## 4 Resource Management in Heterogeneous Network (RM HetNet)
### Power Allocation (PA)
#### 4.1.3 Spectrum Efficiency (SE)
They proved that a simplified version of the problem could be easily solved by applying a linear relaxation, and so they designed a distributed algorithm with low complexity to solve the original NP-hard problem. The simulation result showed that the proposed model performed an excellent tradeoff between SE, EE, and queue length. Furthermore, better results in terms of performance metrics than the state-of-the-art solutions based on the greedy algorithm are achieved. Nonetheless, all base stations used a single frequency band to provide users. Hence, inter-cell interference occurs. In [98], the authors suggested a solution to modify the transmit power of BSs on each of the channels in the fractional frequency reuse scheme after a cell zooming/switch-off event. This is performed to reduce the energy that minimizes the total interference that occurs in relation to the requirements of UEs. After a cell switch-off/zooming event in the network, the fractional frequency reuse scheme requires a multi-objective optimization problem to be formulated to determine the radius of the cell-center region. Along with this, a heuristic algorithm for dealing with this issue is proposed. Adjusting the downlink transmission power of BSs to each user based on the required rate and the result of the heuristic algorithm leads to the formulation of a multi-objective optimization problem. Both EE, SE, and total interference are factors in this optimization problem. Additionally, a meta-heuristic algorithm for dealing with this issue is proposed. According to the research results of this investigation, its EE and SE are significantly superior to those of the previous studies. More specifically, the scheme's EE has increased by 41.95 and 36.60 percent compared to the earlier algorithm. Additionally, the method that has been proposed demonstrates improvements in total interference, SE, and, consequently, total network throughput. Nonetheless, user association and mobility require attention and consideration. To maximize both the EE and SE, [99] suggests an approach to allocating resources. Three strategies were modeled in their system: (1) a load balancing association schedule for the MBS and SBS that uses a range growth association scheme, (2) inter-cell and intra-cell interference management via fractional frequency reuse, and (3) allocating resources in a proportional manner to ensure users' fairness. The binary search algorithm was used to formulate the stochastic optimization problem and find an optimal solution that simultaneously maximized both the EE and the SE. The proposed method was evaluated with a variety of parameters, including power threshold, BS power usage, spectrum partition, and BS density. The results demonstrated that SE and EE could be significantly enhanced. In this method, the binary search algorithm was shown to reduce the amount of time needed to find the optimal value; consequently, the method is appropriate for use in real-life scenarios. However, because the MBS is allocating all of the system resources, a high amount of signaling overhead is to be expected.