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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 is 30 dB. A slot duration of 100 ms per slot and a slot repetition period of 10 slots (or 1 s) is chosen for MBS. This selection provides the best gains for the indoor/slow moving scenarios.

In the United States, the MBS signal is transmitted in the licensed M‐LMS band in the 919.75 MHz to 927.25 MHz frequency range. The M‐LMS signals are required to be compliant with Federal Communications Commission (FCC) Part 90, sub‐part M regulations (see [6]).

The filter shape is chosen to meet the stringent out‐of‐band specifications as per U.S. emission regulations [7]. The spectrum mask is shown in Figure 39.2 for the 2 MHz signal. A similar spectrum mask also applies to the 5 MHz signal.

The transmit spectrum is shaped using the finite impulse response (FIR) filter taken from the MBS Generic ICD [8]. The filter’s amplitude and phase frequency responses are shown in Figure 39.3. The transmit filter is derived from a form of root‐raised cosine (RRC) filter [9]. The shape of the transmit filter is chosen to be similar to GPS C/A code spectrum in the region between the first nulls on either side of the center frequency while at the same time meeting the out‐of‐band (OOB) emissions specification. In addition, the transmit filter is chosen within the spectrum constraints to produce a sharp correlation function without significant sidelobes. Figure 39.4 shows the frequency spectrum of the MBS signal overlaid on top of the GPS signal spectrum shape. Note that the zero on the x‐axis represents the carrier center frequency of the signal. The GPS C/A spectrum has sidelobes in the frequency domain which roll off slowly as a sinc function. The MBS spectrum is spectrally contained with a very strong OOB rejection requirement. Figure 39.5 shows the close‐in spectrum shapes of GPS and the MBS, illustrating the strong OOB rejection of the MBS filter. Given this choice of transmit filter, a receiver that uses a GPS‐shaped matched filter in the receive chain when compared to an MBS‐shaped matched filter will have <0.5 dB loss in sensitivity. This property enables easier implementation of MBS processing in GPS receivers.

The comparison of the correlation function using the full GPS C/A spectrum of 20 MHz with PRN 7 and the MBS 2 MHz signal spectrum with one of the representative PRNs is shown in Figure 39.6. The figure shows clean autocorrelation side lobes for MBS codes relative to GPS code to facilitate multipath mitigation. Observe the slight widening of the correlation function for MBS relative to the correlation function for GPS at the peak in Figure 39.7 due to the spectrally contained nature of the MBS signal. Figure 39.8 shows the close‐in autocorrelation sidelobes. The close‐in MBS sidelobes due to the transmit filter have amplitude < 0.03 (i.e. at least 30 dB below the main peak).

The beacon transmissions are illustrated in Figure 39.9. Each transmission period is 1 second long and is partitioned into 10 slots of 100 ms each. The transmission periods are separated by ∆_T seconds, where ∆_T is greater than or equal to 1. Each transmitter is assigned at least one of 10 slots.

The beacon transmission within a slot is partitioned into a preamble section, a pilot section, and, optionally, a data section. All the three sections use spread‐spectrum signals with BPSK‐spread PRN codes that have a common spreading rate. The sections which are data modulated use BPSK modulation. The preamble section is transmitted by all beacons with a system‐wide common PRN sequence to enable quick synchronization. The pilot and data sections use the PRN spreading sequence allocated for that beacon. The pilot sections are either unmodulated or have known modulation. The pilot sections are meant to be used for ranging and allow the receiver to use coherent integration. The data sections contain the required information bits to facilitate MBS trilateration in a stand‐alone manner without the need for external data. The data may be encrypted to protect against spoofing of the MBS signal and to control receiver access. In order to facilitate TDMA operation, there is a guard period within each beacon transmission slot when the beacon is silent (i.e. does not transmit).

Figure 39.2 Spectral mask for the 2 MHz signal (GLONASS Interface Control Document [2]).

Source: Reproduced with permissions of Russian institute of Space Device Engineering.

Figure 39.3 Amplitude and phase response of the MBS transmit filter as a function of frequency.

Figure 39.4 Frequency spectrum of MBS 2 MHz signal in comparison with GPS C/A code spectrum.

Figure 39.5 Zoomed‐in frequency spectrum of MBS 2 MHz signal in comparison with the GPS C/A code signal spectrum.

Figure 39.6 Correlation function of MBS compared with example of GPS code PRN 7.

A list of possible PN spreading codes used by the MBS is shown in the Appendix of [8].

As discussed above, each beacon transmits a preamble for a certain duration within its slot using a common spreading sequence across the network. The usage of a common spreading sequence allows the receiver to exclusively search for the preamble PRN to acquire the signal. The preamble enables the receiver to obtain frequency and slot synchronization using a relatively small amount of search resources since a single PRN search is only required to cover the frequency search dimension corresponding predominantly to receiver clock ppm uncertainty. Section 39.1.4.2 discusses some more details of how the preamble is used to aid the acquisition process.

Figure 39.7 Correlation function of MBS and GPS at the peak.

Figure 39.8 Correlation function showing close‐in side lobes.

Figure 39.9 Beacon slot structure – preamble, pilot, and data sections.

Figure 39.10 Relationship between GPS and MBS system timing.

The MBS system time is synchronized with GPS system time at an absolute level. Figure 39.10 shows the MBS system time relationship with GPS system time, the relationship between MBS system time and MBS beacon transmissions, as well as the time of arrival (TOA) at the MBS receiver of the MBS beacon signals. As shown in Figure 39.10, MBS time equals GPS time + Fixed Offset. Note that all beacons are synchronized to transmit as per MBS system time. MBS system time is synchronized to the phase center of the transmit antenna. In the MBS signal transmission from the beacon, the peak of the first chip of the first preamble PRN sequence within a slot is aligned to MBS system time. The MBS transmission for beacons in the N‐th slot starts at an offset of (N × 100) ms from the MBS system time second boundaries. Note that any fixed offset between MBS and GPS system time is transmitted through the data packets.

Figure 39.11 shows a block diagram describing signal generation in an MBS transmitter. Note the similarity of the MBS signal generation section to the GPS C/A code transmit section [1]. The logic shown in Figure 39.11 controls the selection of the PRN sequence (system‐wide preamble versus beacon‐specific pilot/data spreading sequence) for different parts of the slot as well as the optional beacon‐specific frequency offset for the pilot/data sections. The data packet content can consist of trilateration information, atmospheric information, timing information, and/or system control messages. The data content may also be encrypted to prevent unauthorized usage of the data stream. A forward error correction/detection scheme is added on the data frames to protect against channel errors. Once the spreading is done, pulse shaping is applied to generate the signal, which is then up‐converted to the MBS frequency and amplified before transmission over the air.

Figure 39.11 Transmitter block diagram.