GNSS Navigation message analysis and perspectives (797930), страница 2
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New demodulation Performance methodology (4/4)• Methodology until now: Providing:a) Data set Error Rate vs Cpre-urban/N0 only for given favorable reception conditionsb) Statistical occurrence values of these favorable reception conditions• Remaining question: Which statistical occurrence values are considered acceptable? Determined by the operational requirementsLink between the statistical occurrence/Error Rate and operational requirements• Low level requirements are defined to achieve this link: reception of a given data setby 1 single satellite during a continuous period of time.ExampleTo determine if a GPS L1C receiver can calculate a continuous valid position during 4consecutive hours, with a probability greater than 95%- 21 -Outline1.
Navigation Message Evolution2. New demodulation performance methodology in mobile channels3. Code Shift Keying (CSK) modulation4. GNSS Signal with variable data rate- 22 -7. Code Shift Keying (CSK) modulation (1/6)CSK Definition: CSK is an orthogonal M-ary modulation since M orthogonal signalingwaveforms are used in order to transmit U = log2(M) bits.• Each waveform (or symbol representing a set of input bits) is obtained from a differentcircular cyclic integer shift of the fundamental PRN code.• M=4 possible different symbols• U=2 bits mapped by a CSK SymbolCSK(N=1, U=2) Increase of the bit rate by a factor of U with respect to a BPSK modulation.• CSK Configuration (N,U): ‘N’ consecutive and identical PRN code shifts, with ‘U’ bitsmapped by each PRN code shift, are used to create the CSK symbolCSK(N=2, U=2) Increase of the bit rate by a factor of (U/N) with respect to a BPSK modulation.- 23 -7.
Code Shift Keying (CSK) modulation (2/6)Why is CSK modulation useful?• Objectives:– Increase the signal data rate– Demodulation of the navigation message in suburban/urban/indoor environmentsCSK vs BPSK modulation for increasing the data rate:BPSKCSKSymbol rate mustbe increased1) If chip rate is maintained, PRN code length is decreased Lossof PRN orthogonal properties2) If PRN code length is maintained, chip rate is increased Signalbandwidth increaseChip rate and PRN code length are maintained.
Symbol rate is maintained or decreasedCSK vs BPSK modulation in harsh environments:BPSKCSKOnly coherent demodulation ispossibleA PLL is unable to estimate the carrier phase forlow /0 values and fast carrier phase variationsCoherent and non-coherent demodulation are possible- 24 -7. Code Shift Keying (CSK) modulation (3/6)CSK characteristics: Demodulator• Demodulator of an orthogonal M-ary modulation implements a bank of matched filters,one filter for each symbol• The matched filter with the largest output value determines the transmitted symbolIn-phaseQuadrature-phase1) Coherent demodulator:• Vector , = 0 , … , −1is used to determine which symbol has been used2) Non-coherent demodulator:•Vector = 0 , … , −1 =,is used to determine which symbol has been used2, 2+ Carrier phase influence is removed byobtaining the module- 25 -7.
Code Shift Keying (CSK) modulation (4/6)CSK characteristics: Coherent demodulation performance in an AWGN channel• Different couples channel code/decoding methods can be implemented:BER vs Eb/N0 for a reference BPSK signal and for aCSK signal with Reed-Solomon Channel CodesBER vs Eb/N0 for a reference BPSK signal and for aCSK signal using GPS L1C subframe 2 LDPC, CDand BICM–ID decoding methods and Mapping A• CSK configuration (N=U/n,U), with the possible largest U value, provides the bestdemodulation performance when the data rate is increased by a factor of ‘n’.• However, the codeword duration increases along with N=U/n- 26 -7.
Code Shift Keying (CSK) modulation (5/6)CSK characteristics: Non-coherent demodulation performance in a mobile channel• BER vs data Cpre-urban/N0 for different CSK configurations (U/n,U), x2 bit rate increase• The simulated mobile Channel is the Perez-Fontan model without change of statesIntermediateShadowing:( = −, =. , = −, )Deep Shadowing:( = −, , =. , = −)• CSK modulated signal can be non-coherent demodulated even for harsh conditions• Due to the carrier phase variations, CSK configurations (U/n,U) with larger U values dono longer provide the best demodulation performance- 27 -7.
Code Shift Keying (CSK) modulation (6/6)CSK Drawbacks:1. The data component cannot be used to acquire or track the signal Requires a pilotcomponent2. The complexity of the receiver increasesCSK Advantages:1. Possibility of implementing a non-coherent demodulation no need to have carrierphase lock2. Increase of the useful data rate without modifying the symbol data rate, the chip rateand the PRN code length as opposite to BPSK modulation.3.
Flexibility of the useful data rate Dynamic change of the number of bits mapped bya CSK symbol.- 28 -Outline1. Navigation Message Evolution2. New demodulation performance methodology in mobile channels3. Code Shift Keying (CSK) modulation4. GNSS Signal with variable data rate- 29 -8. GNSS Signal with variable data rate (1/5)Motivation: The different types of information data transmitted by a GNSS or SBASsystem does not have the same level of importance.• For a GNSS:– Essential information to calculate the user PVT: Ephemeris and Clock Error correc.– Additional information such as ionosphere and troposphere corrections areimportant but not mandatory• For a SBAS:– Integrity information is the most critical information and occupies the majority of thebandwidth (UDREi in current SBAS L1).– Satellite long corrections are important but the receiver could still use the SBASsystem if some are lost• Additional services could be transmitted if more bandwidth was available:– GNSS example: Precise positioning– SBAS example: Information to reach CAT II approachDifferent degrees of importancefor different types of dataDifferent degreesof robustnessDifferent usefuldata rates- 30 -8.
GNSS Signal with variable data rate (2/5)Example of GNSS/SBAS signal structure with variable data rate:• Fundamental idea:– The signal is always divided in fixed blocks of known length, (e.g. 1s)– The characteristics/configuration of each fixed length block varies are/is providedby a signal component or by a pre-planned schedule• Proposed signal is constituted of 4 components:1) Pilot component In charge of tracking, acquiring and synchronizing the fixedlength blocks2) Primary Data component BPSK modulated3) Secondary Data component BPSK/CSK modulated with different CSKconfigurations4) Configuration component (or pre-planned schedule) In charge of providingthe fixed length block configuration of the primary and secondary datacomponents- 31 -8.
GNSS Signal with variable data rate (3/5)Shared property among all signal components:• All signal components have a different and synchronized primary PRN code of thesame period ,Pilot component structure:• Secondary code, , of period equal to , each chip is one primary PRN codeConfiguration component structure:• Secondary code, , of period equal to , each chip is 1 primary PRN code• A (1, U) CSK modulation is applied over the secondary code in order to transmit thedata components configuration of the next fixed block length data components.- 32 -8.
GNSS Signal with variable data rate (4/5)Primary Data component structure:• Each block carries a BPSK modulated signal with constant data rate (BPSKsymbol = 1 PRN code)Secondary Data component structure:• Each block is dynamically BPSK-CSK modulated. It carries either:– A BPSK modulated signal with constant data rate (BPSK symbol = 1 PRN code)– Or a (N,U) CSK configuration where U and N depends on the configurationData rate increase: x1Data rate increase: x5Data rate increase: x2.5- 33 -8. GNSS Signal with variable data rate (5/5)Example:Additional advantages:• In order to further increase/decrease the data rate and thus to furtherdecrease/increase the message robustness:– Combination of data message transmitted at the same time on both components– Channel Coding varies depending on the configuration• A codeword can span more than one fixed length block• The current message can be interrupted An alarm can be emitted each seconds- 34 -Proposal for future GNSS/SBAS signal/message• To design a GNSS/SBAS signal/message with a variable data rate in order to betteradapt the message robustness to the information importance level• To use selectively the CSK modulation in some components of the newsignal/message• To use different channel codes to protect the different types of information• To use the new proposed methodology for analyzing GNSS signals demodulationperformance in mobile channels in order to define a new signal which meets the highlevel operational requirements- 35 -Thank you for your attentionAxel GARCIA-PENA: garcia-pena@recherche.enac.fr- 36 -BACK-UP SLIDES- 37 -1.
Navigation message GPS L1 C/A (1/4)Integrity (1/2):•GPS L1 C/A message integrity is controlled by an extended Hamming code (32,26)•Since each codeword is constituted of 30 bits, 2 bits of the previous word must beused to encoded and decoded the codeword•Theoretically, this code can detect up to 2 errors with a 100% probability•Theoretically, the probability of not being able to detect an error is: 226 == 32 = 2−6 = 2- 38 -1.
Navigation message GPS L1 C/A (2/4)Integrity (2/2):•However, due to the sharing of the last 2 bits between consecutive words, the finalintegrity will depend on the receiver strategy– Example: If Codeword is detected as erroneous, last 2 bits of codeword ( − 1)could be erroneous and thus codeword ( − 1) is also considered as erroneousCarrier Phase ambiguity for demodulation:•EH(32,26) code is not systematic: bit 30 of codeword ( − 1) controls de polarity ofinformation bits of codeword n.•Due to the non systematic property, EH(32,26) is transparent to the carrier phasejumps of pi (except for the first word).Original information bits are recovered- 39 -1.
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