In this study, the authors find the optimum inter-distance between visible light access points (VAPs) in a multi-cell system. This is an important issue to avoid the lack of resources resulting from wiring too few VAPs while minimising the installation cost. This study uses a one-dimensional corridor model suitable for many practical environments such as train aisles and hospital/library corridors. They provide formulas for the average rate achieved per user to determine the optimal inter-VAP distance. These formulas depend on information that is usually available in the design step such as the corridor dimensions, luminaries' parameters, and user density. They consider different power allocation techniques and different frequency reuse factors. The analysis shows that the optimum inter-VAP distance depends mainly on the corridor dimensions and not as much on the communication technique. Higher frequency reuse factors reduce the interference and improve the performance in the case of small cells. Specifically, the results determine the corridor dimensions that are better served by multi-VAP rather than a single centred VAP. Many important design issues are answered throughout this study such as how the ceiling height and the estimated crowdedness in the corridor affect the optimum cell size.

In order to suppress the nonlinearity impact of light-emitting diodes (LEDs) on the performance of visible light communication (VLC), the combined method including companding and predistortion was proposed in this work. The modified companding and predistortion based on the LED characteristic analysis were used corresponding to the output current -input voltage (I-V) and light output-injected current (L-I) nonlinearity, respectively. Using the combined method, the performances of the DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) VLC system were analysed. It is demonstrated that the non-linear distortion of OFDM transmitted signal is alleviated, thus the received signal quality is increased. As a result, the bit error rate (BER) performance of VLC system is improved obviously by using the combined method of companding and predistortion. Furthermore, the applicability of this combined method is also verified by analysing the nonlinearity suppression of the VLC system using different LED. The results reveal that both the impact from LED's I-V and L-I nonlinearity can be alleviated using the proposed method combining from companding and predistortion for DCO-OFDM VLC system.

This study explores how the field-of-view (FOV) of a visible light communications (VLCs) receiver can be manipulated to realise the best signal-to-noise ratio (SNR) while supporting device mobility and optimal access point (AP) selection. The authors propose a dynamic FOV receiver that changes its aperture according to receiver velocity, location, and device orientation. The D-FOV technique is evaluated through modelling, analysis, and experimentation in an indoor environment comprised of 15 VLC APs. The proposed approach is also realised as an algorithm that is studied through analysis and simulation. The results of the study indicate the efficacy of the approach including a 3X increase in predicted SNR over static FOV approaches based on measured received signal strength in the testbed. Additionally, the collected data reveal that D-FOV increases effectiveness in the presence of noise. Finally, the study describes the tradeoffs among the number of VLC sources, FOV, user device velocity, and SNR as a performance metric.

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.

Optical orthogonal codes play an important role in optical code division multiple access networks and optical wireless systems. In this study, by combining combinatorial design and meta-heuristic search algorithm, a mixed construction of -OOCs is proposed. The mixed construction enjoys the benefits of both mathematical methods and search algorithms and can be used to produce OOCs with large and flexible parameters. Numerical results show that some of the OOCs obtained by this construction have more codewords in comparison with the construction method using outer-product matrix.

Here, a high accuracy camera-based visible light indoor positioning system (IPS) has been investigated. A novel two LEDs-based positioning algorithm is proposed aiming at solving the problems of positioning ambiguity with a good trade-off between high accuracy and computational complexity. The simulation results show that the positioning errors of the proposed 2-LED positioning algorithm achieves 2 cm when the resolution of the image sensor (IS) exceeds 2500 pixels/line. A specular reflection cancellation scheme is presented to further reduce the positioning errors when specular reflection interference occurs. The simulation results show that the positioning errors can be reduced to 2.71 cm after using the specular reflection cancellation.

In this paper a novel 2-D range free positioning method which uses visible light communications network architecture is presented. The algorithm performance for lattice and hexagonal LED lighting infrastructure deployment is obtained by computational simulation. The robustness of our method against changes on receiver's field of view and mismatch between the assumed vertical distance and real vertical position is simulated in a room of dimensions 5 × 5 × 2.5 m. A localization interest zone with receiver's height between 0.5 and 1.1 m measured from the floor is considered as variation on vertical distance. Moreover, FOV's ranging from 90° to 130° are used to measure the behaviour of the method when different built-in photo detectors are incorporated in the mobile device. Our algorithm overcomes three traditional localization methods, i.e., convex position estimation, centroid and received signal strength based trilateration when changes in the FOV occurs. In addition to this, it shows better accuracy than other range free algorithms when an assumed height difference mismatch of up to 30 cm exist. The method shows better coverage than range based methods being able to reduce border effects and perform localization with reliable accuracy using only one connected cell and a single photo detector.

Power line communication (PLC) is an emerging field of communication that makes use of the existing power line infrastructure for the transmission of data and power. However, the presence of additive and multiplicative power line noises deteriorate the performance of a PLC system. Background and impulsive noises are the two main categories of additive noise, whereas channel gain accounts for the multiplicative noise. In this study, the authors propose coordinate interleaving (CI) technique and investigate its performance for a PLC system under the combined effect of Nakagami-m additive background noise and Rayleigh channel gain. They assume perfect channel state information at the transmitter and receiver of the PLC link. By interleaving the coordinates of the symbols sent over different channel realisations, CI results in significant improvement in the performance of a PLC system. Both the simulated and analytical results shown in this study reveal the effectiveness of the proposed technique in achieving better performance.

Blind signal detection is very important in various applications such as cognitive radio, spectrum surveillance, and eavesdropping. This gets trickier when one needs to deal with burst signals (as opposed to continuously transmitted signals) in non-cooperative environments. Here, existing methods for signal detection are studied and developed to use for burst detection. For the special case of downlink time division multiple access (TDMA) burst transmission, performances of these detection methods are improved by proposing a blind synchronisation algorithm which is applied to the output result of detection algorithms. The results of Monte–Carlo simulations demonstrate the performance of the proposed detection and synchronisation algorithms.

A portable visible light communication (VLC) system using core-shell CdSe/ZnS quantum dots (QDs) as light converter was presented. The absorption peak wavelength of the fabricated QDs locates at 540 nm, and the emission peak wavelength locates at 570 nm. Owning to a short fluorescent life of QDs, a wide modulation bandwidth as well as a fast data transmission rate can be achieved. A portable transmitter and receiver based on digital signal processors (DSPs) were developed and the related communication experiments were carried out. Under an optimum direct bias voltage of 2.70 V, the achieved maximum communication distance is ∼ 1.3 m, and the maximum transmission rate is 267 Kb/s under the premise that the bit error ratio (BER) is <10⁻³ due to a slow turnover speed of the DSP input/output pin. At a 133 Kb/s data transmission rate, high stability was observed through measuring the BER results over 5 h at the two communication distances of 0.5 and 0.7 m.

This work examines the use of genetic algorithms (GAs) to optimise two different problems regarding positioning applications in wireless communication systems. The first problem involves speeding-up the search for the mobile station (MS) position in cellular networks, when using a database correlation (DCM) technique. While addressing this problem, this work investigates three different approaches for using GA (DcmGaFull, DcmGaBs and DcmGaBS+RTT) and evaluates their performances (in terms of computational complexity and positioning precision) in two different regions. The performance of those methods was compared with those of the standard fall-back methods for emergency call locating in 2G, 3G and 4G networks: cell identity (CID) and enhanced CID (ECID). Method DcmGaBs+RTT, originally proposed in this work, achieved the best performance, both in terms of lower latency and higher precision, in the two test areas. The second problem concerned optimising the deployment of a new BTS in two different test regions, while considering parameters such as coverage, traffic density and CID and ECID positioning error. This was done considering also the mixed objective of weighting the positioning error by the traffic demand, as a higher number of emergency calls are expected to be generated from areas with higher traffic density.

Key pre-distribution in wireless sensor networks is an important area of research due to limited resource availability in sensor nodes. In this study, the authors propose a novel key pre-distribution scheme for wireless sensor networks based on combinatorial design. The proposed scheme is used to assign secret keys to the sensor nodes so that they can securely communicate among themselves. It also aims to decrease the key storage overhead and improve the overall resiliency of the network. They use the ratio of links broken/affected and the ratio of nodes disconnected to measure the resiliency when some sensor nodes are compromised in the network. They observed that the proposed scheme reduces the key storage overhead in the network while maintaining the desired connectivity among all the sensor nodes. Further, the proposed scheme is more resilient than the majority of existing schemes present in the literature.

Channel estimation is one of the most important parts in three-dimensional multiple-input multiple-output (3D MIMO) systems. The characteristics of non-line-of-sight (NLOS) channel and line-of-sight (LOS) channel are different in 3D MIMO systems. If the same channel estimation scheme is used in LOS case as NLOS, the performance of estimation will be bad. In outdoor propagation environment, 3D MIMO channels between closely located antennas share the same delay support in temporal domain. With those prior knowledge, in this study, a new channel estimation scheme is proposed. The proposed scheme can be divided into two processes. First, it is needed to identify the received sounding reference signal whether is LOS or NLOS propagation. Then, different enhanced DFT-based channel estimation schemes are proposed separately according to the identification results. Simulation results verify the proposed algorithm outperforms traditional discrete Fourier transform (DFT)-based channel estimation. At signal-to-noise ratio of 20 dB, the proposed algorithm has 17.7 and 35.7% improvement in NLOS case and LOS case separately in terms of normalised mean squared error compared with traditional DFT-based channel estimation scheme, and is achieved with additional liner complexity.

This study investigates the impact of imperfect channel state information that is caused by feedback delay with time-variant channels and also by channel estimation errors with an opportunistic source-pair selection strategy. The investigation considers multi-user bi-directional wireless relaying networks. The opportunistic source-pair selection strategy selects the appropriate source-pair node among N available nodes. The selection strategy is based on each source pair's maximum sum-rate in each time slot. According to analytical, asymptotic, and Monte–Carlo simulation results, the opportunistic source-pair selection strategy, besides achieving a near-optimal solution for inter-cell interference that is caused by other user-pairs in the system model, also achieves diversity order in high signal-to-noise regimes. Moreover, the opportunistic source-pair selection strategy outperforms the max–min-based selection strategy in such a system model. Results also show that feedback delay degrades the achievable diversity order from N to 0 while affecting coding gain. The results also show that the channel estimation error case does not affect the achievable diversity order, but it does affect the system coding gain in low-signal-to-noise regimes. Moreover, the channel estimation error effects upon system coding gain become negligible at high-signal-to-noise regimes.

The fifth generation upcoming applications require an alternative multicarrier modulation (MCM) scheme rather than the well-known orthogonal frequency division multiplexing scheme. Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) is the most suitable scheme to meet this requirement. However, as an MCM, the high peak to average power ratio (PAPR) is a significant problem which upsets the FBMC/OQAM signal. In the presence of a non-linear high power amplifier, FBMC/OQAM signals with high PAPR can be strongly distorted. To date, the PAPR reduction technique called tone reservation (TR) has been revised and adapted to the overlapping nature of the FBMC/OQAM signal in many research works. In view of that, the authors (i) present two refined TR methods deemed relevant for reducing the PAPR of the FBMC/OQAM and (ii) compare their performances with those of some existing versions of TR PAPR reduction methods.

The authors consider downlink transmission in a multi-cell massive multiple-input and multiple-output (MIMO) system and focus on decreasing the feedback overhead caused by cell cooperation in constant envelope precoding (CEP). In the literature, the single-cell case for CEP with perfect channel state information (CSI) has been studied. Here, considering full cooperation among the cells, they develop CEP for the multi-cell case. They devise a low overhead centralised construction for CEP which employs limited cooperation among the cells, providing higher system throughput. To achieve the minimum feedback overhead, a distributed realisation of CEP is proposed where each cell locally performs CEP. Furthermore, a new optimisation problem is solved to compensate for the effects of pilot contamination. In addition, to reduce the computational complexity, a relaxed iterative form is presented for the limited cooperation and the distributed scenarios. Numerical results show that in the proposed structures, despite a tremendous decrease in the system overhead, the performance confronts merely an insignificant degradation, <10%, compared to the full-cooperation case. Moreover, in the presence of imperfect CSI, the performance of the distributed structure whose imperfect CSI is compensated for, approaches the performance of this structure in the perfect CSI scenario.

A conventional relaying system is based on an orthogonal two-phase transmission protocol where time slots for the source (the first phase) and relays (the second phase) are exclusively allocated to avoid interference. For this reason, the conventional relaying schemes suffer from the reduction in spectral efficiency. In order to overcome this drawback, a parallelised relaying protocol with simultaneous transmission has been proposed. However, simultaneous transmission may not be a good strategy in such a strong cross-relay interference. In this study, the authors propose a parallelised relaying protocol with an opportunistic priority assignment. In the proposed scheme, they consider the cognitive radio technique as a way to improve the spectral efficiency of the conventional parallelised relaying protocol in the cross-relay interference. They evaluate the instantaneous capacity of the proposed scheme through computer simulations and conclude that their scheme can improve the spectral efficiency of the conventional relaying systems significantly.

A new partial transmit sequence (new-PTS) scheme is illustrated in this study. The target behind it is to reduce the peak-to-average power ratio (PAPR) in an orthogonal frequency division multiplexing (OFDM) system. Despite its competitive attributes, the PTS technique is considered computationally expensive due to multiple inverse fast Fourier transforms (IFFT) and the need of a thorough investigation to find the optimal phase factor. The primary concern, thus, is to eliminate the IFFT blocks. In the present study, a remarkable strategy has been followed, mainly relying on analysing the available data in the random access memory. Additionally, the least PAPR value is calculated and its corresponding address is precisely determined; such an address is the side information to be sent to recover the users' original data at the OFDM receiver. Moreover, the effectiveness of the so-called complexity reduction of the new-PTS method is pointed out in order to limit the number of searches that are required to acquire the best PAPR performance, which significantly reduces the computational complexity overhead. Consequently, the numerical analysis and comparative study show the overall high performance, which the proposed PTS scheme offers in respect to both the bit error rate and PAPR reduction.

Cognitive radio (CR) is a rapidly growing technology that can be employed to effectively utilise the radio spectrum. The detection accuracy of the CR user is compromised when a network is under degrading conditions like fading and shadowing effects. Cooperative spectrum sensing (CSS) has been extensively employed to overcome these issues. In CSS, all secondary users communicate with the fusion centre (FC) to share the PU information. Hard fusion schemes are applied to make a correct decision about PU presence at FC; however, such approaches lack reliable decision-making. Hence, there is a need to construct more accurate and reliable fusion schemes. In this study, a neural network (NN)-based decision fusion scheme at FC is used to construct a reliable decision. In the proposed scheme, NN is used as a machine learning tool that is trained using input data and output data to generate the desired fusion model. It has been evaluated through simulation results that the proposed fusion scheme sensing accuracy is much better as compared to conventional fusion schemes and other state-of-the-art schemes proposed in the literature. Further, the proposed fusion scheme is tested using the clustering approach to make it more reliable and accurate.