Advanced global navigation satellite system receiver design (797918), страница 23
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This is important because it allows a single high power amplifier to beimplemented in the transmitting satellite, removing the need for inefficient combiningof separate stages [Dafesh et al 2000]. Constant envelope signals also act to mitigatethe non-linear effects of high power amplifiers, such as spectral re-growth. Therefore,despite the power lost in the IMP, interplex claims to provide a very efficient methodto combine three PRN code signals onto a single carrier. Figure 7-10 shows the resultof interplexing two BOC(1,1) signals (data and pilot) with a third BOC(10,5) signal,using the modulation index, m = 0.825.156GNSS signal generators and the Giove-A satelliteFigure 7-10, Interplex of 2*BOC(1,1) + BOC(10,5) IF signalsThe AltBOC modulation will be used by future Galileo satellites to combine fourPRN code signals onto a single carrier in the Galileo E5 band.
The AltBOCmodulation can be written as follows.S AltBOC (t ) = PA × [c1 (t ) + c2 (t )] × s (t ) × cos(ω Lt )+ P × [c (t ) − c (t )]× ~s (t ) × sin(ω t )A127–11L+ PB × [c3 (t ) + c4 (t )]× ~s (t ) × cos(ωLt )− PB × [c3 (t ) − c4 (t )]× s(t ) × sin(ωLt )s(t), is a in-phase square wave sub-carrier and ~s (t ) is orthogonal square wave subcarrier. The four PRN code sequences have 16 possible combinations, which are usedto control the phase of the sub-carrier waveform. This is accomplished by producing8 phase-shifted versions of the sub-carrier waveform, where the state of the PRNsequences determines which is modulated onto the carrier.
For implementation inhardware, it is simpler to view the AltBOC as an 8-PSK modulation as follows. 2π t 2π tS AltBOC (t ) = sgn sin + θ A cos(θ B ) cos(ω L 5t ) + sgn sin + θ A sin (θ B ) sin (ω L 5t ) Ts Ts7–12157GNSS signal generators and the Giove-A satellite()()θ A ∈ 0, π 4 , π 2 , 3π 4 , θ B ∈ 0,± π 4 ,± π 2 ,± 3π 4 and TS is the sub-carrier period. θ Adefines the timing of 180º phase reversals of the carrier and θ B chooses the pair ofopposite phase points which are hopped between during the chipping interval. θ Aand θ B are set by the 16 possible states of the input sequences using a look-up table,shown in [Kaplan and Hegarty 2006]. Again, this can be achieved in hardware simplyby adjusting the I and Q levels of the DAC for each of the 16 possible states of thefour input signals and the transitions of the sub-carrier (see Appendix I).
A spectrumanalyser plot of an AltBOC(15,10) signal is shown in Figure 7-11.Figure 7-11, AltBOC(15,10) IF signal7.2Digital noise synthesisIn order to enable receiver performance testing in various noise conditions wesynthesise additive noise digitally in the IF signal generator. Dr MS Hodgartprovided the statistical principles required to precisely control the carrier to noisedensity CN0of the source. A representation of the digital noise synthesisimplemented in the IF signal generator is shown in Figure 7-12.158GNSS signal generators and the Giove-A satelliteIFCodegeneratorc(t)f(t)g(t)v(t)NoisegeneratorFigure 7-12, IF noise synthesisThe representations shown in Figure 7-12 are continuous in time.
However, in thedigital FPGA architecture the signal is necessarily discretized in steps, k. We add aninteger noise sequence v[k] with known quasi-Gaussian distribution beforemultiplication with the IF carrier. The quasi-Gaussian integer noise sequence iscreated by addition of random numbers generated using a multiplicative congruentialrandom number generator developed first in [Lehmer 1951]. Lehmer’s algorithm isgenerally considered the benchmark for random number generation satisfyingvirtually all tests of statistical randomness [Park and Miller 1988].
This elegantlysimple algorithm can be implemented to generate random numbers z[k] as follows.z[k + 1] = mod(r × z[k ], q )7–13q is called the modulus and is a large prime integer, the multiplier r is an integer in therange 2 ≤ r ≤ q − 1 . The operation mod(x, q) means the remainder after division of xby q. The designer must also set an initial value or seed n[0] in the range1 ≤ z[0] ≤ q − 1 .
We choose r = 75 = 16807 , q = 2 31 − 1 and a seed n[0] = 1 asrecommended values used in [Park and Miller 1988] and adopted by the well-knownmathematical software Matlab. Figure 7-13 shows a histogram of 50,000 normalisedrandom numbers generated by Equation 7–13.159GNSS signal generators and the Giove-A satelliteFigure 7-13, Histogram of 50,000 numbers from the multiplicative congruential random numbergeneratorA quasi-Gaussian distribution is generated in the IF signal generator by a pipelinedaddition of eight random numbers as follows.7v[k ] = ∑ n[k − i ]7–14i =0Figure 7-14 shows a histogram of the quasi-Gaussian distribution derived fromModelsim simulation of the IF signal generator VHDL code.
The distribution has amean of µ = 128 (zero for an 8-bit DAC) and a standard deviation of σ = 24.292 .Figure 7-14, Histogram of quasi-Gaussian noise generator (15852 points)160GNSS signal generators and the Giove-A satelliteWe define a DAC sampling rate to code rate ratio, K SC =decimation factor K N =f DACfNf DACfCand a noise,where fN is the noise update rate. The PRN codesequence with added noise can then be written as follows.f [k ] = c[k K SC ] + v[k K N ]7–15Where the notation [ ] denotes an ‘integer’ or ‘floor’ to a real number.
The result ofmodulating with a bipolar (±1) IF carrier running at half the DAC sampling rate canthen be written as follows.g [k ] = f [k ]× (− 1)k7–16Theory presented by Dr MS Hodgart (see Appendix F) derives an effective carrier tonoise density for the digital noise synthesis as follows.CN0==2 × A2 × f NσN7–1722A2 × f DAC×KNσ N2where σN is the r.m.s. value of the synthesised noise A is the amplitude level given tothe PRN code sequence, either +A or −A.Figure 7-15 shows a spectrum analyser plot of a BOC(1,1) signal produced by the IFsignal generator with the digital noise generator switched off.
The IF signal has acentre frequency of 20.46 MHz. Figure 7-16 shows a spectrum analyser plot of aBOC(1,1) signal produced by the IF signal generator with the digital noise switchedon. We choose a noise update rate of 1.023 MHz, A = 1, σ N = 24.292 andf DAC = 81.84 MHz, which equates to a carrier to noise density of 38.4 dB-Hz.161GNSS signal generators and the Giove-A satelliteFigure 7-15, Spectrum analyser plot of BOC(1,1) from IF signal generator with no noise.Figure 7-16, Spectrum analyser plot of BOC(1,1) from IF signal generator with noise(C/N0 = 38.4dB-Hz).Receiver performance testing using the IF signal generator across various carrier tonoise levels in shown in Chapter 9.2.This chapter has detailed the implementation of GNSS signal generators in hardware.The design of the SSTL MFUU Galileo signal generator was outlined and itsextension to a prototype IF signal generator used by this research.
The modulationtechniques and representations of current and future GNSS signals are detailed withimplementation examples. Also, the principles of digital noise synthesis for benchtesting of GNSS receivers are given with application to the IF signal generator.1628The SGR receivers and the PIF receiverThis chapter provides a detailed overview of SSTL’s existing receiver designs andidentifies specific areas of improvement that this project has contributed towards. It isnecessary to understand the detailed low-level hardware functions of existing GNSSspace receivers, such as the SSTL receivers in order to address the impact of thefuture GNSS signals and enhancements on the receiver. The design of anexperimental Prototype Intermediate Frequency (PIF) receiver is detailed.
The PIFreceiver provides a development platform for evaluating acquisition and trackingapproaches to the new generation of GNSS signals. The choice of components for thereceiver and the rationale behind them is discussed. Details are also given on thefrequency plan and sampling scheme for the receiver. Finally, a full description of theinner workings of the receiver’s correlator and processor are given with an emphasison the adaptations required for future GNSS signals. The PIF receiver provides ademonstration of the DE BOC tracking technique and details on the receiverprocesses required for implementation of this technique are given.8.1The SSTL receiver hardwareSSTL produces and supplies a wide range of Space GNSS Receivers (SGRs).
Thesereceivers are primarily used for their position, velocity and time (PVT) information,enabling orbit determination for the parent spacecraft. In recent years the receivershave been used to demonstrate novel applications of GNSS in space, such as GNSSattitude determination and ocean reflectometry.Table 8-1 summarizes the SGR range of receivers and highlights the key features ofindividual models.163The SGR receivers and the PIF receiverTable 8-1, SGR receiver modelsReceiverChannelsAntennasMassPower(g)consumptionKey features / applications(W)SGR–05121200.8Miniaturised receiver suitablefor nano-satellitesSGR–1024210505.3Established space receiver withconsiderable heritage.Enhanced version used forGNSS ocean reflectometrySGR–2024411506.3Enabled for 3-axis attitudedeterminationSGR–GEO241 or 225005.0Suitable for GEO and GTOapplicationsThe SGRs are based on a chipset from Zarlink semiconductors [Zarlink 1999] andcomprise of an RF front-end, an ASIC correlator and an ARM processor.
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