On Generalized Signal Waveforms for Satellite Navigation (797942), страница 19
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Given the enormous interest thatGalileo is arising in the GNSS community, we will dedicate a whole chapter to describe howthe Galileo Baseline has evolved in the last years and shortly describe what ideas were in themind of those engineers that made it possible. This chapter aims thus at giving a historicaloverview of how the Galileo Signal and Frequency Plan has evolved over the time.3.2Square-Root Raised Cosine (SRRC) Signalwaveforms for Galileo?The Square-Root Raised Cosine (SRRC) was the first option for the Galileo Signal andFrequency Plan.
It is already long time ago since the proposal was made but these first worksdeeply influenced the evolution of the coming years. At the time when the first analyses onthe future Galileo signals were made [R. De Gaudenzi et al., 2000], the current frequencyband assignments had not taken place yet. Thus, to limit the number of signal and frequencycombinations, a set of seven candidate signal structures was identified, such that each of themcould be independently assigned to a particular frequency. These are shown in Table 3.1.As we can recognize, all the signals from Table 3.1, with the exception of S3, consist of an inphase BPSK modulated spread-spectrum signal and an unmodulated quadrature spreadspectrum pilot that uses a different spreading sequence.
It is also interesting to note thepresence of an unmodulated pilot to achieve robust carrier phase tracking. This idea wouldremain until the final baseline of Galileo as we will see. Indeed, in spite of the fact that theSRRC was quickly abandoned due to its limitations, some original ideas present in Table 3.1were kept until the end.65Galileo Baseline EvolutionTable 3.1.
Galileo Signal Plan proposed in [R. De Gaudenzi et al., 2000]. N/A* indicatesthat the pilot channel is not modulated by dataSignalIdentifierSpreadingcode LengthChippingRate (Mcps)S1102303.069S2102303.069S310233.069S41023015.345S51023015.345S61023015.345S71023015.345ModulationArmInformationRateCodingRateDataRateI7501/21500N/A*QI2501/2500*QN/AI2501/2500Q2501/2500I15001/23000*QIN/A250750Q1/21500*QI500N/A*QI1/2N/A20002/33000*N/AIn addition, the S3 signal used a short spreading code of 1023 chips as the GPS C/A tosupport fast acquisition of the S3 signal.
The driver behind was to use this signal to assistacquisition of the longer codes on other frequencies and thus the implicit assumption was alsothat the S3 signal would always be present regardless of the combination of other signals. Onthe other hand, all other signals were planned to use longer spreading codes of length 10230to provide increased robustness against inter- and intra-system interference. The proposedsignals allowed a strict occupancy for the narrowband and wideband cases respectively.Figure 3.1.
Galileo first Frequency plan for L-band [R. De Gaudenzi et al., 2000]For the purposes of the Galileo Signal Validation Facility (GSVF) project, the frequencybands of interest were assumed to be El, E2, E4 as well as GPS-L1 and GLONASS-G1 asshown in the previous figure.66Galileo Baseline EvolutionThe centre frequency and available bandwidth for the different frequency designators are alsoshown in Table 3.2.
It is important to note that all the signals are band-limited with a squareroot raised cosine filter. The basic signals used a roll-off factor, α = 0.22 although other rolloff factors such as α = 0.28 and α = 0.35 were also under consideration. In chapter 4.8.2 amore detailed description of the Square-Root Raised Cosine (SRRC) modulation is given.Table 3.2. First studied Galileo Frequency bands [R. De Gaudenzi et al., 2000]Frequency DesignatorAvailable Bandwidth [MHz]Centre Frequency [MHz]E14.01598.742E24.01561.098E44.01256.244E520.01202.025E620.01278.750C120.05014.746The Signal Plan presented above was not the only proposed solution from[R.
De Gaudenzi et al., 2000]. In fact additional solutions were proposed as shown in thefollowing table:Table 3.3. Galileo Signal parameters for different options [R. De Gaudenzi et al., 2000]Freq.BandBaselineE1E2E4Option 1G1E2G2Option 2E1E4CTargetC/N0(dB-Hz)EIRP(dBW)CarrierFrequency(MHz)ChipRate(Mcps)DataStreamInfo.BitRate(bps)FEC,Codeddata rate(bps)Codelength,Gold seq.(chips)Codeduration(μs)45454531.030.828.41589.7421561.0981256.2443.0693.0693.069E-NAV’E-NAV’E-NAV’15001500750½, 3000½, 3000½, 1500102310231023333,3333,3333,345454531.030.328.91598.9491561.0981589.10615.3453.06915.345E-NAV’E-NAV’E-NAV’15007501500½, 3000½, 1500½, 300010231023102366,7333,366,745453831.028.434.01589.7421256.2445014.7463.0693.06915.345E-NAV’E-NAV’E-NAV’15007501500½, 3000½, 1500½, 3000102310231023333,3333,366,7Finally, in regards to the Galileo orbit parameters, the following values were suggested.Table 3.4.
Galileo MEO Orbit Parameters [R. De Gaudenzi et al., 2000]Orbital height20,230 kmOrbital plane inclination55°Maximum Doppler Shift4.4-5.2 kHz (L-band)14.7-17.3 kHz (C-band)Maximum Number of satellites in view7-12Elevation angle5°-90°67Galileo Baseline EvolutionAs we can recognize, changes were also made here with respect to the final configuration ofGalileo that we saw in chapter 2.4.1.The Square-Root Raised Cosine (SRRC) modulation comes from the communications worldand was proposed at that time due to its optimum spectral efficiency in the sense that for alimited bandwidth it is capable of transmitting more power than any other signal waveforms,such as the rectangular one. But as we know, the needs of navigation and communication donot always go hand by hand and what is good for the one is not necessarily also good for theother.
The SRRC signal is band-limited as we will thoroughly describe in chapter 4.8.2 andeven though it performs very well for narrow bandwidths the signal would have beenhandicapped from the beginning regarding its navigation performance since no matter howmuch we would broaden the receiver bandwidth, the performance would never be able to getany better. Indeed, this was its major drawback.The intersymbol interference is better with the SRRC waveform than with a rectangularsolution being this a very important aspect to take into account in environments withextremely low SNR as in the case of deep space transmissions.
Nonetheless, the limitedbandwidth of the signal to a very narrow value as shown above, condemned the signal tonever being able to perform any better further away than 3 MHz. Of course the signal wouldbe very good in terms of power transmission for that very narrow bandwidth, but no matterwhat we would do, we would never be able to improve its limited natural performance of3 MHz. The ACF would present a very rounded peak and for navigation purposes this wouldimply a low intrinsic quality in terms of tracking.Other consequences from the band-limited property of the SRRC pulse would be thatcompared to the GPS signals, for example, no improvement due to narrow correlation wouldbe possible. In terms of receiver complexity in comparison with the rectangular waveformsolution an additional degradation was observed.Last but not the least, such a signal would have a degraded antijamming protection.
We willsee more in detail this in the coming chapters but we can already mention here that theantijamming protection against narrowband and wideband interference, may it be due to anintentional or unintentional source, increases as the spectrum of the signal widens.
For thecase of a limited-band signal thus, an important degradation in this regard would be observed.This would be especially critical for the Public Regulated Service (PRS) of Galileo. Tofinalize, it is important to mention that at the time as the studies were made[R. De Gaudenzi et al., 2000], the PRS signal was not even planned yet.68Galileo Baseline Evolution3.3Galileo Baseline of 2002The first tentative Galileo frequency and signal plan alternative to the one that we saw in theprevious chapter was presented in [G.W. Hein et al., 2001] and it slowly became the baselinefor the development of Europe’s satellite navigation system. The Galileo carrier frequency,modulation scheme and data rate of all the 10 Galileo navigation signals as of September2002 had experienced very important changes with respect to the first proposals.
Moreover,the band frequency assignment was not an unknown any more and Galileo was developingsimilar concepts with regards to signal modulation as GPS. This means in other words, thatthe SRRC concept was abandoned and similar signal structures as those of GPS were nowproposed for Galileo too. As we will point out later again, the status was already in a verymature phase and until the final signal plan not many substantial changes were required.The main changes and add-ons with respect to the initial Signal Plan of chapter 3.2 aresummarized in the following lines [G.W. Hein et al., 2001]:••In the lower L-band (i.e. E5a and E5b) the central frequency for E5b was moved to1207.14 MHz in order to minimize possible interference from the Joint TacticalInformation Distribution System (JTIDS) and the Multifunctional InformationDistribution System (MIDS).
All signals on E5a and E5b would be using chip rates of10 Mcps but the modulation scheme for that band was not decided yet. The idea inmind was to have a modulation that allowed processing of very wideband signals byjointly using the E5a and E5b bands. This joint use of the bands has the potential tooffer enormous accuracy for precise positioning with a low multipath as we will see inthe next chapters. This final wideband signal would be the AltBOC modulation.Furthermore, data rates had also been fixed in the baseline of 2002.In the middle (i.e. E6) and upper (i.e.
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