GNSS Navigation message analysis and perspectives (797930), страница 3
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Navigation message GPS L1 C/A (3/4)Message Content Repetition•Some message content is repeated: Ephemeris, Clock Error corrections, etc.•Since there is not an implemented FEC, two bits which a priori contain the sameinformation can be coherently accumulated.•Nevertheless, due to the non-systematic property of EH(32,23) code it is very difficultto guarantee that bits containing the same information have the same value•The coherent accumulation remains marginalMessage Content Distribution•Since there is no implemented FEC, the number of subframes carrying theephemeris does not matter.Subframe synchronization:• Subframe synchronization process is achieved by using some bits of the 1st(Preamble) and 2nd (TOW) codeword of each subframe• However, these bits are not protected by any FEC Less robust to the noise- 40 -1.
Navigation message GPS L1 C/A (4/4)Bit synchronization:• Bit synchronization is necessary prior to subframe synchronization since 20 PRNcodes span 1 data symbol.- 41 -1b. Navigation message GPS L2C and L5 (1/5)Information Message Structure:• Continuous stream of information messages.• Each message is constituted of 300 bits: 276 information bits and 24 parity bits• First information bits are: 8b preamble, 6b SV Id, 6b Message Id• Flexible structure: Message contents (or Id) are determined by time outsMessage structure sent to the channel:• The continuous stream of information messages are fed to an encoder block• The output of the encoder block is sent (after BPSK modulation) to the channel- 42 -1b. Navigation message GPS L2C and L5 (2/5)Forward Error Correction (FEC):• Referred as Inner channel code, it is a convolutional channel code (CC):Code rate = 1/2Polynomial GeneratorsG1 = 171G2 = 133Constraint LengthMinimum free distanceL=7 = 10• Therefore, each information word is converted into 600 coded bits.• A continuous stream of bits is broadcast to the channel• Traditional decoding method is the Viterbi Algorithm• The introduction of a CC makes the navigation message more robust to the noise: For a BER=10-5, about5dB less will be required~5dB- 43 -1b.
Navigation message GPS L2C and L5 (3/5)Integrity (1/2): GPS L2C (synthetiser)• GPS L2C message integrity is controlled by a Cyclic Redundancy Code, CRC-24Q,which generates the 24 parity bits of the information message from its 276 info bits.• CRC-24Q performance:– It detects all single bit, double bit and any odd number of errors per code word– It detects any burst error for which the length of the burst is ≤ 24 bits.– The fraction of error bursts of length b > 24 that are undetected is:• = 2−23 = 1.19 × 10-7, if b = 25 bits.• = 2−24 = 5.96 × 10-8, if b > 25 bits.• To calculate the final integrity performance, the CC output pattern must be taken intoaccount since the CRC-24Q verification is applied on this output.• Whenever a CC commits an error during the correction process, CC will output apattern of errors (consecutive bit errors) instead of independent errors.• Besides, the error burst occurrence probability is inversely proportional to its length, b,• Therefore, the combination of a CC and a CRC is very interesting because, for shortinformation words, the majority of CC output errors will be detected since b ≤ 24 bits- 44 -1b.
Navigation message GPS L2C and L5 (4/5)Integrity (2/2): Comparison GPS L2C and GPS L1 C/A• Although GPS L1 C/A integrity depends on the receiver strategy, GPS L2C integrity ismore robust even for the example given before.• The percentage of integrity dedicated bits is lower for GPS L2C than for GPS L1 C/A• Whereas EH(32,26) imposes the information bits being protected, CRC-24Q does not.• However, the integrity is better checked with a small number of information bits and thecombination with a CC emphasizes this fact.Carrier Phase ambiguity for demodulation:• Since GPS L2C implements a pilot component, the carrier phase ambiguity fordemodulation is removedMessage Content Repetition• Since GPS L2C has a flexible structure and the message must be decoded beforeknowing its ID, the coherent accumulation is not possible.- 45 -1b.
Navigation message GPS L2C and L5 (5/5)Message Content Distribution• Ephemeris and Clock Error Corrections are distributed into 2 messages• This separation into 2 messages is mandatory due to the use of CCMessage synchronization:• Message synchronization process can be achieved by using the preamble bits, SV Idbits and integrity check (CRC-24Q verification).Bit synchronization:• GPS L2C:– Bit synchronization is directly achieved since 1 PRN code spans 1 data symbol– However, synchronization with Viterbi decoder must be achieved for decoding• GPS L5:– Bit synchronization achieved with data component secondary code– Synchronization with Viterbi achieved with pilot component secondary- 46 -1c.
Navigation message GALILEO E1 OS (1/2)Information Message Structure:• Continuous stream of frames, where each frame is constituted of 24 subframes• Each subframe is constituted of 15 pages (or words)• Each page (or word) is constituted of two page partsSubframe:Page (or word):- 47 -1c. Navigation message GALILEO E1 OS (2/2)Forward Error Correction (FEC):• The same channel code as GPS L2C is used to implement the FEC: CC(171,133)• Difference: 6 tail bits are inserted at the end of each page part– Preamble bits are no longer protected by the FEC (with respect to GPS L2C)– Allows the implementation of an interleaverInterleaver:• Rectangular interleaver with dimensions, (30 columns x 8 rows), applied to 1 page• Function: To make the navigation message more robust to its transmission through amobile channel by breaking the burst of errors introduced by the channelReceiver processing difference between GPS L2C and GALILEO E1 OS:•GPS L2C: 1)2)•GAL E1 OS: 1)2)Decoding the received stream of bitsAchieving message synchronizationAchieving page part synchronizationDe-interleaving + Decoding the bits of page part- 48 -1d.
Navigation message GPS L1C (1/2)Forward Error Correction (FEC):• Subframe 1 is encoded with an extendedBCH(52,9) with minimum distance, dmin=20• Subframe 2 is encoded with a LDPC(1200,600)and subframe 3 with a LDPC(548,274)• LDPC are very powerful codes which obtain nearshanon limit performance• Both LDPC codes and BCH(52,9) obtain betterdemodulation performance than the CC(171,133)Interleaver:• Subframe 2 and subframe 3 interleaved together (38 rows x 46 columns) interleaver• Breaks larger burst of errors than GALILEO E1 OS interleaver and distributes theerrors into two subframesFrame synchronization:• Frame synchronization is achieved with the pilot component secondary code- 49 -1d.
Navigation message GPS L1C (2/2)Message Content Repetition• For the first time, the complete information content of a subframe is repeated AND theresulting encoded subframe is the same for a known period of time. Exact suframe 2 replicas are repeated for 2 hours• Subframe 2 replicas can be coherently accumulated and thus the received C/N0 isincreased with each accumulation: /0 = 10 log10 Message Content Distribution• For the first time, CED is carried on the same subframe• In AWGN channel, this property provides additional robustness since just onecodeword/subframe/message must be decoded.- 50 -6.
New demodulation Performance methodology (1/3)•Proposition: Separating the reception message conditions into favorable receptionconditions and unfavorable reception conditions Providing demodulation performance for favorable reception conditions Providing the occurrence rate: Statistical information about the occurrence ofthe message favorable reception conditions.• Occurrence rate:• P0fav : No ‘favorable state message’ has occurredduring the interval of interest 12Normalized Count [%]10P0fav86 Key point: To define a criterion which providesthe ‘favorable state messages’ having the bestpossible demodulation performance (work to do).42X= 0Y = 0.59900510152025Number of favorable states messages in 1 hour (=200 messages)Favorable state occurrence rate, = , for GPS L1C frame using thePrieto channel model with 40° of elevation considering the Prieto channelGOOD states as favorable reception conditions- 51 -6.
New demodulation Performance methodology (2/3)• Methodology until now: providing the Error Rate (ER) of a given data set as a functionof the signal Cpre-urban/N0 only for determined favorable reception conditions andproviding the statistical occurrence values of these 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.• Minimum Required Availability (MRA): The couple GNSS signal/receiver meets theMRA requirement if with probability , the receiver receives at least ONE data set ina given time , associated to a data set error rate .• Absolute Minimum Required Availability (AMRA): Couple GNSS signal/receiver meetsthe AMRA requirement if with probability , the receiver is able to successfullydemodulate at least ONE data set in a given time , for a − 0 value- 52 -6.
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