Variable-rate capacity-approaching LDPC codes for HF communications

The HF propagation channel involves several interrelated phenomena that results in a highly variable medium, which poses a challenge in designing and implementing effective high speed digital communications systems for HF band. The reliability of HF communication is affected by time-varying propagation conditions of HF channel that demands a requirement for coding schemes with adaptive protection capabilities, to combat the ever-changing channel conditions and thus improve the reliability and performance of the channel. The novel idea introduced in this paper, proposes a variable forward error correction scheme using capacity-approaching ‘Low Density Parity Check’ codes that enables the switching between LDPC codes of constant length but variable rates (1/4, 1/2, & 9/10) to provide variable-error protection based on the channel condition at any given time. These codes have been simulated over a Rayleigh fading channel and their capacity approaching performance using the sum-product decoding algorithm is shown.