Microwave Engineering Europe - January/February 2009 - (Page 15)

TEST & MEASUREMENT — FEMTOCELLS 15 GPS: making a play for femtocells By John Kikuchi and Frank Palmer, Agilent Technologies emtocells – small low-power base stations designed for indoor use in residential or small-business environments – are expected to realize tremendous growth in the coming years as they move closer to full commercial deployment. In 2008 Sprint became one of the ﬁrst companies to launch services using a commercial femtocell. Some projections predict that by 2011, over 40 million femtocells will be installed globally with over 100 million subscribers. As the deployment of femtocells progresses there will be increasing demand for femtocellspeciﬁc products including integrated transmitters, receivers and low noise ampliﬁers. It is also expected that the Global Positioning System (GPS) will play an important role. The integration of GPS into the femtocell will facilitate location assistance for emergency E911 calls, provide timing and frequency assistance for system clock functionality, as well as offering location licensing capability. The veriﬁcation of GPS functionality within a femtocell is a critical task for all designers of these systems. Today, GPS receiver manufacturers, OEM integrators and contract manufacturers struggle for standard tests to verify receiver performance. This article discusses the issues that arise when integrating GPS into femtocells and outlines the test and veriﬁcation requirements that this introduces. The femtocell-GPS connection Femtocells are wireless access-points designed for use in the home or small business environment. The femtocell plugs into the customers’ broadband connection and routes calls using VoIP (Figure 1). The femtocell provides the customer with improved call quality and faster data connections, and allows service providers to extend service coverage indoors. This is especially beneﬁcial where access would otherwise be limited or unavailable. Thanks to their faster data speeds and ability to enable better user experiences inside the home, femtocells also encourage adoption of mobile data services. Femtocells face similar requirements to macro base stations for accurate real-time location and timing information. GPS makes F Figure 1: A typical femtocell system with GPS. obtaining this information possible. It also offers a way to make the femtocell smart enough so that it can avoid interfering with the existing infrastructure. GPS can also be used to check that the femtocell is only deployed within the geographical area for which the network operator has a license. GPS relies on a constellation of between 24 and 32 orbiting satellites, arranged such that at least six satellites are always visible from any line-of-sight point on earth. Each satellite broadcasts navigation data that is transmitted using a distinct spread-spectrum code unique to each individual satellite. When correlated by GPS receivers, the transmitted data is used to identify and calculate the signal travel time from each satellite in view and the distance to each satellite based on this travel time. Signals from at least four GPS satellites are used by the GPS receivers to calculate their position; to solve for longitude, latitude, altitude, and time; and subsequently, to determine the GPS receiver’s actual location. Using GPS within a femtocell presents a number of challenges. First, GPS is less reliable in residential/urban environments. Second, if femtocell transmission is not precisely tuned in time and frequency it can interfere with other femtocells and the macro network. Assisted GPS (A-GPS) offers one means of addressing these challenges. It improves the accuracy, TTFF and reliability of the GPS receiver by providing assistance data to the GPS receivers through the cellular communication channels. To be effective testing solutions have to address the needs of GPS and A-GPS. GPS receiver measurements Veriﬁcation of the GPS receiver performance is required to validate its functionality and to evaluate objectively competing GPS IC performance. Veriﬁcation procedures require a controlled environment that facilitates precise repeatability. Generally, using actual GPSsatellite signals received through an antenna does not provide this type of environment. A real-time GPS signal simulation, generated by a metric-grade RF signal generator, offers an excellent starting point for creating a calibrated and repeatable test environment. For example, GPS signals can be created by the Agilent Technologies E4438C ESG vector signal generator with its GPS personality that provides up to eight real-world GPS satellite signals based on pre-conﬁgured scenarios (see Figure 2). This GPS signal simulator provides a number of capabilities, including: multi-satellite GPS conﬁguration (maximum eight satellites); the simulation of real-world scenarios; use of real satellite data (synchronized satellites with Doppler shifts and navigation messages); adjustable number of visible satellites between ● Microwave Engineering Europe ● January/February 2009 www.mwee.com http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - January/February 2009

Microwave Engineering Europe - January 2009 News Contents Comment Using KPIs to Ensure Quality in a Converging Network Amplifier Error Vector Magnitude Characterisation Using High-Speed Modular PXI Instruments GPS: Making a Play for Femtocells Accelerating Global WiMAX Adoption: The Move to Picocell and Femtocell Base Stations Addressing PA Efficiency for Multi-Mode Wideband Handset Applications Wi-Fi: Mobile Feature or Fundamental RAN? Products Calendar

Microwave Engineering Europe - January/February 2009

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