This paper addresses the nonlinear memory effects in the response of typical illumination Light Emitting Diodes (LEDs), in order to enhance the performance of Visible Light Communication (VLC) systems. These LEDs have a limited bandwidth of only several MHz. To reflect the physical mechanisms in the Quantum Well (QW), we describe the LED transient response by a nonlinear dynamic differential equation. Three different mechanisms of the nonlinearity are relevant in the Double Hetero-structure (DH) LEDs, which result in dynamic nonlinearities, that is, a mixture of nonlinearities and memory effects. Hitherto generic pre-distorter and non-linear equalizers have been studied for LEDs. Yet this work shows that recombination rates of photon generation can be translated into an equivalent discrete-time circuit, that can be inverted. This allows us to develop a new pre-distorter with simpler and more efficient structure than previously studied and overly generic approaches. The novel predistorter along with a parameter estimation can effectively overcome LED nonlinearity for high-speed VLC with amplitude-based single carrier modulations, including On-Off Keying (OOK) and Pulse Amplitude Modulation (PAM)-4 systems, and with the multi-carrier Orthogonal Frequency Division Multiplexing (OFDM). We report experimentally obtained eyediagrams, firstly to justify our choice for the LED model on which our nonlinear predistorter have been based, and secondly to verify the effectiveness in enhancing the VLC link performance to the extent predicted by our model.

In this paper, we consider a light fidelity (LiFi)-enabled bidirectional Internet of Things (IoT) communication system, where visible light and infrared light are used in the downlink and uplink, respectively. In order to efficiently improve the energy efficiency (EE) of the bidirectional LiFi-IoT system, non-orthogonal multiple access (NOMA) with a quality-of-service (QoS)-guaranteed optimal power allocation (OPA) strategy is applied to maximize the EE of both downlink and uplink channels. We derive closed-form OPA sets based on the identification of the optimal decoding orders in both downlink and uplink channels, which can enable low-complexity power allocation. Moreover, we propose an adaptive channel and QoS-based user pairing approach by jointly considering users' channel gains and QoS requirements. We further analyze the EE and the user outage probability (UOP) performance of both downlink and uplink channels in the bidirectional LiFi-IoT system. Extensive analytical and simulation results demonstrate the superiority of NOMA with OPA in comparison to orthogonal multiple access (OMA) and NOMA with typical channel-based power allocation strategies. It is also shown that the proposed adaptive channel and QoS-based user pairing approach greatly outperforms individual channel/QoS-based approaches, especially when users have diverse QoS requirements.

LEDs, particularly those used for Visible Light Communications (VLC), have a limited bandwidth, while above their 3 dB bandwidth, the roll-off is relatively gentle. If the modulation bandwidth would be limited to the 3 dB LED bandwidth, the achievable rate would be unacceptably constrained. Hence, effective communication systems need to optimize the use of bandwidth significantly above this 3 dB point. Orthogonal Frequency Division Multiplexing (OFDM) is a popular method to fine-tune the amount of power and constellation as a function of the channel response over different frequencies. Various power and bit loading strategies have been proposed and simulated in literature, but their performance was not captured in expressions. This manuscript derives these for optimal waterfilling, uniform and pre-emphasized power loading for the LED channel, that severely attenuates high frequencies. We also investigate the influence of practical discrete constellations and verify our new results experimentally. Interestingly, simple uniform loading only falls less than 1∼2% short of the throughput achieved by waterfilling, but when we restrict OFDM to discrete QAM constellation sizes, the penalty for uniform loading is 1.5 dB. Inspired by the good performance of uniform power loading, we propose an algorithm to find the best discrete bit loading for uniform power within an optimized band. As pre-emphasis is nonetheless attractive because a flattened channel does not need adaptive sub-carrier loading, we quantify its penalty. This can be modest provided that the system can adapt its transmit bandwidth, thereby adaptively switching upper sub-carriers to zero power.

The requirement of a DC-bias is known to make DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) less energy efficient. This can be improved by asymmetrically clipped optical OFDM (ACO-OFDM), pulse amplitude modulated OFDM (PAM-OFDM) or Flip-OFDM, but these variants use the bandwidth inefficiently. Our trade-off between energy and spectrum efficiency considers a given limited channel bandwidth of the light emitting diode (LED) and then attempts to get the highest throughput per unit of energy. We investigate previous findings that clipped OFDM can be more attractive in a low-SNR regime. More specifically, we consider visible light communication (VLC) in which the average light level, i.e., the bias, is prescribed by illumination requirements, thus comes for free. ACO/PAM/Flip-OFDM can convert the DC-bias into power for communication, but all variants of OFDM, including DCO-OFDM consume extra electrical power. We conclude that in this scenario, advantages attributed to ACO/PAM/Flip-OFDM vanish, as DCO-OFDM outperforms its variants in all SNR conditions, in terms of achieved throughput over a bandlimited channel as a function of extra electrical power required. For hybrid solutions, such as Asymmetrically clipped DC biased Optical OFDM (ADO-OFDM) and Hybrid ACO-OFDM (HACO-OFDM), we optimize a new adaptive power and rate splitting between odd (clipped) and even (biased/clipped) subcarriers to balance power and bandwidth efficiency.

This paper addresses the power penalty in a visible light communication (VLC) emitter that re-uses illumination light emitting diodes (LEDs) for high-speed communications. We quantify the effect of modulation depth for two widely explored modulation schemes in VLC, namely orthogonal frequency division multiplexing (OFDM) and pulse amplitude modulation (PAM). Extra power is dissipated in the LED and in the modulator due to such modulation. Two popular high-speed VLC transmitter topologies, namely a Bias Tee and a Serial FET, are compared in terms of efficiency, extra energy per symbol/bit and complexity. This paper presents a theoretical model and compares it to simulation results. We show that PAM outperforms OFDM in terms of better bit error rate, larger dynamic range for modulation depth, less extra power loss, thus higher power efficiency for communication over an additive white Gaussian noise channel.

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.