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					            SpaceOps 2006, 19-23 June, Rome, Italy




     Combined Advanced Coding & Modulation
      for Future CCSDS High-Rate Missions


    Gian Paolo Calzolari, and
    Enrico Vassallo
                                     ESA/ESOC


1
                                Introduction
       TRADITIONALLY in CCSDS coding and modulation techniques
        have been kept separated from each other and assigned to two
        different WGs.
       Mission designers are (normally) free to select whatever CCSDS
        channel code and couple it with any of the CCSDS modulations.
       This approach is sub-optimal. Already in 1999, CCSDS recognized
        the advantages of combining modulation and coding and introduced
        'Trellis coded modulation' for a specific class of missions
       As more and more missions share the scarce spectrum resource
        with continuously increasing data rate requirements while the on-
        board power remains constrained, it is necessary to extend such
        techniques to the other frequency bands used by the space
        science services.
                                      Combined Advanced Coding and Modulation
                                         for Future CCSDS High-Rate Missions
2
                         The 3 Approaches (1 of 2)
       The first option proposed by the French Agency CNES is to adopt “as is”
        the recently developed DVB-S2 standard for the next generation of
        digital broadcast via satellite.
       The second option by ESA is based on a serial concatenation of a Turbo-
        like code (SCCC) coupled with QPSK, 8PSK, 16APSK and even higher
        order modulations.
         –   Such scheme was proposed by ESA to the DVB-S2 forum in 2004 where it was the
             runner-up due to the complexity of the decoding relative to the selected LDPC while
             achieving basically the same power efficiency. Further work by ESA in 2005 led to the
             discovery of a parallelization method making the decoding simpler than DVB-S2 LDPC by
             at least 30%, thereby promising higher throughput for the same complexity or lower
             complexity for the same data rate.


       Both CNES and ESA approaches require several changes to the various
        layers of CCSDS.

                                                  Combined Advanced Coding and Modulation
                                                     for Future CCSDS High-Rate Missions
3
                    The 3 Approaches (2 of 2)
       The third scheme proposed by NASA is based on the LDPC codes
        being considered by CCSDS channel coding WG, pragmatically
        coupled with QPSK modulation or higher order schemes like 8PSK.
       Being a pragmatic approach based on existing CCSDS standards or
        proposed standards, it is designed to fit CCSDS layers seamlessly
        at the possible expenses of performances.
       Actually NASA has provided two “flavors” with two kinds of LPDC
        codes based on different design approaches.

       The difficulty in finding consensus has led to explore the new
        approach of producing Agency specific “experimental” Orange
        Books that may eventually allow standardization on a faster track.
        In addition, it is planned to increase the future work program for
        the joint effort by coding and modulation delegates in this area.

                                       Combined Advanced Coding and Modulation
                                          for Future CCSDS High-Rate Missions
4
             CNES Proposal for DVB-S2 Adoption
       In spring 2002 the Digital Video Broadcasting (DVB) Project initiated the search
        for a second generation standard for broadband satellite applications: DVB-S2.
       At the end of 2002 a LDPC code based solution proposed by Hughes Network
        Systems was selected and eventually converted into the European Standard
        (Telecommunications series) ETSI EN 302 307. Such scheme is based on a
        pragmatic coupling of LDPC codes with QPSK and 8PSK modulations
       It is expected that hardware for space science applications may become available
        as spin-off of the DVB-S2 market although the cost of the applicable patents
        would have to be taken into account.
       However this standard is not directly applicable to CCSDS data structures and
        CNES, to support their proposal, investigated the main features and performance
        of the channel coding scheme selected by DVB-S2 with respect to frames shorter
        that those under definition by DVB. In their presentation at CCSDS, CNES
        showed the possibility of using frame sizes compatible with CCSDS at expenses of
        performance reduction with respect to DVB-S2 frames.


                                              Combined Advanced Coding and Modulation
                                                 for Future CCSDS High-Rate Missions
5
                                The SCCC scheme proposed by ESA

         It is based on the serial concatenation of an outer 4-state
          systematic recursive rate ½ encoder punctured to rate 2/3, an
          interleaver and an inner 4-state systematic recursive rate ½
          encoder with suitable puncturing to obtain the desired rate
         The code design involves choosing the puncturing patterns matching
          the desired rate
                                                                                                                      The interleaver length
                                                                                                                      is designed in order to
                                                         PUNCT.
                                                          SYST.
                                                          BITS


          CC1      FIX PUNCT.                   CC2
                                                                           ROW-                                       keep the block length
                                                                                                                      on the channel constant
                                 INTERLEAVE                              COLUMN                 MAPPING/
        Rate 1/2        11                    Rate 1/2            P/S
                                      R                                 INTERLEAV              MODULATION
INPUT   4-states        10                    4-states
                                                                            ER                               OUTPUT

                                                                                                                      to 8100 symbols
 BITS
                                                                                                            SYMBOLS

                                                         PUNCT.
                                                         PARITY
                                                          BITS                      ATTACHED                          regardless the
                                                                                                                      modulation cardinality
                                                                                     FRAME
                                                                                     MARKER


                                                                                                                      or the code rate.



                                                                            Combined Advanced Coding and Modulation
                                                                               for Future CCSDS High-Rate Missions
  6
                            Convolutional Encoders
       The SCCC is based on the serial concatenation of two identical 4-state
        systematic recursive rate ½ encoders. The outer convolutional encoder is
        punctured through a fixed scheme to a rate 2/3
       The outer convolutional encoder is punctured through a fixed scheme to a
        rate 2/3
       In order to obtain the desired coding rate, puncturing is performed at
        the output of the inner encoder.       The upper register at the output of the
          u
                                            c1     inner encoder contains the N+2 inner
                                                   systematic bits, which coincide with
                        +                          the interleaved outer code word plus
                                                   the 2 bits terminating the inner
                                                   trellis. The lower register, instead,
                                                   contains the N+2 parity-check bits
                 +      D           D


                                                   generated by the inner encoder. Two
                                            c2     different puncturing algorithms are
                                    +
                                                   used to puncture bits.
                                            Combined Advanced Coding and Modulation
                                               for Future CCSDS High-Rate Missions
7
                            Row-column interleaver
 At the transmitter side a row-column interleaving is used to spread the bits
  belonging to one symbol (pragmatic approach). The interleaving size is equal
  to the size of one codeword and the number of columns is equal to m, where
  m is the efficiency of the modulation scheme.
 The bit-interleaving is such that the bits transmitted with the same
  modulation signal are spread at the output of the inner encoder so that their
                                 affect the decoding process.
  correlation does not adversely MSB
                                       WRITE                  READ


                   Row 0




                 Row 8099



                            Column 0       Column m-1

                                                          Combined Advanced Coding and Modulation
                                                        LSB


                                                             for Future CCSDS High-Rate Missions
8
                 SCCC Modulation formats
                                                                                                                 0111              0011
                                             011             001

                                                                                                    0110                                      0010




                             010                                                000
                                                                                             0100                0101              0001               0000




                                                                                             1100                1101              1001               1000
                             110                                                100
       01   00




                 8PSK                        111              101
                                                                                        16APSK      1110                                       1010


                                                                                                                 1111              1011


                                                                                                                                   101100 001100
                                                                                                                             101110             001110
                                             01111   00111                                                              100110                       000110

                                   01101                       00101                                            100100             101101 001101               000100

QPSK                                         01110   00110                                                 100101
                                                                                                                             101111

                                                                                                                        100111
                                                                                                                                                001111

                                                                                                                                                        000111    000101
       11   10           01100                                              00100
                                     01010                   00010                                                                    101001 001001
                                                                                                       100001     100011         101011           001011     000011     000001

                     01000       01001       01011   00011                      00000                                                                 001010 000010 000000
                                                                    00001                            100000 100010 101010 101000 001000


                                                                                                     110000 110010 111010             111000 011000   011010 010010 010000
                     11000       11001       11011   10011          10001       10000

                                                                                                       110001     110011         111011           011011     010011 010001
                                     11010                   10010                                                                    111001011001
                         11100                                              10100
                                                                                                           110101       110111                           010111   010101
                                             11110   10110

            32APSK                                                                                              110100
                                                                                                                             111111
                                                                                                                                   111101 011101
                                                                                                                                                011111
                                                                                                                                                              010100
                                   11101

                                             11111   10111
                                                               10101
                                                                                        64APSK                          110110                         010110
                                                                                                                             111110              011110
                                                                                                                                   111100 011100


                                                                      Combined Advanced Coding and Modulation
                                                                         for Future CCSDS High-Rate Missions
9
     Supported set of spectral efficiencies
                                       Nominal values                          Interleaver constrained
                                                              Code
                        Es/N0   eta     K       I       N   m Rate     K'         I'    eta'   Es/N0'    delta
                   1     -1.85 0.7254 5,876   8,816 16,200 2    0.36   5,758    8,640 0.7109     -1.96
                   2     -0.85 0.8659 7,014   10,523 16,200 2   0.43   6,958 10,440 0.8590       -0.89    1.07




          QPSK
                   3     0.15 1.0254   8,306 12,461 16,200 2    0.51   8,398 12,600 1.0368       0.22     1.11
                   4     1.15 1.2039   9,752 14,630 16,200 2    0.60   9,838 14,760 1.2146       1.21     0.99
                   5     2.15 1.4012 11,350 17,027 16,200 2     0.70 11,278 16,920 1.3923        2.11     0.90
                   6     3.15 1.6164 13,092 19,640 16,200 2     0.81 13,198 19,800 1.6294        3.21     1.10
                   7     2.15 1.4012 11,350 17,027 24,300 3     0.47 11,278 16,920 1.3923        2.11     -1.10
                   8     3.15 1.6164 13,092 19,640 24,300 3     0.54 13,198 19,800 1.6294        3.21     1.10
          8PSK




                    9    4.15 1.8484 14,972 22,460 24,300 3     0.62 14,878 22,320 1.8368        4.10     0.89
                   10    5.15 2.0958 16,976 25,466 24,300 3     0.70 17,038 25,560 2.1035        5.18     1.08
                   11    6.15 2.3568 19,090 28,637 24,300 3     0.79 19,198 28,800 2.3701        6.20     1.02
                   12    7.15 2.6299 21,302 31,955 24,300 3     0.88 21,358 32,040 2.6368        7.18     0.98
                   13    6.15 2.3568 19,090 28,637 32,400 4     0.59 19,198 28,800 2.3701        6.20     -0.98
          16APSK




                   14    7.15 2.6299 21,302 31,955 32,400 4     0.66 21,358 32,040 2.6368        7.18     0.98
                   15    8.15 2.9133 23,598 35,399 32,400 4     0.73 23,518 35,280 2.9035        8.12     0.94
                   16 9.15 3.2056 25,966 38,951 32,400 4        0.80 25,918 38,880 3.1998        9.13     1.01
                   17 10.15 3.5053 28,392 42,590 32,400 4       0.88 28,318 42,480 3.4960       10.12     0.99
                   18 9.15 3.2056 25,966 38,951 40,500 5        0.64 25,918 38,880 3.1998        9.13     -0.99
          32APSK




                   19 10.15 3.5053 28,392 42,590 40,500 5       0.70 28,318 42,480 3.4960       10.12      0.99
                   20 11.15 3.8111 30,870 46,307 40,500 5       0.76 30,958 46,440 3.8220       11.19     1.07
                   21 12.15 4.1220 33,388 50,084 40,500 5       0.82 33,358 50,040 4.1183       12.14     0.95
                   22 13.15 4.4370 35,940 53,912 40,500 5       0.89 35,998 54,000 4.4442       13.17     1.03
                   23 12.15 4.1220 33,388 50,084 48,600 6       0.69 33,358 50,040 4.1183       12.14     -1.03
          64APSK




                   24 13.15 4.4370 35,940 53,912 48,600 6       0.74 35,998 54,000 4.4442       13.17     1.03
                   25 14.15 4.7555 38,520 57,782 48,600 6       0.79 38,638 57,960 4.7701       14.20     1.02
                   26 15.15 5.0766 41,120 61,682 48,600 6       0.85 41,038 61,560 5.0664       15.12     0.92
                   27 16.15 5.4000 43,740 65,612 48,600 6       0.90 43,678 65,520 5.3923       16.13     1.01


                                                            Combined Advanced Coding and Modulation
                                                               for Future CCSDS High-Rate Missions
10
       Simulated performance of the 27
     spectral efficiencies (AWGN channel)
           1.00E+00
                       142Mbps   171   207       243           278           326       367       421       474        527       581   640        700    764    824    889    954    1,013        1,078

           1.00E-01

                             1     2         3             4         5             6
           1.00E-02
                                                                         7         8         9             10         11              12
           1.00E-03

                                                                                                                 13        14    15         16         17
     BER




           1.00E-04

                                                                                                                                                 18 19        20     21      22
           1.00E-05
                                                                                                                                                                     23 24         25       26       27
           1.00E-06


           1.00E-07


           1.00E-08
                      -2   -1      0         1         2         3           4     5         6         7         8     9         10    11        12    13     14     15     16      17      18      19    20

                                                                                                                 E s /N 0 [dB]



                                                                                                            Combined Advanced Coding and Modulation
                                                                                                               for Future CCSDS High-Rate Missions
11
                NASA Proposal for LDPCC
      NASA   found the answer to bandwidth efficient
       codes in Low Density Parity Check Codes (LDPCC).
      Opposite to the LDPC Codes selected for DVB-S2,
       the NASA proposals consider codes designed to
       fit with traditional CCSDS data structures.
      Researches performed at the Goddard Space
       Flight Center (GSFC) in Maryland and at the Jet
       Propulsion Laboratories (JPL) in California have
       actually ended up into two different “flavors” of
       this approach.

                              Combined Advanced Coding and Modulation
                                 for Future CCSDS High-Rate Missions
12
           Low Density Parity Check Code for
                       Rate 7/8
      The proposal by NASA/GSFC has started in fall 2002
       with the submission to CCSDS Channel Coding Panel 1B
       of a White Paper based on Euclidean Geometry LDPCC.
      The rationale was that this type of codes had shown to
       provide very low error floors and very fast iterative
       convergence, important qualities for near Earth
       applications where very high data rates and high
       reliability are the driving requirements.
      The LDPC code considered by NASA/GSFC is a
       member of a class of codes called Quasi-Cyclic codes.


                                Combined Advanced Coding and Modulation
                                   for Future CCSDS High-Rate Missions
13
                         Construction of GSFC Codes
        The construction of these codes involves juxtaposing smaller circulants (or cyclic
         submatrices) to form a larger parity check or base matrix.
          –   Being a Circulant a square binary matrix where each row is a cyclic shift of the row above
              (degree may be >1), the GSFC matrix of circulant is built such that every row is one bit
              right cyclic shift (where the end bit is wrapped around to the beginning bit) of the
              previous row.
          –   Constructing parity check matrices in this manner produces two positive features: 1. the
              encoding complexity can be made linear with the code length or parity bits using shift
              registers, and 2. encoder and decoder routing complexity in the interconnections of
              integrated circuits is reduced.
        With this approach a “baseline” (8176, 7156) LDPC code has been designed. The
         rate of this code is (7156/8176 = 0.875; i.e. approximately 7/8). A total of 7156
         information bits are used (=894.5 octets).
        The parity check matrix for this code is formed by using a 2 x 16 array of 511 x
         511 square circulants creating a parity check matrix of dimension 1022 x 8176. A
         scatter chart of the parity check matrix for the rate 7/8 LDPC code is shown in
         next slide where every “1” bit in the matrix is represented by a point.

                                                     Combined Advanced Coding and Modulation
                                                        for Future CCSDS High-Rate Missions
14
     Scatter Chart of Parity Check Matrix




                    Combined Advanced Coding and Modulation
                       for Future CCSDS High-Rate Missions
15
                   Performances of GSFC Code




    The curves
     were
     determined
     at GSFC by
     hardware
     simulation.




                             Combined Advanced Coding and Modulation
                                for Future CCSDS High-Rate Missions
16
             Low Density Parity Check Code Family

        The proposal by NASA/JPL includes a complete “family”
         of LDPC Codes identified according to well defined
         criteria.
        The selected code rates are 1/2, 2/3, and 4/5. These
         values are about uniformly spaced by 1 dB on the rate-
         dependent capacity curve for the binary-input AWGN
         channel. The selected (information) block lengths are
         k=1024, k=4096, and k=16384.
        There are 9 combinations of the 3 block lengths with
         the 3 possible code rates providing flexible solutions to
         different mission needs.
                                     Combined Advanced Coding and Modulation
                                        for Future CCSDS High-Rate Missions
17
                    Constant Frame Length
      By choosing to keep the information block length “k”
       constant among family members, rather than the
       codeblock length “n”, the spacecraft’s command and
       data handling system can generate data frames
       without knowledge of the code rate.
      The selected codes are systematic.
      They are based on “Accumulate Repeat Accumulate
       Codes”, precisely Accumulate Repeat-4 Accumulate
       (AR4) codes.



                                Combined Advanced Coding and Modulation
                                   for Future CCSDS High-Rate Missions
18
     The JPL Parity Check Matrix




                Combined Advanced Coding and Modulation
                   for Future CCSDS High-Rate Missions
19
                     Performances of JPL Code
        From left to right
         the performance
         curves for the
         midsize information
         block length codes
         with parameters
         (n=8192, k=4096)
         rate 1/2, (n=6144,
         k=4096) rate 2/3,
         and (n=5120,
         k=4096) rate 4/5.
        The curves were
         determined at JPL
         by hardware
         simulation.


                               Combined Advanced Coding and Modulation
                                  for Future CCSDS High-Rate Missions
20
                                    Orange Books
        Such a wide offer of choices did not make easy to get consensus on
         selecting a single proposal. The difficulty in finding consensus has led to
         explore the new approach of producing Agency specific “experimental”
         Orange Books, i.e. “Experimental Specifications”.
        The "Experimental" designation typically denotes a specification that is
         part of some research or development effort. Its funding and other
         associated resources are normally independently provided by the
         organization that initiates the work.
        This designation therefore allows the work to progress roughly to the
         equivalent technical status of a “Draft Standard” without being actually
         on the Standards Track.
        Experimental work may be rapidly transferred onto the Standards Track
         if a hard requirement emerges, thus shortening the response time in
         satisfying the new customer.

                                             Combined Advanced Coding and Modulation
                                                for Future CCSDS High-Rate Missions
21
                     Why Orange Books?
  Opposite to other fields where the impact of hardware
   solutions is less relevant, in Channel Coding as well as in RF
   and Modulation it is more difficult to reach consensus via
   compromises merging features taken from more proposals.
  Orange Books have the advantage that a period of time
   may elapse allowing to reconsider the available solutions
   at the light of flying space mission and progress.
  Orange Books shall not be seen as way to escape
   discussion and aim for consensus, but as an effort to
   record important work for future verification and re-
   discussion according to progress in requirements and
   technology.
                                Combined Advanced Coding and Modulation
                                   for Future CCSDS High-Rate Missions
22
                                 Conclusion
        The very lively discussion within CCSDS Working Groups has
         confirmed that the achievement of gains in both the
         spectral and the power efficiency domains over conventional
         CCSDS encoding followed by binary and quaternary (BPSK
         and QPSK) PSK modulations is essential.
        Therefore, the planning of work to better investigate the
         applicable requirements, possibly narrowing the mission
         scenarios, is a key point for the future together with the
         inclusion of the modulation aspects to complement coding
         techniques.
        For these reasons, it is planned to increase the future
         CCSDS work program for the joint effort by coding and
         modulation delegates in this area.
                                    Combined Advanced Coding and Modulation
                                       for Future CCSDS High-Rate Missions
23
     Thank you for your attention.


       Questions ?




                Combined Advanced Coding and Modulation
                   for Future CCSDS High-Rate Missions
24

				
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