Crate pccc

Parallel-concatenated convolutional code (PCCC)

This crate implements encoding and decoding functionality for a parallel-concatenated convolutional code (PCCC), commonly referred to as a turbo code. An instance of such a code is used for forward error correction in the 4G LTE standard for wireless broadband communication (see 3GPP TS 36.212).

The encoder for a PCCC comprises the parallel concatenation of two identical recursive systematic convolutional (RSC) encoders, separated by an internal interleaver. The decoder is based on “turbo” iterations between two corresponding soft-input/soft-output a posteriori probability (APP) decoders, separated by an interleaver and deinterleaver.

The encoder and decoder functions handle PCCC encoding and decoding, respectively, while the Interleaver struct models the internal interleaver and associated deinterleaver. The Bit enum represents binary symbol values, and the DecodingAlgo enum the supported decoding algorithms; each variant of the latter holds a u32 representing the number of turbo iterations. The code below illustrates their usage through a toy example.

Examples

use pccc::{decoder, encoder, Bit, DecodingAlgo, Interleaver};
use Bit::{One, Zero};

let code_polynomials = [0o13, 0o15]; // Rate-1/3 PCCC in LTE
let interleaver = Interleaver::new(&[3, 0, 1, 2])?; // Interleaver for 4 information bits
let decoding_algo = DecodingAlgo::LogMAP(8); // Log-MAP decoding, 8 turbo iterations

// Encoding
let info_bits = [One, Zero, Zero, One];
let code_bits = encoder(&info_bits, &interleaver, &code_polynomials)?;
assert_eq!(
    code_bits,
    [
        One, One, One, Zero, One, Zero, Zero, One, Zero, One, Zero, Zero, One, Zero, One, One,
        Zero, Zero, Zero, One, One, One, Zero, Zero
    ]
);

// Decoding
let code_bits_llr = [
    -1.0, -1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0, 1.0, -1.0, -1.0,
    1.0, 1.0, 1.0, -1.0, -1.0, -1.0, 1.0, 1.0,
];
let info_bits_hat = decoder(
    &code_bits_llr,
    &interleaver,
    &code_polynomials,
    decoding_algo,
)?;
assert_eq!(info_bits_hat, [One, Zero, Zero, One]);

The lte module contains encoder and decoder functions specifically for the rate-1/3 PCCC used in the LTE standard; the block size is restricted to the values in Table 5.1.3-3 of 3GPP TS 36.212 (from 40 to 6144 bits), and the interleaver for each of these block sizes is automatically set to the quadratic permutation polynomial (QPP) interleaver prescribed therein. This module also has functions to evaluate the performance of the LTE PCCC over a BPSK-AWGN channel by simulation. Finally, the utils module has some useful functions for such a simulation.

Modules

lte

Encoder and decoder for the rate-1/3 PCCC in LTE

utils

Some useful functions for simulating code performance

Structs

Interleaver

Interleaver for sequences of a given length

Enums

Bit

Enumeration of binary symbol values

DecodingAlgo

Enumeration of PCCC decoding algorithms

Error

Custom error type

Functions

decoder

Returns information bit decisions from PCCC decoder for given code bit LLR values.

encoder

Returns code bits from PCCC encoder for given information bits.