## 4 Resource Management in Heterogeneous Network (RM HetNet)
### Mode Selection (MS)
#### 4.3.2 Dynamic  
There are multiple possible modes in a B5G network. The users are in their proximity area, and they select D2D mode; otherwise, they select cellular mode to help BS. In the dynamic network, the users are free to move in any direction and change their mode when their information is substituted. In [119], the authors considered the MS problem of D2D multi-tier HetNets for a downlink scenario in terms of improving EE. A mathematical representation of the MS problem describes it as an NP-hard problem. The complexity of the optimization problem is reduced and solved using various optimization techniques, all of which are selected based on a load of traffic passing through the network. This paper proposes a novel method for dynamic mode selection based on fuzzy clustering. Depending on the network resource block availability, the proposed model allows D2D users to switch between dedicated and reuse modes in real time dynamically. Based on the received signal strength and the level of interference, the method selects D2D users for dedicated and reuse mode operations. Further, whenever there is a shift in resource block availability, users automatically transition to the mode that best manages the network's optimum performance. The new method's effectiveness is evaluated compared to the traditional methods. The simulation results demonstrate that the proposed approach is superior to the conventional methods in terms of EE and the number of connected D2D users. In addition, the EE of downlink transmission is improved by deploying the D2D underlying multi-tier HetNet. Nevertheless, the greater physical distance between devices leads to a user being banned. In [120], the authors explored the MS issue in the downlink scenario of multi-hop cellular communication and D2D communication over HetNets. They propose a new approach for MS in HetNets that makes use of greedy multi-hop cellular networks. The proposed system selects a mode of communication after weighing its advantages and disadvantages. In addition, the proposed approach switched between the conventional cellular mode, the multi-hop cellular mode, and the greedy multi-hop cellular mode. Further, the proposed approach combines the selection of the optimum number of hops into the MS process. Multi-hop cellular and single-hop cellular modes are contrasted with the proposed approach. The simulation results demonstrate that opportunistic multi-hop cellular network communications significantly improve the performance of future networks. Further, the proposed approach selects the best MS to maximize system throughput, EE, and network capacity. Nonetheless, signaling overhead is not considered. In [121], to enhance the performance of cellular networks, the authors explored an MS problem for D2D communication for each uplink and downlink scenario. A new MS scheme is proposed to reduce the network signaling overhead at each BS in which the D2D mode is stimulated based on the distance between users. MS in mobile D2D scenarios is further facilitated by a tractable analytical approach that accounts for relative motion. They formulated the threshold distance for D2D communication, the successful transmission probability, the probability of using D2D mode, and the minimum amount of time spent in D2D mode. The evaluation of the D2D performance while considering mobility primarily focuses on these parameters. Monte Carlo simulations were performed to verify the analytic results and evaluate the proposed method's advantages. As evidenced by the determined minimum for D2D residence time, the results confirmed the importance of the system's dynamic performance regarding user mobility. However, the interference between the D2D and CU is not considered in the proposed scheme. In [122], the authors discussed the MS issue in the downlink scenario of D2D and multi-hop cellular communication over HetNets. They suggested a context-aware MS strategy that can identify and select the optimal mode for each device in various deployment and operating conditions.