Optical wireless communication (OWC) is emerging as a potential technology for next-generation communication systems. However, there are few measurements of the optical channel, which limits the full understanding of the benefits and challenges of the technology. Here, we report the first channel measurements in a realistic environment using a visible laserbased channel sounder. The OWC channel sounder comprises two swept-frequency$\mathbf{4 0 5 - n m}$laser sources and a highly sensitive silicon photomultiplier mounted at the back of a narrow-field-of-view telescope. Channel characteristics in terms of multipath and bandwidth at different elevation and azimuthal angles can be measured for channel bandwidths of up to 900 MHz.

Radio-frequency (RF) systems are employing higher carrier frequencies (into the THz bands) leading to more limited link budgets. Above a particular RF carrier frequency, optical wireless communications (OW) systems can have a more favourable link budget, due to the different frequency dependence of signal propagation. In this paper, we present a study that estimates the RF carrier frequency at which this occurs, referred to as the cross-over RF frequency. The results show that there exists a cross-over RF frequency above which the OW system outperforms the RF system in terms of BER, which under LED background light can be as low as a few hundreds of GHz. Different receivers and background lighting conditions are also considered. A silicon photomultiplier (SiPM) detector is shown to outperform an avalanche photodiode (APD) detector in this background light conditions, while the latter becomes superior in higher sunlight conditions emphasising the crucial need for an optical filter for the SiPM.

Visible Light Communications (VLC) allow new solutions and applications thanks to the advantages of optical wireless communications. It can be used on any mobile phone device, since these already bear the required hardware component (LED in the mobile screen), and as a result inmediate application is possible. The technological principle of VLC in controlled access is based on the use of the mobile LED (screen), which can transmit codes by modulating the LED light in the sub-second domain and offered in an App, and an optical receiver in the device to be accessed, which is able to decode and verify the codes sent. Given the universal integration of mobile devices in our society, LightKey project is an intuitive and multi-purpose solution for any user, and is able to ensure a secure and personal controlled access. For industrial applications LightKey can become a middleware between users and a host. This implies that LightKey will not store any personal data, but will communicate with the host, mainly by wireless communication, and the host will be responsible for validating the codes received.

Visible light communication (VLC) transceivers suffer from optical interference and ambient light saturation, which severely affect the reception of optical signals. In this paper, we discuss various techniques for the suppression of ambient light in electronic, optical, communication and wavelength domains with focus on VLC systems for vehicular communication. Designing transceivers for outdoor VLC systems is challenging due to the dynamic environment and the time-varying nature of interference from different types of ambient light sources. This is a unique study that addresses the problem of optical interference in multiple physical domains and offers qualitative insights in a systematic manner.

This work studies the quality of service (QoS) performance of wireless networks that integrate light fidelity (LiFi) and wireless fidelity (WiFi). While the hybrid network is potential for improving network capacity, load balancing becomes essential and challenging due to the nature of heterogeneous access points (APs). A number of studies have been conducted to address this issue, focusing on maximising the network capacity with user fairness constraints. However, in practice, QoS metrics including packet loss ratio and latency are important to network services. In this paper, QoS-driven load balancing is studied for hybrid LiFi and WiFi networks (HLWNets) in two scenarios: single-AP association (SA) and multi-AP association (MA). In each case, an optimisation problem is formulated to minimise the packet loss ratio and latency, and a low-complexity iterative algorithm is proposed to solve the problem. Results show that the novel methods, especially MA, can effectively balance the traffic loads among the APs and improve the QoS performance. In addition, the more subflows the better performance MA provides. Targeting the same level of QoS, MA can achieve a system throughput up to 160% higher than the signal strength strategy and 130% higher than the proportional fairness load balancing.

Combining the high transmission speed of light fidelity (LiFi) and the ubiquitous coverage of wireless fidelity (WiFi), hybrid LiFi and WiFi network (HLWNet) has attracted intensive research interest. While such a network can boost the system capacity, it faces a challenging issue in handover since the coverage areas of LiFi and WiFi completely overlap each other. Also, LiFi has a relatively short coverage range from a single access point (AP). As a result, HLWNets are susceptible to frequent handovers, in terms of both handovers between LiFi and WiFi and handovers between two LiFi APs. Therefore handover skipping was introduced, which enables handovers between two non-adjacent APs. Conventional handover skipping schemes rely on knowledge about the user's trajectory, and are difficult to implement in practice. Alternatively, the rate of change in reference signal received power (RSRP) can be used to reflect the user's moving direction. Based on this, an adaptive handover scheme is proposed in this paper, which varies the user's network preference according to the user's speed. Results show that in comparison with the standard and trajectory-based schemes, the proposed approach can improve user throughput by up to about 120% and 30%, respectively.

Light fidelity (LiFi) is a relatively new wireless communication technology that exploits the optical spectrum. Compared to wireless fidelity (WiFi), LiFi is densely deployed with each access point (AP) covering an area only a few meters in diameter. Also, LiFi users are susceptible to intermittent light-path blockages. Meanwhile, the user is served by a single AP in conventional LiFi systems. This would cause frequent handovers for LiFi users, resulting in a degradation in quality of service. In this paper, parallel transmission is investigated for LiFi, which is named PT-LiFi. With a delicate design of the transmitter and the receiver, PT-LiFi enables multiple LiFi APs to serve the user simultaneously. Particularly, data transmission continues without interruption when the user is losing connectivity to some of the connected APs. Resource allocation is studied for the PT-LiFi system, and a novel load balancing method is proposed to jointly allocate resource across the APs. Results show PT-LiFi can make efficient use of the densely deployed LiFi APs and provide a flexible way of load balancing. Compared with a conventional LiFi system, the proposed method can increase user throughput by up to 150% and improve user fairness by up to 15%.

This work investigates handover in hybrid light fidelity (LiFi) and wireless fidelity (WiFi) networks (HLWNets). In such a network, the handover process becomes challenging due to two main factors: i) the relatively short coverage range of a single access point (AP), and ii) the largely overlapping coverage areas of different networks. As a result, HLWNets are susceptible to frequent handovers. To reduce the handover rate, the concept of handover skipping (HS) was introduced, which enables handovers between non-adjacent APs. However, conventional HS methods rely on knowledge about the user's trajectory, which is not readily available at the AP. In this paper, a novel HS scheme is proposed on the basis of reference signal received power (RSRP) and its rate of change, with an adaptive network preference adopted. Since RSRP is commonly used in the existing handover schemes, the proposed method requires no additional signalling between the user and the AP. Simulation results show that the new method can effectively reduce unnecessary handovers, especially those between LiFi and WiFi. Compared to the standard and trajectory-based handover schemes, the proposed method improves network throughput by up to about 120% and 30%, respectively.

Using optical communications for smart dust applications enables small size of transceivers and offers a potentially large power advantage over RF. This paper presents an optically powered transceiver, which consists of on-chip solar cells, an optical receiver, a storage capacitor, and a passive transmitter formed by a liquid crystal (LC) modulator and a modulated retro-reflector (MRR). The transceiver circuit system has been fabricated using a standard 0.18-$ {\mu }\text{m}$CMOS process and the circuit area is$1 {\times }1.7$mm2excluding the MRR. Measurement results show that the complete transceiver can successfully work for 1-kb/s downlink and 10-b/s uplink under 0.5-V voltage supply and consume 53 nA totally. This indicates the transceiver can work at least in a 10-m range. It is successfully demonstrated that the transceiver scavenged the power from a 670-nm modulated laser beam sent by a base station (BS), extracted clock signal and the encoded data from the beam, decoded two designed instructions, switched theLCin the MRR and sent responses back to the BS. To the author’s best knowledge, this is the first time to present that an on-chip solar-cell powered transceiver realizes the two-way optical wireless communications for Internet of Things applications.

Considering reflected light in physical layer security (PLS) is very important because a small portion of reflected light enables an eavesdropper (ED) to acquire legitimate information. Moreover, it would be a practical strategy for an ED to be located at an outer area of the room, where the reflection light is strong, in order to escape the vigilance of a legitimate user. Therefore, in this paper, we investigate the impact of multipath reflections on PLS in visible light communication in the presence of randomly located eavesdroppers. We apply spatial point processes to characterize randomly distributed EDs. The generalized error in signal-to-noise ratio that occurs when reflections are ignored is defined as a function of the distance between the receiver and the wall. We use this error for quantifying the domain of interest that needs to be considered from the secrecy viewpoint. Furthermore, we investigate how the reflection affects the secrecy outage probability (SOP). It is shown that the effect of the reflection on the SOP can be removed by adjusting the light emitting diode configuration. Monte Carlo simulations and numerical results are given to verify our analysis.

Visible light communications (VLC) has the potential to play a major part in future smart home and next generation communication networks. There is significant ongoing work to increase the achievable data rates using VLC, to standardize it and integrate it within existing network infrastructures. The future of VLC systems depends on the ability to fabricate low cost transceiver components and to realize the promise of high data rates. This paper reports the design and fabrication of integrated transmitter and receiver components. The transmitter uses a two dimensional individually addressable array of micro light emitting diodes (μLEDs) and the receiver uses an integrated photodiode array fabricated in a CMOS technology. A preliminary result of a MIMO system implementation operating at a data rate of 1 Gbps is demonstrated. This paper also highlights the challenges in achieving highly parallel data communication along with the possible bottlenecks in integrated approaches.

The design, simulation and practical implementation for a bidirectional 1.25 Gbit/s indoor optical wireless communications system has been presented. It is a part of the European community funded hOME Gigabit Access project (OMEGA). The line-of-sight (LOS) system uses angle-diversity transceivers enabling discrete beam steering. Each transceiver uses three transmitting and receiving elements giving an overall field of view of ∼25×8° and a transmission range of 3 m. Measurement shows that the system can operate at a bit error rate below 10−9 without channel coding and the handover between cells is <400 ns. Detail of a demonstrator of high-definition (HD) video embedded in gigabit ethernet stream using this system is also reported.

The authors take a look at the existing 3G systems in service and investigate the capabilities of 4G, and while the theoretical throughput of these cellular systems is expected to be high, the future promises to offer more technological improvements and innovations.

The Myth: Sixty years of research following Shannon's pioneering paper has led to telecommunications solutions operating arbitrarily close to the channel capacity—“flawless telepresence” with zero error is available to anyone, anywhere, anytime across the globe.

Nonlinearities can drastically degrade the performance of OFDM (orthogonal frequency division multiplexing) based optical wireless (OW) communication systems using intensity modulation (IM) of the optical carrier. The light emitting diode (LED) transfer function distorts the signal amplitude and forces the lower signal peaks to be clipped at the LED turn-on voltage (TOV). Additionally, the upper signal peaks can result in optical output degradation. The induced distortion can be controlled by optimizing the bias point (BP) of the LED and/or backing-off the signal power modulating the LED. In this paper, the obtained experimental results using a hardware demonstrator for OW OFDM transmission based on field programmable gate array (FPGA) and off-the-shelf analog components are presented. The conducted measurements for the bit-error performance focus on determining the optimum BP and optimizing the OFDM signal amplitude to obtain best performance. In this context, the experimental bit-error ratio (BER) is obtained as a function of the LED BP and the RMS (root mean square) OFDM signal across the LED.

Future Home Access Networks (HANs) should operate at Gbit/s and provide data services that are as straightforward to use as an electrical outlet or any other utility. This is an extremely challenging goal, and an EU funded project 'Home Gigabit Access Networks' (OMEGA) aims to achieve this. In this project a significant part is to realize bi-directional Gigabit infrared (IR) optical wireless communications. In this paper we introduce the overall approach to the HAN, and outline the challenges and approaches required to implement these IR Gigabit links. Results from a proof-of-concept 1.25Gbit/s light-of-sight (LOS) IR link are shown, as well as designs for the first IR link demonstration.

Complete electronic and micro-mechanical systems can now be fabricated on the scale of hundreds of microns. Implementing radio frequency wireless communications with such 'smart dust' is challenging, due to the power required and the small size of any antennas that can be implemented. Optical wireless communications, using a modulated retro-reflector at the smart dust has the advantages of low-power consumption and highly directive channels that allow long communications ranges. In this paper we report the design and implementation of a communications system that uses a base station to communicate with, and power, smart dust motes, over ranges of 10s of metres. A base station that uses holographic beamsteering is described, and dust motes that use silicon ICs to provide communications, power and modulation control. Results indicate the dust mote will operate at a range of over 30m from the base station.

Solid-state lighting is a rapidly growing area of research and applications, due to the reliability and predicted high efficiency of these devices. The white LED sources that are typically used for general illumination can also be used for data transmission, and Visible Light Communications (VLC) is a rapidly growing area of research. One of the key challenges is the limited modulation bandwidth of sources, typically several MHz. However, as a room or coverage space would typically be illuminated by an array of LEDs there is the potential for parallel data transmission, and using optical MIMO techniques is potentially attractive for achieving high data rates. In this paper we investigate non-imaging and imaging MIMO approaches: a non-imaging optical MIMO system does not perform properly at all receiver positions due to symmetry, but an imaging based system can operate under all foreseeable circumstances. Simulations show such systems can operate at several hundred Mbit/s, and up to Gbit/s in many circumstances.

In this letter, we propose a novel receive antenna selection algorithm based on cross entropy optimization to maximize the capacity over spatially correlated channels in multiple-input multiple-output (MIMO) wireless systems. The performance of the proposed algorithm is investigated and compared with the existing schemes. Simulation results show that our low complexity algorithm can achieve near-optimal results that converge to within 99% of the optimal results obtained by exhaustive search. In addition, the proposed algorithm achieves near-optimal results irrespective of the mutual relationship between the number of transmit and receive antennas, the statistical properties of the channel and the operating signal-to-noise ratio.

Power consumption is an important design consideration for wireless sensor networks (WSNs). Hybrid radio frequency/free space optical (RF/FSO) WSNs have the potential to reduce this power consumption substantially. This paper studies the performance of the RF/FSO WSN by comparing its network lifetime against the lifetime of a traditional RF-only WSN. Two types of networks are considered: reconfiguring and non-reconfiguring networks. The results show that for the range of network parameters considered, the RF/FSO WSN lasts at least twice as long as its RF-only counterpart. This paper also discusses parameter selection for the best RF/FSO network coverage.

Two of the key components of sensor motes are a power source and a communications receiver. One approach to creating very small motes is to use free space optical communication and to scavenge power from the optical downlink. In this paper photodiodes are described that were made on a standard CMOS process and generate electrical power from the downlink beam. These photodiodes can be used to power the optical receiver that converts the modulation on the downlink to a bit stream.

Fibre to the home and other ‘last-mile’ transmission technologies provide end-user data rates of at least 100 of Mbit/s. These technologies are currently deployed around the world, and in the short term gigabit-class home access networks will be required if this capacity is to be fully used, and new services are to be developed. In order to meet this goal, the Home Gigabit Access project (OMEGA, EU FP 7-1) aims at a range of transmission techniques and intelligent control. A significant part of this effort is devoted to two areas of optical wireless communications.

Solid-state lighting is a rapidly developing field. White-light and other visible LEDs are becoming more efficient, have high reliability and can be incorporated into many lighting applications. Recent examples include car head-lights based on white LEDs, and LED illumination as an architectural feature. The prediction that general illumination will use white LEDs in the future has been made, due to the increased energy efficiency that such an approach may have. Such sources can also be modulated at high-speed, offering the possibility of using sources for simultaneous illumination and data communications. Such visible light communications (VLC) was pioneered in Japan, and there is now growing interest worldwide, including within bodies such as the Visible Light Communications Consortium (VLCC) and the Wireless World Research Forum (WWRF). In this paper we outline the basic components in these systems, review the state of the art and discuss some of the challenges and possibilities for this new wireless transmission technique.

The application of coherent detection to indoor optical free-space communications is considered here. Analytical expressions are derived for a wide line-of-sight (W-LOS) optical link which determine the coverage area and the required photo-detection area given different binary digital transmission schemes and a given bit rate. The coverage area is maximised for systems using both metal–semiconductor–metal (MSM) and PIN photo-detectors. Results show that MSM photo-detectors, due to their inherently lower capacitance per unit photo-detection area, result in a greater coverage area than their PIN counterparts. To demonstrate the viability of implementing coherent detection indoors, a 200 Mb/s LOS optical link which uses coherent detection is also reported.

Abstract—A low complexity user scheduling algorithm based on a novel adaptive Markov chain Monte Carlo (AMCMC) method is proposed to achieve the maximal sum capacity in an uplink multiple-input multiple-output (MIMO) multiuser system. Compared with the existing scheduling algorithms, our algorithm is not only more efficient but also converges to within 99 % of the optimal capacity obtained by exhaustive search. We demonstrate the convergence of the proposed scheduling algorithm and study the tradeoff between its complexity and performance. Index Terms—Adaptive Markov chain Monte Carlo (AM-CMC), multiple-input multiple-output (MIMO), multiuser selec-tion, scheduling, sum capacity. I.

Orthogonal netted radar transmitter signals require a very low aperiodic autocorrelation peak sidelobe level (PSL), low aperiodic cross-correlation, and a good resilience to small Doppler shifts. A new set of polyphase sequences is presented with good correlation properties as well as resilience to Doppler shifts. These sequences are built using numerical optimization based on correlation properties. A structural constraint is imposed on the optimized polyphase sequences, which maintains Doppler tolerance. Cross entropy (CE) technique is used to optimize the sequences. Correlation and Doppler results are compared with best-known sequences on various merit factors and shown to be superior

Abstract-Due to the complementary nature of radio and optical wireless (OW) both in capacity and coverage, the combined use of both for data transmission could have advantages over a single media. However, big technical challenges for vertical-handover (VHO) strategy arise for such an integrated system. According to different interruption types and traffic modes, two basic VHO schemes can be applied: immediate VHO (I-VHO) and dwell VHO (D-VHO). This paper proposes a novel fuzzy-logic (FL)-based decision-making algorithm for VHO, which is capable of combining the merits of both schemes to achieve excellent handover in terms of packet transfer delay for all the cases considered here. The strength of FL in handling uncertain and conflicting decision metrics is exploited. Since excessive transfer delay results in disrupted connection and corrupted service, the proposed FL-based VHO decision-making algorithm has the potential to provide a better quality of service (QoS) to users in future wireless broadband communications.

The indoor infrared wireless channel is nonstationary and subject to bursty errors. In such an environment, multiple access protocols play an essential role in system performance. Two novel polling schemes are proposed, which explore the use of beam blockage sensing and signalling, and make channel reallocation according to the channel state. Compared to other classic medium access control (MAC) protocols, it has significant performance improvement on both system throughput and average transfer delay.

A novel approach to the evaluation of bit error rate performance for directive optical wireless links, which takes into account shot noise, fluorescent lighting and intersymbol interference (ISI) is presented. The evaluation technique is based on nonclassical Gauss quadrature rules, with the fluorescent light and the ISI from high pass filtering being modeled mathematically. As an application of this method the BER performance of NRZ-OOK and Manchester modulation has been evaluated and numerical results are presented. It is shown that Manchester modulation is quite robust in combating fluorescent lighting interference and its performance is not critical on the selection of the HPF cutoff frequency.

Currently, there is considerable interest in infrared as an alternative medium to radio for indoor wireless communications, particularly for computer networking applications. This paper describes a novel IR-LAN link topology that employs an optical spatial-angular mapping technique which minimizes link losses and facilitates high link rates, whilst maintaining a useful coverage area. Blocking probability effects and overall system performance are examined in terms of aggregate transmission rates.