[Sea-sa] Background materials for today's SEA-SA SCCS-ARD discussion

Tue Mar 2 22:36:17 UTC 2021

Dear SCCS-ARD sub-team,

During today’s SEA-SA SCCS-ARD discussion we spent quite a period of time discussing the challenges in create a reasonably compact, and also accurate, table that reflects the currently documented set of configurations that are made available by the suite of space data link, coding, synchronization, modulation, RF (and optical), and physical layer signaling standards.  There are many situations where there is no one, simple, statement, or even set of statements, that can be made.  We have had to resort to a tabular presentation, Table 6-8 in Sec 6 on protocols, to address this.   A copy of this table is attached, along with the “cheat sheet” of notes that encode the cells in this table.

Any standards that are expected to come into being within the next 6-12 months, but that are not yet final, are highlighted in yellow.  We hope these are final before we publish this document, but all of those dates are still rather uncertain.

Note that uplink is separate from downlink, that RF coding and modulation is separate from optical coding and modulation, and that SCCC and DVB-S2 (which both contain coding, modulation, and physical layer signaling in a single standard) are separated from the “normal” CCSDS standards that break these into three separate layers.  The new Variable Coding and Modulation (VCM) spec that is now in progress is also shown as a separate layer.  This VCM spec is related to the “bottom” parts of the DVB and SCCC specs, but it is different from them in distinct ways.

It became clear during discussion that most of those on the call were unfamiliar with the details and complexities represented in this table.  Furthermore, most are unfamiliar with the complexities inherent in the “3-layer sandwich” that SCCC and DVB present, and with how they compare with the “normal” CCSDS link layer, coding, synch, modulation, physical layer and RF stack.  I have attached a presentation that some of us constructed in order to make sure that we understood what those relationships are.  It is named “SEA high rate comm issue 1Mar21” and is attached here.  This is a statement of the recent issues and also a set of diagrams comparing these different protocol sets.  It does not address optical comm.

It should be noted that the “bottom” part of the DVB and SCCC specs includes a specialized set of physical layer signaling mechanisms.   These are not present in normal CCSDS protocol stacks, where any choices that are made for different coding, synchronization, and modulation combinations are made “by management”.  That phrase “by management” means that the mission manages these choices manually, outside of the protocols themselves, that the protocol layers contain no “signals” as to which choices were made, and that any changes to the coding and modulation must be agreed to and managed out of band, by pre-agreement.

In the DVB and SCCC, and in the new draft CCSDS VCM spec (CCSDS 431.1-b-1) which is attached here as a CESG draft spec, a physical layer signaling mechanism is introduced.  VCM is defined as “variable coded modulation, VCM: A method to adapt the transmission scheme to channel conditions following a predetermined schedule. ”.  This includes two separate physical layer structures:  1) the “Pilot Symbols” and 2) the encoded and modulated data symbols.  The CCSDS 431.1 spec describes two different VCM “types”.  Type 1 uses the DVB-S2 VCM pilot symbol and data symbol length approach, Type 2 uses the SCCC VCM pilot symbol and data symbol length approach.  These pilot symbols are, in both cases, just short blocks of 7 bits, protected by a linear code and BPSK modulation (see attached Table from Annex E).  Five of these bits are used to identify one of the 32 possible sets of code and modulation pairs that are applied to the encoded and modulated symbols that follow the pilot.

Where these DVB Type 1, SCCC Type 2, and CCSDS Type 1 or 2 schemes differ is in the length of the symbol strings and the sets of code/modulation pairs that are allowed.

  *   DVB-S2 has its own set shown in Table 3-4.  It allows different code rates, from 1 / 4 (0.25) up to 9 / 10 (0.9), different input lengths from 2992 up to 58112 bits, different modulations (QPSK, 8-PSK, 16 & 32-APSK) and its own set of DVB-S2 codes that are patented.
  *   SCCC has its own set shown in Table 3-3.  It allows different code rates, from 0.36 up to 0.9, different input lengths from 5758 up to 43678 bits, the same set of modulations (QPSK, 8-PSK, 16 & 32-APSK) and its own set of SCCC codes that are patented.
  *   The CCSDS VCM has its own set shown in Table 3-2.  It allows different (CCSDS standard) code rates, from 1 / 6 (0.16) up to 223/255 (0.875), different (CCSDS standard) input lengths from 1748 up to 16384 bits, the same set of modulations plus BPSK (BPSK, QPSK, 8-PSK, 16 & 32-APSK) and the standard LDPC codes.

You can see that these are similar, and that the modulation set largely overlaps, but they are different.  In all cases specialized equipment will be needed in the RF front ends to handle the pilot symbols and the continually changing coding and modulation .  The other difference is that the CCSDS VCM expects to signal a pre-planned set of code & modulation changes, but the SCCC and DVB-S2 also include adaptive coding and modulation (ACM), which uses signals sent back from the receiver to the sender.  To quote from SCCC, CCSDS 131x2b1d1, Sec 3.2.7:
NOTE –
Changes of the value of the information block size K are done by a system to adjust the modulation and coding schemes. This is achieved through, e.g., one of the following approaches: the ground receiver provides the signal quality estimation (or prediction) through a feedback channel (e.g., via telecommand) or the change of modulation and coding schemes is pre-scheduled for each satellite pass based on geometrical information (elevation angle).
So the SCCC may use a feedback loop, but no specific protocol appears to be specified for this.  The DVB-S2 standard, as adapted for CCSDS, makes essentially the same statement.   The full ETSI DVB-S2 spec, however, defines an actual feedback protocol that is, in my opinion, only of use over a near Earth (or at least a “local”) communications path where the RTLT is sufficiently short to allow requests  for data rate changes to be responded to.  This is not appropriate for use in deep space where the RTLT may be measures in 10’s of minutes or tens of hours.  They also bring substantial added complexity which, in the general case, may not be worth the added cost of engineering, testing, etc unless the mission is a) in a near Earth orbit, and b) can make use of available commercial parts.

As I suggested during the webex, I think we must treat the following groups of standards separately, because to do otherwise will overly complicate the core of the CCSDS standard suite, that I estimate meets 95% of the users.

  1.  The “CCSDS standard” suite of link layer, coding, synchronization, modulation, and RF standards
  2.  A subsection on the Optical coding and modulation standards that slot in underneath the normal link layer protocols, along with a brief description
  3.  A separate subsection on the VCM and the associated SCCC and DVB-S2 “omnibus” standards that replace the standard CCSDS coding, synchronization, modulation and add physical layer signaling.

If anyone has issues with this approach please bring them up now.  I think this is the only sensible way to handle this issue of these very different approaches to the lower layer protocols.

Thanks, Peter

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