# Special Issue on Light Communications: Latest Advances and Prospects
## 3 Indoor Systems  
In this area, 10 excellent papers were accepted, of which 9 are on the different aspects of visible light communications (VLC). The first paper by [PERSON], et al. [12] presents a mobile receiver that uses several photodetectors to provide omnidirectional receiving capability. The photodetectors are fixed to truncated pyramid walls. The receiver was evaluated experimentally for indoor application using bit error rate measurements and comparingsignal-to-noise ratio parameters for various photodetector numbers. The next two papers discuss the importance of using synchronization for the orthogonal frequency division multiplexing (OFDM) scheme and a pre-equalizer in VLC systems. [PERSON], et al. [13] have used a single pilot sequence to achieve both frame synchronization and the peak-to-average power ratio (PAPR) reduction in the optical OFDM scheme. The two direct-current biased optical (DCO)-OFDM and asymmetrically clipped (ACO)-OFDM are discussed in this research. They show that this scheme gives precise frame synchronization at the SNR as low as 4 dB. [PERSON] and co-authors [14] have aimed to extend the limited bandwidth of phosphorescent white LEDs for a multiple-input single-output (MISO) VLC system introducing a pre-equalizer circuit, with results demonstrating a 28% bandwidth enhancement. Moreover, they proposed a model for LED arrangement to improve illumination uniformity along the room. The next paper discusses the interference suppression in a MIMO VLC system. [PERSON], et al. [15] have proposed the use of a liquid-crystal display (LCD) as a receiver-side filter unit to focus the field-of-view of the receiver on a specific light source and thereby suppress interference. Interference due to the ambient light, modulated interference and multi-aperture interference are introduced and signal-to-interference ratio improvements are derived using experimental results. For MIMO communications, the potential of the proposed interference reduction method is also demonstrated. The following two papers have focused on the visible light-based positioning for indoor applications. The first paper by [PERSON], et al. [16] proposes a new frequency modulated VLC system for localization in underground mining, which is based on a three-dimensional trilateration localization scheme. The obtained results show the superiority of the proposed method in comparison with the traditional short range radio frequency technologies. [PERSON], et al. [17] proposed a VLC-based positioning method using dual-function machine learning algorithms. Their predicted results show that the proposed solution increases positioning accuracy under the negative effect of multipath reflections and decreases the computational time. The last 4 papers in this section have focused mostly on the resource allocation and energy efficiency in VLC. [PERSON], et al. [18] have investigated the power domain non-orthogonal multiple access (PD-NOMA) method to enhance the spectral efficiency of VLC systems downlink. The bit error rate (BER) and the achievable rate performance of the non-cooperative and cooperative PD-NOMA under a perfect channel state information are investigated and moreover, analytical BER estimation is verified by Monte Carlo simulation. [PERSON] and co-authors [19] have proposed a low-complexity multi-cell resource allocation algorithm with a near-optimal system throughput to resolve the conflict between the high system throughput and low complexity of indoor VLC ultra-dense networks. By the simulation results, authors show that the proposed method achieves an improvement of 57% in performance in terms of the average system throughput and improvement of 67% in performance in terms of the quality of service (QoS) guarantee against the required data rate proportion allocation method. [PERSON], et al. [20] have proposed a non-contention bandwidth assignment protocol called the adaptive polling medium access control (APMAC) protocol for optical wireless networks.