Yi Jiang

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.