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A Higher Data Rate for the Galileo E6 Signal Nigel Hoult GNSS Signal 2007, 24th-25th April 2007

Thales Research

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A Higher Data Rate for the Galileo E6 Signal Nigel Hoult GNSS Signal 2007, 24th-25th April 2007

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    1. Thales Research & Technology (UK) Ltd. A Higher Data Rate for the Galileo E6 Signal Nigel Hoult GNSS Signal 2007, 24th-25th April 2007

    2. Thales Research & Technology (UK) Ltd. Contents Motivation Current E6 Spectrum Constraints Options for Increasing Data Rate Options for Signal Combination Impact on Tracking Error Detection and Correction Conclusions

    3. Thales Research & Technology (UK) Ltd. Motivation The Galileo E6 CS signal does not carry navigation data i.e. ephemeris, almanac, clock correction… Therefore, the whole data capacity is available for navigation-related commercial services However, the user data rate is only about 500bps (per satellite) 1kbps before FEC, CRC, sync header, etc. An increased data rate, even just for users in good propagation conditions (e.g. maritime) would be beneficial commercially The target is a user data rate of 5kbps or more

    4. Thales Research & Technology (UK) Ltd. Current E6 Signal

    5. Thales Research & Technology (UK) Ltd. Constraints No change to existing signal modulation No change to total power or power of PRS and CS pilot All necessary clock signals to be derivable by division of the master clock frequency, 120 x 1.023MHz An integer number of spreading code chips per data bit Code length will be chosen such that this is also an integer number of code repeats Raw data rate to be a multiple of 1kbps Not absolutely necessary if existing signal replaced, but data rate should still be a multiple of all the other Galileo data rates

    6. Thales Research & Technology (UK) Ltd. Options for Offering a Higher Data Rate Increase the data rate of the existing E6-B signal Add a new signal (“E6-D”) This must steal some power from the E6-B signal

    7. Thales Research & Technology (UK) Ltd. Option 1 – Modify Existing Signal Chip rate of E6-B is 5115kbps = 3 x 5 x 11 x 31 ? Possible data rates are 3, 5, 11, 15, 31, 33, 55 … kbps Bearing in mind the need for FEC and the limitations of the link budget, feasible rates are 11 and 15kbps Pros: Relatively simple to implement Cons: All E6 CS users have to rely on the new signal Therefore they will all pay a penalty in either impaired link budget, greater receiver complexity, or both

    8. Thales Research & Technology (UK) Ltd. Option 2 – Add a New Signal Main gaps in spectrum are at ±5 MHz Suggests BOC(5,n) For this to exist, n must divide into 2 x 5 Constraints require 1023 x n to be an integer i.e. n = 5, 10/3, 2, 5/3, 1, 10/11 or 2/3

    9. Thales Research & Technology (UK) Ltd. Data Rate Data rate must be a multiple of 1kbps and a factor of chip rate Possible values 10, 11, 15, 22, 31, … kbps But not all combinations allowed

    10. Thales Research & Technology (UK) Ltd. Signal Combination Options – 1 Addition in amplitude – like CBOC on the E1 signal

    11. Thales Research & Technology (UK) Ltd. Signal Combination Options – 2 Time-multiplexing – like TMBOC on GPS L1C Three variants: chip-by-chip, bit-by-bit, page-by-page

    12. Thales Research & Technology (UK) Ltd. Impact on Existing Signal * - Also constrains CS/PRS power ratio and increases IM power Extra benefits of page by page multiplexing Both signals can use the same modulation Split between signals can be changed easily and dynamically Therefore page by page multiplexing was selected

    13. Thales Research & Technology (UK) Ltd. Impact on Receiver Tracking - 1 The E6 CS signal cannot be used alone for navigation Hence the main use will be as part of an MCAR approach Therefore it is carrier phase tracking that is of interest Such receivers will probably track data and pilot Pilot tracking unaffected by increased data rate Data tracking degraded because coherent integration time reduced Overall impact depends how the two are combined Equal weight: easy, but gives sub-optimum performance (can even be worse than tracking just the pilot) Optimum: difficult, because the standard deviations of data and pilot tracking need to be known (and vary with C/No) The best receivers will probably fall between these extremes

    14. Thales Research & Technology (UK) Ltd. Impact on Receiver Tracking - 2

    15. Thales Research & Technology (UK) Ltd. Error Detection and Correction – 1 Current E6 CS approach Rate ½ constraint length 7 convolutional code for correction 5% block failure rate at ~30.5dB C/No 24 bit CRC for detection 6 x 10-8 probability of undetected error on random data High rate CS signal (11 or 15kbps before FEC) Existing convolutional code not good enough 42% to 100% block failure rate at 40.9dBHz C/No But more modern approaches are now feasible Turbo codes – used in 3G cellular etc. LDPC codes – proposed for GPS L1C Both of these are more complex, but only by a modest factor Present CRC approach still adequate Undetected error probability unchanged

    16. Thales Research & Technology (UK) Ltd. Error Detection and Correction – 2 Example – turbo code

    17. Thales Research & Technology (UK) Ltd. Conclusions It is feasible to offer a 5.5 or 7.5kbps user data rate service on E6, the only impact on users of the current service being a reduced data rate The scheme proposed offers a flexible split between current and higher rate The impact on receiver tracking, with optimum weight combining, is less than 2dB An improved FEC scheme will be required for the new service, but the complexity increase of this is modest

    18. Thales Research & Technology (UK) Ltd. Thank you for your attention

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