An increasing number of industrial Internet-ofthings applications requires wireless transmission of control and data packets. A combination of optical wireless communication (also denoted as LiFi), for deterministic wireless access, and powerline communication, as a cost-effective backbone, is considered promising to enable easy installation of small wireless cells. This paper reports on work in progress from an initial measurement campaign to characterize the cascaded LiFi-over-powerline link in physical layer. The combined channel and noise have been studied in a practical setup for the first time. The frequency response was measured up to 100 MHz under various channel conditions to capture multi-path and attenuation effects across different cable lengths and free-space distances. Also, interference and noise were evaluated up to 300 MHz, revealing contributions from both the PLC and LiFi segments.

This paper proposes handover-aware scheduling solutions for multi-cell small-and large-scale visible-light communication networks, enabling soft handover. For this, we coordinate the transmissions at the access points (APs), serving the users in different time slots based on their locations with respect to the AP coverage areas for an efficient utilization of the resources. For scenarios where coverage in large areas is needed, we additionally propose clustering solutions to decrease the handover rate. Compared with non-coordinated schemes, the proposed soft handover techniques offers improved performance in terms of user achievable throughput and link reliability.

In this paper, we propose and evaluate a hybrid visible light communication (VLC)/radio-frequency (RF) network architecture, where a VLC access point serves two user equipments (UEs), which also act as RF relays in order to extend the network’s coverage to a third UE outside the VLC cell. The proposed protocol is inspired by uplink rate-splitting multiple access to efficiently route the messages to the UEs. In more detail, the proposed protocol utilizes the nuances of the specific network architecture to efficiently utilize the wireless resource blocks for both coverage and throughput. The protocol is then optimized by maximizing the minimum achievable rate. Simulation results show that the proposed method achieves superior results compared with a more conventional benchmark scheme, that is also optimized under the same constraints.

Optimizing the energy efficiency (EE) of wireless networks is one of the key priorities in the design of beyond 5G mobile technologies. In this pursuit, the use of new frequency bands, in combination with advanced multiple access protocols and cooperative communications strategies, has recently shown promising results. To this end, this paper investigates an indoor wireless network that aggregates communication resources in visible light and radio-frequency (RF) bands, taking advantage of the complementary aspects of the two technologies. More specifically, a non-orthogonal multiple access (NOMA) scheme is introduced for the visible light communication (VLC) downlink, such that cell-edge users experiencing a weak VLC signal enhance their aggregated data rate with the aid of cooperative communications over RF sidelinks (i.e., device-to-device links). The optimal resource allocation strategy over both VLC and RF bands is derived aiming at EE maximization based on the Dinkelbach’s algorithm and successive convex approximation. Additionally, for the sake of flexibility, a weighted EE metric is proposed for the characterization of the aggregated VLC/RF network performance. Simulation results are provided to validate the proposed analysis, revealing the impact of various design and system parameters, such as the weighting factors, quality of service requirements, and channel conditions.

This paper considers the use of infrared-based optical wireless communications for multi-user uplink wireless body-area networks. We propose the use of optical-orthogonal frequency division multiple access (O-OFDMA) signaling to manage the multiple access (MA) requirement for relatively high-rate medical applications. In particular, we consider asymmetrically clipped O-OFDMA and analyze its performance in terms of bit-error-rate and outage probability in the presence of multi-user time synchronization errors. These latter may occur due to mobility and random transmitter orientations, and will impact the link performance by inducing MA interference (MAI). We show that the effect of MAI increases with increasing the data rate. For instance, for 1 and 2 Mbps data rates, to achieve a target bit-error-rate of 10−3, the link distance is limited to 1.9 and 1 m, respectively, compared to 2.2 and 1.8 m in the absence of MAI.

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.

We propose a quantized feedback-based differential signaling scheme for 2 × 1 free space optical (FSO) multiple-input single-output communication systems. The analytical bit error rate (BER) for the proposed differential signaling scheme is derived and compared with the existing differential signaling scheme for FSO systems. The asymptotic BER of the proposed scheme is also determined and it is shown by using the asymptotic BER expression that the proposed scheme achieves full spatial diversity of two. Moreover, the effect of the feedback error on the performance of the proposed scheme is studied and it is demonstrated by simulation-based and analytical results that the proposed scheme significantly outperforms a previously proposed differential signaling scheme in terms of turbulence fades reduction for tolerable feedback errors.

We propose an analytical approach to evaluate the performance of space‐diversity free‐space optical (FSO) systems over correlated Gamma–Gamma (ΓΓ) fading channels. To do this, we firstly derive an infinite series representation for the moment generating function (MGF) of the sum of arbitrarily correlated ΓΓ random variables (RVs). Then, a closed‐form approximate expression for the probability density function (PDF) of the sum of ΓΓ RVs is obtained from the MGF with the aid of Padé approximation. The performance of the FSO system is then evaluated based on this PDF. Through some numerical results we illustrate the accuracy of the proposed method and further discuss its limitations.

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.

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.

We propose a semi-blind channel estimator based on superimposed pilots for the case of single-carrier single-user MIMO systems. The proposed estimator is based on the expectation maximization (EM) algorithm. Numerical results show that even with this semi-blind estimator, the classical approach of time-multiplexed pilots still outperforms the superimposed approach, except for a large number of receive antennas and not too short frames.

We consider channel estimation in multiple-input multiple-output (MIMO) systems using iterative detection at the receiver. Space-time bit-interleaved coded modulation (BICM) and soft-input soft-output maximum a posteriori (MAP) symbol detection and decoding are considered. Channel coefficients are updated at each iteration of the detector using a semi-blind estimation approach based on the expectation maximization (EM) algorithm. We first show that a "classical" and non-optimized EM implementation, as already proposed in some previous works, gives a biased estimate of the channel coefficients. We then try to optimize the EM implementation and propose a modification to it that provides an unbiased channel estimate and leads to a better convergence of the iterative detector. We show that considerable improvement in the receiver performance can be obtained by using our proposed modified unbiased (MU) EM algorithm, especially for large number of transmit antennas and short training sequences. We also show that when MIMO signal detection is strongly asymmetric in the sense of too few receive antennas, the EM-based channel estimation may be of little interest. Moreover, we consider a simple semi-blind estimation scheme, based on hard decisions on reliable decoded data bits, and compare its performance with the EM based estimation methods

We consider channel re-estimation based on the EM (expectation maximization) algorithm in an iterative MIMO detector based on parallel interference cancellation, also known under the name of turbo-BLAST. While providing an appropriate formulation of EM, we show that a previously proposed EM implementation gives a biased estimate, and propose modifications in the EM implementation in order to obtain an unbiased estimate. Testing the proposed algorithms by simulations, we show that the achieved improvement is considerable for important diversity orders.

We study the sensitivity of a special iterative detection scheme, called turbo-equalizer, to an SNR (signal-to-noise ratio) mismatch at the receiver. For turbo equalization, the same algorithm as for SISO (soft-input soft-output) decoding is used for equalization at the first iteration. This turbo equalizer uses a MAP (maximum a posteriori) or Viterbi based SISO equalizer at the first iteration, and employs a simple interference canceller in the next iterations. It is shown that the sensitivity to an SNR mismatch is more for larger encoder memory length, as well as for difficult-to-equalize (highly delay-dispersive) channels. For the case that no information is available on SNR, a simple online SNR estimation technique is proposed, which gives satisfying results.

Use of antenna arrays at both sides of the communication link can result in high channel capacities provided that the propagation medium is rich scattering. Previous studies on MIMO wireless structures have mostly considered Rayleigh fading conditions. Here a study is made on the capacity of Rician channels; it is shown that depending on the number of antennas, the MIMO capacity achieved under the condition of Rician fading may be greater or smaller than the capacity in the case of Rayleigh fading. In particular, for a large number of antennas, Rician channels exhibit smaller capacity than Rayleigh channels. On the other hand, for Rician channels, if the line-of-sight component is not very significant, the increase in capacity by an increase in the number of antennas can still be considerable.

MIMO channels can attain high capacities provided that the propagation medium contains enough scatterers. The capacity of MIMO channels can further be increased by performing optimal power distribution over transmit antennas. This, in turn, necessitates knowledge of the channel at the transmitter. It is shown in this paper that for low signal-to-noise ratios (SNR) and small number of antennas, this increase in capacity can be of interest. Rayleigh fading conditions are considered.