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.
