Читать книгу Position, Navigation, and Timing Technologies in the 21st Century - Группа авторов - Страница 45

Broadcast and wide area networks include networks designed for localization purposes, such as GPS, and broadcast networks not originally intended for localization purposes, such as television broadcast signals [17], cellular phone networks [18], and FM radio signals [19].

As the signal properties and geometrical arrangement of the digital TV broadcast network have been designed to penetrate indoors, they offer significantly greater indoor coverage than GPS‐based solutions. For instance, [17] proposed using synchronization signals already present in the Advanced Television Signal Committee (ATSC) standard for compliant digital TV signals to perform indoor localization. Emitters of digital television are synchronized with GPS time, allowing the data to be time‐stamped, which can be useful for distance estimation with ToA techniques (Section 37.5.1.2). Digital TV signals also have a wide bandwidth of 5–8 MHz that can theoretically help reduce multipath mitigation. However, the weak density of terrestrial emitters causes the direct signal to arrive at low elevation angles near the horizon. As such, only 2D positioning is feasible, and multipath is severe because the direct signal is usually blocked.

Similar to digital TV networks, cellular networks have a wider range than, say, Wi‐Fi signals, and can also be used for indoor localization, much like with Wi‐Fi. With the Federal Communications Commission’s Enhanced‐911 (E‐911) mandate, cellular networks include positioning information as part of their standards. The E‐911 mandate requires mobile phones to be locatable within a 50 m accuracy for 67% of emergency calls. Such accuracies are usually achievable with the help of GPS signals. However, an accuracy of 50 m is insufficient for indoor areas. Therefore, cellular networks such as 2G/GSM, 3G UMTS, 4G LTE, and emerging standards must be utilized with other technologies for finer‐grained localization resolution in indoor environments.

FM radio is another possible candidate, utilizing frequency‐division multiple access (FDMA) to split the wireless band into a number of separate frequency channels that are used by stations. FM band ranges and channel separation distances vary in different regions, but the pervasiveness of FM signals can enable their use for indoor localization. Typically, radio waves operating in the frequency band 87.5 to 108.0 MHz are part of the FM spectrum. Due to the passive nature of the client devices, FM can be used in sensitive areas where other RF technologies are prohibited for safety or security reasons. Unfortunately, FM signals lack timing information, which limits their use in certain localization techniques (such as the time‐based trilateration techniques discussed in Section 37.5.1.2).