text: Managing interference in a network of macrocells underlaid with femtocells presents an important, yet challenging problem. A majority of spatial (frequency time) reuse based approaches partition the users based on coloring the interference graph, which is shown to be suboptimal. Some spatial time reuse based approaches schedule the maximal independent sets (MISs) in a cyclic, (weighted) round-robin fashion, which is inefficient for delay-sensitive applications. Our proposed policies schedule the MISs in a non-cyclic fashion, which aim to optimize any given network performance criterion for delay-sensitive applications while fulfilling minimum throughput requirements of the users. Importantly, we do not take the interference graph as given as in existing works; we propose an optimal construction of the interference graph. We prove that under certain conditions, the proposed policy achieves the optimal network performance. For large networks, we propose a low-complexity algorithm for computing the proposed policy. We show that the policy computed achieves a constant competitive ratio (with respect to the optimal network performance), which is independent of the network size, under wide range of deployment scenarios. The policy can be implemented in a decentralized manner by the users. Compared to the existing policies, our proposed policies can achieve improvement of up to 130 in large-scale deployments.
  @cite_38: 
  @cite_4: This paper proposes a decentralized interference control scheme for an OFDMA (Orthogonal Frequency Division Multiple Access) cellular scenario where multiple macrocells and femtocells are deployed. In particular, we model the distributed resource allocation problem by means of a potential game, which is demonstrated to converge to a unique and pure Nash equilibrum. In this game, the femto base stations are the players, their actions are the resource blocks and the corresponding power levels to be allocated for downlink transmission, and the utility is designed to guarantee coexistence with the macro system, guaranteeing a fair tradeoff between femto and macro systems' performances. To do that, the utility function does not only consider the capacity of the femtocells, but also the different sources of inter-system interference: macrocell to femtocell, femtocell to femtocell, and femtocell to macrocell. The game is solved by applying a better response dynamic, which first selects the best resource blocks allocation policy for the femtousers, and then selects the most appropriate power levels for each resource block, through the solution of the corresponding convex optimization problem. Simulation results show that the introduction of femtocells in the scenario increases the total system capacity, and that the considered utility function provides an improvement in macro system performance of up to 40 , while reducing the femto capacity only up to 7 , with respect to the well known iterative waterfilling game.
  @cite_1: We propose a new distributed power control scheme for closed access femtocell networks. Since femto APs can be deployed unpredictably in a macro cell area, interference to macro users and interference between femto APs in down-link (DL) cause a severe problem. In this paper, we describe the system model using a noncooperative game model. We formulate a payoff function to provide fairness and to minimize interference. At the same time, we prove that this function can be appeared as a supermodular type. Our power control algorithm based on the game model can be applied to the decentralized environment. This novel power control algorithm leads transmission power to reach a steady state condition, i.e. Nash Equilibrium (NE). Through mathematical analysis and numerical results, we show that our proposed power control algorithm has several good characteristics.
  @cite_32: Femtocells are low-power, very small-service-area (e.g. home or office environment) cellular base stations that will significantly impact the cellular landscape in the next several years. One of the most important open technical issues related to femtocells concerns the impact on system performance of different policies regarding who is allowed to connect to a femtocell. In the present paper, interaction between mobile stations (MS) that are near to, but not necessarily communicating with, femtocells is explored. It is shown that an adaptive femtocell access policy that takes specific account of the instantaneous loads on the network can lead to improved performance over a completely open, or completely closed approach.
  @cite_24: In a two tier cellular network - comprised of a central macrocell underlaid with shorter range femtocell hotspots - cross-tier interference limits overall capacity with universal frequency reuse. To quantify near-far effects with universal frequency reuse, this paper derives a fundamental relation providing the largest feasible cellular Signal-to-Interference-Plus-Noise Ratio (SINR), given any set of feasible femtocell SINRs. We provide a link budget analysis which enables simple and accurate performance insights in a two-tier network. A distributed utility- based SINR adaptation at femtocells is proposed in order to alleviate cross-tier interference at the macrocell from cochannel femtocells. The Foschini-Miljanic (FM) algorithm is a special case of the adaptation. Each femtocell maximizes their individual utility consisting of a SINR based reward less an incurred cost (interference to the macrocell). Numerical results show greater than 30 improvement in mean femtocell SINRs relative to FM. In the event that cross-tier interference prevents a cellular user from obtaining its SINR target, an algorithm is proposed that reduces transmission powers of the strongest femtocell interferers. The algorithm ensures that a cellular user achieves its SINR target even with 100 femtocells cell-site (with typical cellular parameters) and requires a worst case SINR reduction of only 16 at femtocells. These results motivate design of power control schemes requiring minimal network overhead in two-tier networks with shared spectrum.
  @cite_31: This paper proposes two interference mitigation strategies that adjust the maximum transmit power of femtocell users to suppress the cross-tier interference at a macrocell base station (BS). The open-loop and the closed-loop control suppress the cross-tier interference less than a fixed threshold and an adaptive threshold based on the noise and interference (NI) level at the macrocell BS, respectively. Simulation results show that both schemes effectively compensate the uplink throughput degradation of the macrocell BS due to the cross-tier interference and that the closed-loop control provides better femtocell throughput than the open-loop control at a minimal cost of macrocell throughput.
  @cite_10: This paper proposes a decentralized model for the allocation of modulation and coding schemes, subchannels and transmit power to users in OFDMA femtocell deployments. The proposed model does not rely on any exchanged information between cells, which is especially useful for femtocell networks. Coordination between femtocells is achieved through the intrinsic properties of minimising transmit power independently at each cell, which leads the network to self-organize into an efficient frequency reuse pattern. This paper also provides a two-level decomposition approach for solving this intricate resource assignment problem that is able to find optimal solutions at cell level in reduced periods of time. System-level simulations show a significant performance improvement in terms of user outages and network capacity when using the proposed distributed resource allocation in comparison with scheduling techniques based on uniform power distributions among subcarriers.
  @cite_20: 
summary: The first and most widely-used interference management policies @cite_1 @cite_20 @cite_10 @cite_4 @cite_38 @cite_31 @cite_24 are based on power control. In these policies, all the UEs in the network transmit at a @math power at all time (provided that the system parameters remain the same) in the entire frequency band Although some power control policies @cite_1 go through a transient period of adjusting the power levels before converging to the optimal power levels, the users maintain the constant power levels after the convergence. . When the cross channel gains among BSs and UEs are high, simultaneous transmissions at the same time and in the same frequency band will cause significant interference among cells. Such strong interference is common in macrocells underlaid with femtocells. For example, in @cite_32 it is shown that that interference from MUEs near the FBS severely affects the uplink transmissions of FUEs. Also, in offices and apartments, where FBSs are installed close to each other, inter-cell interference is particularly strong. In contrast, our proposed solutions mitigate the strong interference by letting only a subset of non-interfering UEs to transmit at the same time.