This paper attempts to clarify the difference between visible light communication (VLC) and light-fidelity (LiFi). In particular, it will show how LiFi takes VLC further by using light emitting diodes (LEDs) to realise fully networked wireless systems. Synergies are harnessed as luminaries become LiFi attocells resulting in enhanced wireless capacity providing the necessary connectivity to realise the Internet-of-Things, and contributing to the key performance indicators for the fifth generation of cellular systems (5G) and beyond. It covers all of the key research areas from LiFi components to hybrid LiFi/wireless fidelity (WiFi) networks to illustrate that LiFi attocells are not a theoretical concept any more, but at the point of real-world deployment.

What is LiFi

Light-Fidelity (LiFi) takes visible light communication (VLC) further by using light emitting diodes (LEDs) to realise fully networked wireless systems. Synergies are harnessed as lights become LiFi attocells resulting in enhanced wireless capacity for the Internet-of-Things (IoT), 5G and beyond.

http://www.homepages.ed.ac.uk/hxh/Li-Fi_PAPERS/15_cheng_ecoc_invited.pdf

In this paper, we present a comprehensive channel modeling and characterization study for visible light communications. Our study is based on ray tracing, which allows for an accurate description of the interaction of rays emitted from the lighting source within a specified confined space. Contrary to existing works, which are mainly limited to ideal Lambertian sources and purely diffuse reflections, our approach is capable of obtaining channel impulse responses (CIRs) for any nonideal sources, as well as specular and mixed specular–diffuse reflections. Furthermore, we can precisely reflect the presence of objects (e.g., furniture) and wavelength-dependent reflection characteristics of surface materials (e.g., ceilings, floor, walls, and furniture) in a channel study. As case studies, we consider a number of indoor environments with various dimensions and different surface materials, i.e., plaster, gloss paint, wood, aluminum metal, and glass. We further consider various scenarios with different transmitter specifications (i.e., single versus multiple transmitters and array type) and receiver specifications (i.e., location and rotation). For each environment, we obtain CIRs and present a channel characterization study where channel parameters, such as channel DC gain, root mean square (RMS) delay spread, coherence bandwidth, and mean excess delay, are obtained. We also make one-to-one comparisons between infrared and visible-light CIRs for the same environments to emphasize the differences between two optical bands.

Channel Modeling and Characterization for Visible Light Communications

In fifth generation wireless networks, the escalating teletraffic and energy consumption has necessitated the development of green communication techniques in order to further enhance both the system’s spectral efficiency and energy efficiency In the past few years, the novel index modulation (IM) has emerged as a promising technology that is widely employed in wireless communications In this paper, we present a survey on IM in order to provide the readers with a better understanding of its principles, advantages, and potential applications We start with a comprehensive literature review, where the concept of IM is introduced, and various existing IM schemes are classified according to their signal domains, including the frequency domain, spatial domain, time domain and channel domain Then the principles of different IM-aided systems are detailed, where the transceiver design is illustrated, followed by descriptions of typical systems and corresponding performance evaluation A range of challenges and open issues on IM are discussed before we conclude this survey

/pdf/novel-index-modulation-techniques-a-survey-4py1xcreox.pdf

Novel Index Modulation Techniques: A Survey

The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined.

/pdf/a-review-of-gallium-nitride-leds-for-multi-gigabit-per-4fgs6zvkvk.pdf

A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications

Unipolar communications systems can transmit information using only real and positive signals. This includes a variety of physical channels ranging from optical (fiber or free-space), to RF wireless using amplitude modulation with non-coherent reception, to baseband single wire communications. Unipolar OFDM techniques enable to efficiently compensate frequency selective distortion in the unipolar communication systems. One of the leading examples of unipolar OFDM is asymmetric clipped optical OFDM (ACO-OFDM) originally proposed for optical communications. Flip-OFDM is an alternative approach that was proposed in a patent, but its performance and full potentials have never been investigated in the literature. In this paper, we first compare Flip-OFDM and ACO-OFDM, and show that both techniques have the same performance but different complexities (Flip-OFDM offers 50% saving). We then propose a new detection scheme, which enables to reduce the noise at the Flip-OFDM receiver by almost 3dB. The analytical performance of the noise filtering schemes is supported by the simulation results.

Flip-OFDM for Unipolar Communication Systems

Unipolar communications systems can transmit information using only real and positive signals. This includes a variety of physical channels ranging from optical (fiber or free-space), to RF wireless using amplitude modulation with non-coherent reception, to baseband single wire communications. Unipolar OFDM techniques can efficiently compensate frequency selective channel distortion in unipolar communication systems. One of the leading example of unipolar OFDM is asymmetric clipped optical OFDM (ACO-OFDM) originally proposed for optical communications. Flip-OFDM is an alternative approach that was proposed in a patent, but its performance and full potentials have never been investigated in the literature. In this paper, we first compare Flip-OFDM and ACO-OFDM, and show that both techniques have the same performance but different complexities. In particular, Flip-OFDM offers 50% saving in hardware complexity at the receiver over ACO-OFDM. We then propose a new detection scheme, which enables to reduce the noise at the Flip-OFDM receiver by almost 3dB. The analytical performance of the noise filtering schemes is supported by the simulation results.

We consider two uniploar OFDM techniques for optical wireless communications: asymmetric clipped optical OFDM (ACO-OFDM) and Flip-OFDM. Both techniques can be used to compensate multipath distortion effects in optical wireless channels. However, ACO-OFDM has been widely studied in the literature, while the performance of Flip-OFDM has never been investigated. In this paper, we conduct the performance analysis of Flip-OFDM and propose additional modification to the original scheme in order to compare the performance of both techniques. Finally, it is shown by simulation that both techniques have the same performance but different hardware complexities. In particular, for slow fading channels, Flip-OFDM offers 50% saving in hardware complexity over ACO-OFDM at the receiver.

Flip-OFDM for optical wireless communications

Self-heterodyne OFDM (self-het OFDM) is known to provide complete immunity against frequency-offset and phase noise, with a much lower RF frontend complexity, when compared to conventional OFDM techniques. Self-het OFDM is considered to be a promising physical layer technology for millimeter-wave RF communications, where the implementation of low complexity stable oscillators is technically difficult. Although self-het OFDM has great potential, it has only been studied for additive white Gaussian noise and two-ray channel models. In this paper, we analyze the performance of self-het OFDM for general frequency selective channels and show that the standard self-het OFDM undergoes an outage if the RF carrier is affected by deep fading. In order to avoid this, we introduce a new technique called smart carrier positioning. We show both analytically and by simulation that the smart carrier positioning can improve the diversity order and the performance of standard self-het OFDM by approximately 4dB at bit error rate of 10^{-2}. In addition, we investigate the optimum power allocation between the carrier and the OFDM subcarriers under frequency selective conditions.

/pdf/self-heterodyne-ofdm-transmission-for-frequency-selective-286pua02ae.pdf

Self-Heterodyne OFDM Transmission for Frequency Selective Channels

Self-heterodyne orthogonal frequency-division multiplexing (self-het OFDM) is a promising physical-layer technique for millimeter-wave and terahertz RF communication due to its simple RF front end and complete immunity against frequency offset and phase noise. In this paper, we proposed a multiple-input–multiple-output (MIMO) self-het OFDM with the adaption of the smart carrier positioning (SCP) technique. At the transmitter, a space–time block code (STBC) is used to produce the coded information symbols to be transmitted on each antenna over the self-het OFDM subcarriers. At the receiver, a simple nonlinear detection is adopted at each receive antenna. The achievable diversity order of such setting is analyzed, and it is found that with the adaptation of the SCP technique, the diversity loss in comparison to the conventional coherent MIMO-OFDM can be compensated. We further derive analytically both lower and upper bounds on the diversity order of the proposed scheme, and simulations will be shown to illustrate the performance of such combination.

Nirmal Fernando

Papers

Flip-OFDM for Unipolar Communication Systems

Flip-OFDM for Unipolar Communication Systems

Flip-OFDM for optical wireless communications

Self-Heterodyne OFDM Transmission for Frequency Selective Channels

MIMO Self-Heterodyne OFDM