In channels with a wide available bandwidth, M-ary pulse amplitude modulation (M-PAM) with a small number of signal levels is a popular modulation method. The capacity of such optical channels has been investigated before as a function of the energy per bit and a limitation in average or peak optical power. However, in practice, the transmit energy consumption can be a complicated function of the signal waveform. In this work, we study a channel in which, firstly, the power associated with radiating a signal level is not a quadratic function of the signal amplitude, because the emitter may not act as a linear ohmic resistor. Secondly, level transitions consume additional power, for instance, because an intrinsic capacitance in the emitter diode junction needs to be charged up quickly. Light-emitting diodes (LED)s or lasers used in optical wireless communications are prime examples. We optimize their source entropy for a limited power budget. This not only leads to asymmetric probabilities for signal levels, as known from previous theory, but also introduces a dependency on previous signal probabilities. For example, our results show that in an energy-constrained system that employs on-off keying (OOK), the conditional probability of transmitting a logical 1 following a logical 0 should be lower than that of transmitting a second 1 after an initial 1.

To allow a good signal-to-noise ratio (SNR) over a wide bandwidth for signals arriving from an unknown angle, many optical wireless communication (OWC) systems need a wide-aperture high-bandwidth detector. However, the physical properties of a photodiode (PD) challenge the design for high bandwidth and a large detection area, simultaneously. This problem is also known as the bandwidth versus area trade-off of an amplified PD receiver. In this paper, we study the limitations of a single-PD OWC receiver on throughput, particularly on bandwidth, photon capturing area, and electrical noise, to formulate a model that evaluates and optimizes the throughput of a communication link for different signal modulation schemes. We optimize the signal power spectral density (PSD) of pulse-amplitude modulation (PAM) and of DC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) modulation for throughput, considering that large modulation bandwidths suffer disproportionally from noise enhancement in typical receiver amplifiers. This worsens if large PDs are used, but we show that to a large extent, the corresponding stronger signal can compensate for the increased noise. We study the optimum detector size, in combination with adaptations of the spectrum and, in particular, the maximum frequency of the modulation. While the limitations of photonic emitters have been extensively studied before, this is one of the first papers that models how the receiver limits the throughput of PAM and OFDM.

Light mainly propagates along a line-of-sight without the rich multipath scattering that can be exploited in radio frequency (RF) multiple-input multiple-output (MIMO) systems. To ensure outage-free communication MIMO can nonetheless be used, provided that multiple ceiling nodes are spatially distributed. Angular-selective client nodes can to some extent, but not perfectly, separate spatial streams coming from different ceiling points. In addition, many optical wireless communication (OWC) links have a low-pass frequency response. Because of these properties, indoor distributed (D)-MIMO OWC differs in many aspects from RF links. This paper explores how OWC D-MIMO systems can best be operated. The optimization of how to spread power over spatial streams and frequencies comes to different settings than typically seen for radio. We evaluate algorithms by an analytical foundation and by simulation based on a commonly accepted channel model. We derive a simple criterion to select the number of spatial streams which can be used. This also can serve as a selector for switching between spatial diversity and spatial multiplexing transmission modes. Moreover, we derive a simple formula for how to best spread power among spatial streams. Besides theoretically optimum solutions, we propose a very-near-optimum algorithm that is highly efficient for practical systems in rapidly varying channels.

This paper addresses the question of whether an optical wireless communication (OWC) LiFi system preferably uses LEDs near their most efficient operating point, which is preferred for environmentally-friendly highly efficient illumination, or that one should preferably drive the LED into its droop regime to speed up its response. In fact, additional non-radiative recombination speeds up the LED response to modulation but spoils efficiency. We introduce the differential (Internal and External) Quantum Efficiency (dIQE; dEQE), as a particular derivative of quantum efficiencies. It quantifies the efficiency at which a current modulation translates into an optical signal. We optimize the current density, thus how deeply the LED is to be driven into droop, using DC-offset (DCO) OFDM throughput expressions for the first–order low–pass LED response. Short– range small–coverage systems prefer high current densities to achieve very high throughputs. LiFi with wide opening angles, e.g. covering tens of square meters are better off with a high dIQE, thus lower current densities.

In this paper we report, for the first time, measured performance results of the concatenation between two different plastic optical fibre (POF) fronthaul approaches and distributed multiple-input multiple-output (D-MIMO) LiFi using commercial G.hn chipsets. These integrated circuits (IC)s are capable of performing, among other features, MIMO signal processing, orthogonal frequency-division multiplexing (OFDM) modulation, and adaptive bit-loading, which are important features for LiFi systems. The use of commercially available ICs with the aforementioned features, accelerates the mass market adoption of LiFi technology. We consider the POF fronthaul approaches: spatial division multiplexing (SDM) and wavelength division multiplexing (WDM). POFs are interesting for industrial scenarios due to its electromagnetic interference immunity, their low weight and cost. The measured throughput results show that a WDM POF fronthaul, using a red light-emitting diode (LED) and a green laser diode (LD), has ample throughput to support the DMIMO LiFi link, which proves the viability of the use of POFs as fronthaul for D-MIMO LiFi systems. Our results give input for possible enhancements to the ITU G.vlc standard G.9991.

Recently, the coherent optic wireless communication (OWC) has received extensive attention due to its superiority over traditional intensity modulated direct detection (IMDD) systems. Yet, the Doppler effect could be a showstopper for coherent OWC which is sensitive to the frequency offset and spread. Thus we introduce a new modulation scheme, which called orthogonal time frequency space (OTFS) modulation, into the coherent OWC to solve the Doppler problem. OTFS transforms traditional time-varying channel into delay-Doppler (DD) domain, which ensures all transmit symbols experience an almost identical and slowly varying sparse channel. Thus full channel diversity in time and frequency can be obtained when a suitable receiver is used. In addition, most of the research on Doppler effect only consider the random Doppler spread and average frequency shift to the signal, while ignoring the spectral spread caused by the frequency-dependent Doppler shift of a wideband signal such as radar signals and terahertz signals. To accurately model the Doppler effect, it is necessary to calculate the Doppler frequency shift according to the frequency bin of each sub-carrier, thereby the overall frequency offset and spectral spread are included. This manuscript proposes a Doppler channel model based on frequency-domain subband partition (FDSP), and for the first time, this wideband Doppler channel model is applied into wideband OTFS and orthogonal frequency division multiplexing (OFDM) systems. We carried out simulation for OTFS and OFDM under different velocities. Simulation results show that the OTFS can resist high Doppler frequency shift in high mobility scenarios, and in the case of large subcarrier spacing, OTFS is less affected by the Doppler frequency shift of each subcarrier than OFDM.