Robust Power Control In Two-tier Femtocell Networks With Uncertain Channel Gains

Femtocell also called “home data service network”, has characteristics of low-power, low-cost, and high-service. Femtocell technology plays a pivotal role in solving the problem of Macrocell’s edge coverage, reducing the cost of cellular coverage, and improving the wireless communication quality. However, the random deployment of Femtocells leads to serious interference problem, so interference management in two-tier Femtocell networks is a significant issue. And, power control is an important method to deal with the interference. In the existing power control alrorithms, the majority are based on determined wireless environment. But, in practice, the wireless communication environment is unstable, which makes the performance of the communication system unsatisfied with the existed power algorithm. Therefore, for the uncertain communication environment of two-tier Femtocell networks, robust power control algorithms are researched in our paper.Firstly, for the case of uncertainty of users’ instantaneous signal to intrtference plus noise ratio(which is affected by fast fading), a robnust optimization problem based on probability threshold constraint is estanblished. And a robust power control algorithm based on adjustment of maximum power is proposed. The algorithm achieves cross-tier interference management by adjusting Femtocell users’ maximum power during the power iteration. Therefore, the algorithm guarantees the macrocell user’s communication and optimization of Femtocell users’ transmission power. In addition, to utilize the network resource reasonably, we propose an admission control algorithm to conduct access management for Femtocell users whose communication qualities are not satisfied.Secondly, for the case of uncertain channel gains, which are affected by path loss, shadowing and Rayleigh fading, we conduct structural decomposition of users’ signal to intrtference plus noise ratio and a robust optimization problem based on worst-case probability constraint is set up. Then according to the optimization problem, a robust iterative algorithm based on standard interference function is proposed. The power of Femtocell users and Macrocell user is allocated jointly by the algorithm with less communication overhead. Furthermore, the Monte Carlo method is used to verify the robustness of the scheme. The simulation results show that the power based on the algorithm can adapt to the complex channel environment, i.e., the power algorithm has strong robustness.Finally, for the case of uncertain channel gains, which are changing slowly in a certain closed interval, a robust distributed power control algorithm is proposed. The power of Femtocell users and Macrocell user are allocated jointly by the algorithm. Since the algorithm can obtain the interference without information exchanging, then the power allocation is executed in a perfectly distributed manner. Moreover, admission control is introduced to protect the uplink communication of the Macrocell user. Simulation results show that, the signal to intrtference plus noise ratio of users can be guaranteed in a high probability in the dynamic environment, which can not affect the users’ normal communication. Therefore, the power algorithm is verified to be robust. Also, the simulation demonstrates that the power algorithm is more stable than the FM algorithm, and that the admission control is effective.