are described in Section 39.1.3. An example receiver architecture and the associated challenges of implementing a receiver to process terrestrial‐based signals is described in Section 39.1.4. Section 39.1.5 describes the assisted mode of operation of an MBS receiver. The use of MBS signals for time and frequency synchronization is described in Section 39.1.6. Various industry standards that reference MBS signals are shown in Section 39.1.7. The chapter concludes with third‐party performance test results in Section 39.1.8 and a conclusion in Section 39.1.9.
39.1 Metropolitan Beacon System (MBS)
The MBS belongs to the class of downlink broadcast terrestrial system. In the context of Third Generation Public Partnership (3gpp) standards, a class of locations systems termed Terrestrial Beacon Systems (TBS) has been defined of which MBS is considered an instantiation. The system aims to provide radio signals that can be used to locate the position of the receiver in indoor and dense urban areas where GPS availability is limited. While the system is intended to be complementary to GPS, the system can also be deployed so that the UE can use only MBS signals for position location.
The MBS beacon network is a network of low‐cost, highly synchronized beacons. The network is purpose‐built for positioning, achieving a good geometry of beacons for a receiver anywhere in the network. Network planning and design are done in such a way as to overcome the near‐far problem common to terrestrial systems. In addition, the signal design is done in a manner that facilitates multipath resolution. In order to facilitate accurate altitude estimation, the MBS network also utilizes differential barometric techniques to determine altitude precisely.
The MBS beacons transmit all the information required for trilateration as part of the MBS signal, which allows receivers to estimate 3D position without the need for additional external information. The information may be encrypted to control access and prevent unauthorized usage of the signal.
39.1.1 System Description
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.
39.1.2 Signal Description
Terrestrial Beacon Systems can transmit a variety of different waveforms. In 3GPP, an Orthogonal Frequency Division Multiplexing–Multiple Access (OFDMA) waveform and a Code Division Multiple Access (CDMA)‐based waveform are described. The MBS utilizes a CDMA type of waveform such as GPS. In addition to the type of waveform, the waveforms could have different bandwidths with greater bandwidth proving capability to better resolve multipath. 3GPP defines two forms of signals: TB1 (referred to as the 2 MHz MBS signal in the following) and TB2 (referred to as the 5 MHz MBS signal in the following). TB1 is exactly compatible with the GPS waveform [1], and TB2 is compatible with other GNSS constellations such as BeiDou [3]. The goal of the signal design is to minimize the impact on mass‐market receivers and minimize the modifications that need to be made in the chipsets to support terrestrial signals.
The MBS signal consists of one or more direct‐sequence spread spectrum signals, each spreading its carrier spectrum with a pseudorandom (PN) sequence. One of the key factors in the MBS signal design was to keep the structure very similar to GNSS signals to facilitate reuse of GNSS receiver hardware. GPS [1], Glonass [2], and BeiDou [3] use BPSK spreading for the civil ranging code. In keeping with the same signal structure, it was decided to use BPSK spreading for MBS. In addition, spreading codes very similar to GNSS codes were selected for MBS.
Terrestrial systems suffer from the near‐far problem due to which a receiver close to a beacon cannot easily detect other beacons further away. The MBS system was designed to overcome this problem using a twofold approach. The scheme can be thought of as a combination of time‐division multiple access (TDMA) and code‐division multiple access (CDMA). The beacons in a local area are allocated different slots to avoid simultaneous transmission. The slots may be reused in a larger geographic area. In addition, the beacons within a slot are allocated with spreading codes that have good cross‐correlation properties. Further improvement in cross‐correlation may be obtained using an optional frequency offset.
Time‐division multiple access is achieved through slotted transmission. The slot duration is selected to enable enough processing gain for a range measurement using one slot worth of samples. When using GPS codes, the code duration is 1 ms, and the processing gain from one code duration
