# A Brief Overview of Energy Efficiency Resources in Emerging Wireless Communication Systems
## 4 Optical Wireless Communication Schemes
The system proved to be both power efficient and cost-efficient; though the initial cost was high, the operational cost was lesser. The problem of assembling several users was not directly settled. In 2018, through the notion of cell-free massive MIMO [78] and encouraged by the dense-luminaries' facility, a dense VLC system enabled by densely distributed LEDs was proposed. In order to provide for several users simultaneously, dense VLC is used to fit the beam spots according to the system dynamics. The complicacy in adaptation was dwindled by designing a heuristic algorithm which was a proposal to make NLOS VLC harmonize with the distributed transmitters. In order to optimize the system throughput, an optimalo5 policy was formulated for the power allocation among the distributed LEDs. While keeping up with the specifications for uniform illumination, the outcome proves that dense VLC can enhance the rate at which data are transmitted and the efficiency of the power [79]. Many works have been done around optical wireless communication, but much work has not been concentrated on the best modulation technique with spectral and energy efficiency [4, 19]. One of the best modulation techniques in wireless communication is orthogonal frequency-division multiplexing (OFDM) [80, 81, 82, 83]. OFDM has remarkable features like optimum transmission channel management, low intercarrier interference (ICI), minimal intersymbol interference (ISI), etc. In July 2010, the authors of [84] analyzed the impact of OFDM on an OWC system. It helped to see the effect of clipping in the LED transmitter front-end and the relationship between the modulation order and SNR degree, which are two important factors to consider when using OFDM in an OWC system [25, 85]. A variant of OFDM for optimal spectral and energy efficiency was presented in [8]. Another remarkable modulation technique [86] for OWC was introduced just the next year, 2011--spatial modulation. Spatial modulation complements the drawback of a reduced data rate in OFDM, which was investigated previously to efficiently manage sub-carriers positioning for maximum spectral performance [87]. Subsequently, OFDM and spatial modulation have combined OWC [88] and MIMO [89]. In 2017, spatial modulation for the VLC system in MIMO-OFDM was first analyzed by authors in [90], which provided a better data rate than other schemes. The work helped to minimize spectral attenuation that has largely been previously encountered. A slight improvement, an improved detection algorithm, was added in the following year [91], which further improved the overall efficiency of the system. One drawback in the last two investigations is that the configuration of the transmitter and the receiver were limited to a \(4\times 4\) setup and a multiple of it. In 2019, a polar form was analyzed [92] for the same modulation configuration improved SNR saving. In that same year, the authors improved on the drawback of the setup configuration in their work [93]. The most recently improved work [94] on this technique by the same authors was done in 2020. One major problem envisaged in VLC is the security of critical user information. The key challenges in physical layer security for VLC are outlined in [95]. Figure 6 shows a typical illustration of a scenario where an eavesdropper appears to narrow its FoV in an attempt to wiretap an information-bearing signal. This scenario tries to minimize the reception of jamming signals [96]. Here, a typical VLC system with a jamming strategy is demonstrated. The electron device (ED) is boosted with a narrow field-of-view (FoV) photodiode (PD) and oriented directly toward the information LED transmitter, as described in Figure 6[95]. As seen in Figure 6, the LEDs assume a uniform distribution on a square lattice. Specifically, one legitimate user and one eavesdropper could be positioned in the region under investigation. The nearest LED to the legitimate user is selected to transmit the information-bearing signal. In this case, the other LEDs pretend to be jammers.