This paper proposes a novel integrated visible light communication (VLC) and radio frequency (RF) system using an angular diversity receiver (ADR) for Internet of Things (IoT) sensor networks in industrial contexts. The prototype, including the transceiver’s analog front end (AFE), is designed for microcontroller unit (MCU) operation and is implemented for experimental demonstrations. The VLC coverage of the proposed system is improved with a coverage rate of 50% compared to 37% in our previous work, and the physical layer security (PLS) performance is also enhanced with double security capacity (SC) compared to a single photodiode (PD) receiver. Furthermore, to evaluate the feasibility of industrial IoT (IIoT) applications, real-time sensor network applications are demonstrated using the proposed integrated VLC-RF system. Experiments of real-time sensor transmission, image transmission and integration of the robot arm system indicate benefits in a practical scenario by showing millisecond-order latency via VLC, such as response time, round-trip time (RTT) and one-way latency.

Misalignment poses a significant challenge in optical wireless communication (OWC) links, disrupting the trajectory of collimated light beams and causing continual beam displacement at the receiver end. This paper introduces a low-cost and robust real-time beam tracking system that enables the receiver to accurately track the beam and align itself. The approach leverages a novel configuration of low-cost tracking photodiodes (PDs) arranged around a high-bandwidth central data PD, combined with a differential intensity detection algorithm and real-time two-axis position adjustment. The results tested in real-time OWC system show that the tracking system effectively mitigates misalignment, yielding an average signal-to-noise ratio (SNR) enhancement of approximately 80 dB in free-space experiments.

Optical wireless communication (OWC) has been considered a promising solution for overcoming the spectrum limitations of RF in wireless communication. However, studies on OWC systems, particularly on actual real-time prototyping, are limited and only focus on simulation and experiments using waveform generators and oscilloscopes. Meanwhile, the data rate requirements for optical communication systems are rapidly increasing, approaching the Gbps range. Therefore, the development of real-time prototypes with high data speeds is essential and requires super sample rates (SSR), necessitating signal processing blocks capable of processing multiple samples in parallel within a single clock cycle while operating at very high clock frequencies. To address these requirements, this work proposes a parallel implementation of an orthogonal frequency division multiplexing (OFDM) baseband processor on an FPGA for OWC systems. The proposed system operates with a bandwidth of 320 MHz, achieving a maximum raw data rate of 720 Mbps at 16-QAM 3/4 modulation. This performance is comparable to that of the IEEE 802.11be WiFi 7 standard. The proposed system is applicable not only to underwater OWC but also for indoor OWC (e.g. Li-Fi), broadening its potential use cases in next-generation wireless communication networks.

Light Fidelity (Li-Fi) is the complementary technology to the RF based communication in 6G. It uses optical spectrum which is completely free and unregulated. Sampling frequency offset (SFO) is an issue related to the mismatch between TX and RX clock oscillators. However, this issue is often assumed to be ideal in many Li-Fi research. In this work, we aim to address this issue by proposing an FPGA implementation of SFO compensator module for OFDM system. We optimize an SFO compensation method so that it can be implemented in FPGA efficiently in terms of resource usage. We evaluate the module by integrating it with the OFDM processor, Linux TCP/IP stack, and tested with TCP data. Real-time experiment results demonstrate that with our SFO compensator, the EVM of our Li-Fi system can be decreased up to 6.7% and 1.4% for QPSK and 16-QAM, respectively. Frame lost is decreased by 22.42% and TCP data rate is increased by 8.25x.

This work presents a comparative study of frequency-based modulation techniques for visible light communication (VLC). The study focuses on frequency shift keying (FSK) and frequency shift chirp modulation (FSCM) schemes, considering their performance in bandwidth-limited channels. FSCM exhibits superior performance over FSK in scenarios characterised by bandwidth limitations and frequency-selective channels, making it an energy-efficient choice. In two experiments conducted in free-space and underwater channels, FSCM yielded higher data rates with lower energy requirements than FSK. These results corroborate the simulation findings of performance in band-limited channels.

A Rydberg sensor is an atomic-based sensor which has great potential for detection of radio frequency (RF) communication signals. It has recently been studied as an alternative to conventional antennas to detect analogue or digital RF signals providing benefits of high sensitivity and very wideband frequency tuning. In addition, adaptive antenna arrays are a widely utilized technique for improving the directivity of reception and power gain, mitigating interference. Here, we demonstrate a new approach to design optimal antenna arrays of multiple Rydberg-atom based sensors. This design will be based on a regular layout of positions of Rydberg sensors for modularity. We then operate iterative random search of array geometry patterns to identify the best configuration for the performance of the antenna array in terms of directivity. This algorithm permits an overall multi-band antenna array design which is composed of multiple optimal sensors sub-arrays working at different carrier frequencies,

This article presents the performance analysis of angular diversity receivers (ADRs) and proposes a novel ADR system for visible light communication (VLC) in Internet of Things (IoT) sensor networks. The numerical analysis applying the combining techniques indicates that any combination of ADR types and diversity techniques can enhance performance. Therefore, a frustum-type ADR is implemented in the experimental demonstration with the equal gain combining (EGC) for signal-to-noise (SNR) enhancement. The ADR and the transceiver’s analog front end (AFE) are designed and prototyped. The proposed ADR system shows a 4 dB increase in received SNR under two access points. The bit error rate (BER) is also improved, and the link coverage is more than doubled. Additionally, to show the feasibility of the IoT application, a real-time sensor network system using the ADR is demonstrated. This experimental duplex system indicates the benefits in a practical scenario by showing packet error rate (PER) and response time.

In this experimental study, data transmission over step-index plastic optical fiber (SI-POF) is implemented with multilevel pulse amplitude modulation (PAM-M) scheme using a resonant cavity light emitting diode (RC-LED) as the optical transmitter. An artificial neural network (ANN) based equalizer is used in the RC-LED SI-POF system to jointly mitigate channel distortion and system non-linearities. Furthermore, the ANN equalizer's performance is compared to state-of-the-art equalizers used for the system - the Volterra equalizer and the decision feedback equalizer (DFE). The ANN equalizer offers the best bit rates compared to the others for the system with high non-linearities. For instance, at a BER of about 10−3, the ANN equalizer results in a bit rate of 780 Mbps, 710 Mbps, and 650 Mbps with PAM-2, PAM-4 and PAM-8 schemes, respectively. However, the Volterra equalizer results in a bit rate of 720 Mbps, 660 Mbps, and 580 Mbps with these PAM schemes, respectively. And with the DFE, the bit rates are 610 Mbps, 510 Mbps, and 120 Mbps, respectively.

This paper presents the performance analysis of angular diversity receivers (ADRs) in visible light communication (VLC) for multi-cell networks. Different ADR configurations are compared through numerical analysis for indoor down-link scenarios, they are: square pyramid (SP), square frustum (SF), hexagonal pyramid (HP) and hexagonal frustum (HF). Additionally, three receive diversity techniques are applied: selection combining (SC), equal gain combining (EGC) and maximum ratio combining (MRC). It is shown that the signal-to-interference-plus-noise ratio (SINR) and bit error rate (BER) is improved when the frustum type ADR with SC is used. Our results also indicate that the HP and HF ADR, which has 6 photo detectors (PDs) on the side, do not improve the BER performance compared to the SP and SF type with 4 PDs on the side when SC is applied.

This paper presents an experimental study of water-to-air optical camera communication (OCC) in the presence of wavy surface conditions. It evaluates the system performance in terms of the signal-to-noise ratio (SNR), considering various camera exposure times and regions of interest (ROI) in captured images. The results reveal a noticeable reduction in the SNR due to the presence of surface waves. However, the system performance is improved by considering the average value of several illuminated pixels produced by the point spread function as the ROI. Furthermore, the statistical characteristics of the received optical signals in the presence of surface waves are assessed.

This paper presents the performance analysis of angular diversity receivers (ADRs) in visible light communication (VLC) multiple-input multiple-output (MIMO) networks. The performance of different ADR configurations is compared through simulation and numerical analysis for indoor downlink scenarios: square pyramid (SP), square frustum (SF), hexagonal pyramid (HP) and hexagonal frustum (HF). It is shown that the bit error rate (BER) performance versus ADR rotation angles improves when the hexagonal type ADR is used. Our results also indicate that the HF ADR, which has the largest number of photodetectors (PDs) with seven, has the widest coverage for multi-stream.

In this paper, multi-tier LiFi networks are studied in terms of energy efficiency (EE) and spectral efficiency (SE), which are crucial metrics for LiFi system design. We derived a closed-form expression of the user association probability for different tiers using stochastic geometry based Poisson Voronoi Tessellation (PVT) LiFi network. The performance metrics of the network, EE and SE, are analyzed in terms of different parameters such as transmit power and Lambertian index. Performance evaluations and numerical results show that multi-tier LiFi networks have an optimum transmit power in which EE is maximized. Besides, increasing the transmit power does not increase SE after passing a threshold point. The resulting trade-off between EE and SE is presented.

This paper analyzes a new and accurate overthe-air time synchronization (TS) method for industrial LiFi networks which achieves synchronization via the exchange of timestamps between nodes. The necessity for accurate TS will become increasingly important with future networks due to its connection with industrial applications. Over-the-air TS vision is to achieve accuracy of the order of 1 µs. In this technique, timestamps are transmitted using optical signals which are used for the estimation of the time-of-arrival (TOA). To this end, the Cramer-Rao Lower Bound (CRLB) is computed as the theoretical limit on the performance and accuracy. In this way, the effects of distance, optical power, and semi-angle at half illuminance of the transmitter are investigated. Calculations and comparison show that the proposed technique can be efficiently used for TS in future wireless networks.

This paper presents an experimental comparison of pulse amplitude modulation (PAM), carrierless amplitude and phase (CAP), and quadrature amplitude modulation-orthogonal frequency division multiplexing (QAM-OFDM) in a turbid underwater optical wireless communications (UOWC) channel. It is shown that the transmission rates of PAM and CAP are maintained at around 180 Mbps even as turbidity increases. However, the highest transmission rate of 472 Mbps is achieved using 16-QAM-OFDM in tap water. Finally, bit power loading (BPL) is applied to further improve the performance of QAM-OFDM, yielding an increased maximum transmission rate of 557 Mbps in tap water.

Using a single low-power LED, we present a probabilistic shaped (PS) VLC system with near Shannon capacity transmission rates. For the channel conditions under consideration and a single 21 MHz −3 dB modulation bandwidth LED, probabilistic shaped symbols resulted in Gbps transmission rates. Comparatively, the PS resulted in above 27 % higher transmission speed than the widely used adaptive bit-power loading algorithm under the same channel conditions.

This paper presents a two-tier LiFi network and analyses the cross-tier handover rate between the primary and secondary cells. Based on the semiangle at half illuminance of the primary and secondary cells, we propose coverage model for the secondary cells. Using stochastic geometry, closed-form expressions are derived for the cross-tier handover rate and sojourn time in terms of the received optical signal intensity, time-to-trigger and user mobility. The analytical models are validated with simulation results.

This work proposes a dual pairwise coding (DPWC) technique for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) systems. DPWC is a technique designed to mitigate the signal-to-noise ratio (SNR) imbalance created in the received information symbols due to the light emitting diodes' bandwidth limitations and the strong correlation of typical indoor VLC MIMO channels. With singular value decomposition precoding, the MIMO VLC channel is decomposed into independent parallel sub-channels. The DPWC technique is then applied to further improve the system performance by pairing and interleaving OFDM subcarriers and MIMO sub-channels with imbalanced SNR. The error performance of the proposed DPWC technique is investigated in detail via simulation and experimental demonstration. Its effectiveness with different degrees of bandwidth-limitation (data rate) and channel correlation is presented, and the results are compared with other variants of the pairwise coding technique. DPWC provides performance improvement with a significant reduction in the SNR required to achieve a specific bit error rate (BER).

Orthogonal frequency-division multiplexing (OFDM) is a popular multi-carrier technique used in many digital communication systems such as wireless fidelity (Wi-Fi), long term evolution (LTE) and power line communication systems. It can be designed using fast Fourier transform (FFT) or wavelet transform (WT). The major drawback in using WT is that it is computationally inefficient. In this study, we introduce a simple and computationally efficient WT, harmonic wavelet transform, for OFDM signal processing. The new WT uses the orthogonal basis functions of conventional FFT-OFDM except that it involves translation and dilation of the input signal; the new wavelets is referred to as harmonic wavelets (HW). When compared with pilot-assisted OFDM system in terms of reduction in the peak-to-average power ratio, the results show that HW-OFDM outperforms FFT-OFDM by 3 dB at 10-4CCDF (complementary cumulative distribution function). Over Rayleigh fading channel with additive white Gaussian noise (AWGN), the bit error ratio of both FFT-OFDM and HW-OFDM perfectly matched, showing that the proposed HW-OFDM is better in terms of peak-to-average power ratio reduction.

Generalised spatial carrierless amplitude and phase modulation (GS-CAP) is developed in this paper to improve the spectral efficiency of the conventional CAP in indoor visible light communication. The proposed GS-CAP preserves the attractive low complexity transceiver of CAP by exploiting the spatial domain for performance improvement. Bit error rate (BER) analysis is derived for the performance evaluation of the proposed GS-CAP and is shown to be in excellent agreement with the simulation results. The impact of different system parameters, such as the number of active light emitting diodes (LEDs) and the number of receiving photodiodes (PDs), on the performance of GS-CAP are investigated. In addition, a precoding technique is developed for improving the BER performance of GS-CAP when the channel gains are similar. Finally, it is found that for different GS-CAP system configurations with the same spectral efficiency, the configuration with the smallest number of active LEDs results in optimum power efficiency and BER performance.

In this study, we investigate the performance of split-complex multi-layer perceptron (SC-MLP) equalizer for a step index plastic optical fibre (SI-POF) communication link employing the carrierless amplitude and phase (CAP) modulation scheme. The SC-MLP equalizer is proposed to mitigate the effect of intersymbol interference (ISI) and non-linearity in the system. It is demonstrated that the SC-MLP offers significantly superior bit error rate (BER) performance compared to the complex-valued transversal equalizer and the complex-valued Volterra equalizer in a highly non-linear SI-POF system. For example, for a 10 m SI-POF, a data rate of 15 Gbps is attainable at a BER of 10-3with SC-MLP compared to 10 Gbps with the complex-valued transversal equalizer using CAP-64.

In this study, data transmission over step-index plastic optical fiber (SI-POF) is demonstrated using orthogonal frequency division multiplexing (OFDM) scheme with sub-carrier pairwise coding (PWC). To investigate the performance of OFDM with PWC, a simulation study is implemented with QPSK and 16-QAM format using the Gaussian theoretical frequency response for a 60 m SI-POF channel. For a data rate of 200 Mbps with QPSK-OFDM, the study shows that compared to equalization, PWC offers about 1.5 dB improvement at a bit-error-rate (BER) of 10-3. An experiment is then set-up with a 60 m SI-POF that uses an eye-safe resonant cavity light emitting diode (RC-LED) as the optical source. The experiment shows that at a bit rate of 200 Mbps with QPSK-OFDM, PWC offers a 1.82 dB improvement in SNR at a BER of 10-3. For the same data rate with 16-QAM-OFDM, PWC offers about 1.1 dB Q2-factor improvement for a modulation index of 0.8. The study therefore shows that PWC improves the BER performance of an SI-POF link that uses OFDM.

This paper presents an analytical investigation into the performance of an underwater optical wireless cellular network based upon the non orthogonal multiple access (NOMA) scheme in the presence of weak underwater turbulence. The performance metrics used are the probability that all nodes in a network can achieve a connection and the total cell capacity. Coverage probability and cell capacity are solved numerically and the results presented. Further it is shown that the strength of turbulence affects the maximum data rate supported by a cell as well as the coverage probability. In a NOMA cell however the number of users is shown to be a bigger limiting factor than turbulence.

This work investigates the impact of timing offset on the performance of an optical spatial modulation (OSM) technique termed spatial pulse position modulation (SPPM). SPPM combines optical spatial shift keying (OSSK) with pulse position modulation (PPM), exploiting the spatial domain to improve spectral the efficiency of PPM. In most works related to OSM, a perfect synchronization between the transmit and receive units is assumed. However, timing offset can result from clock jitter and variation in propagation delay of the transmit paths. Timing offset introduces intersymbol interference which can degrade system performance, hence the need to investigate its effect. Using the union bound technique, we derive the analytical upper bound on the symbol error rate (SER) of an SPPM-based optical wireless communication system that is impaired by timing offset. The analysis is validated via closely matching simulation results. Results show that error performance degrades as timing offset increases. For a small range of timing offset values, OSSK shows a better tolerance compared to SPPM.

In this paper, the performance of optical spatial modulation (OSM) in the indoor multipath optical wireless channel is investigated. Multipath propagation of the transmitted signal results in the temporal dispersion which causes intersymbol interference (ISI) especially in high-speed communications. The OSM schemes have been investigated in line-of-sight (LOS) channels. However, given the recent trend in Gigabits/s communication, there is a need to investigate the impact of the neglected higher-order reflections. Two variants of OSM are explored as case studies: optical space shift keying and spatial pulse position modulation. The multipath-induced ISI is modeled to account for the spreading of the transmitted signal, and the analytical upper bound on the symbol error rate of both OSM schemes in the multipath channel is derived. The derived analytical bounds are validated by closely matching simulation results. Furthermore, using the spatial distributions of the multipath-induced power penalty, the LOS channel response and the delay spread across the room, we demonstrate how the interaction between these parameters impacts error performance. Multipath-induced ISI has a significant adverse impact on the performance of OSM schemes, particularly on the detection of the activated transmitter as the incurred ISI can alter the channel gain of the received symbol.

We propose and experimentally demonstrate for the first time, M-PAM transmission through a hybrid POF/VLC downlink using off-the-shelf components. The link consists of 60 m SI-POF and 1 m VLC and with the aid of a blue filter, the link has an effective bandwidth of 17 MHz. Conventional adaptive decision feedback equaliser (DFE) and multi-layer perceptron DFE (MLP-DFE) are also implemented and their bit error rate (BER) performances are measured and compared with offline (MATLAB) implementation. For the conventional DFE, the achievable data rates at a BER of 10-3are 60, 91.5 and 80 Mbps for M = 2, 4 and 8 respectively. Higher data rates are achieved for MLP-DFE at the same BER of 10-3as we obtained 61.5, 95 and 85 Mbps for M = 2, 4 and 8 respectively. Thus, we show that MLP-DFE offers superior BER performance than conventional DFE for higher PAM level.

In this paper, a linear adaptive least mean square (LMS) fractionally spaced equalizer (FSE) is proposed to jointly mitigate the severe inter-symbol interference (ISI) and timing jitter encountered by carrierless amplitude and phase modulation (CAP) in visible light communication (VLC) systems. The performance of FSE is compared to that of its counterpart, the symbol spaced equalizer (SSE), at the forward error control (FEC) bit error rate (BER) limit. It is shown that at a signal-to-noise ratio (SNR) of 20 dB and FEC BER limit of 3 × 10-3, FSE is able to achieve a bit rate of 95 Mb/s (spectral efficiency η of 14.6 bits/s/Hz) while only 30 Mb/s (η of 4.6 bits/s/Hz) is possible with SSE using CAP-64. It is observed that the FSE performance is insensitive to timing jitter while SSE performance suffers severe degradation. The results therefore indicate that FSE is better suited for CAP-based VLC systems than the widely used SSE.

In this work, we design an infrared protocol (IRP) for light emitting diode (LED) based indoor positioning. The designed IRP compensates for the shortcomings of other existing protocols when applied to the multiple LED estimation indoor positioning model (MLEM). MLEM uses overlap of LED beams to increase accuracy of positioning. The overlap sets up a multipoint-to-point optical communication channel. The existing protocols which are designed for point-to-point links, when modified to suit the MLEM overlapping region, show a high positioning time between 3 s and 4.5 s. These values are not desirable for real time tracking. A new protocol is therefore designed to reduce the positioning time. The protocol is implemented in an experimental MLEM design using ATmega 328 microcontroller hardware. The experimental results show the new protocol reduces the positioning time to 0.5 s.

This paper experimentally investigates the use of a single input multiple output (SIMO) free space optical (FSO) communication system in an indoor laboratory controlled turbulence chamber. Two receiver circuits are used with a single laser input. The current outputs of two receivers were combined using equal gain combining (EGC) scheme and compared with single input single output (SISO) FSO system. A weak turbulence was generated within the atmospheric chamber and link performances are measured using ß-factor and bit error rate (BER) for On-Off Keying (OOK) modulation scheme. The results demonstrated that the simplest form of SIMO can improve the performance of the FSO communication channel under weak turbulent condition.

This paper examines the effect of synchronization error on the performance of spatial modulation (SM) technique in optical wireless communication (OWC) systems. SM exploits the deployment of multiple transmitters by encoding user information on their spatial domain. In most works related to SM, a perfect synchronization among these multiple transmitters is assumed. However, synchronization error can result from multipath propagation in OWC channel, and clock jitter and variation in propagation delay of each transmitter. Synchronization error degrades system performance and hence the need to investigate its effect. Using union bound technique, and defining synchronization errors as timing offsets in the received signals, we derive the symbol error rate for space shift keying (SSK), generalized SSK (GSSK), SM, and generalized SM (GSM) schemes, and we validate our analysis with tightly matched simulation results. Results show that degradation in performance increases with synchronization error. While SSK is tolerant for a small range of synchronization error, GSSK, SM, and GSM are significantly impaired. Our results also demonstrate the dependence of SM on channel gain values. We observe that the lower the channel gain of the transmitter in which synchronization error occurs, the lesser the impact of the synchronization error on the system performance.

In this paper, we investigate the performance of a spatial modulation scheme that combines the energy efficiency of pulse position modulation (PPM) with the spectral efficiency of generalized space shift keying (GSSK) for optical wireless communication system. The combined scheme is called generalized spatial pulse position modulation (GSPPM). We present the error performance analysis and validate the results through simulation. Error performance results show that the upper bound derived in the theoretical analysis is tight for all cases when the symbol error rate is below 0.1. The influence of parameters such as channel path gain and number of PPM slots on GSPPM's performance is illustrated. GSPPM is compared with some of the existing spatial modulation techniques, and results show that GSPPM provides an attractive trade-off between energy and spectral efficiency.

In this paper, the performance of non-orthogonal multiple access (NOMA) is characterized in a downlink visible light communication system for two separate cases. In the case of guaranteed quality of service (QoS) provisioning, we derive an analytical expression of the system coverage probability and show the existence of optimal power allocation coefficients on two-user paired NOMA. In the case of opportunistic best-effort service provisioning, we formulate a closed-form expression of the ergodic sum rate, which is applicable for arbitrary power allocation strategies. The probability that NOMA achieves higher individual rates than OMA is derived. Also, we give an upper bound of the sum rate gain of NOMA over OMA in the high signal-to-noise ratio regime. Both the theoretical and simulation results prove that the performance gain of NOMA over OMA can be further enlarged by pairing users with distinctive channel conditions. We also find out that the choice of light emitting diodes (LEDs) have a significant impact on the system performance. In the case of guaranteed QoS provisioning, the LEDs with larger semi-angles have better performance; while in the case of opportunistic best-effort service provisioning, the LEDs with 35° semi-angle give nearly optimal performance.

We present recent progress on visible light communication systems using polymer light-emitting diodes as the transmitters and a commercial silicon photodetector as the receiver. In this paper, we use transmitters at red, green, and blue wavelengths to investigate the maximum on-off keying link performance of each device type as the first steps toward a wavelength-division multiplexed link. We show that a total transmission speed of 13 Mb/s is achievable when considering the raw bandwidth of each of the RGB PLEDs. Such a rate represents a 30% gain over previously demonstrated systems. Further capacity improvement can be achieved using high performance artificial neural network equalizer offering a realistic prospect for transmission speeds up to 54.9 Mb/s.

In this paper, we experimentally demonstrate a multiband carrierless amplitude and phase modulation format for the first time in VLC. We split a conventional carrierless amplitude and phase modulated signal into m subcarriers in order to protect from the attenuation experienced at high frequencies in low-pass VLC systems. We investigate the relationship between throughput/spectral efficiency and m, where m = {10, 8, 6, 4, 2, 1} subcarriers over a fixed total signal bandwidth of 6.5 MHz. We show that transmission speeds (spectral efficiencies) of 31.53 (4.85), 30.88 (4.75), 25.40 (3.90), 23.65 (3.60), 15.78 (2.40), and 9.04 (1.40) Mb/s (b/s/Hz) can be achieved for the listed values of m, respectively.

The advent of data intensive applications on the mobile devices is putting a strain on the available RF spectrum. Recently, visible light wireless communications has become a desirable alternative due to the low cost of light emitting diode (LED) devices which can offer data rates in the Gigabit per second range while providing lighting. In addition, visible light belongs to the unlicensed part of spectrum which is easily reusable due to the fact that light does not penetrate walls. Various modulation techniques have been adopted for optical wireless because negative signals (or complex signals) cannot be used due to the fact that light intensity is a positive quantity. Orthogonal frequency division multiplexing (OFDM) has been successfully modified for optical wireless but it suffers from high peak-to-average-power ratio (PAPR). High PAPR causes LED to operate in nonlinear region which leads to performance degradation. We discuss a technique to reduce PAPR while keeping averaging power constraint which is important for the purpose of lighting.

In this paper the error performance analysis of a low complexity, multiple transmitter generalised space shift keying (GSSK) signalling technique for short range indoor visible light communications is presented. In an N_t transmitter system, this signalling technique is capable of achieving a spectral efficiency of N_t bits/s/Hz. The GSSK system has a higher spectral efficiency than the conventional on-off keying (OOK) and pulse position modulation (PPM) techniques. The GSSK transmitter is much simpler than that of an equivalent regular pulse amplitude modulation (PAM) system with similar spectral efficiency. An analytical expression for the symbol error rate (SER) is presented and verified using simulations. The receiver is based on the maximum likelihood criterion and using multiple photodetectors (PDs) in the receiver improves the error performance. Within the room and the transmitter-receiver configuration under consideration, with four PDs, a gain of 6 dB is attainable over the single PD system for the case of 2 and 3 bits/s/Hz. Considering a fixed link range with N_t = 2 and at a symbol error rate of 10-6, using wide half angle LEDs with Φ1/2= 60° requires about 12 dB less in electrical signal-to-noise ratio compared with Φ1/2= 30°. The beam directivity is however of much lower effect when the room under consideration is equiped with sufficient LEDs to provide full coverage. The GSSK technique can also support dimming control by using different pulse widths. This, however, is at a price of reduced spectral efficiency.

Spatial Modulation (SM) is a relatively new tech- nique to include information bits in the spatial domain, combining multiple-input multiple-output (MIMO) and digital modulation techniques. The increase on the overall data rate is achieved by activating only one transmitter at any time, whereas the index of the active emitter represents data bits. At the receiver side the active transmitter index is estimated based on the channel gain for the different paths. In this work we show the behaviour of an optical SM wireless communications system. A fully operative test system consisting of four transmitters and one receiver has been implemented and tests have been conducted to obtain the symbol error rate (SER) for different link distances. Results show the strong relation between the error rate achieved and the relative channel gain differences between all channels rather than the individual signal-to-noise ratio (SNR).

In this work, the performance of binary phase shift keying (BPSK) subcarrier intensity-modulated terrestrial free-space optical (FSO) communication link is analysed in the presence of noise and atmospheric turbulence. To ameliorate channel fading caused by the atmospheric turbulence, we propose a subcarrier time delay diversity (STDD) scheme in which the delayed versions of the original data are re-transmitted on different subcarriers. In short range links, a single re-transmission scheme results in an estimated gain of up to 4.5 dB in received irradiance. With sufficient link margins, short range FSO links might be able to cope without the use of any diversity techniques but for links spanning over 1 km, turbulence induced fading effect must be compensated for. In this study, the authors show that the subcarrier STDD is a viable scheme for this purpose.