IEEE Communications Magazine • October 2008 58 0163-6804/08/$25.00 © 2008 IEEE INTRODUCTION Due to advances in wireless technology, cell phones have become indispensable in everyday life. To improve user experience is the never- changing goal of the wireless industry. A com- mon complaint with current cellular systems is their imperfect coverage, especially indoors. Due to the high penetration loss from walls, the sig- nal strength received from an outdoor base sta- tion (BS) inside a building may be too low to attain acceptable performance. Since BSs cannot increase transmission power unlimitedly to enhance received signals in outage areas, a bet- ter solution may be to use additional devices to cover these locations. The traditional approach is to deploy additional BSs, which is economical- ly inefficient because of the tedious location finding process and the high backhaul and imple- mentation cost. Thus, a new concept, femtocell, is proposed by the cellular industry. A direct interpretation of femtocell is a small coverage area such as a house or small office, home office (SOHO). An existing exam- ple of femtocell coverage is the WiFi access point (AP) whose typical coverage region is up to 100 m in radius. The success of WiFi has inspired cellular operators to cover a small area with a low-cost AP-like device. In the current cellular industry definition, femtocell can refer to either the coverage area or the device itself. This article uses the term Femto-AP for the device, which is a simplified low-power device that utilizes cellular technology (2G/3G/WiMAX) with IP backhaul through a local broadband connection, such as digital sub- scriber line (DSL), cable, or fiber. Ideally, the Femto-AP should be a low-cost simple plug- and-play device like a WiFi AP. Through deployment of a large number of Femto-APs, significant gain in areal capacity and indoor coverage can be achieved, which is detailed in later sections. In addition, Femto- AP deployments have several advantages over other technologies. First, they are a cost-effec- tive solution for indoor access since they are more likely to be deployed at places that need them most, and being a consumer device, the cost of a Femto-AP is expected to be under $200 [1]. Wireless operators save on backhaul costs since Femto-AP traffic is carried over wired residential broadband connections that connect to the IP backbone. The consumer can expect improved data speeds and service quali- ty, and longer battery life as it is no longer nec- essary to connect to outdoor macro/micro BSs. Furthermore, Femto-APs enable the conver- gence of landline and mobile services since the same handheld device can be used to access the ABSTRACT Femtocells are viewed as a promising option for mobile operators to improve coverage and provide high-data-rate services in a cost-effective manner. The idea is to overlay low-power and low-cost base station devices, Femto-APs, on the existing cellular network, where each Femto-AP provides high-speed wireless connection to sub- scribers within a small range. In particular, Femto-APs can be used to serve indoor users, resulting in a powerful solution for ubiquitous indoor and outdoor coverage, using a single access technology such as WiMAX. In this arti- cle we consider a WiMAX network deploying both macro BSs and Femto-APs, where it is assumed that Femto-APs have wired backhaul such as cable or DSL and operate on the same frequency band as macro BSs. Simulation results show that significant areal capacity (throughput per unit area) gain can be achieved via intense spatial reuse of the wireless spectrum. In addi- tion, Femto-APs improve indoor coverage, where the macro BS signal may be weak. Moti- vated by the gains in capacity and coverage offered by femtocells, we review the state of the art of this “infant” technology, including use cases and network deployment scenarios, techni- cal challenges that need to be addressed, and current standardization and industry activity. WIMAX: A TECHNOLOGY UPDATE Shu-ping Yeh and Shilpa Talwar, Intel Corp Seong-Choon Lee, KT Heechang Kim, Telcordia WiMAX Femtocells: A Perspective on Network Architecture, Capacity, and Coverage