## 4 Resource Management in Heterogeneous Network (RM HetNet)
### Spectrum Allocation (SA)
#### 4.4.2 Millimeter Wave (mmWave)  
Beyond 5G and 6G networks will need to be able to handle massive amounts of data, and both cellular network densification and mmWave communications hold great promise as possible solutions. In addition to improving the SE and EE of the next-generation wireless networks, HetNet can enable network densification in a cost-efficient manner. HetNet architecture and mmWave bands can be integrated to realize the goals of massive bandwidth and network densification. Furthermore, mmWave communications are more susceptible to blockages such as shadows and building walls and are also affected by atmospheric conditions compared to communications in low-frequency bands. In [133], the author identified transmission scheduling for downlink transmission OFDMA-based mmWave in HetNet. Specifically, a different SA can use the data rate on each subcarrier. Consideration of the impact of mmWave beam directivity on resource optimization is incorporated into the derivation of signal power and interference. They formulated a joint optimization problem of UA, subcarrier allocation, power control, and SA to maximize the sum rate. This joint optimization problem is partitioned into two subproblems for a more effective solution. Joint optimization of users, SA, and subcarrier allocation for fixed power allocation is the first subproblem to be addressed. The second subproblem addressed power allocation for a fixed number of users, SA, and a subcarrier. They suggested a novel multiple SAs cooperative-based method to achieve optimal scheduling by solving the two subproblems. The simulation results demonstrated that compared to the standard algorithms, the proposed multi-SAs cooperative algorithm has better transmission performance regarding the total rate and power efficiency. The optimal iteration number for the proposed algorithms is shown to be relatively low. Nevertheless, user fairness needs more attention and consideration. In [134], the joint optimization problem of resource allocation in a cellular network with mmWave and traditional wave has been investigated. A cross-layer optimization problem is formulated for this innovative architecture to optimize EE by balancing SA and route selection. Two subproblems have been identified and addressed individually, allowing this problem to be resolved. In the first part of the resource allocation problem, they discovered it was non-convex and introduced four solutions using stochastic algorithms with a given route selection. According to the new model specifications, both SA and power allocation are coded independently. Then, the four resource allocation methods, including the genetic algorithm and the differential evolution algorithm, are discussed, through which the SA and power allocation jointly enhance the SE and EE. Regarding the second component of the routing issue, they developed it as a linear programming problem and solved it with an optimization toolbox while managing to keep the resource allocation constant. The simulation results show that the EE and SE of the mmWave network can be improved by simultaneously optimizing resource allocation and path selection on a cross-layer basis. Furthermore, network throughput can be increased by perfectly locating the route without overloading any of the BSs. Nonetheless, user mobility is not considered. In [135], they proposed a new approach for licensed SA to allocate the entire 28 GHz mmWave spectrum to each BS to perform its small cells for each building to avoid co-channel interference. Hence, increasing spectrum availability and utilization. Because most data are produced inside, the full mmWave SA provided to a BS is regarded as being reused for small indoor cells. They introduced a frequency-domain and time-domain method for avoiding co-channel interference and deriving its optimality conditions. They determine the average capacity, SE, EE, and cost efficiency for licensed SA and carry out extensive numerical and simulation results.