## 5 Overview of 5G-Enabling Technologies
### Key 5G-Enabling Technologies
#### 5.1.2 The 5G Massive MIMO  
The Multiple Input Multiple Output (MIMO) technology is another critical component of the wireless communication system. Massive MIMO is a type of multi-user MIMO technology in which user equipment with only one antenna receives signals on the same time-frequency resource from another single base station outfitted with many antennas [50; 74; 75]. More than 100 antennas and dozens of antenna ports installed in a base station can be used in a distributed or centralized manner in massive MIMO. Technologies such as channel estimation, reference signal design, multi-user scheduling mechanism, receiving algorithm, and channel information feedback should be optimized and improved for each type of MIMO [76]. The usage of several antennas at the sender and receiver end is made possible by MIMO technology to separate data streams and offer virtual paths onto individual antennas [75; 77], even though conventional MIMO performed poorly, its connectivity is less reliable, and its throughput is limited [9]. These issues have been addressed through the use of various MIMO upgrades, such as network MIMO, single-user MIMO (SU-MIMO), and multiuser MIMO (MU-MIMO). Nonetheless, these novel developments are insufficient for end-user applications. Massive MIMO technologies enable 5G networks to differentiate themselves from competitors. By deploying several thousands of broadcast and receive antennas at 5G base stations, 5G massive MIMO will be significantly improved. In addition, when radiated energy efficiency is increased by approximately 100 times, the capacity of 5G networks can increase up to 10 times by employing massive MIMO technology [78]. Figure 5 presents a diagram of the operation of a massive MIMO system. Various solutions for improving the energy efficiency advantages provided by massive MIMO for 5G have been reported [78; 79; 80; 81; 82; 83]. The research made use of massive MIMO technology and offered a comprehensive explanation of the massive MIMO energy consumption model. The Energy Efficiency advantages of huge MIMO systems have been examined using a variety of methodologies. Several methods for improving 5G technology through common EE-maximization techniques for traditional massive MIMO systems were discussed, including reducing the number of antennas, performing simple tasks in real-time at the base station (BS), minimizing power amplifier losses, and reducing the need for Radio Frequency (RF) chains. The authors also described the uplink and downlink services for a 5G massive MIMO system [84; 85]. The work maintained a performance matrix that calculates how pilot contamination affects various performance metrics. Furthermore, various applications of massive MIMO were examined, including Orthogonal Frequency Division Multiplexing (OFDM) systems, small cells, higher frequency bands, and massive MIMO IEEE 802. Finally, the work looked at some critical techniques for improving 5G performance through massive MIMO. An energy-efficient system-sharing strategy based on large-scale MIMO systems was reported in [86], where the costs of circuit energy and transmission energy for resource allocation were considered. Optimization approaches were used for an energy-efficient resource-sharing system to optimize the energy efficiency for individual QoS and energy constraints. The authors also investigated the BS configuration, which incorporates both homogeneous and heterogeneous UEs. During simulations, they stated that boosting energy  
Figure 5: Massive MIMO Scenario. efficiency heavily depends on the total number of transmit antennas. They emphasized that the maximum energy efficiency was achieved when the BS was configured with 100 antennas serving 20 UEs. Various strategies for MIMO next-generation wireless communications were suggested in the work of [13; 74].