Interference Mitigation Using Uplink Power Control for Two-Tier Femtocell Networks

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.

💡 Research Summary

This paper addresses the critical challenge of cross-tier interference in two-tier femtocell networks, where consumer-deployed indoor femtocells share the licensed spectrum with an existing macrocell network. The uplink interference from femtocell users to the macrocell base station (BS) can severely degrade macrocell performance. To mitigate this, the authors propose two novel and autonomous interference mitigation strategies centered on dynamically adjusting the maximum transmit power limit (P’_max) of femtocell users.

The first strategy is an open-loop control scheme. In this method, a femtocell user autonomously estimates the path loss to surrounding macrocell BSs by measuring received downlink signal powers. It identifies the macrocell BS (k*) that would be most susceptible to its interference (having the minimum path loss, L_min). The user then calculates its maximum allowed transmit power (P_max,OL) as the product of L_min and a predetermined, fixed interference threshold (I_th,k*). This threshold is derived from system parameters like a target interference-to-noise ratio and the number of active femtocell users affecting BS k*. This approach allows femtocells to self-limit their interference without any real-time feedback from the macrocell network, making it simple to implement.

Recognizing a key limitation of the open-loop method—its inability to adapt to the actual interference conditions at the macrocell BS—the authors propose a second, more sophisticated closed-loop control scheme. This method incorporates real-time network state information. Along with determining L_min and k*, the femtocell user also receives periodic broadcasts of the current Noise and Interference (NI) level from macrocell BSs. The core innovation lies in making the interference threshold I_th,k(n) adaptive. It is calculated based on a comparison between the current NI level (N_I_k(n)) and a reference level (N_I_k(0)) measured just before the femtocell user became active. If the NI level increases (indicating rising interference), the threshold remains strict to protect the macrocell. Conversely, if the NI level decreases (e.g., due to reduced macrocell traffic), the threshold is relaxed, allowing the femtocell user to transmit with higher power. The maximum power limit for the closed-loop scheme (P_max,CL) is then calculated using this adaptive threshold and L_min.

The performance of both schemes is evaluated via system-level simulations based on CDMA2000 1xEV-DO Rev. A parameters. Key performance metrics are the Degradation Ratio of Macrocell Throughput (DRMT) and the Achievement Ratio of Femtocell Throughput (ARFT). In a single-femtocell scenario, results demonstrate that both proposed schemes drastically reduce macrocell throughput degradation compared to a conventional fixed maximum power approach, consistently achieving a DRMT of less than 5% across various distances and wall penetration losses. Crucially, the closed-loop control scheme provides significantly better femtocell throughput (higher ARFT) than the open-loop control, while incurring only a minimal additional cost to macrocell throughput. This superior performance stems from the closed-loop method’s ability to opportunistically increase femtocell transmit power when macrocell interference is low, leading to more efficient overall spectrum utilization.

In conclusion, this paper presents two practical and effective uplink power control strategies for autonomous interference management in femtocell networks. The open-loop method offers a simple, standalone solution, while the closed-loop method provides a smarter, adaptive approach that balances macrocell protection with enhanced femtocell capacity by leveraging real-time network feedback, offering a valuable framework for future self-organizing networks.