GPS: making a play for femtocells

Femtocells - small low-power base stations designed for indoor use in residential or small-business environments - are expected to grow tremendously in the coming years as they march closer to commercial deployment.

Evidence of this abounds. In 2008, for example, Sprint became the first to launch services using a commercial femtocell and that’s just the beginning.

In-Stat projects that by 2011, over 40 million femtocells will be installed globally with 101.5 million subscribers. As the market opportunity for femtocells increases, so too will the demand for femtocell-specific products like integrated transmitters, receivers and low-noise amplifiers. Global positioning systems are yet another component that will become increasingly important in the femtocell, enabling location assistance for emergency calls, timing and frequency assistance to provide system clock functionality and a location licensing capability (see Figure 1).

 

Figure 1: A typical femtocell system with GPS.

With its adoption into the femtocell, verification of GPS functionality has become an increasingly important task. Today, GPS receiver manufacturers, OEM integrators and contract manufacturers struggle for standard tests to verify receiver performance.

Typically, this verification is accomplished though the use of a GPS RF satellite simulation.

The E4438C is a high-performance, general-purpose RF signal source that is capable of not only creating these GPS signals, but other wireless standards associated with femtocells (eg, HSDPA, W-CDMA, GSM/EDGE, WiMAX, CDMA2000, TD-SCDMA and UMTS standards) as well.

Femtocells, wireless access-points or home base stations plug into the user's broadband connection and route calls using VoIP. This provides the user with better call quality and faster data connections, and allows service providers to extend service coverage indoors, especially where access would otherwise be limited or unavailable.

Due to their faster data speeds and ability to enable better user experiences inside the home, femtocells also encourage adoption of mobile data services.

Like macro base stations on a hilltop, femtocells face similar requirements for accurate real-time location and timing information.

GPS makes obtaining this information possible. It also offers a way to make the femtocell smart enough so that it can avoid interfering with existing infrastructure and can only be used within the geographical area for which the network operator has a licence.

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 which 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 longitude, latitude, altitude and time, and to determine the receiver’s actual location.

Verification of the receiver performance is required to validate its functionality and to objectively evaluate competing GPS IC performance. Verification procedures require a controlled environment that facilitates precise repeatability.

Generally, using actual GPS-satellite signals received through an antenna do not provide this type of environment. A real-time GPS signal simulation, generated by a metric-grade RF signal generator, offers a starting point for creating a calibrated and repeatable test environment.

 

Figure 2: The Agilent Technologies E4438C ESG.

GPS signals can be created by the E4438C ESG vector signal generator with its GPS personality which provides up to eight real-world satellite signals based on preconfigured scenario files (see Figure 2). This simulator provides a number of capabilities, including:

• Multi-satellite GPS configuration (maximum eight satellites);
• Simulation of real-world scenarios;
• Real satellite data (synchronised satellites with Doppler shifts and navigation messages);
• Adjustable number of visible satellites between one and eight;
• Automation of signal generation through SCPI commands.

Five measurements in particular are especially helpful in verifying receiver performance. These measurements can be made with the E4438C and include:

• Time to first fix. The time interval between the GPS receiver start-up (power-up) and the first-valid navigation 3D-data point, derived from the simulation. GPS receiver TTFF measurements may include cold, warm and hot start configurations.
• Receiver sensitivity. A measure of the signal strength carrier to noise ratio (C/No) under various power levels, as well as the power level and C/No ratio level at which the 3D location fix is lost.
• Static navigation accuracy. The accuracy of the GPS receiver location fix with respect to the simulated location.
• Reacquisition time. The time required to reacquire a navigation fix following a short blockage of all GPS signals during normal operation.
• Radiofrequency interference. The receiver’s ability to operate in the presence of interfering (jamming) signals that may be received through its input. Note that a second RF source is required to provide the interference signal.

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 receiver by providing assistance data to the receivers through the mobile communication channels.

Technologies like A-GPS are breaking down the barriers to using GPS in femtocells. Yet, its adoption demands an effective functional A-GPS receiver test to ensure service providers that A-GPS operation will not interfere with mobile phone calls.

Here, again, the E4438C ESG signal generator with GPS personality can be used, along with the company’s 8960 wireless communications test set, to perform the types of functional testing needed to meet service provider proof requirements (see Figure 3).

Figure 3: This diagram shows the connections required to set up the mobile device A-GPS functional test system. Both the 8960 and E4438C are connected to the PC controller via GPIB. The DUT is coupled to the instruments via antennas connected at the RF ports of each of the 8960 and E4438C.

As GPS takes on a more prominent role in femtocell systems, verification of the GPS or A-GPS receiver’s performance becomes critical. The ability to simulate GPS signals easily with a high-performance RF source like the E4438C provides great flexibility in creating an accurate and repeatable test environment for evaluating GPS receivers used in femtocells.

John Kikuchi, application expert, and Frank Palmer, wireless marketing program manager, Agilent Technologies.