On Generalized Signal Waveforms for Satellite Navigation (797942), страница 39
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DLL tracking threshold for the DP Discriminator with a DLL loopbandwidth of 0.5 Hz and a chip spacing of δ =0.1 (left) and δ =0.2 (right)It is interesting to note that for a coherent integration time of 0.5 seconds and a spacing of0.1 chips, the improvement in sensitivity of BOC(1,1) and MBOC(6,1,1/11) with respect toBPSK(1) is even more spectacular. Indeed, BOC(1,1) performs approximately 4.6 dB betterthan BPSK(1), while MBOC outperforms BPSK(1) by 6.5 dB.If we repeat the figures above for a DLL loop bandwidth of 0.1 Hz, we can see that althoughthe difference in sensitivity of the various signals reduces, for a coherent integration of0.5 seconds and a spacing of 0.1 chips, BOC(1,1) is still 4.2 dB better than BPSK(1) andMBOC(6,1,1/11) approximately 1.7 dB better than BOC(1,1).168GNSS Signal StructureFigure 4.64.
DLL tracking threshold for the DP Discriminator with a DLL loopbandwidth of 0.1 Hz and a chip spacing of δ=0.1 (left) and δ =0.2 (right)Once the sensitivity performance of the different signals has been compared as a function ofthe DLL loop bandwidth and the spacing, the next step is to assess the behaviour of thesesignals as a function of the coherent integration time.Figure 4.65.
DLL tracking threshold for the DP Discriminator with a coherentintegration time of 20 ms and a chip spacing of δ =0.1 (left) and δ =0.2 (right)As we can see, for a DLL loop bandwidth of 0.5 Hz and a chip spacing of 0.1, BOC(1,1) has asensitivity approximately 3.5 dB higher than that of BPSK(1) while MBOC(6,1,1/11) is betterthan BOC(1,1) by 1.4 dB.Figure 4.66. DLL tracking threshold for the DP Discriminator with a coherentintegration time of 500 ms and a chip spacing of δ =0.1 (left) and δ =0.2 (right)169GNSS Signal StructureFurthermore, when the coherent integration time is increased to 500 ms, the sensitivityimprovement of BOC(1,1) turns to be even clearer, resulting in a improvement ofapproximately 4.6 dB for BOC(1,1) with respect to BPSK(1) and of 1.9 dB for MBOC withrespect to BOC(1,1).From the figures above we can clearly recognize that the tracking threshold decreases as theintegration time increases and the DLL loop bandwidth decreases.
We can also see that theobtained values seem to be very low compared to those obtained in real applications.However, it must be noted that ideal conditions were assumed in the simulations.The following tables summarize the results for a chip spacing of 0.1 chips with DP and EMLPdiscriminators. Similar tables could have also been derived for a spacing of 0.2 chips showingalways the superiority of MBOC against BOC(1,1) and C/A Code.Table 4.6. DLL Tracking Threshold [dB-Hz] using the DP discriminator δ = 0.1 chipsDLL Tracking Threshold [dB-Hz]DP discriminatorCoherentintegrationtime [ms]10201005001000Loop bandwidth [Hz]Signal0.10.250.51BPSK(1)17.189619.693121.753123.9894BOC(1,1)14.420116.713118.553720.5185MBOC(6,1,1/11)13.279815.509617.281619.1553BPSK(1)16.052518.742820.979123.4025BOC(1,1)13.131615.543417.508219.6268MBOC(6,1,1/11)11.946114.271316.145018.1493BPSK(1)13.989417.230119.859422.5642BOC(1,1)10.518513.355215.697018.1742MBOC(6,1,1/11)9.155311.836914.046016.3974BPSK(1)12.869716.444519.068821.3663BOC(1,1)8.707311.997114.489716.7134MBOC(6,1,1/11)7.056310.184612.587814.7558BPSK(1)12.564216.058518.356019.5899BOC(1,1)8.174211.479413.703114.9092MBOC(6,1,1/11)6.39749.577511.745512.9294Once we have computed the DLL tracking threshold for the case of infinite bandwidth, thenext step should be to employ different assumptions on the receiver bandwidth.
Differentconfigurations have been analyzed delivering however similar results to those provided inprevious lines.170GNSS Signal StructureThe previous results show the ideal sensitivity values when all the potential sources of errorare eliminated. As we know, increasing the total integration time does not only require extracomplexity but implies other real challenges. Indeed, there are several problems inherent tolonger coherent integration times, which are mainly related to the fact that the longer thecoherent integration is, the more likely it will be that the signal conditions change during theintegration period.
Among others, the existence of frequency errors, non-ideal normalizationsin the discriminator and the change of signal power during the integration would be the mainsources of additional errors. They were not considered in the simulations above, since we areinterested here in finding the theoretical limit, no matter how this is realized in reality. Thesame comment applies for the normalization of the discriminator. In fact, as explained in[O. Julien, 2005], the effect of normalization in the discriminator would be another factor totake into account for more realistic simulations.Table 4.7.
DLL Tracking Threshold [dB-Hz] for an EMLP discriminator with δ = 0.1EMLP discriminatorDLL Tracking Threshold [dB-Hz]with a spacing of 0.2 chipsLoop bandwidth [Hz]Integrationtime [ms]10201005001000Signal0.10.250.51BPSK(1)17.278119.769521.817324.0394BOC(1,1)14.731817.001518.817020.7487MBOC(6,1,1/11)13.824516.023617.761819.5899BPSK(1)16.132318.807021.029123.4380BOC(1,1)13.426615.806717.738419.8161MBOC(6,1,1/11)12.468814.751516.579618.5248BPSK(1)14.039417.261219.878822.5755BOC(1,1)10.748713.530115.826018.2612MBOC(6,1,1/11)9.589912.190614.325516.6008BPSK(1)12.889116.453919.074221.3695BOC(1,1)8.836312.072514.536716.7432MBOC(6,1,1/11)7.335810.365012.707514.8350BPSK(1)12.575516.063916.063919.5924BOC(1,1)8.261211.526413.732914.9323MBOC(6,1,1/11)6.60089.697211.824712.9918The figures obtained above extend over very long coherent integrations. In fact, no datachannel could in reality reach such values unless external sources were used, what showsclearly the superiority of the pilot channel for these purposes.
Indeed, the introduction of pilotchannels by Galileo and the modernized GPS can be considered as one of the maincontributions to the navigation. Navigation and communication applications require ofdifferent needs and the use of pilot signals in GNSS in the future is clear proof of that.171GNSS Signal Structure4.7.8MBOC Interference with other GNSSesInteroperability and compatibility have been hot issues in the design of Galileo since thebeginning. Indeed, as more systems join the select club of countries with their own navigationsystem, the more important these concepts have become.
As we have seen in chapter 2, theglobal system of systems that GPS, GLONASS, Galileo, the Japanese Quasi Zenith SatelliteSystem (QZSS), the Chinese Compass and the Indian Regional Navigation Satellite System(IRNSS) might become one day makes this chapter of major interest.As defined in, [S. Wallner et al., 2005] and [S. Wallner et al., 2006] and, interoperabilityrefers to the ability of civil U.S. and foreign space-based PNT services to be used together toprovide better capabilities compared with those that would be achieved relying solely on oneservice or signal.In June 2004, the United States and the European Union signed a historical Agreement on thecommon use of shared frequencies, setting up a complete methodology to assess theGPS/Galileo radio frequency compatibility. More details on the theoretical framework can befound in Appendix M. Based on the mathematical ideas gathered in the work,[S.
Wallner et al., 2005] have carried out simulations with smooth spectra and with real codes.According to the results, the degradation from GPS on Galileo and of Galileo on GPS is lowerthan 0.25 dB proving thus that both systems can perfectly coexist. Moreover, the introductionof QZSS will lead to an increase of the intersystem interference in the visibility region ofQZSS that will never be higher than 0.07 dB. We show next the degradation values forBOC(1,1) when the analytical model is employed.Figure 4.67. Maximum C/N0 Degradation due to Intersystem Interference caused by theGPS L1 Signals on Galileo [S.
Wallner et al., 2005]. Minimum: 0.186 dB, mean 0.214 dBand maximum 0.243 dB172GNSS Signal StructureIf the same model is applied to the MBOC baseline, we can see that this contributes to aneasier compatibility since the interference reduces in all considered scenarios. In fact, foraverage scenarios the typical figures are far lower than the 0.25 dB mentioned above.The additional reduction of interference that MBOC provides is direct consequence of thebetter Spectral Separation Coefficients (SSC) of the signal. This confirms the greatimportance of this instrument to assess the degradation and overlapping among differentsignals. In the next figure we show the reduction of the maximum C/N0 degradation thatresulted from changing the baseline from BOC(1,1) to the final MBOC(6,1,1/11).Figure 4.68.
Reduction of the maximum C/N0 Degradation due to IntersystemInterference when MBOC is used instead of BOC(1,1) [S. Wallner et al., 2005].Minimum: 0.016 dB, mean 0.018 dB and maximum 0.023 dBEqually, if we include the effect of QZSS, the following results are obtained.Figure 4.69. Maximum C/N0 Degradation due to Intersystem Interference caused by theGPS L1 and QZSS Signals on Galileo [S.
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