is a spread‐spectrum signal similar to the 2 MHz signal. The 5 MHz signal provides better multipath resolvability due to its wider bandwidth. The code duration of the 5 MHz signal is the same as the 2 MHz signal, whereas its code and code length (2046) are chosen similar to GNSS signals such as BeiDou to facilitate GNSS receiver reuse. The 5MHz signal may optionally contain data modulation. The 5 MHz signal is synchronized at the beacon antenna along with the 2 MHz signal potentially allowing trilateration with a mix of 2 MHz and 5 MHz signals. When both 2 MHz and 5 MHz signals are available from the same beacon, it may be preferable to acquire using the 2 MHz signal due to its shorter code length.
39.1.4 Receiver Architecture
The MBS signal structure is designed to be similar to GPS so that significant reuse of the GNSS chipsets is possible. Potentially, the entire GPS baseband processing can be reused for MBS processing. One of the key differences between MBS and GPS/GNSS is the dynamic range of the terrestrial MBS signals as compared with satellite signals. In addition, MBS is a slotted system, and the receiver signal strengths can be above the receiver noise floor, necessitating an automatic gain control (AGC) with a fast response. On the other hand, GNSS signals are generally CDMA/FDMA with no slotting, and, moreover, the signals are always well below the receiver noise floor. Section 39.1.4.1 discusses the signal dynamic range, and gain control requirements for MBS, followed by Section 39.1.4.2, which discusses acquisition, tracking, and ranging of the MBS signal, and Section 39.1.4.3, which discusses position calculation using MBS ranges.
39.1.4.1 Signal Dynamic Range and Gain Control
The MBS signal is licensed to transmit from beacons at a maximum power of 30W ERP as per Federal Communications Commission (FCC) rules Part 90 rules Subpart M [6]. Given that the MBS network is a terrestrial network, the detectable signal dynamic range can be much larger than a cellular system because of the receiver’s ability to process signals below its thermal noise floor. Since different beacons can be received in different slots, the received signal strength can potentially change from the high signal level to the low signal level (and vice versa) in adjacent slots. An AGC loop with a fast response is required that responds to the received signal strength indicator (RSSI) changes in a fraction of a Gold code time.
39.1.4.2 Signal Acquisition, Tracking, and Ranging
The MBS signal can be acquired using similar acquisition hardware as GPS receivers. However, there are differences arising from the time‐slotted structure of the MBS that requires a different acquisition sequence. The MBS signal search space (similar to GPS) consists of PRN, frequency, and code phase. One additional dimension in a TDMA system is slot alignment. The system‐wide preamble portion transmitted by every beacon simplifies the search in the frequency and slot time alignment dimensions so that the search can be completed using low search resources with a single preamble PRN.
The frequency dimension of the search is dominated by the receiver clock ppm uncertainty since Doppler (in contrast to GNSS systems such as GPS) is relatively small. For example, a moving object at 200 kmph directly in the direction of an MBS beacon will experience a Doppler of about ±175Hz (< 0.2 ppm). Just as in a GPS/GNSSS receiver, when external fine time assistance information (such as from the modem) is not available, search over the full code duration needs to be done in the MBS receiver.
As an illustration, the well‐understood GPS search space shown in Figure 39.12 is compared with the MBS search space shown in Figure 39.13. The GPS acquisition search space basically consists of three dimensions: code phase, frequency, and PRN. The code phase search space is defined by the 1 ms CA code, whereas the frequency search space is a combination of satellite Doppler, user Doppler, and receiver clock uncertainty, as shown in Figure 39.12. In assisted GPS, this search space is reduced, potentially, in all three dimensions through rough knowledge of user position, GPS time, and satellite ephemeris/almanac information [10].
Figure 39.12 GPS search space.
The MBS search space also consists of the same three dimensions. However, the search space is effectively reduced to two dimensions when using the preamble for initial acquisition. The optional modulation pattern on the preamble (see Section 9.2 of [8]) can be used to facilitate a more robust slot alignment.
The preamble acquisition enables coarse frequency and code phase acquisition, which reduces pilot/data PRN search requirements. Once the initial preamble acquisition has been performed, the beacon‐specific PRNs need to be searched. The search space in code phase and frequency is reduced to the intersection of the gray boxes in Figure 39.13 (a) and leads to a search space as shown in Figure 39.13 (b) for the beacon PRN search. Once a beacon is detected, the search space for other beacons can be reduced, since their relative ranges in a terrestrial system will always be below the code duration of 1 ms.
Once the beacon pilot/data signal acquisition is complete, the range measurements as well as the trilateration data can be extracted. Ranging is normally done using the pilot section with known modulation, but can also be done using the data section.
Estimating the TOA in terrestrial channels from the beacon is a different challenge when compared to a GPS satellite channel. The channel responses are quite complex due to blockage, diffraction, and reflection from a variety of obstacles creating a mix of LoS and NLoS paths. Figure 39.14 shows sample measured correlation functions measured at the receiver when using a direct‐sequence spread MBS transmission waveform of bandwidth 2MHz. The measurements were carried out in outdoor rooftop locations. The purpose of these figures is to illustrate various common channel scenarios in static outdoor terrestrial scenarios. In the figures, the red vertical line represents the TOA of the true LOS path, whereas the green line represents the TOA of the detected earliest path in the receiver. The x‐axis represents distance in meters, and the y‐axis represents the magnitude of the correlation function. Figure 39.14(a) shows the measured correlation function in the case where the LOS is clearly detectable so that the green and red vertical lines overlap each other. Figure 39.14(b) shows a correlation function for the NLoS scenario with a strong early NLoS path, and Figure 39.14 (c) shows a correlation function for the NLoS scenario with a weak early NLoS path. Note that in both cases the earliest path is not detectable, as shown by the green vertical line (representing the estimated TOA of the earliest detectable path) being to the right of the red vertical marker (true LoS TOA). Observe that in Figure 39.14(b), the earliest detectable path is actually stronger, whereas in Figure 39.14(c), the earliest detectable path is actually weaker.
