[Sea-sa] Table 6-8: Background materials for today's SEA-SA SCCS-ARD discussion

Wed Mar 10 15:08:53 UTC 2021

Dear Peter,
        please find here attached a few comments on the two files 
addressing Table 6-8 and the “cheat sheet” of notes that encode the cells 
in this table.
About the "separations" you mentions, the following is understood (with no 
comment now):
A) uplink is separate from downlink ==> RAF, RCF, and ROCF columns refer 
to downlink, F-CLUTU and FF-CSTS columns refer to uplink
B) RF coding and modulation is separate from optical coding and modulation 
==> only one row addresses optical comms (coding + physical layers) while 
two rows are dedicated to TM Coding 131.0 and RFM 401.0
C) 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 ==> two rows 
are dedicated to SCCC and DVB-S2 standards
D) 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. ==> There is a dedicated row fro VCM (though the 
table seems to miss an orizontal line after RFM).

With respect to VCM (now published) it does not look correct to me to 
state that it is related to the “bottom” parts of the DVB and SCCC specs.
Actually it should be considered having an equivalent behaviour to SCCC 
and DVB-S2.
However let's look at the table row by row (more or less).

1) N4 statement is incorrect. RAF as specified today does not handle 
variable length frames. Even if formally the service provision could 
handle it, the production part is specified to support only fixed length 
frames.
2) AOS - it looks that the C2 here means OK unless lower layer is TC 
Coding. Would the shorter formulation referring to TC or a remark/note 
help the reader? Most likely that meaning is true for all occurrences of 
C2.
3) Fixed Length USLP - What is R3?
4) TC SDLP - the box for RAF shall be empty.
5) Variable Length USLP - currently not allowed for downlink. RAF cannot 
support it at the time being. What is the sense of C3 for RAF and RCF? 
Does anybody plan to use TC coding for the downlink of USLP variable 
length frames? What is R4?
6) Why the second sentence of N5 refers to RAF and not to CLTU?
7) VCM - Which O? I think that VCM can also use HOMs and should be treated 
as SCCC and DVB-S2

The reading and evaluation  of this Table is very painful and confusing 
and most likely other comments could easily be produced.

In addition to this, I would like to better focus some misunderstanding 
about the handling of VCM (intended as methods and not as 431.0-B)  and 
ACM.
The receiver does not change for VCM and ACM and does not depend on the 
duration of the time interval in which a ModCod is applied.
What changes is how the transmitter decides to apply changes of ModCod. A 
ModCod may remain constant for days. weeks, months (e.g. based on seasons, 
whether forecast, etc.)
In case of ACM, the expectation is for a process external to the receiver 
that - based on the SNR estimation etc, something provided by all 
receivers on the market - decides how to instruct the transmitter to 
change ModCod at intervals of second or (most likely) minutes. This kind 
of side process (i.e. external to the VCM/ACM mechanism per-se)  in a 
typical space to ground configuration may interact with the sending 
spacecraft e.g sending command with Forward Frame Service.

Best regards

Gian Paolo 

From:   "Shames, Peter M\(US 312B\) via SEA-SA" <sea-sa at mailman.ccsds.org>
To:     "SEA-SA" <sea-sa at mailman.ccsds.org>
Date:   02-03-21 23:37
Subject:        [Sea-sa] Background materials for today's SEA-SA SCCS-ARD 
discussion
Sent by:        "SEA-SA" <sea-sa-bounces at mailman.ccsds.org>

[attachment "431x1b0_CESG_Approval.pdf" deleted by Gian Paolo 
Calzolari/esoc/ESA] 

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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