Читать книгу Position, Navigation, and Timing Technologies in the 21st Century - Группа авторов - Страница 126

The MBS beacon system is normally deployed as a wide‐area system and has coverage footprint similar to a wide‐area cellular network. The wide‐area beacons are co‐located on cell towers or rooftops and, typically, use an omnidirectional antenna. Localized MBS beacons can also be deployed in localized target areas to augment performance. The MBS beacons are autonomous and generate signals locally on the beacon; therefore, they have minimal backhaul requirements that are primarily restricted to configuring and monitoring (telemetry services).

The MBS radio signal may consist of multiple signals with different bandwidths to provide different quality of position. When multiple spread‐spectrum signals are used as part of the MBS signal, they can be transmitted simultaneously in a frequency‐multiplexed or time‐interleaved manner. Frequency multiplexing would be preferred in a deployment where dedicated spectrum is available, whereas time‐interleaving may be preferable when the MBS may need to share the spectrum with other deployed systems.

In order to minimize synchronization error impact on ranging and trilateration, the radio signals transmitted by the beacons are synchronized at the antenna, both by design and construction. The synchronization design is done in such a way that the relative timing of beacons in any given area is stable across time. The beacons are also synchronized in an absolute manner to standard GPS time. The fact that beacons are synchronized to GPS time means that receivers can be built to extract GPS time from MBS beacon signals even in deep‐indoor environments where GPS may not be available for applications such as small cell synchronization and timing of financial trading transactions.

Since the MBS network is purpose‐built for positioning, it is designed and deployed to provide sufficient detectable beacons with good beacon geometry for receivers across the MBS coverage area. One of the key problems in systems that use terrestrial radio signals for trilateration is the near‐far problem (also referred to as hear‐ability problem) in which a nearby beacon makes detection of good‐quality signals from far‐away beacons difficult. Through a combination of signal and network design, the MBS system is designed to overcome this problem. The details of signal and network design are discussed in the following sections.

One of the other challenges in terrestrial systems is the presence of multipath. Multipath scenarios can include line‐of‐sight (LoS) scenarios where the LoS signal is detectable and NLoS scenarios where the LoS signal is too weak or not detectable. The MBS system provides good trilateration performance through a combination of signal design (to help resolve LoS when available) and network design (that provides additional good beacon measurements in heavy‐multipath NLoS environments to facilitate appropriate trilateration to aid performance).

The MBS system uses barometric techniques that can allow the receiver to estimate altitude within a floor level (approximately 3 m). In indoor applications, determining receiver altitude at floor level accuracy enables several new use cases.