# A Brief Overview of Energy Efficiency Resources in Emerging Wireless Communication Systems
## 1 Introduction
Simultaneous illumination and communication are dual features offered by an optical wireless system, making it an alluring system widely adopted. It has numerous applications, such as street lights becoming internet access points, LEDs of driving cars having a real-time exchange of information which can help avoid overhead accidents, airplane cabin communication, etc. [11]. The constraining of energy at its nodes poses a need to integrate good performance alongside consuming less energy for each of its circuits/components. These components include the modulator, LED driver, photodetector, amplifier, and demodulator. They help to achieve an overall optimum energy consumption [12]. In constraining energy at nodes, a good modulation technique is needed. Modulation helps the constraining of signals to predetermined frequencies. An energy and spectrally efficient modulation technique in optical wireless communication systems is the orthogonal frequency division multiplexing (OFDM) and its variants [13, 14, 15]. The amount of information transmitted per unit of power and bandwidth in a communication system is defined as its energy and spectral efficiency. OFDM effectively fulfils these features by using different methods like the Fast Fourier Transform (FFT) to override several modulation restrictions [16]. One more effective method for achieving energy efficiency in a wireless network is sleeping modes. Sleep mode involves the selective turning on and off of different components of the base station. The selection is based on different factors chosen by the operator, which is how often the components are used per day. So, when some components are turned off, less energy than usual is utilized, and therefore energy is saved. Utilizing this technique requires careful management of the sleep mode such that it is quick to respond to its function without oversleeping so as not to dwinddle the effectiveness of the system [17]. By integrating the above-explained techniques, an energy-efficient framework can thus be formulated within a heterogeneous network. Services that provide wireless communication will be utilized in a broader range in the future, which would cause a large consumption of energy [18]. Two important metrics for wireless communication are spectral efficiency and energy efficiency [19; 20]. One major challenge in optical wireless communication is providing an energy-efficient, trusted, and guaranteed quality-of-service (QoS) [21]. The low price of light-emitting diodes, fast switching and their characteristic energy efficiency has enabled its widespread use for several purposes, such as car indicators, lighting systems, traffic data, and more [22; 23]. One crucial factor to consider in LED lights is dimming [9]. An LED has a forward current, IF, which is the current that gets to the cathode coming through the anode of the LED light so that there can be enough current for powering the device in the forward bias [24]. Therefore, a linear relationship exists between the illumination of the LED and its forward current [25]. As such, LEDs are referred to as current-driven devices. Thus, dimming of LED lighting can be defined as controlling/influencing its forward current [26; 27; 28; 29; 30]. One method of dimming is a continuous modification of the current until half of the maximum current is achieved, which would give a 50% brightness. Pulse width modulation (PWM) is an alternative technique applied for dimming. Here, the brightening and dimming are done by increasing and reducing the pulse width, respectively, in which the amount of time the LED light is on is referred to as the duty cycle. Following the limited coverage distance of LEDs, to expand the coverage regions in an indoor VLC environment, holographic light shaping diffusers (LSDs) with suitable angles are utilized. This provides uniform power distribution and stands as a significant breakthrough in the design of VLC cellular systems [31]. In general, this review mostly examined recent literature on the proposed topic.