Farhad Meshkati

Interference management and performance analysis of UMTS/HSPA+ femtocells

Femtocells are low-power cellular base stations that operate in licensed spectrum. They are typically deployed indoors to improve coverage and provide excellent user experience, including high data rates. Cellular operators benefit from reduced infrastructure and operational expenses for capacity upgrades and coverage improvements. Femtocells also bring unique challenges, such as unplanned deployment, user installation, restricted access, and interoperability with existing handsets and network infrastructure. Although femtocells may cause some interference to other users in the network, with the use of proper interference management techniques, this can be well controlled. We present interference management techniques for both downlink and uplink of femtocells operating based on 3GPP Release 7 standards (also known as HSPA+). Femtocell carrier selection and femtocell DL Tx power self-calibration are proposed as key interference management methods for downlink. For uplink interference management, adaptive attenuation at the femtocell and limiting the Tx power of the femtocell users are proposed. Different interference models and their analysis are presented. In addition, coverage performance and capacity results are presented to quantify the benefits of femtocells. We demonstrate that in addition to coverage enhancements, significant capacity improvements are achieved on both downlink and uplink when femtocells are deployed in 3G UMTS/HSPA+ networks.

Uplink Interference Management for HSPA+ and 1xEVDO Femtocells

Femtocells are low power cellular base stations typically deployed indoors in residential and enterprise environments as well as hotspots in order to improve voice and high rate data coverage and provide excellent user experience. The cellular operator benefits from reduced infrastructure deployment costs for capacity upgrades and coverage improvements. While improving performance, femtocells may cause some interference to other users in the network. However, with the use of proper interference management techniques, this interference may be well controlled. This paper focuses on uplink (UL) interference management techniques for 3G femtocell deployments. On the UL, the challenge is the presence of large uncontrolled interference from nearby users not associated by the femtocell that result in high noise rise (rise-over-thermal, RoT), that may lead to poor femto user experience. Femtocell users that can not be power controlled due to their very close proximity to femtocells may also cause high noise rise levels. An algorithm is proposed for both HSPA+ and 1xEVDO femtocells to desensitize the receiver when uncontrolled interference is detected, ensuring robust UL performance with minimal impact on the macro network. It is demonstrated through system level simulations that in addition to superior performance experienced by femtocell users, the macro users also benefit significantly from offloading traffic load to the femto network.

Mobility and Capacity Offload for 3G UMTS Femtocells

Femtocells are low-power cellular base stations that are typically deployed indoors in residential, enterprise or hotspots settings. Femtocell deployments provide excellent user experience through better coverage for voice and very high data throughputs. In this paper, we focus on 3G UMTS femtocells and analyze the impact of femtocells on idle-mode mobility and UE battery life. Detailed dynamic simulations are presented to quantify the impact of femtocells on the number of cell searches, cell reselections and location area updates for both femtocell users and macro users. It is shown that femtocells reduce the number of intra-frequency and inter-frequency searches for femtocell users while increasing the number of searches for macro users. In addition, methods for facilitating capacity offload to femtocells are presented. In particular, cell reselection parameter optimization, use of beacons and enhancements of UE search algorithms are described as possible methods for facilitating capacity offload to femtocells. The tradeoffs between capacity offload and UE battery life are also evaluated.

Downlink Transmit Power Calibration for Enterprise Femtocells

Downlink transmit (Tx) power calibration is necessary in femtocell deployments to achieve a good tradeoff between downlink femtocell coverage and interference to other macrocells and femtocells. Developing a practical Tx power calibration scheme that provides excellent performance in different deployment scenarios is challenging. In this paper, we present an enterprise deployment framework that consists of coverage planning and Tx power calibration using technician assistance. The Tx power calibration algorithms presented here use mobile feedback to learn key information such as surrounding RF environment, required coverage range, and interference impact to other cells to adapt to different deployments. Both centralized and distributed Tx power calibration algorithms are evaluated for multi-femto enterprise. It is shown that the distributed algorithm, is simple, efficient and performs very close to the centralized algorithm. Furthermore, over-the-air field tests highlight the practical efficacy of the distributed algorithm in providing excellent femtocell coverage indoors with minimal leakage to outdoors.

Benefits of transmit and receive diversity in enterprise femtocell deployments

In this paper, we study the benefits of transmit and receive diversity for enterprise UMTS femtocell deployments. Indoor enterprise femtocell deployments face a single-path slow fading wireless environment that may lead to frequent hard handovers in the boundary region of neighboring femtocells and consequent degradation in the voice quality experienced by the users. In the absence of soft-handover (SHO) support, transmit diversity at the femtocell can combat single-path fading channel. We demonstrate through over-the-air tests that transmit diversity is very effective in reducing the number of hard handovers and therefore results in significant improvement in voice quality for enterprise users. On the uplink, we study system stability using an analytical approach. We derive analytical conditions for system stability with and without receive diversity at the femtocells. Using this analytical framework, benefits of receive diversity in maintaining system stability and preventing uplink power racing between neighboring femtocells are quantified. It is shown that, in the absence of SHO, receive diversity is very effective in maintaining system stability by preventing potential uplink power racing caused by inter-femto interference.

Transmit power self-calibration for residential UMTS/HSPA+ femtocells

Downlink transmit power calibration is critical for femtocell deployment to achieve a good tradeoff between downlink coverage and interference impact to a co-channel macrocell network. In this paper, we study the performance of a femtocell power calibration algorithm that uses mobile reports to learn key information such as required coverage region for its own users and amount of interference to non-allowed users to fine-tune the femtocell downlink transmit power. It is shown that such a scheme is able to adapt to different deployment scenarios and provides better control of coverage-interference tradeoff than that provided by algorithms based on Network Listen Module alone.

Downlink interference management techniques for residential femtocells

Downlink transmit (Tx) power calibration techniques are vital in self-organizing femtocell networks to achieve a good tradeoff between femtocell coverage and interference impact to nearby macrocells and femtocells. In this paper, we present two power calibration techniques that adapt to a wide variety of residential deployment scenarios: (1) Over a long term, Tx power is adapted by learning required coverage range, RF profile inside the home and interference outside using feedback from home users and macro users; (2) Tx power is adapted in real-time based on short term dynamic variations in traffic such as arrival of active macro users (guest or passer-by). The presence of nearby active macro users is detected by sensing their uplink channel. The algorithms are shown to provide superior performance compared to conventional techniques in wide variety of deployment scenarios.

Uplink interference management techniques for 3G femtocells

In this paper, we present a comprehensive study of uplink interference management for 3G femtocell deployments. Focusing on co-channel deployment, where both femtocells and macrocells operate on the same carrier, femto-to-macro, macro-to-femto and inter-femto uplink interference scenarios are identified. To effectively control uplink interference in residential and enterprise femtocell deployments, Macro Aware noise Rise Setting (MARS) and Controlled LImit on uplink Power (CLIP) algorithms are introduced. MARS maximizes the femtos tolerance to out-of-cell interference by properly setting the femto noise rise threshold based on the location of the femto in the macrocell. CLIP dynamically limits the femto user Tx power or rate to control the interference to macrocell. The effectiveness of the proposed algorithms is demonstrated via detailed system-level simulations for enterprise and residential femtocell deployments.