gazagnaire.org / ocaml-scc
CCSDS Synchronization and Channel Coding (131.0-B, 231.0-B)
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README.md

ocaml-scc — CCSDS Synchronization and Channel Coding#

Pure OCaml implementation of the CCSDS Synchronization and Channel Coding sublayer per CCSDS 131.0-B (TM) and CCSDS 231.0-B (TC).

Overview#

The SCC sublayer sits between the Physical Layer and the Data Link Protocol sublayer. It handles frame synchronization and forward error correction:

   Space Packet / TC Frame / TM Frame
              |
   +----------v-----------+
   |  Data Link Protocol   |  ocaml-tc, ocaml-tm, ocaml-aos, ...
   +-----------+-----------+
              |
   +----------v-----------+
   |  Synchronization &    |  ** this package **
   |  Channel Coding (SCC) |
   +-----------+-----------+
              |
   Physical Layer (RF)

Modules#

Synchronization (Scc.Sync)#

Module Standard Description
Scc.Sync.Cltu 231.0-B CLTU encode/decode with BCH(63,56)
Scc.Sync.Asm 131.0-B ASM marker (0x1ACFFC1D) insert/detect
Scc.Sync.Cltu_sync 231.0-B Stream parser for CLTU extraction
Scc.Sync.Asm_sync 131.0-B Stream parser for ASM-framed data

Channel Coding (Scc.Coding)#

Module Standard Description
Scc.Coding.Randomizer 131.0-B §9 LFSR pseudo-random sequence
Scc.Coding.Reed_solomon 131.0-B §4 RS(255,223) with interleaving I=1..8
Scc.Coding.Convolutional 131.0-B §3 Rate 1/2, K=7 (G1=0x79, G2=0x5B)
Scc.Coding.Turbo 131.0-B §6 Parallel concatenated + BCJR/MAP
Scc.Coding.Ldpc 131.0-B §7 Belief propagation decoder

The generic algorithms live in standalone packages (ocaml-reed-solomon, ocaml-viterbi, ocaml-turbo, ocaml-ldpc). This package provides the CCSDS-specific parameterizations.

Installation#

Install with opam:

opam install scc scc-eio

If opam cannot find the packages, they may not yet be released in the public opam-repository. Add the overlay repository, then install them:

opam repo add samoht https://tangled.org/gazagnaire.org/opam-overlay.git
opam update
opam install scc scc-eio

Usage#

(* TC uplink: frame -> CLTU. *)
let uplink tc_frame = Scc.Sync.Cltu.encode tc_frame

(* TM downlink: randomize + RS encode + convolutional encode. *)
let downlink frame =
  let r = Scc.Coding.Randomizer.create () in
  Scc.Coding.Randomizer.apply r frame 0 (Bytes.length frame);
  let codeword = Scc.Coding.Reed_solomon.encode ~interleave:I5 frame in
  Scc.Coding.Convolutional.encode codeword

(* Stream parsing: extract CLTUs from raw bytes. *)
let feed_cltus state raw_bytes =
  let new_state, result = Scc.Sync.Cltu_sync.feed state raw_bytes in
  (new_state, result)

Eio Transport (scc-eio)#

The scc-eio package provides Eio flow-based send/recv:

let serve ~source ~sink ~process_tc tm_frame =
  (* Receive CLTU-framed TC from a raw byte stream. *)
  (match Scc_eio.Cltu.recv source with
  | Ok frame -> process_tc frame
  | Error `Closed -> ()
  | Error (`Transport _) -> Fmt.epr "transport error@."
  | Error (`Error msg) -> Fmt.epr "decode error: %s@." msg);
  (* Send ASM-framed TM. *)
  match Scc_eio.Asm.send sink tm_frame with
  | Ok () -> ()
  | Error (`Error msg) -> Fmt.epr "send error: %s@." msg