Abstract In this paper, an improved method to arrange the light emitting diodes (LED) on the ceiling for an indoor visible light communication (VLC) system is proposed. More precisely, a LiFi‐based connection is considered where several LEDs are placed on the ceiling of an office and communicate with several receiver (user) located randomly within the room area. It is proposed to find the optimal location of the LEDs by minimizing the outage probability at the user's location. Both cases of static and mobile users are addressed. First, the closed‐form expression for the outage probability at the user's location is derived. Then, by minimizing the average outage probability, the optimal location of the LEDs is found. The numerical results show that the proposed LED arrangement outperforms the classically used uniform LED arrangement in terms of average outage probability at the receiver and reduces the necessary signal‐to‐noise ratio (SNR) to achieve a target average outage probability by about 3 and 2.2 dB for mobile and static users, respectively. They also show that the performance superiority of the proposed method over classical arrangement remains valid even under large estimation errors.

Abstract In this work, the physical layer security performance of an underlay cognitive buffer‐aided relay network over the general Nakagami‐ m fading channel model is addressed, where a primary source communicates with a destination via a decode‐and‐forward relay node in the presence of an eavesdropper and a secondary communication system. The relay is equipped with a finite‐size storage equipment or buffer which can store the sequence of received data packets from source and forward it immediately or subsequently in an adequate time‐slot. In this way, the buffer relay enables to adopt a link selection scheme in order to detect the best available link, (i.e. the one with higher secrecy rate) among the available data at source or at the relay buffer. The closed‐form expression for the secrecy outage probability (SOP), which is a widely adopted performance metric in the considered scenario, is also derived. Numerical results confirm the accuracy of the derived closed‐form analytic expression for SOP and indicates the interaction between secrecy performance and the Nakagami channel parameter m . In particular, it is shown that the SOP experiences a significant drop as the Nakagami channel parameter increases for a predefined target secrecy rate.

The main objective of this study consists in reducing the correlated background radiations along with minimising the bit error rate (BER) for a multiple aperture free‐space optical (FSO) communication system based on differential signalling scheme. Instead of performing the conventional equal gain combining (EGC) of the two wavelengths, here, the authors propose to efficiently combine them at the receiver by using non‐equal channel‐dependent weighting factors. To this aim, they first derive the analytic expression of the BER of the considered system, which is necessary for optimising the background radiations. Second, by considering ON–OFF keying modulation, they optimise the decision threshold and the weighting factors of the two wavelength by minimising the BER. The performance merit of the proposed scheme is contrasted to the zero threshold scheme and to the EGC scheme in terms of the average BER, the average signal‐to‐noise ratio (SNR) and the background rejection at the receiver. They show that the proposed scheme can improve the SNR up to 17 dB, based on the correlation coefficient of the background radiations at two wavelengths. Moreover, simulation results confirm the superiority of the proposed scheme in terms of achievable BER compared to the aforementioned conventional methods.

Physical layer (PHY) security is an important issue in wireless communication systems. In this study, the authors investigate the interactions between the PHY security performance and the spectrum access technique in a cooperative cognitive radio (CR) network. In the classical spectrum access, if the measured energy on a given subchannel is less than a threshold, the subband is dedicated for CR transmission. Here, they consider a probabilistic spectrum access (PSA), which assigns to each primary subchannel a probability which is determined according to the measured energy on that subchannel. In addition, both spectrum access and CR power allocation are performed simultaneously. They formulate the general outage probability and some important PHY security performance metrics, such as average secrecy rate, secrecy outage probability, and probability of positive secrecy rate in terms of the spectrum access and optimal power allocation functions. The authors' numerical analysis indicates that the PSA method achieves a superior PHY security and outage performance compared to the classical threshold‐based spectrum sensing and power allocation method.

The authors propose an optimal joint power control and multi‐band spectrum sensing scheme for cognitive radio (CR) systems to increase the spectral efficiency and to overcome the spectrum‐sensing errors, in the presence of primary user emulation attacker (PUEA). This optimisation problem is solved to maximise the opportunistic CR data rate under interference and power budget constraints. To solve the formulated optimisation problem, they convert it to a convex problem, and then derive an optimal strategy for joint power control and spectrum access in the considered CR system. They show that the optimal strategy follows a quasi‐underlay signaling, which is equipped with spectrum sensing. They evaluate the performance of the optimised scheme for the CR system in the presence of PUEA, and compare it with the conventional underlay scheme. For more comparison, they compare the proposed approach to that of a similar system, which does not consider the PUEA in the formulation of the joint spectrum sensing and power allocation. They show that the proposed optimal joint power control and spectrum sensing can significantly improve the system performance in terms of average achievable CR data rates compared with the considered conventional methods.

The hybrid cognitive radio (CR) system which combines the two classical interweave and underlay approaches has an improved spectrum efficiency compared to conventional CR systems. In this study, the authors apply the joint spectrum access and power control in a hybrid CR system in the presence of threshold estimation errors. They first formulate the performance metrics of the hybrid CR system under the optimal joint spectrum access and power control strategies by taking into account the threshold estimation errors. Then they show that the threshold estimation error decreases the probability of detecting the primary user (PU) while it increases the false‐alarm probability. Next, they show that the threshold estimation error degrades the performance of the hybrid CR system in terms of outage probability, average achievable data rate and the average interference introduced to the PU. To overcome these performance degradations, they propose a modified spectrum access formulation based on the probabilistic spectrum access scheme, which reduces the impact of threshold estimation uncertainty on the performance of the hybrid CR system. Simulation results indicate that the proposed spectrum access can compensate the performance degradation of a CR system up to 65% for a detection probability of 0.7.

We consider the use of free-space optical communication for fast moving platforms such as high-speed trains, where the sampling clock offset is randomly changing, in addition, the receiver does not have any information on the instantaneous channel fading coefficient. By employing multiple samplers at the receiver, we propose a class of sequence detection methods for the case of On-Off keying (OOK) signaling without using any training sequence. First, we study maximum likelihood-based detection, which has a relatively high-computational complexity. Second, by employing generalized likelihood ratio test, we propose a more practical blind sequence detection method of reduced complexity. To further reduce the computational complexity, third, we propose a novel scheme that uses two wavelengths at the transmitter and differential blind detection at the receiver. Fourth, to benefit from diversity gain with this differential scheme, we consider the use of sufficiently different wavelengths along with sufficient spatial separation between the transmitters and/or the receivers, where we propose an efficient blind detection method. The pros and cons of the proposed detection methods are contrasted through numerical results and their processing loads are compared.

Recently, the use of multi-rotor (MR) unmanned aerial vehicles (UAVs) has emerged as a promising solution for establishing flexible free-space optical communication links. We address, in this paper, the accurate channel modeling to assess the benefits of MR UAV-based deployment for such links. In particular, in the absence of active tracking subsystems, we derive statistical models for ground-to-UAV, UAV-to-UAV, and UAV-to-ground links over both Gamma-Gamma and log-normal atmospheric turbulence models. Unlike previous works on this topic, our proposed model considers the joint effect of atmospheric turbulence along with position and angle-of-arrival fluctuations. The high accuracy of the proposed analytical models is verified by comparing numerically solved and Monte Carlo simulation results in terms of link outage probability. We further discuss the impact of different transmitter/receiver parameters and their optimization in view of maximizing the link availability.

In amplify‐and‐forward wireless relay networks, channel state information is required at the destination for signal detection when coherent distributed space‐time coding is performed at the relays. The authors consider in this study signal detection in the case of imperfect partial channel knowledge at the destination. For the case of maximum a posteriori signal detection, they propose a modified detector formulation that takes the variance of channel estimation errors into account, and reduces the impact of channel uncertainty on the receiver performance. They show that the proposed modification permits a significant performance improvement, while increasing only slightly the receiver's complexity. It is of special interest for non‐constant modulus signal constellations of relatively large size, as well as for relatively low pilot powers.

We consider a cognitive two-way relay network which consists of two transceivers (primary users) and multiple cognitive terminals. The two transceivers transmit their data toward the cognitive terminals which sense the spectrum constantly. If the primary users are in operation, the cognitive terminals act as relay nodes by multiplying the received signal by a beamforming coefficients and then broadcasting the so-obtained signal to the transceivers. When the primary users are not in operation, the cognitive terminals communicate with a base station (BS) to increase the overall throughputs. Our aim is to optimally calculate the beamforming weight coefficients so as to minimize the total transmit power of the two transceivers and the cognitive radios subject to three constraints on the received signal-to-noise-ratios at the two transceivers and at the cognitive BS. Simulation results show that the sum-rate of the cognitive two-way relay network is increased while the total transmit power is minimized.

Classical systems using hierarchical modulation (such as DVB-SH) involve a "high-priority" (HP) and a "low-priority" (LP) bit stream that are separately and independently encoded before being mapped on non-uniformly spaced constellation points, leading to different levels of error protection. However, an inherent drawback of this scheme is the severe performance degradation of the less protected LP stream. In this paper, we propose a concatenated encoding scheme that mixes the two streams in order to make the encoding of the LP stream dependent on the well protected HP stream. Consequently, at the receiver side, the reliable information on HP bits provided by the HP decoder is naturally involved in the decoding process of the LP stream and acts as "soft pilots". We have exploited this feature inside a soft decoding mechanism in order to improve the LP decoding performance while keeping the HP decoding performance unchanged. Numerical results presented in the case of the DVB-SH standard show that the proposed system outperforms the classical approach in terms of LP bit error rate without a considerable increase in the receiver complexity.

The optimal decoder achieving the outage capacity under imperfect channel estimation is investigated. First, by searching into the family of nearest neighbor decoders, which can be easily implemented on most practical coded modulation systems, we derive a decoding metric that minimizes the average of the transmission error probability over all channel estimation errors. Next, we specialize our general expression to obtain the corresponding decoding metric for fading MIMO channels. According to the notion of Estimation-induced outage (EIO) capacity introduced in our previous work and assuming a block Rayleigh-fading channel, we characterize the maximal achievable information rates using Gaussian codebooks associated to the proposed decoder. These achievable rates are compared to the rates achieved by the classical mismatched maximum likelihood (ML) decoder and the ultimate limits given by the EIO capacity. Numerical results show that the derived metric provides significant gains, in terms of achievable EIO rates and bit error rate (BER), in a bit interleaved coded modulation (BICM) framework, without introducing any additional decoding complexity. However, the achievable rates of such metric are still far from the EIO capacity.

Assuming imperfect channel estimation, we propose an improved detector for orthogonal frequency-division multiplexing (OFDM) systems over a frequency-selective fading channel. By adopting a Bayesian approach involving the statistics of the channel estimation errors, we formulate an improved maximum-likelihood (ML) detection metric taking into account the characteristics of the channel estimates. As a first step, we propose a modified iterative detector based on a maximum a posteriori receiver formulation which reduces the impact of channel uncertainty on the decoder performance, by an appropriate use of this metric. The results are compared to those obtained by using a detector based on a mismatched ML metric, which uses the channel estimate as if it was the perfect channel. In a second step, we calculate the information rates achieved by both the improved and mismatched ML detectors, in terms of achievable outage rates. These outage rates are compared to those provided by a theoretical (not practical) decoder defined as the best decoder in the presence of channel estimation errors. Numerical results over both uncorrelated Rayleigh fading channels and realistic ultra wideband channels show that the improved detector outperforms the classically-used mismatched approach in terms of bit error rate and achievable outage rates, without any increase in the receiver complexity.

The optimal decoder achieving the outage capacity under imperfect channel estimation is investigated. First, by searching into the family of nearest neighbor decoders, which can be easily implemented on most practical coded modulation systems, we derive a decoding metric that minimizes the average of the transmission error probability over all channel estimation errors. This metric, for arbitrary memoryless channels, achieves the capacity of a composite (more noisy) channel. Next, according to the notion of estimation-induced outage capacity (EIO capacity) introduced in our previous work, we characterize maximal achievable information rates associated to the proposed decoder. The performance of the proposed decoding metric over uncorrelated Rayleigh fading MIMO channels is compared to both the classical mismatched maximum-likelihood (ML) decoder and the theoretical limits given by the EIO capacity (i.e. the best decoder in presence of channel estimation errors). Numerical results show that the derived metric provides significant gains, in terms of achievable information rates and bit error rate (BER), in a bit interleaved coded modulation (BICM) framework, without introducing any additional decoding complexity.

Under imperfect channel estimation, we propose an improved iterative detector for multiple-input multiple-output (MIMO) systems employing the simple spatial multiplexing or V-BLAST space-time scheme and orthogonal frequency-division multiplexing (OFDM). The detector is based on soft interference cancellation and linear minimum mean-square error filtering which takes into account the error on channel estimation. Numerical results, presented for the case of Rayleigh block fading, demonstrate that our proposed detector can achieve significant performance improvement compared to the classically-used mismatched detector. This improvement is obtained with almost no increase in the receiver complexity.