Researches On Distributed Optimization Algorithms For Ultra-dense Small Cell Networks

The development of wireless communications has provided a great convenience for people’s life. As the commercialization of the fourth generation of mobile communications, the researches on the fifth generation of mobile communications(5G) in 2020 and beyond have attracted much more attentions from both academics and industries. As one of the key technologies in 5G, ultra-dense small cell networks are becoming one of the hot topics around the world. In ultra-dense small cell networks, the coverage range of each cell becomes much smaller due to the reduced distance between the transmitters and receivers. In addition, ultra-dense small cell networks can increase frequency reusing and thus improve the spectrum efficiency and energy efficiency. However, with the increase of the network scale, the information to be processed will increase and the process will be more difficult. In addition, the information exchange among different base stations brings a lot of signal overhead. Therefore, distributed optimization algorithms in ultra-dense small cell networks are studied in this paper, the main contributions of this paper are as follows:Firstly, the resource allocation problem in downlink small cell networks is investigated and a distributed resource allocation scheme is proposed. In this proposed scheme, the weighted sum rate of the system is maximized by optimizing the power allocation and subcarrier allocation. The resource allocation problem in ultra-dense networks is in nature a large scale optimization problem. By using an advanced decomposition algorithm, the original optimization problem is decomposed into a series of independent sub-problems. The optimal solution of the original problem is obtained by solving these sub-problems in parallel. The proposed algorithm is compared with the conventional resource allocation methods and simulation results demonstrate the better performance of the proposed algorithm.Then a chunk-based resource allocation problem in ultra-dense small cell networks is studied. In this method, the continuous subcarriers are grouped into a chunk. The resource allocation overhead is greatly reduced by allocating different chunks to users. By jointly optimizing the power allocation and chunk allocation, the throughput of the network is maximized. By simulation, under the situation that the system performance is not degraded, the number of subcarriers is first determined. And by comparing the proposed method with conventional methods, the good performance is shown.Finally, a beamforming design scheme in ultra-dense small cell networks is proposed. In the proposed scheme, the channel estimation error is considered, and the power of the desired signal is maximized, while the probability that the leakage interference is larger than a certain value is less than a given threshold. Also, in order to reduce the computation complexity, a low complexity distributed beamforming algorithm is derived. The performance of the proposed method is proved by simulation.