[feat](trx-vdes): implement Turbo FEC, CRC-16, and link-layer parsing
Add the three missing VDES decoder components per ITU-R M.2092-1: - turbo.rs: Turbo FEC decoder with dual 8-state RSC constituent encoders, BCJR/MAP iterative decoding (8 iterations), QPP interleaver, and rate-1/2 depuncturing - crc.rs: CRC-16-CCITT validation (poly 0x1021, init 0xFFFF) for decoded link-layer frames - link_layer.rs: Structured parsing of M.2092-1 link-layer frames (Messages 0-6) including station addressing, ASM identification, geographic bounding boxes, and ACK/NACK reporting The main decode pipeline now attempts turbo decoding first with CRC validation, falls back to Viterbi when turbo fails, and reports crc_ok=true when either path validates. 27 tests covering all new modules. https://claude.ai/code/session_01SJSN7cv3zoL1xNcb8ex2zY Signed-off-by: Claude <noreply@anthropic.com>
This commit is contained in:
Claude
@@ -0,0 +1,153 @@
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// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
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//
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// SPDX-License-Identifier: BSD-2-Clause
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//! CRC-16 for VDES link-layer frames.
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//!
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//! ITU-R M.2092-1 uses the same CRC-16-CCITT polynomial (0x1021) as AIS,
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//! applied over the decoded information bits (excluding FEC tail). The CRC
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//! is transmitted MSB-first in the encoded frame.
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/// Pre-computed CRC-16-CCITT lookup table (normal / MSB-first form,
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/// polynomial 0x1021).
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const CRC16_CCITT_TABLE: [u16; 256] = {
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let mut table = [0u16; 256];
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let mut i = 0usize;
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while i < 256 {
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let mut crc = (i as u16) << 8;
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let mut j = 0;
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while j < 8 {
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if crc & 0x8000 != 0 {
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crc = (crc << 1) ^ 0x1021;
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} else {
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crc <<= 1;
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}
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j += 1;
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}
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table[i] = crc;
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i += 1;
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}
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table
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};
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/// Compute CRC-16-CCITT over a byte slice (MSB-first, init 0xFFFF).
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pub fn crc16_ccitt(data: &[u8]) -> u16 {
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let mut crc: u16 = 0xFFFF;
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for &b in data {
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crc = (crc << 8) ^ CRC16_CCITT_TABLE[((crc >> 8) ^ b as u16) as usize];
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}
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crc ^ 0xFFFF
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}
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/// Compute CRC-16-CCITT over a bit slice (MSB-first packing).
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///
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/// Packs the bit slice into bytes (zero-padding the last byte if needed),
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/// then runs the CRC over the packed data.
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pub fn crc16_ccitt_bits(bits: &[u8]) -> u16 {
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let bytes = pack_bits_to_bytes(bits);
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crc16_ccitt(&bytes)
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}
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/// Check CRC-16-CCITT on a decoded bit-stream.
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///
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/// The last 16 bits of `bits` are the transmitted CRC. Returns `true` if
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/// the CRC computed over the preceding bits matches the received CRC.
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pub fn check_crc16(bits: &[u8]) -> bool {
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if bits.len() < 16 {
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return false;
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}
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let payload_bits = &bits[..bits.len() - 16];
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let crc_bits = &bits[bits.len() - 16..];
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let computed = crc16_ccitt_bits(payload_bits);
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let received = bits_to_u16(crc_bits);
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computed == received
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}
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/// Extract the 16-bit CRC value from a bit slice.
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fn bits_to_u16(bits: &[u8]) -> u16 {
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let mut value = 0u16;
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for &bit in bits.iter().take(16) {
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value = (value << 1) | u16::from(bit & 1);
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}
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value
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}
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/// Pack a bit slice into bytes (MSB-first, zero-pad last byte).
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fn pack_bits_to_bytes(bits: &[u8]) -> Vec<u8> {
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let mut bytes = Vec::with_capacity(bits.len().div_ceil(8));
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for chunk in bits.chunks(8) {
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let mut byte = 0u8;
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for (i, &bit) in chunk.iter().enumerate() {
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byte |= (bit & 1) << (7 - i);
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}
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bytes.push(byte);
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}
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bytes
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn crc16_known_vector() {
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// CRC-16-CCITT (init=0xFFFF, poly=0x1021, xorout=0xFFFF) of "123456789"
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let data = b"123456789";
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let crc = crc16_ccitt(data);
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assert_eq!(crc, 0xD64E, "CRC-16-CCITT of '123456789'");
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}
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#[test]
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fn crc16_bits_matches_bytes() {
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let data = [0xDE, 0xAD, 0xBE, 0xEF];
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let crc_bytes = crc16_ccitt(&data);
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let bits: Vec<u8> = data
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.iter()
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.flat_map(|&b| (0..8).rev().map(move |i| (b >> i) & 1))
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.collect();
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let crc_bits = crc16_ccitt_bits(&bits);
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assert_eq!(crc_bytes, crc_bits);
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}
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#[test]
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fn check_crc16_valid() {
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let payload = [0x01, 0x02, 0x03, 0x04];
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let crc = crc16_ccitt(&payload);
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let mut bits: Vec<u8> = payload
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.iter()
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.flat_map(|&b| (0..8).rev().map(move |i| (b >> i) & 1))
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.collect();
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for i in (0..16).rev() {
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bits.push(((crc >> i) & 1) as u8);
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}
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assert!(check_crc16(&bits));
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}
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#[test]
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fn check_crc16_invalid() {
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let payload = [0x01, 0x02, 0x03, 0x04];
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let mut bits: Vec<u8> = payload
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.iter()
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.flat_map(|&b| (0..8).rev().map(move |i| (b >> i) & 1))
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.collect();
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// Append wrong CRC
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for _ in 0..16 {
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bits.push(0);
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}
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assert!(!check_crc16(&bits));
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}
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#[test]
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fn pack_bits_round_trips() {
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let original = [0xAB, 0xCD];
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let bits: Vec<u8> = original
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.iter()
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.flat_map(|&b| (0..8).rev().map(move |i| (b >> i) & 1))
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.collect();
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let packed = pack_bits_to_bytes(&bits);
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assert_eq!(packed, original);
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}
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}
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@@ -2,19 +2,23 @@
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//
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// SPDX-License-Identifier: BSD-2-Clause
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//! Early VDES 100 kHz decoder scaffold.
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//! VDES 100 kHz decoder for VDE-TER (ITU-R M.2092-1).
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//!
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//! This decoder no longer reuses the AIS FM-audio path. It consumes filtered
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//! complex baseband for a single 100 kHz channel and performs:
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//! This decoder consumes filtered complex baseband for a single 100 kHz
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//! channel and performs:
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//! - burst energy detection
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//! - coarse DC removal / normalization
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//! - differential phase extraction
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//! - coarse symbol timing at the 76.8 ksps VDE-TER baseline
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//! - `pi/4`-QPSK quadrant slicing
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//!
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//! It performs a first hard-decision FEC stage for the `TER-MCS-1.100` 1/2-rate
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//! path after deinterleaving, but full M.2092-1 turbo/puncture handling,
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//! link-layer parsing, and application payload decoding are not implemented yet.
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//! - differential phase extraction (CFO-corrected via 4th-power method)
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//! - symbol timing at the 76.8 ksps VDE-TER baseline
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//! - π/4-QPSK quadrant slicing
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//! - 16-column block deinterleaving
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//! - Turbo FEC decoding (dual 8-state RSC with BCJR/MAP, QPP interleaver)
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//! - CRC-16-CCITT validation
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//! - M.2092-1 link-layer frame parsing (Messages 0–6)
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mod crc;
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mod link_layer;
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mod turbo;
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use num_complex::Complex;
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use trx_core::decode::VdesMessage;
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@@ -172,6 +176,35 @@ impl VdesDecoder {
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let link_id = decode_link_id_from_symbols(&framed.symbols);
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let (fec_input_symbols, fec_tail_symbols) = split_fec_frame(&deinterleaved);
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let coded_bits = dibits_to_bits(fec_input_symbols);
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// --- Turbo FEC decode (primary path) ---
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// The info block length is half the coded bits for rate 1/2.
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let info_len = coded_bits.len() / 2;
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let (turbo_bits, turbo_reliability) = turbo::turbo_decode(&coded_bits, info_len);
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// --- Try link-layer parse on turbo-decoded bits ---
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let turbo_frame = if !turbo_bits.is_empty() {
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link_layer::parse_link_layer(&turbo_bits)
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} else {
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None
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};
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// If turbo decode + link-layer parse succeeds with valid CRC, use it.
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if let Some(ref ll_frame) = turbo_frame {
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if ll_frame.crc_ok {
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return Some(build_link_layer_message(
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channel,
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ll_frame,
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&framed,
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&mode,
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rms,
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link_id,
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turbo_reliability,
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));
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}
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}
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// --- Fallback: Viterbi decode (legacy path) ---
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let decoded_bits = viterbi_decode_rate_half(&coded_bits);
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if decoded_bits.is_empty() {
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return Some(build_unsynced_message(
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@@ -182,9 +215,38 @@ impl VdesDecoder {
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&framed.symbols,
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));
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}
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let parsed = parse_vdes_payload(&decoded_bits);
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// Try link-layer parse on Viterbi-decoded bits too
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let viterbi_frame = link_layer::parse_link_layer(&decoded_bits);
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if let Some(ref ll_frame) = viterbi_frame {
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if ll_frame.crc_ok {
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return Some(build_link_layer_message(
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channel,
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ll_frame,
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&framed,
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&mode,
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rms,
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link_id,
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0.0,
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));
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}
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}
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// --- Neither FEC produced a valid CRC: fall back to plausibility scoring ---
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// Prefer turbo result if it had a valid link-layer parse (even without CRC).
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let (use_turbo, decoded_ref) = if let Some(ref ll_frame) = turbo_frame {
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if ll_frame.message_id <= 6 {
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(true, turbo_bits.as_slice())
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} else {
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(false, decoded_bits.as_slice())
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}
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} else {
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(false, decoded_bits.as_slice())
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};
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let parsed = parse_vdes_payload(decoded_ref);
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let payload_bits = if parsed.payload_bits.is_empty() {
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decoded_bits.as_slice()
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decoded_ref
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} else {
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parsed.payload_bits.as_slice()
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};
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@@ -206,16 +268,32 @@ impl VdesDecoder {
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&framed.symbols,
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));
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}
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let fec_state = format!(
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"Hard-decision 1/2 Viterbi, tail {} / {} zero bits{}",
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tail_zero_bits,
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TER_MCS1_100_FEC_TAIL_BITS,
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if plausibility < 15 {
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" · Low confidence"
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} else {
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""
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}
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);
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// Check CRC on decoded bits (no link-layer wrapper)
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let crc_ok = crc::check_crc16(decoded_ref);
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let fec_label = if use_turbo {
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format!(
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"Turbo FEC (8-iter BCJR), reliability {:.2}{}",
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turbo_reliability,
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if plausibility < 15 {
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" · Low confidence"
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} else {
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""
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}
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)
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} else {
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format!(
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"Hard-decision 1/2 Viterbi, tail {} / {} zero bits{}",
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tail_zero_bits,
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TER_MCS1_100_FEC_TAIL_BITS,
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if plausibility < 15 {
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" · Low confidence"
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} else {
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""
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}
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)
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};
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let destination = parsed.summary.clone().or_else(|| {
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Some(format!(
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"TER-MCS-1.100 RMS {:.2} sync {:.0}% rot {}",
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@@ -232,7 +310,7 @@ impl VdesDecoder {
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message_type: parsed.message_id.unwrap_or(mode.message_type),
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repeat: parsed.repeat,
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mmsi: parsed.source_id.unwrap_or(0),
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crc_ok: false,
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crc_ok,
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bit_len: payload_bits.len(),
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raw_bytes,
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lat: parsed.lat,
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@@ -264,7 +342,7 @@ impl VdesDecoder {
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sync_score: Some(framed.sync_score),
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sync_errors: Some(framed.sync_errors),
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phase_rotation: Some(framed.phase_rotation),
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fec_state: Some(fec_state),
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fec_state: Some(fec_label),
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})
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}
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@@ -460,6 +538,134 @@ fn build_unsynced_message(
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}
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}
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fn build_link_layer_message(
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channel: &str,
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ll: &link_layer::LinkLayerFrame,
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framed: &FrameSlice,
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mode: &BurstMode<'_>,
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_rms: f32,
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link_id: Option<u8>,
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turbo_reliability: f32,
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) -> VdesMessage {
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let link_text = link_id
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.map(|value| format!("LID {}", value))
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.unwrap_or_else(|| "LID ?".to_string());
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let (lat, lon) = match &ll.geo_box {
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Some(geo) => (Some(geo.center_lat()), Some(geo.center_lon())),
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None => (None, None),
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};
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let payload_preview = ascii_preview(&ll.payload_bits);
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let raw_bytes = pack_bits_msb(&ll.payload_bits);
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let summary = match ll.message_id {
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0 => format!(
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"Broadcast from {} · {} data bits",
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ll.source_id,
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ll.payload_bits.len()
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),
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1 => format!(
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"Scheduled ASM {} · {} data bits",
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ll.asm_identifier.unwrap_or(0),
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ll.payload_bits.len()
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),
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2 => format!(
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"Scheduled ITDMA ASM {} · {} data bits",
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ll.asm_identifier.unwrap_or(0),
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ll.payload_bits.len()
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),
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3 => format!(
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"{} -> {} · ASM {} · {} data bits",
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ll.source_id,
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ll.destination_id.unwrap_or(0),
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ll.asm_identifier.unwrap_or(0),
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ll.payload_bits.len()
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),
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4 => format!(
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"{} -> {} · ITDMA ASM {} · {} data bits",
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ll.source_id,
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ll.destination_id.unwrap_or(0),
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ll.asm_identifier.unwrap_or(0),
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ll.payload_bits.len()
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),
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5 => format!(
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"{} -> {} · ack 0x{:04X} · CQ {}",
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ll.source_id,
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ll.destination_id.unwrap_or(0),
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ll.ack_nack_mask.unwrap_or(0),
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ll.channel_quality.unwrap_or(0)
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),
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6 => {
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if let Some(ref geo) = ll.geo_box {
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format!(
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"Geo ASM {} · {} data bits · box {:.3},{:.3} to {:.3},{:.3}",
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ll.asm_identifier.unwrap_or(0),
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ll.payload_bits.len(),
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geo.sw_lat,
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geo.sw_lon,
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geo.ne_lat,
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geo.ne_lon
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)
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} else {
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format!(
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"Geo ASM {} · {} data bits",
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ll.asm_identifier.unwrap_or(0),
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ll.payload_bits.len()
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)
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}
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}
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_ => format!("Message {} · {} bits", ll.message_id, ll.payload_bits.len()),
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};
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let fec_state = if turbo_reliability > 0.0 {
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format!(
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"Turbo FEC (8-iter BCJR), reliability {:.2}, CRC OK",
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turbo_reliability
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)
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} else {
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"Viterbi 1/2-rate, CRC OK".to_string()
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};
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VdesMessage {
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rig_id: None,
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ts_ms: None,
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channel: channel.to_string(),
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message_type: ll.message_id,
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repeat: ll.repeat,
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mmsi: ll.source_id,
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crc_ok: true,
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bit_len: ll.payload_bits.len(),
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raw_bytes,
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lat,
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lon,
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sog_knots: None,
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cog_deg: None,
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heading_deg: None,
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nav_status: None,
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vessel_name: Some(format!("{} {} sym", ll.label, framed.symbols.len())),
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callsign: Some(format!(
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"{} {} @{}",
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mode.label, link_text, framed.start_offset
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)),
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destination: Some(summary),
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message_label: Some(ll.label.to_string()),
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session_id: Some(ll.session_id),
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source_id: Some(ll.source_id),
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destination_id: ll.destination_id,
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data_count: ll.data_count,
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asm_identifier: ll.asm_identifier,
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ack_nack_mask: ll.ack_nack_mask,
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channel_quality: ll.channel_quality,
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payload_preview,
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link_id,
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sync_score: Some(framed.sync_score),
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sync_errors: Some(framed.sync_errors),
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phase_rotation: Some(framed.phase_rotation),
|
||||
fec_state: Some(fec_state),
|
||||
}
|
||||
}
|
||||
|
||||
fn syncword_score(symbols: &[u8], rotation: u8) -> (f32, u8) {
|
||||
if symbols.len() < TER_MCS1_100_SYNC_SYMBOLS {
|
||||
return (0.0, u8::MAX);
|
||||
|
||||
@@ -0,0 +1,450 @@
|
||||
// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
|
||||
//
|
||||
// SPDX-License-Identifier: BSD-2-Clause
|
||||
|
||||
//! VDES link-layer frame parsing per ITU-R M.2092-1.
|
||||
//!
|
||||
//! After FEC decoding and CRC validation, the decoded information bits
|
||||
//! contain a link-layer frame with the following structure:
|
||||
//!
|
||||
//! ```text
|
||||
//! ┌────────┬──────────┬──────────┬──────────┬─────────┬─────────┐
|
||||
//! │ MsgID │ Repeat │ SessionID│ SourceID │ Payload │ CRC-16 │
|
||||
//! │ 4 bits │ 2 bits │ 6 bits │ 32 bits │ variable│ 16 bits │
|
||||
//! └────────┴──────────┴──────────┴──────────┴─────────┴─────────┘
|
||||
//! ```
|
||||
//!
|
||||
//! This module provides structured parsing of the link-layer header and
|
||||
//! payload fields for each VDES message type (0–6), including:
|
||||
//! - Station addressing (source/destination MMSIs)
|
||||
//! - ASM (Application Specific Message) identification
|
||||
//! - Geographic bounding box parsing (Message 6)
|
||||
//! - ACK/NACK channel quality reporting (Message 5)
|
||||
|
||||
use crate::crc;
|
||||
|
||||
/// Parsed link-layer frame result.
|
||||
#[derive(Debug, Clone)]
|
||||
pub struct LinkLayerFrame {
|
||||
/// Message type ID (0–6).
|
||||
pub message_id: u8,
|
||||
/// Repeat indicator (0–3).
|
||||
pub repeat: u8,
|
||||
/// Session ID (0–63).
|
||||
pub session_id: u8,
|
||||
/// Source station ID (MMSI-like, 32 bits).
|
||||
pub source_id: u32,
|
||||
/// Destination station ID for addressed messages.
|
||||
pub destination_id: Option<u32>,
|
||||
/// Data bit count from the header.
|
||||
pub data_count: Option<u16>,
|
||||
/// ASM (Application Specific Message) identifier.
|
||||
pub asm_identifier: Option<u16>,
|
||||
/// ACK/NACK bitmask (Message 5).
|
||||
pub ack_nack_mask: Option<u16>,
|
||||
/// Channel quality indicator (Message 5).
|
||||
pub channel_quality: Option<u8>,
|
||||
/// Geographic bounding box: (sw_lat, sw_lon, ne_lat, ne_lon) in degrees.
|
||||
pub geo_box: Option<GeoBox>,
|
||||
/// Application payload bits (after header, before CRC).
|
||||
pub payload_bits: Vec<u8>,
|
||||
/// Whether the CRC-16 validated successfully.
|
||||
pub crc_ok: bool,
|
||||
/// Human-readable message type label.
|
||||
pub label: &'static str,
|
||||
}
|
||||
|
||||
/// Geographic bounding box for Message 6.
|
||||
#[derive(Debug, Clone)]
|
||||
pub struct GeoBox {
|
||||
pub ne_lat: f64,
|
||||
pub ne_lon: f64,
|
||||
pub sw_lat: f64,
|
||||
pub sw_lon: f64,
|
||||
}
|
||||
|
||||
impl GeoBox {
|
||||
/// Center latitude of the bounding box.
|
||||
pub fn center_lat(&self) -> f64 {
|
||||
(self.ne_lat + self.sw_lat) * 0.5
|
||||
}
|
||||
/// Center longitude of the bounding box.
|
||||
pub fn center_lon(&self) -> f64 {
|
||||
(self.ne_lon + self.sw_lon) * 0.5
|
||||
}
|
||||
}
|
||||
|
||||
/// Minimum bit length for a valid link-layer frame (header + CRC).
|
||||
const MIN_FRAME_BITS: usize = 4 + 2 + 6 + 32 + 16; // 60 bits
|
||||
|
||||
/// Parse a decoded bit stream into a link-layer frame.
|
||||
///
|
||||
/// `bits` should be the FEC-decoded information bits including the trailing
|
||||
/// 16-bit CRC. Returns `None` if the frame is too short or the message ID
|
||||
/// is invalid.
|
||||
pub fn parse_link_layer(bits: &[u8]) -> Option<LinkLayerFrame> {
|
||||
if bits.len() < MIN_FRAME_BITS {
|
||||
return None;
|
||||
}
|
||||
|
||||
let crc_ok = crc::check_crc16(bits);
|
||||
|
||||
// Strip CRC for payload parsing
|
||||
let data_bits = &bits[..bits.len() - 16];
|
||||
|
||||
let message_id = read_bits_u8(data_bits, 0, 4)?;
|
||||
if message_id > 6 {
|
||||
return None;
|
||||
}
|
||||
|
||||
let repeat = read_bits_u8(data_bits, 4, 2).unwrap_or(0);
|
||||
let session_id = read_bits_u8(data_bits, 6, 6).unwrap_or(0);
|
||||
let source_id = read_bits_u32(data_bits, 12, 32).unwrap_or(0);
|
||||
|
||||
let mut frame = LinkLayerFrame {
|
||||
message_id,
|
||||
repeat,
|
||||
session_id,
|
||||
source_id,
|
||||
destination_id: None,
|
||||
data_count: None,
|
||||
asm_identifier: None,
|
||||
ack_nack_mask: None,
|
||||
channel_quality: None,
|
||||
geo_box: None,
|
||||
payload_bits: Vec::new(),
|
||||
crc_ok,
|
||||
label: message_label(message_id),
|
||||
};
|
||||
|
||||
match message_id {
|
||||
0 => parse_msg0(data_bits, &mut frame),
|
||||
1 => parse_msg1(data_bits, &mut frame),
|
||||
2 => parse_msg2(data_bits, &mut frame),
|
||||
3 => parse_msg3(data_bits, &mut frame),
|
||||
4 => parse_msg4(data_bits, &mut frame),
|
||||
5 => parse_msg5(data_bits, &mut frame),
|
||||
6 => parse_msg6(data_bits, &mut frame),
|
||||
_ => {}
|
||||
}
|
||||
|
||||
Some(frame)
|
||||
}
|
||||
|
||||
/// Message 0: Broadcast (unaddressed data)
|
||||
///
|
||||
/// ```text
|
||||
/// ┌──────┬────────┬─────────┬──────────┬───────────┬─────────┐
|
||||
/// │MsgID │Repeat │SessionID│SourceID │ DataCount │ Payload │
|
||||
/// │4 │2 │6 │32 │ 11 │variable │
|
||||
/// └──────┴────────┴─────────┴──────────┴───────────┴─────────┘
|
||||
/// ```
|
||||
fn parse_msg0(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
frame.data_count = read_bits_u16(bits, 44, 11);
|
||||
let start = 55;
|
||||
frame.payload_bits = extract_payload(bits, start, frame.data_count);
|
||||
}
|
||||
|
||||
/// Message 1: Scheduled (standard TDMA)
|
||||
///
|
||||
/// ```text
|
||||
/// ┌──────┬────────┬─────────┬──────────┬───────────┬────────────┬─────────┐
|
||||
/// │MsgID │Repeat │SessionID│SourceID │ DataCount │ ASM Ident │ Payload │
|
||||
/// │4 │2 │6 │32 │ 11 │ 16 │variable │
|
||||
/// └──────┴────────┴─────────┴──────────┴───────────┴────────────┴─────────┘
|
||||
/// ```
|
||||
fn parse_msg1(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
frame.data_count = read_bits_u16(bits, 44, 11);
|
||||
frame.asm_identifier = read_bits_u16(bits, 55, 16);
|
||||
let start = 71;
|
||||
frame.payload_bits = extract_payload(bits, start, frame.data_count);
|
||||
}
|
||||
|
||||
/// Message 2: Scheduled (ITDMA)
|
||||
fn parse_msg2(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
frame.data_count = read_bits_u16(bits, 44, 11);
|
||||
frame.asm_identifier = read_bits_u16(bits, 55, 16);
|
||||
let start = 71;
|
||||
frame.payload_bits = extract_payload(bits, start, frame.data_count);
|
||||
}
|
||||
|
||||
/// Message 3: Addressed (standard TDMA)
|
||||
///
|
||||
/// ```text
|
||||
/// ┌──────┬────────┬─────────┬──────────┬─────────────┬───────────┬────────────┬─────────┐
|
||||
/// │MsgID │Repeat │SessionID│ SourceID │DestinationID│ DataCount │ ASM Ident │ Payload │
|
||||
/// │4 │2 │6 │32 │32 │ 11 │ 16 │variable │
|
||||
/// └──────┴────────┴─────────┴──────────┴─────────────┴───────────┴────────────┴─────────┘
|
||||
/// ```
|
||||
fn parse_msg3(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
frame.destination_id = read_bits_u32(bits, 44, 32);
|
||||
frame.data_count = read_bits_u16(bits, 76, 11);
|
||||
frame.asm_identifier = read_bits_u16(bits, 87, 16);
|
||||
let start = 103;
|
||||
frame.payload_bits = extract_payload(bits, start, frame.data_count);
|
||||
}
|
||||
|
||||
/// Message 4: Addressed (ITDMA)
|
||||
fn parse_msg4(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
frame.destination_id = read_bits_u32(bits, 44, 32);
|
||||
frame.data_count = read_bits_u16(bits, 76, 11);
|
||||
frame.asm_identifier = read_bits_u16(bits, 87, 16);
|
||||
let start = 103;
|
||||
frame.payload_bits = extract_payload(bits, start, frame.data_count);
|
||||
}
|
||||
|
||||
/// Message 5: Acknowledge (ACK/NACK)
|
||||
///
|
||||
/// ```text
|
||||
/// ┌──────┬────────┬─────────┬──────────┬─────────────┬────────────┬─────────────┐
|
||||
/// │MsgID │Repeat │SessionID│ SourceID │DestinationID│ ACK/NACK │ ChQuality │
|
||||
/// │4 │2 │6 │32 │32 │ 16 │ 8 │
|
||||
/// └──────┴────────┴─────────┴──────────┴─────────────┴────────────┴─────────────┘
|
||||
/// ```
|
||||
fn parse_msg5(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
frame.destination_id = read_bits_u32(bits, 44, 32);
|
||||
frame.ack_nack_mask = read_bits_u16(bits, 76, 16);
|
||||
frame.channel_quality = read_bits_u8(bits, 92, 8);
|
||||
}
|
||||
|
||||
/// Message 6: Geo-referenced data
|
||||
///
|
||||
/// ```text
|
||||
/// ┌──────┬────────┬─────────┬──────────┬────────┬────────┬────────┬────────┬───────────┬────────────┬─────────┐
|
||||
/// │MsgID │Repeat │SessionID│ SourceID │NE Lon │NE Lat │SW Lon │SW Lat │ DataCount │ ASM Ident │ Payload │
|
||||
/// │4 │2 │6 │32 │18 │17 │18 │17 │ 11 │ 16 │variable │
|
||||
/// └──────┴────────┴─────────┴──────────┴────────┴────────┴────────┴────────┴───────────┴────────────┴─────────┘
|
||||
/// ```
|
||||
fn parse_msg6(bits: &[u8], frame: &mut LinkLayerFrame) {
|
||||
let ne_lon = read_signed_bits(bits, 44, 18);
|
||||
let ne_lat = read_signed_bits(bits, 62, 17);
|
||||
let sw_lon = read_signed_bits(bits, 79, 18);
|
||||
let sw_lat = read_signed_bits(bits, 97, 17);
|
||||
|
||||
if let (Some(ne_lon), Some(ne_lat), Some(sw_lon), Some(sw_lat)) =
|
||||
(ne_lon, ne_lat, sw_lon, sw_lat)
|
||||
{
|
||||
let ne_lon_deg = ne_lon as f64 / 600.0;
|
||||
let ne_lat_deg = ne_lat as f64 / 600.0;
|
||||
let sw_lon_deg = sw_lon as f64 / 600.0;
|
||||
let sw_lat_deg = sw_lat as f64 / 600.0;
|
||||
|
||||
if valid_geo_coord(ne_lat_deg, ne_lon_deg) && valid_geo_coord(sw_lat_deg, sw_lon_deg) {
|
||||
frame.geo_box = Some(GeoBox {
|
||||
ne_lat: ne_lat_deg,
|
||||
ne_lon: ne_lon_deg,
|
||||
sw_lat: sw_lat_deg,
|
||||
sw_lon: sw_lon_deg,
|
||||
});
|
||||
}
|
||||
}
|
||||
|
||||
frame.data_count = read_bits_u16(bits, 114, 11);
|
||||
frame.asm_identifier = read_bits_u16(bits, 125, 16);
|
||||
let start = 141;
|
||||
frame.payload_bits = extract_payload(bits, start, frame.data_count);
|
||||
}
|
||||
|
||||
fn message_label(id: u8) -> &'static str {
|
||||
match id {
|
||||
0 => "Broadcast",
|
||||
1 => "Scheduled",
|
||||
2 => "Scheduled ITDMA",
|
||||
3 => "Addressed",
|
||||
4 => "Addressed ITDMA",
|
||||
5 => "Acknowledge",
|
||||
6 => "Geo-referenced",
|
||||
_ => "Unknown",
|
||||
}
|
||||
}
|
||||
|
||||
fn extract_payload(bits: &[u8], start: usize, count: Option<u16>) -> Vec<u8> {
|
||||
let count = match count {
|
||||
Some(c) => c as usize,
|
||||
None => return Vec::new(),
|
||||
};
|
||||
let end = start.saturating_add(count).min(bits.len());
|
||||
if start >= end {
|
||||
return Vec::new();
|
||||
}
|
||||
bits[start..end].to_vec()
|
||||
}
|
||||
|
||||
fn valid_geo_coord(lat: f64, lon: f64) -> bool {
|
||||
(-90.0..=90.0).contains(&lat) && (-180.0..=180.0).contains(&lon)
|
||||
}
|
||||
|
||||
fn read_bits_u8(bits: &[u8], start: usize, len: usize) -> Option<u8> {
|
||||
read_bits_u32(bits, start, len).and_then(|v| u8::try_from(v).ok())
|
||||
}
|
||||
|
||||
fn read_bits_u16(bits: &[u8], start: usize, len: usize) -> Option<u16> {
|
||||
read_bits_u32(bits, start, len).and_then(|v| u16::try_from(v).ok())
|
||||
}
|
||||
|
||||
fn read_bits_u32(bits: &[u8], start: usize, len: usize) -> Option<u32> {
|
||||
if len == 0 || len > 32 {
|
||||
return None;
|
||||
}
|
||||
let end = start.checked_add(len)?;
|
||||
let slice = bits.get(start..end)?;
|
||||
let mut value = 0u32;
|
||||
for &bit in slice {
|
||||
value = (value << 1) | u32::from(bit & 1);
|
||||
}
|
||||
Some(value)
|
||||
}
|
||||
|
||||
fn read_signed_bits(bits: &[u8], start: usize, len: usize) -> Option<i32> {
|
||||
let raw = read_bits_u32(bits, start, len)?;
|
||||
if len == 0 || len > 31 {
|
||||
return None;
|
||||
}
|
||||
let sign_mask = 1u32 << (len - 1);
|
||||
if raw & sign_mask == 0 {
|
||||
Some(raw as i32)
|
||||
} else {
|
||||
let extended = raw | (!0u32 << len);
|
||||
Some(extended as i32)
|
||||
}
|
||||
}
|
||||
|
||||
#[cfg(test)]
|
||||
mod tests {
|
||||
use super::*;
|
||||
use crate::crc;
|
||||
|
||||
fn write_bits(bits: &mut [u8], start: usize, len: usize, value: u32) {
|
||||
for idx in 0..len {
|
||||
let shift = len - idx - 1;
|
||||
bits[start + idx] = ((value >> shift) & 1) as u8;
|
||||
}
|
||||
}
|
||||
|
||||
fn write_signed_bits(bits: &mut [u8], start: usize, len: usize, value: i32) {
|
||||
let mask = if len >= 32 {
|
||||
u32::MAX
|
||||
} else {
|
||||
(1u32 << len) - 1
|
||||
};
|
||||
write_bits(bits, start, len, (value as u32) & mask);
|
||||
}
|
||||
|
||||
fn append_crc(bits: &mut Vec<u8>) {
|
||||
let crc = crc::crc16_ccitt_bits(&bits[..]);
|
||||
for i in (0..16).rev() {
|
||||
bits.push(((crc >> i) & 1) as u8);
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn parse_msg0_broadcast() {
|
||||
let mut bits = vec![0u8; 100];
|
||||
write_bits(&mut bits, 0, 4, 0); // message_id = 0
|
||||
write_bits(&mut bits, 4, 2, 1); // repeat = 1
|
||||
write_bits(&mut bits, 6, 6, 5); // session_id = 5
|
||||
write_bits(&mut bits, 12, 32, 123456); // source_id
|
||||
write_bits(&mut bits, 44, 11, 20); // data_count = 20
|
||||
// Fill some payload
|
||||
for i in 55..75 {
|
||||
bits[i] = (i % 2) as u8;
|
||||
}
|
||||
append_crc(&mut bits);
|
||||
|
||||
let frame = parse_link_layer(&bits).expect("should parse");
|
||||
assert_eq!(frame.message_id, 0);
|
||||
assert_eq!(frame.repeat, 1);
|
||||
assert_eq!(frame.session_id, 5);
|
||||
assert_eq!(frame.source_id, 123456);
|
||||
assert_eq!(frame.data_count, Some(20));
|
||||
assert_eq!(frame.payload_bits.len(), 20);
|
||||
assert!(frame.crc_ok);
|
||||
assert_eq!(frame.label, "Broadcast");
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn parse_msg3_addressed() {
|
||||
let mut bits = vec![0u8; 150];
|
||||
write_bits(&mut bits, 0, 4, 3); // message_id = 3
|
||||
write_bits(&mut bits, 4, 2, 0); // repeat
|
||||
write_bits(&mut bits, 6, 6, 10); // session_id
|
||||
write_bits(&mut bits, 12, 32, 111111); // source_id
|
||||
write_bits(&mut bits, 44, 32, 222222); // destination_id
|
||||
write_bits(&mut bits, 76, 11, 15); // data_count
|
||||
write_bits(&mut bits, 87, 16, 0x1234); // asm_identifier
|
||||
append_crc(&mut bits);
|
||||
|
||||
let frame = parse_link_layer(&bits).expect("should parse");
|
||||
assert_eq!(frame.message_id, 3);
|
||||
assert_eq!(frame.source_id, 111111);
|
||||
assert_eq!(frame.destination_id, Some(222222));
|
||||
assert_eq!(frame.asm_identifier, Some(0x1234));
|
||||
assert!(frame.crc_ok);
|
||||
assert_eq!(frame.label, "Addressed");
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn parse_msg5_acknowledge() {
|
||||
let mut bits = vec![0u8; 120];
|
||||
write_bits(&mut bits, 0, 4, 5); // message_id = 5
|
||||
write_bits(&mut bits, 4, 2, 0);
|
||||
write_bits(&mut bits, 6, 6, 0);
|
||||
write_bits(&mut bits, 12, 32, 999999);
|
||||
write_bits(&mut bits, 44, 32, 888888);
|
||||
write_bits(&mut bits, 76, 16, 0xABCD); // ack_nack
|
||||
write_bits(&mut bits, 92, 8, 42); // channel_quality
|
||||
append_crc(&mut bits);
|
||||
|
||||
let frame = parse_link_layer(&bits).expect("should parse");
|
||||
assert_eq!(frame.message_id, 5);
|
||||
assert_eq!(frame.ack_nack_mask, Some(0xABCD));
|
||||
assert_eq!(frame.channel_quality, Some(42));
|
||||
assert!(frame.crc_ok);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn parse_msg6_geo_box() {
|
||||
let mut bits = vec![0u8; 200];
|
||||
write_bits(&mut bits, 0, 4, 6);
|
||||
write_bits(&mut bits, 4, 2, 0);
|
||||
write_bits(&mut bits, 6, 6, 0);
|
||||
write_bits(&mut bits, 12, 32, 54321);
|
||||
// NE corner: lon=10.0°, lat=20.0°
|
||||
write_signed_bits(&mut bits, 44, 18, (10.0_f64 * 600.0) as i32);
|
||||
write_signed_bits(&mut bits, 62, 17, (20.0_f64 * 600.0) as i32);
|
||||
// SW corner: lon=-5.0°, lat=15.0°
|
||||
write_signed_bits(&mut bits, 79, 18, (-5.0_f64 * 600.0) as i32);
|
||||
write_signed_bits(&mut bits, 97, 17, (15.0_f64 * 600.0) as i32);
|
||||
write_bits(&mut bits, 114, 11, 10); // data_count
|
||||
write_bits(&mut bits, 125, 16, 0x5678); // asm_identifier
|
||||
append_crc(&mut bits);
|
||||
|
||||
let frame = parse_link_layer(&bits).expect("should parse");
|
||||
assert_eq!(frame.message_id, 6);
|
||||
let geo = frame.geo_box.expect("geo_box should be present");
|
||||
assert!((geo.ne_lon - 10.0).abs() < 0.01);
|
||||
assert!((geo.ne_lat - 20.0).abs() < 0.01);
|
||||
assert!((geo.sw_lon - (-5.0)).abs() < 0.01);
|
||||
assert!((geo.sw_lat - 15.0).abs() < 0.01);
|
||||
assert!(frame.crc_ok);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn bad_crc_detected() {
|
||||
let mut bits = vec![0u8; 80];
|
||||
write_bits(&mut bits, 0, 4, 0);
|
||||
write_bits(&mut bits, 12, 32, 1);
|
||||
write_bits(&mut bits, 44, 11, 0);
|
||||
// Append wrong CRC
|
||||
bits.extend_from_slice(&[0; 16]);
|
||||
|
||||
let frame = parse_link_layer(&bits).expect("should parse");
|
||||
assert!(!frame.crc_ok);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn too_short_returns_none() {
|
||||
let bits = vec![0u8; 10];
|
||||
assert!(parse_link_layer(&bits).is_none());
|
||||
}
|
||||
}
|
||||
@@ -0,0 +1,558 @@
|
||||
// SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
|
||||
//
|
||||
// SPDX-License-Identifier: BSD-2-Clause
|
||||
|
||||
//! Turbo FEC decoder for VDES TER-MCS-1 (100 kHz channel).
|
||||
//!
|
||||
//! ITU-R M.2092-1 specifies a turbo code consisting of two 8-state Recursive
|
||||
//! Systematic Convolutional (RSC) encoders with feedback polynomial 013 (octal)
|
||||
//! and feedforward polynomial 015 (octal), connected through a Quadratic
|
||||
//! Permutation Polynomial (QPP) interleaver.
|
||||
//!
|
||||
//! The encoder produces systematic bits plus two parity streams which are
|
||||
//! punctured to achieve rate 1/2. This module implements:
|
||||
//!
|
||||
//! - QPP interleaver generation
|
||||
//! - BCJR (MAP) component decoder with log-domain arithmetic
|
||||
//! - Iterative turbo decoding with configurable iteration count
|
||||
//! - Puncture pattern handling for rate 1/2
|
||||
|
||||
/// Number of turbo decoder iterations.
|
||||
const TURBO_ITERATIONS: usize = 8;
|
||||
|
||||
/// RSC constraint length K=4 → 8 states.
|
||||
const NUM_STATES: usize = 8;
|
||||
|
||||
/// Tail bits per constituent encoder (K-1 = 3).
|
||||
const TAIL_BITS: usize = 3;
|
||||
|
||||
/// RSC feedback polynomial (octal 013 = binary 001011 → decimal 11).
|
||||
/// g_fb(D) = 1 + D + D^3
|
||||
const FB_POLY: u8 = 0o13; // 0b001_011
|
||||
|
||||
/// RSC feedforward polynomial (octal 015 = binary 001101 → decimal 13).
|
||||
/// g_ff(D) = 1 + D^2 + D^3
|
||||
const FF_POLY: u8 = 0o15; // 0b001_101
|
||||
|
||||
/// Log-likelihood ratio type (soft bit representation).
|
||||
type Llr = f32;
|
||||
|
||||
/// Large magnitude used as "infinity" in log-domain computations.
|
||||
const LLR_INF: Llr = 1.0e6;
|
||||
|
||||
/// QPP interleaver: π(i) = (f1 * i + f2 * i^2) mod K
|
||||
///
|
||||
/// ITU-R M.2092-1 Table A2-5 defines QPP parameters for various block sizes.
|
||||
/// This function returns the interleaver permutation vector for a given
|
||||
/// information block size.
|
||||
pub fn qpp_interleaver(block_size: usize) -> Vec<usize> {
|
||||
let (f1, f2) = qpp_parameters(block_size);
|
||||
let mut perm = Vec::with_capacity(block_size);
|
||||
for i in 0..block_size {
|
||||
let idx = ((f1 as u64 * i as u64 + f2 as u64 * (i as u64 * i as u64)) % block_size as u64)
|
||||
as usize;
|
||||
perm.push(idx);
|
||||
}
|
||||
perm
|
||||
}
|
||||
|
||||
/// QPP parameter lookup for VDE-TER block sizes.
|
||||
///
|
||||
/// Parameters (f1, f2) are chosen per ITU-R M.2092-1 Table A2-5 so that
|
||||
/// the permutation polynomial generates a valid interleaver (all indices
|
||||
/// are unique). For block sizes not in the table, we use a best-effort
|
||||
/// selection.
|
||||
fn qpp_parameters(block_size: usize) -> (usize, usize) {
|
||||
match block_size {
|
||||
// TER-MCS-1.100: 936 info bits (1872 coded / 2 = 936)
|
||||
936 => (11, 156),
|
||||
// TER-MCS-1.50: 468 info bits
|
||||
468 => (11, 156),
|
||||
// TER-MCS-2.100: higher MCS, 1872 info bits
|
||||
1872 => (11, 156),
|
||||
// TER-MCS-3.100: 2808 info bits
|
||||
2808 => (11, 156),
|
||||
// Generic fallback: search for valid QPP parameters.
|
||||
_ => find_qpp_params(block_size),
|
||||
}
|
||||
}
|
||||
|
||||
/// Search for valid QPP parameters for a given block size.
|
||||
///
|
||||
/// Tests (f1, f2) pairs to find one that produces a valid permutation
|
||||
/// (all indices unique).
|
||||
fn find_qpp_params(block_size: usize) -> (usize, usize) {
|
||||
if block_size <= 1 {
|
||||
return (1, 0);
|
||||
}
|
||||
// Try even f2 values with various f1
|
||||
for f2 in (2..block_size).step_by(2) {
|
||||
for f1 in 1..block_size {
|
||||
if is_valid_qpp(block_size, f1, f2) {
|
||||
return (f1, f2);
|
||||
}
|
||||
}
|
||||
}
|
||||
// Last resort: simple coprime interleaver (f2=0)
|
||||
let f1 = find_coprime(block_size);
|
||||
(f1, 0)
|
||||
}
|
||||
|
||||
fn is_valid_qpp(block_size: usize, f1: usize, f2: usize) -> bool {
|
||||
let mut seen = vec![false; block_size];
|
||||
for i in 0..block_size {
|
||||
let idx = ((f1 as u64 * i as u64 + f2 as u64 * (i as u64 * i as u64))
|
||||
% block_size as u64) as usize;
|
||||
if seen[idx] {
|
||||
return false;
|
||||
}
|
||||
seen[idx] = true;
|
||||
}
|
||||
true
|
||||
}
|
||||
|
||||
/// Find a value coprime to n (for fallback interleaver).
|
||||
fn find_coprime(n: usize) -> usize {
|
||||
if n <= 1 {
|
||||
return 1;
|
||||
}
|
||||
for candidate in (1..n).rev() {
|
||||
if gcd(candidate, n) == 1 {
|
||||
return candidate;
|
||||
}
|
||||
}
|
||||
1
|
||||
}
|
||||
|
||||
fn gcd(mut a: usize, mut b: usize) -> usize {
|
||||
while b != 0 {
|
||||
let t = b;
|
||||
b = a % b;
|
||||
a = t;
|
||||
}
|
||||
a
|
||||
}
|
||||
|
||||
/// Depuncture rate-1/2 turbo-coded stream.
|
||||
///
|
||||
/// ITU-R M.2092-1 rate-1/2 puncture pattern for TER-MCS-1:
|
||||
/// - Even positions: systematic + parity1 (encoder 1 output)
|
||||
/// - Odd positions: systematic + parity2 (encoder 2 output)
|
||||
///
|
||||
/// The transmitted stream alternates: [sys, p1, sys, p2, sys, p1, sys, p2, ...]
|
||||
///
|
||||
/// Input: received LLRs (positive = likely 0, negative = likely 1)
|
||||
/// Output: (systematic, parity1, parity2) LLR vectors
|
||||
pub fn depuncture_rate_half(received_llrs: &[Llr], info_len: usize) -> (Vec<Llr>, Vec<Llr>, Vec<Llr>) {
|
||||
let mut systematic = vec![0.0; info_len];
|
||||
let mut parity1 = vec![0.0; info_len];
|
||||
let mut parity2 = vec![0.0; info_len];
|
||||
|
||||
// Rate 1/2: for each info bit, we have 2 coded bits.
|
||||
// Puncture pattern: [sys_i, p1_i] for even i, [sys_i, p2_i] for odd i
|
||||
// This means parity1 is available for even indices, parity2 for odd.
|
||||
let mut rx_idx = 0;
|
||||
for k in 0..info_len {
|
||||
if rx_idx < received_llrs.len() {
|
||||
systematic[k] = received_llrs[rx_idx];
|
||||
rx_idx += 1;
|
||||
}
|
||||
if k % 2 == 0 {
|
||||
// Parity from encoder 1
|
||||
if rx_idx < received_llrs.len() {
|
||||
parity1[k] = received_llrs[rx_idx];
|
||||
rx_idx += 1;
|
||||
}
|
||||
// Parity2 is punctured (erasure = 0.0 LLR, no information)
|
||||
} else {
|
||||
// Parity from encoder 2
|
||||
if rx_idx < received_llrs.len() {
|
||||
parity2[k] = received_llrs[rx_idx];
|
||||
rx_idx += 1;
|
||||
}
|
||||
// Parity1 is punctured
|
||||
}
|
||||
}
|
||||
|
||||
(systematic, parity1, parity2)
|
||||
}
|
||||
|
||||
/// Convert hard bits (0/1) to LLRs.
|
||||
///
|
||||
/// Uses a fixed reliability magnitude. 0 → +RELIABILITY, 1 → -RELIABILITY.
|
||||
pub fn hard_bits_to_llr(bits: &[u8]) -> Vec<Llr> {
|
||||
const RELIABILITY: Llr = 2.0;
|
||||
bits.iter()
|
||||
.map(|&b| if b == 0 { RELIABILITY } else { -RELIABILITY })
|
||||
.collect()
|
||||
}
|
||||
|
||||
/// Main turbo decoder entry point.
|
||||
///
|
||||
/// Takes the received coded bits (hard decision), the information block
|
||||
/// length, and returns decoded information bits + a confidence metric.
|
||||
///
|
||||
/// Returns `(decoded_bits, avg_reliability)` where avg_reliability is the
|
||||
/// mean absolute LLR of the final decisions (higher = more confident).
|
||||
pub fn turbo_decode(coded_bits: &[u8], info_len: usize) -> (Vec<u8>, f32) {
|
||||
let received_llrs = hard_bits_to_llr(coded_bits);
|
||||
turbo_decode_soft(&received_llrs, info_len)
|
||||
}
|
||||
|
||||
/// Soft-input turbo decoder.
|
||||
pub fn turbo_decode_soft(received_llrs: &[Llr], info_len: usize) -> (Vec<u8>, f32) {
|
||||
if info_len == 0 {
|
||||
return (Vec::new(), 0.0);
|
||||
}
|
||||
|
||||
let interleaver = qpp_interleaver(info_len);
|
||||
let deinterleaver = invert_permutation(&interleaver);
|
||||
|
||||
let (sys_llr, par1_llr, par2_llr) = depuncture_rate_half(received_llrs, info_len);
|
||||
|
||||
// Interleaved systematic bits for decoder 2
|
||||
let sys_interleaved: Vec<Llr> = interleaver.iter().map(|&i| sys_llr[i]).collect();
|
||||
|
||||
// Extrinsic information passed between decoders
|
||||
let mut extrinsic_1_to_2 = vec![0.0_f32; info_len];
|
||||
let mut extrinsic_2_to_1 = vec![0.0_f32; info_len];
|
||||
|
||||
let mut final_llr = vec![0.0_f32; info_len];
|
||||
|
||||
for _iter in 0..TURBO_ITERATIONS {
|
||||
// --- Decoder 1 (natural order) ---
|
||||
let apriori_1: Vec<Llr> = deinterleaver
|
||||
.iter()
|
||||
.map(|&i| extrinsic_2_to_1[i])
|
||||
.collect();
|
||||
let aposteriori_1 = bcjr_decode(&sys_llr, &par1_llr, &apriori_1);
|
||||
// Extrinsic = aposteriori - systematic - apriori
|
||||
for k in 0..info_len {
|
||||
extrinsic_1_to_2[k] = aposteriori_1[k] - sys_llr[k] - apriori_1[k];
|
||||
}
|
||||
|
||||
// --- Decoder 2 (interleaved order) ---
|
||||
let apriori_2: Vec<Llr> = interleaver
|
||||
.iter()
|
||||
.map(|&i| extrinsic_1_to_2[i])
|
||||
.collect();
|
||||
let aposteriori_2 = bcjr_decode(&sys_interleaved, &par2_llr, &apriori_2);
|
||||
for k in 0..info_len {
|
||||
extrinsic_2_to_1[k] = aposteriori_2[k] - sys_interleaved[k] - apriori_2[k];
|
||||
}
|
||||
|
||||
// Combine for final decision (deinterleave decoder 2 output)
|
||||
for k in 0..info_len {
|
||||
let deint_apost2 = aposteriori_2[deinterleaver[k]];
|
||||
final_llr[k] = sys_llr[k] + extrinsic_1_to_2[k] + deint_apost2
|
||||
- sys_llr[k]
|
||||
- extrinsic_1_to_2[k];
|
||||
// Simplified: final = systematic + extrinsic from both decoders
|
||||
final_llr[k] = sys_llr[k] + apriori_1[k] + (aposteriori_1[k] - sys_llr[k] - apriori_1[k]);
|
||||
}
|
||||
}
|
||||
|
||||
// Final decision: combine all information
|
||||
for k in 0..info_len {
|
||||
let apriori_1: Llr = if let Some(&di) = deinterleaver.get(k) {
|
||||
extrinsic_2_to_1[di]
|
||||
} else {
|
||||
0.0
|
||||
};
|
||||
let aposteriori_1 = sys_llr[k] + apriori_1 + extrinsic_1_to_2[k];
|
||||
final_llr[k] = aposteriori_1;
|
||||
}
|
||||
|
||||
let decoded: Vec<u8> = final_llr
|
||||
.iter()
|
||||
.map(|&llr| if llr >= 0.0 { 0 } else { 1 })
|
||||
.collect();
|
||||
|
||||
let avg_reliability = if info_len > 0 {
|
||||
final_llr.iter().map(|l: &f32| l.abs()).sum::<f32>() / info_len as f32
|
||||
} else {
|
||||
0.0
|
||||
};
|
||||
|
||||
(decoded, avg_reliability)
|
||||
}
|
||||
|
||||
/// Invert a permutation vector.
|
||||
fn invert_permutation(perm: &[usize]) -> Vec<usize> {
|
||||
let mut inv = vec![0usize; perm.len()];
|
||||
for (i, &p) in perm.iter().enumerate() {
|
||||
if p < inv.len() {
|
||||
inv[p] = i;
|
||||
}
|
||||
}
|
||||
inv
|
||||
}
|
||||
|
||||
/// BCJR (MAP) decoder for a single RSC constituent encoder.
|
||||
///
|
||||
/// Inputs:
|
||||
/// - `systematic`: channel LLRs for systematic bits
|
||||
/// - `parity`: channel LLRs for parity bits
|
||||
/// - `apriori`: a priori LLRs (extrinsic from other decoder)
|
||||
///
|
||||
/// Returns: a posteriori LLRs for each information bit.
|
||||
#[allow(clippy::needless_range_loop)]
|
||||
fn bcjr_decode(systematic: &[Llr], parity: &[Llr], apriori: &[Llr]) -> Vec<Llr> {
|
||||
let n = systematic.len();
|
||||
if n == 0 {
|
||||
return Vec::new();
|
||||
}
|
||||
|
||||
let total_len = n + TAIL_BITS;
|
||||
|
||||
// Extend parity for tail section
|
||||
let mut par_ext = vec![0.0_f32; total_len];
|
||||
par_ext[..parity.len().min(total_len)].copy_from_slice(&parity[..parity.len().min(total_len)]);
|
||||
|
||||
// --- Forward recursion (alpha) ---
|
||||
// alpha[t][s] = log P(state_t = s, y_1..t)
|
||||
let mut alpha = vec![vec![-LLR_INF; NUM_STATES]; total_len + 1];
|
||||
alpha[0][0] = 0.0; // Start in state 0
|
||||
|
||||
for t in 0..total_len {
|
||||
let sys_llr = if t < n {
|
||||
systematic[t] + apriori.get(t).copied().unwrap_or(0.0)
|
||||
} else {
|
||||
0.0 // Tail: force to zero state
|
||||
};
|
||||
|
||||
for s in 0..NUM_STATES {
|
||||
if alpha[t][s] <= -LLR_INF + 1.0 {
|
||||
continue;
|
||||
}
|
||||
for input in 0..=1u8 {
|
||||
let (next_state, parity_bit) = rsc_transition(s, input);
|
||||
let sys_metric = if input == 0 {
|
||||
sys_llr / 2.0
|
||||
} else {
|
||||
-sys_llr / 2.0
|
||||
};
|
||||
let par_metric = if parity_bit == 0 {
|
||||
par_ext[t] / 2.0
|
||||
} else {
|
||||
-par_ext[t] / 2.0
|
||||
};
|
||||
let branch = sys_metric + par_metric;
|
||||
alpha[t + 1][next_state] = log_sum_exp(alpha[t + 1][next_state], alpha[t][s] + branch);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
// --- Backward recursion (beta) ---
|
||||
let mut beta = vec![vec![-LLR_INF; NUM_STATES]; total_len + 1];
|
||||
beta[total_len][0] = 0.0; // End in state 0 (after tail)
|
||||
|
||||
for t in (0..total_len).rev() {
|
||||
let sys_llr = if t < n {
|
||||
systematic[t] + apriori.get(t).copied().unwrap_or(0.0)
|
||||
} else {
|
||||
0.0
|
||||
};
|
||||
|
||||
for s in 0..NUM_STATES {
|
||||
for input in 0..=1u8 {
|
||||
let (next_state, parity_bit) = rsc_transition(s, input);
|
||||
if beta[t + 1][next_state] <= -LLR_INF + 1.0 {
|
||||
continue;
|
||||
}
|
||||
let sys_metric = if input == 0 {
|
||||
sys_llr / 2.0
|
||||
} else {
|
||||
-sys_llr / 2.0
|
||||
};
|
||||
let par_metric = if parity_bit == 0 {
|
||||
par_ext[t] / 2.0
|
||||
} else {
|
||||
-par_ext[t] / 2.0
|
||||
};
|
||||
let branch = sys_metric + par_metric;
|
||||
beta[t][s] = log_sum_exp(beta[t][s], beta[t + 1][next_state] + branch);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
// --- LLR computation ---
|
||||
let mut output_llr = vec![0.0_f32; n];
|
||||
for t in 0..n {
|
||||
let sys_llr_t = systematic[t] + apriori.get(t).copied().unwrap_or(0.0);
|
||||
let mut prob_0 = -LLR_INF;
|
||||
let mut prob_1 = -LLR_INF;
|
||||
|
||||
for s in 0..NUM_STATES {
|
||||
if alpha[t][s] <= -LLR_INF + 1.0 {
|
||||
continue;
|
||||
}
|
||||
for input in 0..=1u8 {
|
||||
let (next_state, parity_bit) = rsc_transition(s, input);
|
||||
if beta[t + 1][next_state] <= -LLR_INF + 1.0 {
|
||||
continue;
|
||||
}
|
||||
let sys_metric = if input == 0 {
|
||||
sys_llr_t / 2.0
|
||||
} else {
|
||||
-sys_llr_t / 2.0
|
||||
};
|
||||
let par_metric = if parity_bit == 0 {
|
||||
par_ext[t] / 2.0
|
||||
} else {
|
||||
-par_ext[t] / 2.0
|
||||
};
|
||||
let gamma = sys_metric + par_metric;
|
||||
let metric = alpha[t][s] + gamma + beta[t + 1][next_state];
|
||||
|
||||
if input == 0 {
|
||||
prob_0 = log_sum_exp(prob_0, metric);
|
||||
} else {
|
||||
prob_1 = log_sum_exp(prob_1, metric);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
output_llr[t] = prob_0 - prob_1;
|
||||
}
|
||||
|
||||
output_llr
|
||||
}
|
||||
|
||||
/// RSC encoder state transition.
|
||||
///
|
||||
/// Given current state and input bit, returns (next_state, parity_output).
|
||||
///
|
||||
/// The RSC encoder uses:
|
||||
/// - Feedback polynomial: g_fb = 1 + D + D^3 (octal 013)
|
||||
/// - Feedforward polynomial: g_ff = 1 + D^2 + D^3 (octal 015)
|
||||
///
|
||||
/// State is the shift register content (3 bits for K=4).
|
||||
fn rsc_transition(state: usize, input: u8) -> (usize, u8) {
|
||||
let s = state as u8;
|
||||
|
||||
// Feedback: XOR of input with feedback taps
|
||||
let feedback = input ^ parity_of(s & (FB_POLY >> 1));
|
||||
|
||||
// New state: shift in the feedback bit
|
||||
let next_state = (((s << 1) | feedback) & 0x07) as usize;
|
||||
|
||||
// Parity output: feedforward taps applied to new register contents
|
||||
let reg_with_input = (feedback << 3) | s;
|
||||
let parity = parity_of(reg_with_input & FF_POLY);
|
||||
|
||||
(next_state, parity)
|
||||
}
|
||||
|
||||
/// Compute parity (XOR of all set bits) of a byte value.
|
||||
fn parity_of(val: u8) -> u8 {
|
||||
(val.count_ones() as u8) & 1
|
||||
}
|
||||
|
||||
/// Numerically stable log-sum-exp: log(exp(a) + exp(b)).
|
||||
///
|
||||
/// Uses the Jacobian logarithm approximation for speed, with a correction
|
||||
/// table for improved accuracy.
|
||||
fn log_sum_exp(a: Llr, b: Llr) -> Llr {
|
||||
if a <= -LLR_INF + 1.0 {
|
||||
return b;
|
||||
}
|
||||
if b <= -LLR_INF + 1.0 {
|
||||
return a;
|
||||
}
|
||||
let max = a.max(b);
|
||||
let diff = (a - b).abs();
|
||||
// Correction term: log(1 + exp(-|diff|))
|
||||
let correction = if diff > 5.0 {
|
||||
0.0
|
||||
} else {
|
||||
(1.0 + (-diff).exp()).ln()
|
||||
};
|
||||
max + correction
|
||||
}
|
||||
|
||||
#[cfg(test)]
|
||||
mod tests {
|
||||
use super::*;
|
||||
|
||||
#[test]
|
||||
fn qpp_interleaver_is_valid_permutation() {
|
||||
for &size in &[468, 936, 1872] {
|
||||
let perm = qpp_interleaver(size);
|
||||
assert_eq!(perm.len(), size);
|
||||
let mut seen = vec![false; size];
|
||||
for &idx in &perm {
|
||||
assert!(idx < size, "index {} out of range for size {}", idx, size);
|
||||
assert!(!seen[idx], "duplicate index {} for size {}", idx, size);
|
||||
seen[idx] = true;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn rsc_transition_state_zero_input_zero() {
|
||||
let (next, par) = rsc_transition(0, 0);
|
||||
assert_eq!(next, 0);
|
||||
assert_eq!(par, 0);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn rsc_transition_all_states_valid() {
|
||||
for state in 0..NUM_STATES {
|
||||
for input in 0..=1u8 {
|
||||
let (next, par) = rsc_transition(state, input);
|
||||
assert!(next < NUM_STATES);
|
||||
assert!(par <= 1);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn turbo_decode_all_zeros() {
|
||||
let info_len = 40;
|
||||
// Encode all-zeros: systematic=0, parity=0 for both encoders
|
||||
let coded_len = info_len * 2;
|
||||
let coded_bits = vec![0u8; coded_len];
|
||||
let (decoded, reliability) = turbo_decode(&coded_bits, info_len);
|
||||
assert_eq!(decoded.len(), info_len);
|
||||
// All-zeros input should decode to all zeros
|
||||
assert!(decoded.iter().all(|&b| b == 0), "decoded: {:?}", decoded);
|
||||
assert!(reliability > 0.0);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn turbo_decode_handles_empty() {
|
||||
let (decoded, reliability) = turbo_decode(&[], 0);
|
||||
assert!(decoded.is_empty());
|
||||
assert_eq!(reliability, 0.0);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn log_sum_exp_correctness() {
|
||||
let a = 2.0f32;
|
||||
let b = 3.0f32;
|
||||
let expected = (a.exp() + b.exp()).ln();
|
||||
let result = log_sum_exp(a, b);
|
||||
assert!((result - expected).abs() < 0.01, "got {}, expected {}", result, expected);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn invert_permutation_round_trips() {
|
||||
let perm = qpp_interleaver(40);
|
||||
let inv = invert_permutation(&perm);
|
||||
for (i, &p) in perm.iter().enumerate() {
|
||||
assert_eq!(inv[p], i);
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn depuncture_produces_correct_lengths() {
|
||||
let info_len = 100;
|
||||
let coded = vec![0u8; info_len * 2];
|
||||
let llrs = hard_bits_to_llr(&coded);
|
||||
let (sys, p1, p2) = depuncture_rate_half(&llrs, info_len);
|
||||
assert_eq!(sys.len(), info_len);
|
||||
assert_eq!(p1.len(), info_len);
|
||||
assert_eq!(p2.len(), info_len);
|
||||
}
|
||||
}
|
||||
Reference in New Issue
Block a user