Source-linked AI summary
Interference Mitigation Using Uplink Power Control for Two-Tier Femtocell Networks
Han-Shin Jo, Cheol Mun, June Moon, Jong-Gwan Yook
TL;DR
Cross-tier interference degrades macrocell uplink throughput in two-tier femtocell networks. The paper proposes two maximum-transmit-power control schemes for femtocell users; simulations show both compensate the degradation, while closed-loop control improves femtocell throughput at minimal macrocell cost.
Problem
Cross-tier interference causes uplink throughput degradation at the macrocell BS.
Method
The paper proposes open-loop and closed-loop schemes in which femtocell users adjust their maximum transmit power to suppress cross-tier interference.
Results
Both schemes effectively compensate macrocell BS uplink throughput degradation, while closed-loop control provides better femtocell throughput than open-loop control.
Takeaways & Limitations
Closed-loop control offers superior femtocell throughput relative to open-loop control at a minimal cost of macrocell throughput.
Abstract
from arXiv · showhide
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.
I. INTRODUCTION
Femtocells share licensed spectrum with macrocell networks, making uplink cross-tier interference control essential. The paper proposes open-loop and closed-loop maximum-transmit-power controls that mitigate this interference while preserving throughput.
- Motivation: Femtocells share operators’ licensed spectrum with macrocell networks, so limiting femtocell-user interference at the macrocell BS is essential for uplink deployment.The paper also notes that femtocells must sense their radio environments and self-configure because operators cannot control their locations.
- Related work: Earlier interference-control methods require site-specific parameter tuning and are therefore unsuitable for femtocells needing automatic radio-resource self-configuration.Prior approaches include transmit-power control and time hopping with antenna sectoring; one SIR-based scheme also omits wall-penetration loss.
- Proposed approach: The paper proposes open-loop and closed-loop techniques in which femtocell users adjust their maximum transmit power to suppress cross-tier interference.The proposed strategies target femtocell users’ transmit-power limits rather than relying on manual site-specific optimization.
- Proposed approach: Open-loop control keeps cross-tier interference below a fixed threshold, whereas closed-loop control uses an adaptive threshold based on noise and interference at the macrocell BS.The closed-loop threshold reflects the NI level measured at the macrocell BS.
- Results: Both schemes compensate macrocell uplink-throughput degradation, while closed-loop control delivers superior femtocell throughput at a very low macrocell-throughput cost.The paper presents these findings as simulation results.
II. SYSTEM MODEL
The system models a two-tier CDMA network in which macrocell and femtocell users share spectrum, creating cross-tier interference. Conventional uplink power control derives transmit power from propagation loss, required SINR, and noise-interference conditions, but its fixed maximum power cannot adapt to current interference.
- Network and propagation model: The modeled network contains macrocells and uniformly distributed in-building femtocells, with radio links classified as outdoor, indoor, or outdoor-to-indoor.Propagation loss is modeled using ITU and COST231 path-loss models with log-normal shadowing.
- Conventional uplink power control: Conventional uplink open-loop control estimates propagation loss from received downlink power and broadcast macrocell-BS EIRP.The required transmit power also uses the target SINR and average noise-interference level at the BS.
- Conventional uplink power control: The user transmit power is limited by a maximum value Pmax after deriving the required power from propagation and SINR conditions.Pmax is constrained by the spectrum emission mask and error vector magnitude requirements.
- Interference motivation: Because conventional Pmax is fixed, it cannot adapt to changing interference conditions.The fixed limit may restrict femtocell users more than necessary when macrocell-BS noise and interference is low, degrading femtocell uplink throughput.
- Interference motivation: Additional femtocell-user interference raises macrocell-BS noise and interference, which degrades macrocell uplink throughput.Adjusting Pmax according to cross-tier interference is intended to limit this rise and compensate the degradation.
III. UPLINK POWER CONTROL IN A FEMTOCELL
The paper proposes femtocell-user uplink power control that adjusts the maximum transmit power according to cross-tier interference using open-loop and closed-loop techniques.
- Proposed control framework: The proposed scheme adjusts femtocell-user maximum transmit power Pmax as a function of cross-tier interference.Two variants are considered: open-loop and closed-loop control.
A. Open-Loop Control for Pmax
Open-loop control limits femtocell-user transmit power using a predetermined cross-tier interference threshold derived from propagation loss to the most affected macrocell BS. It guarantees an interference bound but does not use the actual macrocell-BS NI level.
- Open-loop control: The open-loop controller adjusts Pmax so the femtocell user's additional cross-tier interference remains below a predetermined maximum allowable level.The macrocell BS identifies the number of active femtocell users associated with it and broadcasts that information to femtocell users.
- Open-loop estimation: Each femtocell user estimates propagation losses to neighboring macrocell BSs from received signal powers and broadcast EIRP information.The approach can use time-averaged received powers in FDD and instantaneous powers in TDD because of channel reciprocity.
- Open-loop estimation: The macrocell BS with minimum propagation loss is treated as most affected by the femtocell user's cross-tier interference.The user estimates this worst-case BS without monitoring every neighboring BS.
- Open-loop control: The resulting transmit-power rule guarantees that the maximum cross-tier interference does not exceed the maximum allowable interference power of each macrocell BS.The user transmit power is computed using the general power rule with Pmax replaced by Pmax,OL.
- Open-loop limitation: Because open-loop control ignores the actual NI level at each macrocell BS, it can limit femtocell-user power more than necessary when NI is low.This unnecessarily reduces femtocell uplink throughput.
B. Closed-Loop Control for Pmax
Closed-loop control adapts the allowable cross-tier interference using macrocell-BS NI measurements before and after femtocell activation. It lowers femtocell power when NI rises and can increase allowable interference when NI falls, improving femtocell throughput while controlling macrocell NI.
- Closed-loop operation: Closed-loop control adjusts Pmax using both estimated femtocell-user cross-tier interference and the macrocell-BS NI level.Macrocell BSs monitor uplink interference and broadcast NI levels to femtocell users through wired femtocell BS connections.
- Closed-loop operation: The procedure records an initial NI level NIk(0) before femtocell activation and updates NIk(n) after activation.The updated level includes additional cross-tier interference from the active femtocell user.
- Adaptive threshold: The allowable interference Ith,k(n) is determined from the current and initial NI levels using the system-specific parameter β.The rule distinguishes the initial step, NI increases, and NI decreases relative to NIk(0).
- Adaptive threshold: When NI increases, the controller lowers femtocell-user maximum transmit power to keep the macrocell-BS total NI under the target condition.When NI decreases below its initial level, Ith,k(n) increases, allowing higher femtocell transmit power and improving femtocell throughput.
- Closed-loop outcome: The closed-loop transmit-power rule uses Pmax,CL(n) based on the allowable interference at the macrocell BS with minimum propagation loss.The resulting femtocell-user transmit power enhances femtocell uplink throughput while keeping total macrocell-BS NI under control.
IV. PERFORMANCE EVALUATION
The evaluation uses CDMA2000 1xEV-DO Revision A parameters, distributed macrocell and femtocell users, fast fading channels, and proportional-fair scheduling. Transmit-power updates occur at 150 Hz, with performance evaluated under the specified bandwidth, frequency, antenna, and interference-threshold settings.
- Simulation setup: Each sector contains 10 uniformly distributed macrocell users, while each 50-meter building contains four uniformly distributed femtocell users and one centered femtocell BS.
- Simulation setup: Users connect to the macrocell BS offering the lowest path loss, and each femtocell BS is located at the center of its building.
- Simulation setup: Physical-layer packets span four contiguous slots, each slot lasting 1.667 ms, under proportional-fair scheduling.
- Channel and control modeling: Transmit-power updates occur at 150 Hz, and desired and interference links use modified Jakes fast-fading channels.
- Evaluation conditions: The open-loop and closed-loop schemes are compared using an identical interference threshold, with N0WF set to -109 dBm.
A. Single femtocell
For a single femtocell, the proposed controls are evaluated against fixed maximum transmit power using macrocell degradation and femtocell achievement ratios. Both controls outperform fixed power, while closed-loop control offers better femtocell performance with minimal macrocell-throughput degradation, especially under stronger cross-tier interference.
- Evaluation framework: The study compares fixed maximum power of 23 dBm with open-loop and closed-loop controls as distance D and external wall loss Le vary.
- Macrocell performance: DRMT decreases as D increases because cross-tier interference at the macrocell BS decreases.
- Macrocell performance: Both proposed controls consistently outperform fixed maximum power, with gains increasing as D and Le decrease.
- Macrocell performance: Both controls achieve DRMT below 0.05 regardless of D and Le.
- Femtocell performance: Closed-loop control provides better ARFT than open-loop control, while its macrocell-throughput degradation remains minimal compared with open-loop control.
- Femtocell performance: The closed-loop advantage increases as D and Le decrease, when cross-tier interference dominates the macrocell's noise-and-interference level.
B. Multiple femtocells
With multiple femtocells, increasing the number of femtocell BSs per macrocell intensifies cross-tier interference and reduces femtocell throughput. Closed-loop control provides larger gains for femtocell-edge users, while open-loop control remains favorable for average throughput and requires less overhead.
- Multiple-femtocell effects: Increasing the number of femtocell BSs per macrocell causes more severe cross-tier interference and reduces femtocell throughput.
- Macrocell performance: The two proposed schemes keep macrocell interference below the threshold irrespective of the number of femtocell BSs, unlike fixed maximum power.
- User-throughput performance: Macrocell 5% user throughput follows a trend similar to average macrocell throughput because cross-tier interference commonly affects all corresponding macrocell users.
- Control comparison: The closed-loop gain is larger for 5% femtocell user throughput because users near the macrocell BS and with low wall loss benefit more from its higher allowed transmit power.
- Control comparison: Considering both tiers, closed-loop control is more effective for 5% user throughput, whereas open-loop control is favorable for average throughput.
- Control comparison: Closed-loop control gains over open-loop control reach maximally 8% for average femtocell throughput and 120% for femtocell 5% user throughput.
- Control overhead: Closed-loop control additionally requires femtocell users to know the macrocell BS's NI level, requiring downlink broadcast overhead.
V. CONCLUSION
The paper proposes open-loop and closed-loop maximum-transmit-power controls that suppress cross-tier interference using fixed and adaptive thresholds, respectively. Simulations show that both compensate macrocell uplink-throughput degradation, while closed-loop control improves femtocell throughput at minimal macrocell cost.
- The study proposes open-loop and closed-loop schemes for controlling femtocell users' maximum transmit power.
- Open-loop control suppresses cross-tier interference under a fixed threshold, whereas closed-loop control uses an adaptive threshold based on the macrocell BS's actual NI level.
- Both schemes effectively compensate the macrocell BS's uplink-throughput degradation caused by cross-tier interference.
- Closed-loop control provides better femtocell throughput than open-loop control at minimal macrocell-throughput cost.
UPLINK DATA RATE FORMAT
The figures compare uplink data-rate or throughput behavior across fixed-power, open-loop, and closed-loop control schemes under varying network conditions.
- Fig. 2 compares DRMT LT versus D for fixed maximum-power transmission, closed-loop control, and open-loop control at Li = 0 dB.
- Fig. 3 compares ARFT AT versus D for the closed-loop and open-loop control schemes at Li = 0 dB.
- Fig. 4 plots average throughput against the number of femtocell BSs per macrocell, M, for the closed-loop and open-loop control schemes.
- Fig. 5 plots 5% user throughput against the number of femtocell BSs per macrocell, M, for the closed-loop and open-loop control schemes.