Power allocation schemes in OFDM-based femtocell networks

Orthogonal frequency division multiplex-based femtocell networks in downlink with two-tier interference are considered in this paper. In order to satisfy the quality of service of macrocell users, a capacity maximization problem with the total and the upper power constraints by allocating power to subcarriers for a user in femtocell systems becomes important. In this paper, the optimal power allocation is derived by using the Lagrangian technique. Based on the analysis of the Karush-Kuhn-Tucker conditions, subcarriers can be classified into two sets with different power allocation strategies according to the upper power constraint. One set of subcarriers is allocated with upper power, and the other set of subcarriers is processed by using the waterfilling approach. A direct linear search scheme is presented to achieve the optimal performance by finding the threshold over the channel states of all subcarriers to determine the two sets. In order to reduce the computational load, a reduced complexity scheme is designed for the optimal solution by utilizing the relationship of the allocated power in the two sets of subcarriers. Unlike the schemes in iterative manners, a fast power allocation scheme with a near-optimal performance is also developed. Simulation results reveal that the proposed schemes outperform the existing scheme. The efficacy of the proposed schemes is demonstrated in the simulations.