Crate ssdv_fec

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§SSDV systematic erasure FEC

This crate implements a systematic erasure FEC scheme for SSDV. The FEC scheme is based on a Reed-Solomon code over GF(2¹⁶) used as a fountain-like code. This idea was first described in the blog post An erasure FEC for SSDV by the author of this crate.

Given an SSDV image formed by k SSDV packets, the FEC encoder can generate up to 2¹⁶ different SSDV packets identified by a packet ID from 0 to 2¹⁶-1. The packets with IDs from 0 to k-1 are called “systematic packets” and are the same as the k packets of the original image. The remaining packets are called “FEC packets”. Each packet can be generated on demand according to the needs of the transmitter. The large amount of 2¹⁶ distinct packets than can be generated provides a virtually limitless source of packets. The receiver can recover the original SSDV image from any set of k distinct packets.

This implementation of the FEC scheme uses 218-byte SSDV packets following the format used by Longjiang-2, which omits the sync byte, packet type and callsign fields (but includes them implicitly in the generation of the CRC-32).

The crate supports no_std and the implementation is designed with small microcontrollers in mind. The GF(2¹⁶) arithmetic only uses two tables of 256 bytes each that are included in the .rodata section. The FEC encoder and decoder work with externally provided slices, giving freedom as to how to perform memory allocation, and do the computations in-place when possible. The memory required for encoding corresponds to a buffer containing the k SSDV packets of the original image, and a buffer containing the packet being encoded. The memory required for decoding corresponds to a buffer containing at least k distinct received SSDV packets, and another buffer where the k SSDV packets that compose the original image can be written. Besides these buffers, the algorithms use only a small amount of stack space.

A simple CLI application that can perform encoding and decoding can be built with the cli feature, which is enabled by default.

Modules§

  • cli
    CLI application.

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