Expert Advice: Location by Database
Jun 01, 2012
Source: Radio-Frequency Pattern Matching GPS World
Radio-frequency pattern matching (RFPM) is the engine that enables the use of mobile-phone signals to locate wireless devices in any environment, including dense downtown areas and indoors. This exciting technology leverages the power of the database to improve location accuracy to within 50 meters in even the toughest signal environments. Significant advances in RFPM technology have been made over the last 10 years. The system described here is deployed in more than 24 wireless networks to provide the location of E-911 callers and help save lives. For simplicity, we focus on the RFPM using signal strengths even though the technology also works with arrival times, signal-to-noise ratios, differential signal strengths and any signal parameter that varies in a predictable fashion over the coverage area.
Like GPS, RFPM is based on correlation. However, it does not correlate a received spread-spectrum code with a replica code stored in the receiver. Rather, it correlates the signal strength of cell-phone signals measured by the roving phone to a database that contains a map of those signal strengths for the covered area. Consider Figure 1. It shows this key correlation operation. As shown, the database contains a k-vector for each location within the covered area, where the k elements give the estimated strength for the k mobile phone signals that can be received at the given grid point. These k-vectors are typically stored over a 10- or 30-meter grid. This grid of predicted signal strengths is built in advance and is updated only when the topography of the wireless network changes. Thankfully, base stations do not generally move!