link by providing closed-form expressions of the outage probability and the average bit error rate for on-off keying assuming indirect modulation/direct detection at the receiver. A multiuser scenario with several users placed inside the coverage volume is examined as well. A series of numerical results is also depicted to provide valuable insights into how the random terminal placement affects the link performance.

More often than not, transmitters (Txs) in wireless networks operate using directional radiation patterns where the main radiation lobe creates a three-dimensional conic-shaped volume. In this context, any receiver (Rx) placed inside it can establish a strong communication link. Since the Rx location is not always known, it is desirable to investigate its random position effect to describe the link performance in a reliable manner. As in most cases, Rxs lie far away from the apex where the Tx is placed, we investigate the random location problem inside a truncated conic shape where the cone arises as a limiting case. To this end, we deduce a set of statistical metrics, including the random distance from the apex to a random point position. As an application, we focus on the outage performance of an unmanned aerial vehicle (UAV) communications scenario susceptible to Nakagami-$m$fading and derive novel closed-form expressions for the composite model.

Abstract The performance of free‐space optics links strongly depends on scintillation effects caused by the atmospheric turbulence appearance. A widely adopted way to counterbalance the expected deterioration due to scintillation is to employ cooperative and multi‐hop arrangements. However, the use of multiple relays increases significantly the complexity of the mathematical models used to describe the overall performance of the system. Taking the latter into account, we approximate a generic multiple relay‐assisted free‐space optical configuration with a dual‐hop link in order to simplify the performance analysis. To this end, we provide new mathematical derivations for the evaluation of the outage probability and the average bit error rate by assuming typical turbulence models.

Terrestrial free‐space optical (FSO) communication systems with subcarrier intensity modulation have experienced a particular research attention in the recent past. However, their performance strongly degrades in the presence of atmospheric turbulence, pointing errors, and phase noise impairments. In order to overcome these limitations, the authors consider a receiver diversity scheme of a typical subcarrier phase‐shift keying (PSK) system and investigate the performance by means of the average symbol error probability (ASEP). They assume a wide range of turbulence conditions, non‐zero boresight pointing errors, and phase noise strengths described through the gamma‐gamma, Beckmann, and Tikhonov distributions, respectively. Novel approximate ASEP expressions are derived for single‐input single‐output and single‐input multiple‐output (SIMO) configurations. Appropriate numerical results are depicted and validated by Monte Carlo simulations.

The aim of this paper is to develop link layer transmission schemes for hybrid optical wireless (OW)/radio frequency (RF) systems, with constraints on both per-link and total average power consumption at the transmitter. In this context, we adopt a timeslot structure with a queue for storing the data packets for transmission and model the hybrid channel as an erasure channel with parameters varying along the time-slots according to a Markov chain. Then, a stochastic optimization problem is formulated, where intelligent decisions regarding the number of the packets admitted in the queue and the power levels used in every link, are taken by the hybrid transmitter in each slot. The objective of this formulation is to design a control policy that maximizes the transmitter throughput, while satisfying the power constraints as well. A solution is offered by using the Lyapunov optimization framework and an on-line transmission algorithm is developed. The proposed transmission algorithm takes decisions based only on the status of the queue and the statistical parameters of the OW/RF channel in each time-slot, without requiring any knowledge of the underlying Markov chain of the channel process, or the statistics of the packet arrival process. Furthermore, in order to alleviate the requirement for full feedback at the transmitter, i.e., feedback for every successfully received packet, which is critical for the accurate queue update, we extend our analysis and incorporate reduced-feedback coding schemes. The proposed transmission policy in this scenario still meets the throughput objective and satisfies the power consumption constraints, while a tradeoff between transmission delays and feedback requirements is revealed.

This paper investigates link layer transmission schemes for hybrid optical wireless (OW)/radio frequency (RF) systems with constraints on both per-link and total power consumption at the transmitter. Assuming timeslot structure and using a queue for storing the random data packet arrivals, we formulate a stochastic optimization problem where intelligent control decisions are taken by the hybrid transmitter in each slot, regarding the number of the packets admitted in its queue and the power levels used in each link. The aim of this formulation is to design a control policy that maximizes the transmitter throughput, while satisfying its power constraints as well. A solution is offered by using the Lyapunov optimization framework. An on-line transmission algorithm is derived, which takes control decisions based only on the status of the transmitter queue and the erasure probabilities of the OW and RF links. The proposed algorithm meets the desired throughput objective by efficiently exploiting both links, while provides explicit power consumption guarantees.

The transmission of multiple wireless signals over optical links has attained a great research interest nowadays. In case where optical fibers are difficult to be deployed, or installation cost is prohibited, optical wireless systems provide an efficient alternative means. In this paper, we consider the WiMAX (IEEE802.16) standard and construct a simple but adequate scenario to investigate radio signal transmission over terrestrial optical wireless channels. An appropriate system architecture is adopted and a channel model, which entails some of the most critical impairments of the optical channel, i.e., attenuation, turbulence, pointing error effects, as well as of the RF channel, i.e., path loss, shadowing, and Rayleigh fading, is taken into account. The overall link budget and a closed-form of the outage probability of the system are deduced. Several analytical results are depicted using a realistic set of parameter values, to lend a helpful insight to the performance of the proposed architecture.

The performance of optical wireless systems deteriorates to a large extent from the presence of turbulence and pointing error effects. To meet the typical bit error rate (BER) targets for reliable communications within the practical ranges of signal-to-noise ratio, error control coding schemes are often proposed. This paper investigates the error performance for convolutional coded on-off keying free-space optical systems through symbol by symbol interleaved channels characterized by strong turbulence and/or pointing error effects. We consider several channel types and derive exact analytical expressions for the pairwise error probability. These expressions are applied to obtain upper bounds on the BER performance using the transfer function technique.

In this paper, we propose the Inverse Gaussian (IG) distribution, as a less complex alternative to Log-normal (LN), to describe turbulence-induced fading in free-space optical (FSO) systems operating in weak turbulence conditions and/or in the presence of a large amount of aperture averaging. By conducting goodness of fit tests, we define the range of values of the scintillation index, where the two distributions approximate each other, with a certain significance level. The efficiency of the new model is pointed out by deriving analytical expressions for the calculation of the bit-error rate of two typical FSO systems; an intensity-modulation/direct detection FSO system with M-ary pulse position modulation and a heterodyne FSO system with differential phase shift keying. Numerical examples are provided to clearly illustrate the accuracy of the proposed approach in the weak turbulence regime.

Optical wireless, also known as free-space optics, has received much attention in recent years as a cost-effective, license-free and wide-bandwidth access technique for high data rates applications. The performance of free-space optical (FSO) communication, however, severely suffers from turbulence-induced fading caused by atmospheric conditions. Multiple laser transmitters and/or receivers can be placed at both ends to mitigate the turbulence fading and exploit the advantages of spatial diversity. Spatial diversity is particularly crucial for strong turbulence channels in which single-input single-output (SISO) link performs extremely poor. Atmospheric-induced strong turbulence fading in outdoor FSO systems can be modeled as a multiplicative random process which follows the K distribution. In this paper, we investigate the error rate performance of FSO systems for K-distributed atmospheric turbulence channels and discuss potential advantages of spatial diversity deployments at the transmitter and/or receiver. We further present efficient approximated closed-form expressions for the average bit-error rate (BER) of single-input multiple-output (SIMO) FSO systems. These analytical tools are reliable alternatives to time-consuming Monte Carlo simulation of FSO systems where BER targets as low as 10-9are typically aimed to achieve.

Free-space optical (FSO) communication has received much attention in recent years as a cost-effective, license-free and wide-bandwidth access technique for high data rates applications. The performance of FSO communication, however, severely suffers from turbulence-induced fading caused by atmospheric conditions. Multiple laser transmitters and/or receivers can be placed at both ends to mitigate the turbulence fading and exploit the advantages of spatial diversity. Spatial diversity is particularly crucial for strong turbulence channels in which single-input single-output (SISO) link performs extremely poor. Atmospheric-induced strong turbulence fading in outdoor FSO systems can be modeled as a multiplicative random process which follows the K distribution. In this paper, we investigate the error rate performance of FSO systems for K-distributed atmospheric turbulence channels and potential advantages of spatial diversity deployments at the transmitter and/or receiver. Our results demonstrate significant diversity gains of multiple transmitter/receivers deployment in FSO channels. We further present efficient approximated closed-form expressions for the average bit-error rate (BER) of multiple-input single-output (MISO) and single-input multiple-output (SIMO) FSO systems. These analytical tools are reliable alternatives to time-consuming Monte Carlo simulation of FSO systems where BER targets as low as 10-9are typically aimed to achieve.

In this paper, we study the performance of multihop free-space optical (FSO) wireless systems over turbulence-induced fading channels. The analysis is carried out for systems employing amplify-and-forward (AF) or decode-and-forward (DF) relays and for turbulence channels which can be modeled by the Gamma-Gamma distribution. An exact analytical expression for the end-to-end outage probability of AF systems is obtained, while a closed-form expression of DF systems is derived. The average bit-error probability of a dual-hop FSO system employing a DF relay is studied as a special case. Numerical examples are also presented to illustrate the proposed analysis and to further investigate the effects of the turbulence severity on the multihop FSO systems' performance.

This paper presents a neural network model for routing in the space segment of a Satellite Personal Communication System. At first, a proper energy function is constructed from the constraints of the system, which are clearly indicated. The combinatorial nature of the problem is clearly shown and hence a properly modified Hopfield neural network can be easily applied to provide increased survivability and thus improve security.

The demand for more efficient and fast channel allocation techniques in cellular systems increases day by day. Borrowing channel assignment (BCA) was introduced in the literature as a compromise between the classic fixed and dynamic channel allocation schemes. This paper examines the behavior of three heuristic BCA techniques based alternatively on a Hopfield neural network, an efficient evolutionary algorithm named combinatorial evolution strategy (CES) and a third heuristic which combines the basic advantages of the two above computational intelligence methods. By considering some specific assumptions that follows an ideal cellular mobile model, BCA is formulated as a combinatorial optimization problem. The above heuristics have been extensively applied to solve efficiently such problems in the past. Simulation results, derived for uniform and nonuniform traffic load conditions, are used to compare these BCA schemes each other as also with other well-established allocation techniques.

Modern cellular mobile communications systems are characterized by a high degree of capacity. Consequently, they have to serve the maximum possible number of calls while the number of channels per cell is limited. The objective of channel allocation is to assign a required number of channels to each cell such that both efficient frequency spectrum utilization is provided and interference effects are minimized. Channel assignment is therefore an important operation of resource management and its efficient implementation increases the fidelity, capacity, and quality of service of cellular systems. Most channel allocation strategies are based on deterministic methods, however, which result in implementation complexity that is prohibitive for the traffic demand envisaged for the next generation of mobile systems. An efficient heuristic technique capable of handling channel allocation problems is introduced as an alternative. The method is called a combinatorial evolution strategy (CES) and belongs to the general heuristic optimization techniques known as evolutionary algorithms (EAs). Three alternative allocation schemes operating deterministically, namely the dynamic channel assignment (DCA), the hybrid channel assignment (HCA), and the borrowing channel assignment (BCA), are formulated as combinatorial optimization problems for which CES is applicable. Simulations for representative cellular models show the ability of this heuristic to yield sufficient solutions. These results will encourage the use of this method for the development of a heuristic channel allocation controller capable of coping with the traffic and spectrum management demands for the proper operation of the next generation of cellular systems.