# A Brief Overview of Energy Efficiency Resources in Emerging Wireless Communication Systems
## 3 Energy Efficiency Techniques in Wireless Communication Systems
### Energy-Efficient Transmission Techniques  
The growth of future wireless systems will be impeded by the accumulation in the radio frequency (RF) range (3 kHz-300 GHz), except more parts of the spectrum are accessible. There is an alluring technology that has a broader frequency range compared to the RF spectrum. It has a lot of uses, such as lighting facilities in buildings for advanced speed data communication, aeroplane cabins having higher communication speeds, vehicle to vehicle communication, trains, and underwater transmission and reception. The system is called a visible light communication system [54]. It can serve the purpose of both communication and illumination, therefore taking away additional transmission power that would have been used. Direct detection and intensity modulation are utilized mainly by OWC systems [54]. A leading research university in Japan, Keio University, partnered with Nakagawa laboratory, was the first to propose the first VLC system [55]. The desire for advanced network provisioning has waxed stronger; VLC techniques can solve these demands. Several issues surround VLC systems, including uplink connectivity, optimum multiple access approaches, optimum modulation technique, mobility, mitigating ISI standardization, dimming control and illumination [56,57]. Multicarrier modulation, multicolour, and baseband modulation can be applied in OWC [11]. Multiplexing techniques most widely considered are WDM and OFDM [11].  
Recent Advances in Energy-Efficient Transmission  
The authors in [58] used a minimal abstraction layer to demonstrate the essential working parts of a wireless communication system for efficiency, which reduces the delayed/prolonged transmission time rate between several nodes, thereby saving power. The limitation to its use is the standardization of existing communication protocols. In [59], [PERSON] and [PERSON] proposed a technique for load balancing by a methodical apportioning of data traffic to links available in a wireless communication network. Its inadequacy was  
Figure 5: Microcells installed cell towers, which improve coverage in environments with limited signal reception.  
the inability to be carried out in real life and its consideration of only the local parameters of each node.  
The authors of [60] demonstrated a technique that accommodated low power consumption, low latency, and high reliability with simplicity in wireless communication, which was achieved using non-orthogonal multiple access (NOMA). In OFDM, signals are transmitted at discrete frequencies, while NOMA uses the same frequency spectrum but has distinct power measures. The advantage of NOMA is that it has high spectral efficiency, as the same frequency spectrum can be utilized several times. This technique permits multiple connections to the identical frequency spectrum. An evaluation of some precoding techniques was carried out in the work. One benefit of the technique is the reuse of frequency. A significant setback to this technique is the difficulty of maintaining the BER functionality by giving a good estimation algorithm.  
The authors in [61] analyzed that using the Kalman filter could measure the fastest path in a wireless sensor network and the input data noise. The Chinese remainder theorem (CRT) could split the nodes with the Kalman filter. Procuring a threshold for the CRT components is a drawback of this technique. The Kalman filter addressed energy efficiency because imperfections and unwanted signals are removed at the receiver end by producing an optimum from consideration of a group of them.  
The work in [32] showed a promising and remarkable method for reducing the overall consumption of energy by using sleep mode techniques. The advantage is the automatic energy-saving of base stations at non-peak hours; as such, the energy is not wasted, thus causing energy efficiency. The limitation is that the technique was restricted to only cellular networks. Table 1 provides relevant requirements of the RF/Optical wireless communication techniques [11].