MBOC The new optimized spreading modulation (797940), страница 3
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The primaryobjective has been to improve tracking performance inmultipath. In addition, other characteristics have also beenconsidered, including code tracking, initial synchronization foracquisition, spreading code performance, and losses fornarrowband receivers.Multipath PerformanceSince performance in multipath involves a combination ofsignal design and receiver processing, several differentprocessing approaches have been considered.
Furthermore,since new ideas for multipath mitigation processing areemerging, signal characteristics that appear to benefit theseadvanced multipath mitigation techniques are also considered.Fig. 5. Multipath Error Envelope for NELP Processing,BW=24 MHz (4 pole Butterworth), Δτ=24.4 nsecMultipath Performance with Noncoherent Early-LateProcessingEvaluation of early-late processing performance is based on astatic model with one direct and one reflected path, with amultipath to direct path signal power ratio (MDR) that isindependent of delay. This model does not provide for theprobability distribution of (reflected) path delay or theattenuation associated with each delay value.
The resultsshown here employ a MDR of –6 dB. The receiver is assumedto have a four- or six-pole Butterworth band-limiting filterwith –3 dB points at the stated bandwidth (BW). The filter isassumed to be phase-equalized so that the group delay isconstant. Non-coherent early-late processing (NELP) isemployed.The results are provided as pairs of graphs for eachcombination of receiver processing parameters and differentsignals. The first graph is an error envelope showingmaximum and minimum bias error (computed over all relativephases between the multipath and the direct path), for eachdelay.
Many of these error envelopes have oscillatorycomponents. The second graph is of the so-called runningerror. This is computed from the area enclosed within themultipath error envelope and averaged over the range ofmultipath delays from zero to the plotted delay values.Fig. 6. Average Error for NELP Processing, BW=24 MHz(4 pole Butterworth filter), Δτ=24.4 nsecFig.
7 and Fig. 8 show the corresponding results for 24 MHzprecorrelation bandwidth, with a narrower early-late spacingΔτ=12 ns, corresponding to d=0.0125 (proportion of aspreading code interval). In these figures, the multipath errorenvelope for a BPSK-R(10) spreading modulation has alsobeen provided. Note that the MBOC spectrum provides errorenvelopes that are smaller than those for BPSK-R(10) for thesmall values of path length delays (less than ~120 nsec). Thisis the range of delays that are most common in many urbanenvironments and have lower values of attenuation (typicallyless than 20-30 dB).Fig.
5 shows the multipath error envelope for the receiverconfiguration of most interest, with 24 MHz pre-correlation(double-sided) bandwidth and narrow early-late spacing ofΔτ=24.4 nsec, corresponding to a fraction d=0.025 of a 1.023MHz spreading code chip period. Fig. 6 shows thecorresponding running average error, showing that bothMBOC waveforms provide typically smaller average errorsthan either BOC(1,1) or BOC(2,2) waveforms. One of thewaveform options, TMBOC(6,1,4/33), shows an average errorless than those of any other option for all delays.
An importantfeature of all the MBOC waveforms is that the error envelopediminishes at smaller path length delay values than forBOC(1,1) or BOC(2,2). At longer path length delay values,888Fig. 10. Average Error for NELP Processing, BW=24 MHz(6 pole Butterworth filter), d=0.05 chipsFig. 7. Multipath Error Envelope for NELP Processing,W=24 MHz (6 pole Butterworth filter), d=0.0125 chipsFig. 11 and Fig. 12 show corresponding results for a narrowerBW=12 MHz, with Δτ=48.9 nsec (d=0.05 chips).
The averageerror of the CBOC and TMBOC waveform options aretypically smaller than those for BOC(1,1) or BOC(2,2). Theaverage errors for TMBOC or CBOC(6,1,4/33) are smallerthan those for any other choice for all multipath delays.Fig. 8. Average Error for NELP Processing, BW=24 MHz(6 pole Butterworth filter), d=0.0125 chipsFig. 9 and Fig. 10 show results for BW=24 MHz, with earlylate spacing of Δτ=48.9 nsec (d=0.05).
The running averageerror of the MBOC waveforms are typically smaller than thosefor the BOC(1,1) or BOC(2,2) options. The error envelope forthe MBOC(6,1,4/33) waveforms (TMBOC or CBOC) issmaller than for all other options.Fig. 11. Multipath Error Envelope for NELP Processing,BW=12 MHz (6 pole Butterworth filter), d=0.05 chipsFig. 9. Multipath Error Envelope for NELP Processing,BW=24 MHz (6 pole Butterworth filter), d=0.05 chipsFig.
12. Average Error for NELP Processing, BW=12 MHz(6 pole Butterworth filter), d=0.05 chips889TMBOC and CBOC options are the same or larger than theRMS bandwidth for BOC(1,1) for all signal bandwidths, andlarger or almost as large as those for BOC(2,2) for signalbandwidths greater than approximately 12 MHz. (Highperformance receivers would be expected to use bandwidthsmuch greater than 12 MHz.)The results for narrow correlator processors show thatTMBOC(6,1,4/33) provides slightly smaller errors than for theCBOC(6,1,1/11) spreading symbol. This indicates that there isan advantage in placing all the BOC(6,1) spreading symbols inthe pilot for certain applications. In every case examined, theaverage errors for TMBOC(6,1,4/33) and CBOC(6,1,1/11) aresmaller than those for BOC(2,2) for all delays.Multipath Performance with Double-Delta ProcessingLike early-late processing, double-delta multipath mitigationprocessing is a known processing technique that was designedfor BPSK-R spreading modulations, but may be applied tomore advanced modulations as well.
The double-deltatechnique considered in this section processes every edge.Smaller multipath error envelopes may be obtained fromTMBOC and CBOC options by masking the BOC(6,1)spreading symbols in the receiver replica, so that onlyBOC(1,1) symbols are processed. This resulting code trackingSNR after this masked symbol replica (MSR) processing,when compared to the code tracking SNR that would beobtained from an all BOC(1,1) pilot, would be a fraction of adB (0.4, 0.6, or 0.9 dB, depending upon time seriesimplementation) lower.
The difference in tracking error wouldbe very small compared to other error sources, and allspreading symbols would be used for data demodulation andcarrier tracking, thus making use of all the available power.Fig. 13. Multipath Error Envelope for Double-DeltaProcessing, BW=24 MHz (4 pole Butterworth filter),Early-Late Spacings of 24.4 nsec and 48.9 nsecFig. 13 and Fig. 14 show the multipath errors resulting fromdouble-delta processing with the same multipath propagationmodel used previously. In these figures, the BW=24 MHz,outer early-late spacing of 48.9 nsec, and inner early-latespacing of 24.4 nsec. With MSR processing, the multipatherror envelopes for the MBOC options are the same as thosefor BOC(1,1), whilst those from BOC(2,2) are consistentlylarger. The multipath errors from double-delta processing aremuch smaller than those from early-late processing.Fig.
14. Average Error for Double-Delta Processing,BW=24 MHz (4 pole Butterworth filter), Early-LateSpacings of 24.4 nsec and 48.9 nsecPerformance of Advanced Multipath ProcessingA variety of advanced multipath mitigation techniques areevolving to provide improved performance. It is expected thatfurther advances will be possible with new forms of spreadingmodulations. There is no specific metric which provides forthe comparison of signals for advanced mitigation techniques,we have considered two.
The first of these is the root-meansquare (RMS) bandwidth of the spreading symbol, defined by+BWrms ( f lim ) =f lim2∫f 2 ⋅ Gˆ ( f ) ⋅ dfUsing a bandwidth of 12 MHz with one of the MBOC signaloptions would provide greater RMS bandwidth than using a 24MHz bandwidth with BOC(1,1). If receivers use bandwidthsless than approximately 12 MHz, they would lose a fraction ofa dB of signal power with TMBOC or CBOC, compared toBOC(1,1).(9)f− lim2where Gˆ (f ) is normalized for unit power over the signalbandwidth being used, and f lim is the double sided receiverpre-correlation bandwidth.Figure 15 shows the RMS bandwidth of the four spreadingmodulations for a given receiver bandwidth assumed to haverectangular bandwidths. The RMS bandwidths for theFig. 15.
RMS bandwidth vs. two-sided receiver bandwidth890A second measure of performance for advanced multipathmitigation is the number of waveform transitions in a coderepeat interval. These are affected by the spreading symbolrate, the carrier offset frequency and the organization of theBOC(6,1) and BOC(1,1) components.
A detailed analysis ofthis will not be given here. However, for the various optionsconsidered here, there is a gain of between 2.0 dB and 3.5 dBdepending upon the specific waveform implementation used.Summary of Multipath PerformanceThe multipath performance metrics indicate that TMBOC andCBOC options can be processed to obtain smaller multipatherrors than BOC(1,1) for early-late processing. For the doubledelta processor, the multipath errors for the proposedspreading symbol waveforms are the same as for BOC(1,1)and better than BOC(2,2).
Both TMBOC and CBOCwaveforms provide better potential for advanced multipathmitigation processing than BOC(1,1).Fig. 16. Comparison of Crosscorrelation Sidelobes for L1CSpreading Code PerformanceThe new L1 GALILEO OS and GPS L1C spreading codefamily members have been designed for reduced side-lobelevels in auto- and cross-correlation functions.One of the metrics used to select the BOC(1,1) and BOC(6,1)spreading symbols as waveform partners [13] is that these areorthogonal.
This can be used to improve the auto- and crosscorrelation performance. Therefore part of the design processfor TMBOC implementations will be to select the locations inthe code sequence where BOC(6,1) spreading symbols areplaced. Judicious placement introduces zeros into thecorrelations at certain delays, providing a unique opportunityfor additional control over the correlation functions.Fig. 17. Comparison of Autocorrelation Sidelobes for L1Cprovide long battery life.
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