## 4 Resource Management in Heterogeneous Network (RM HetNet)
### User Association (UA)
#### 4.2.1 Signal to Interference Noise Ratio (SINR)
Conventional UA methods that rely on SINR unfairly prioritize the high-power MBS while starving the low-power SBS. In the research that has been conducted on this topic, numerous attempts to arbitrarily allocate users to SBS have met with only minimal levels of success. The authors take a novel approach by employing second-order statistics of user data. These statistics provide a more accurate representation of shifts in user rates. They suggested a new method for UA with the appropriate BS by adding the total network load standard deviation. In order to achieve this objective, they comprehensively search for the optimal user-equipment combinations that yield a global minimum in the standard deviation. This represents the optimal distribution of users across each MBS or SBS. In addition, they developed novel expressions for EE and coverage probability, both of which are useful for quantitatively assessing the system performance. The simulation results showed that the proposed methods effectively reduce traffic load while also increasing data rate, average EE, and coverage probability. Nonetheless, the impact of imperfect CSI needs to be analyzed. In [105], the authors investigated the UA problem of D2D-enabled HetNet Based on parameters such as SINR, path loss, and remaining battery life for each device in the network acting as a node, they derive the Nash equilibrium for games involving each pair of devices to find optimal associations in D2D HetNet using game theory. They demonstrated that the best possible D2D linkages between any two devices could be formed, thereby maintaining the highest possible QoS. As a result of this, it was discovered that the proposed method achieves optimal results while simultaneously possessing a Jain fairness index that is able to compete with that of other methods of a comparable nature that have been illustrated. Thus, the current work demonstrates that a formulation for D2D link formation in 5G wireless networks based on an evolutionary game can aid in selecting suitable link pairs to boost the network's performance. Nonetheless, a distributed open radio access network controller needs to be considered to help devices make an optimal link. In [106], the authors developed semi-closed formulas of network key metrics such as rate coverage, coverage probability, and network latency in ultra-dense heterogeneous networks by using a backhaul-aware user association approach based on SINR. Specifically, the analytical findings of outage probability are minimized by considering all available access and backhaul connections within the user-connectable range of MBS and SBS. The user experiences an outage only if the access link or backhaul link, which together provides the highest possible throughput, fails. Moreover, to find a more feasible deployment of SBS, the theoretical analysis and numerical results examined the impact of the proportion of MBS and the SBS density ratio on network performance metrics. Simulation results revealed that, under backhaul constraints, the coverage probability of the backhaul-aware user association scheme outperformed that of the traditional user association method. Nonetheless, the proposed scheme is limited to a single MBS and single SBS scenario.