On this basis, the available spectrum has minimal resources and cannot deliver the 1000\(\times\) expansion in users and devices with high data requirements [39]. Additionally, the technologies and techniques that followed Long-Term Evolution (LTE), LTE-Advanced (LTE-A), and its predecessors cannot provide prosperous results for future wireless communication [40]. Therefore, new spectrum resources and advanced technologies must be determined to address the upcoming high data requirement for low latency and extend the services to another decade and even more [41]. Nevertheless, the simultaneous operation of new technologies in a cellular network structure results in many management issues [42]. Standardization organizations, research institutions, and governments of various countries focus on 5G spectrum strategies and their effective usage [43]. The International Telecommunication Union (ITU), European Telecommunications Standards Institute (ETSI), and Federal Communications Commission (FCC) are the respective standardization organizations in the telecommunications industry that focus on designing new mobile communication technologies [44, 45]. Future cellular communication networks are considered ultra-dense; thus, a robust and frugal network must be designed to accommodate numerous users with fairness in high throughput and perform spectrum management systematically [46, 47]. To enhance the spectral efficiency of cellular networks, an ultra-dense small cell design can help achieve the objectives of future 6G cellular networks [48]. Concurrent operations of macro-cells and low-power small cells, such as picocells, femtocells, and RNs with wired and wireless backhaul links, are known as HetNet [49]. Adding the structure of many small cells conveys two positive outcomes: (1) it reduces the load on a BS where numerous users compete to access the resources, and (2) it helps use the frequency spectrum efficiently [47]. Thus, this method substantially improves the throughput for each user and provides fairness among the users. However, ultra-dense small cell systems face some serious challenges, such as interference, which is a major issue in HetNet deployment [50]. In addition, supporting mobility in small cells leads to the increasing cost of installation, maintenance, and backhaul structure [51]. While researching on the MIMO technology for 6G goals, M-MIMO undoubtedly supports power utilization and spectrum management efficiencies [52; 53]. Conventional MIMO is equipped with two to four antennas, whereas M-MIMO can use tens and hundreds of antennas for the simultaneous transmission and reception of multiple signals over the same channel [54]. This method greatly boosts the network capacity and accommodates the maximum number of users and devices on the same frequency band, thereby reducing the usage of spectra. To maximize M-MIMO properties effectively, a technique called beamforming can be used to deliver the desired results [55; 56]. The use of CR, which plays a vital role in spectrum management, is another novel idea. CR is an intelligent wireless network technology that can automatically detect available channels in a cellular spectrum. It can be implemented with the M-MIMO system to enable communication and execute simultaneously, besides improvised cellular operational performance [57]. M-MIMO is efficiently handy for networks to which many users connect (i.e., IoT) [58; 59]. Another feature of current 5G communication is access to the mmWave spectrum [60]. However, previously, it was considered unsuitable for cellular communication because of its propagation characteristics, inability to travel over long distances due to its short wavelength, and penetration losses due to objects and large structures and adverse atmospheric conditions [61]. However, the densification of small cell networks and the parallel operation of innovative methods make the use of the mmWave frequency band possible [62].