A log-likelihood ratio for improved receiver performance for VLF/LF communication in atmospheric noise

Receiver-improving techniques are important for submarine communication at the LF/VLF bands, due to very demanding reception conditions. At the surface, the interference environment is dominated by the atmospheric noise, which is highly impulsive in character and may impede the reception of the radio signals. To handle the demanding channel and the impulsive interference environment, error correction and an adapted receiver are necessary. In this paper, we propose a log-likelihood ratio (LLR) as soft output from the demodulator suitable for atmospheric noise. The radio system is assumed to use minimum shift keying (MSK) and a low parity density check (LDPC) code. It is shown that the proposed interference-adapted LLR improves the performance substantially in atmospheric noise compared to when the LLR is designed for AWGN. The performance is also compared to a solution, where the soft output from the demodulator is simplified to a limiter, and to a solution when a larger system bandwidth is used in combination with a limiter. It is concluded that the proposed interference-adapted LLR achieves the best performance in the comparison, although the performance could probably be further improved, when compared to the obtained Shannon capacity of this particular interference.