## 3 Related Works
#### Spectral Efficiency Enhancement  
Spectral efficiency refers to the amount of information that can be transmitted over a given amount of spectrum, while channel capacity refers to the maximum amount of information that can be transmitted over a communication channel. Increasing the SE of a system can lead to an increase in the channel capacity, as more information can be transmitted over the same amount of spectrum. Some common SE improvements techniques include:  
* Multi-carrier modulation (MCM) techniques such as Orthogonal Frequency Division Multiplexing (OFDM), which divide the bandwidth into multiple sub-carriers and transmit data simultaneously on each sub-carrier. * The mMIMO technology that uses multiple antennas to transmit and receive data simultaneously, increasing the data rate and improving the quality of the signal. * Adaptive Modulation and Coding (AMC) that selects the best modulation scheme and coding rate based on the channel conditions to maximize the SE. * Channel equalization which compensates for channel impairments, such as attenuation and distortion, to improve the signal quality and increase the data rate. * Interference Management techniques such as power control, beamforming, and interference cancellation which reduce interference from other signals and improve the signal quality. * Cognitive Radio which uses advanced signal processing techniques to dynamic allocation of spectrum resources to optimize SE while avoiding interference with other users. Overall, these techniques are used in various communication systems, including wireless communication systems, to increase data rates, improve the quality of the signal, and make more efficient use of available bandwidth. Recently, an increasing number of research attention have been done on OFDM-IM (the combination of conventional OFDM with indexed modulation (IM) to provide a trade-off between SE and EE. In [56], a new concept of sparsely indexing modulation (SIM) is used to enhance SE. Another new combination of spacetime electromagnetic models of Tx/Rx antennas with OFDM, leading to the EM-OFDM, is proposed to enhance antenna SE by ratios up to 300% [57]. A promising and innovative digital modulation technology to provide tradeoff between SE and EE is Spatial modulation (SM), investigated in [58]. The primary goal behind SM is to transmit extra information using ON/OFF states of transmit antennas to reduce the implementation cost by minimizing radio-frequency chains. The authors in Refs. [46; 47; 48] have focused on massive-MIMO techniques--but not all in one. Similarly, the authors in Refs. [50; 51] have \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Authors \& Ref.** & **mMIMO** & **Beamforming** & **D2D** & **Small Cell** & **mmWave** \\ \hline [PERSON] et al. [46] & ✓ & ✓ & ✗ & ✓ & ✓ \\ [PERSON] et al. [47] & ✓ & ✗ & ✓ & ✗ & ✗ \\ [PERSON] et al. [48] & ✓ & ✗ & ✓ & ✓ & ✗ \\ [PERSON] et al. [49] & ✗ & ✗ & ✗ & ✗ & ✓ \\ [PERSON] et al. [50] & ✗ & ✗ & ✓ & ✗ & ✗ \\ [PERSON] et al. [51] & ✓ & ✗ & ✓ & ✗ & ✓ \\ [PERSON] et al. [42] & ✓ & ✗ & ✓ & ✗ & ✓ \\ [PERSON] et al. [36] & ✗ & ✗ & ✗ & ✗ & ✓ \\ [PERSON] et al. [52] & ✓ & ✓ & ✗ & ✓ & ✓ \\ [PERSON] et al. [53] & ✗ & ✗ & ✗ & ✓ & ✓ \\ [PERSON] et al. [54] & ✓ & ✗ & ✗ & ✗ & ✓ \\ [PERSON] et al. [55] & ✓ & ✗ & ✗ & ✗ & ✓ \\ [PERSON] et al. [This Work] & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A comparative overview of existing surveys on 5G key enabling techniques. focused on D2D communication as a suitable candidate for 5G networks. The authors in [51] have focused on mmWave, D2D communication, and mMIMO. In [36], the authors only discussed mmWave as a promising candidate for 5G key enabling techniques. The authors in Refs. [54; 55] have focused on mMIMO and mmWave. Therefore, this comprehensive survey is motivated to yield broad and thorough information about all important 5G key enabling techniques for the improvement of channel capacity and SE.