New perspectives for coded continuous phase modulations for narrowband waveforms: Iterative versus non-iterative solutions

Abstract

NarrowBand Waveforms (NBWF) are often used in VHF or UHF tactical communications. For these kinds of waveforms, low latency and robust data rates result in short codeword lengths that are challenging in terms of channel coding. Usually, serially concatenated convolutional code and continuous phase modulation (CC-CPM) schemes are considered in the context of NBWF. When evaluating the achievable rates, CC-CPM schemes show a 1 to 4 dB maximum margin. In this paper, we investigate on two new sparse graph-based channel coding strategies, trying to reduce this degradation for the achievable rate. Some implementation issues with short codeword lengths are also addressed. To this end, we first consider a serially concatenated CPM scheme where the outer code is an optimized Low-Density Parity-Check code (LDPC) based on some recently introduced methods for this kind of applications. The optimized LDPC scheme exhibits a good bit error rate when binary CPMs are used. This new design comes with lower computational complexity and greater flexibility. Second, in order to avoid the cubersome of iterative detection and decoding, we investigate on recently introduced precoded CPM that can achieve near optimal performance, referred to us as Pragmatic CPM (P-CPM). For this kind of precoded CPM schemes, we will show that associated EXIT chart curves have the flat property. It means that, to achieve good performance, P-CPM can be used with standard capacity approaching codes like LDPC Accumulate Repeat Jagged Accumulate (ARJA) or Turbo Codes without the need for iterative decoding. Bit error rate simulations confirm that P-CPM is a reliable alternative NBWF scheme compatible with versatile modern channel codes. As P-CPM is non-iterative, implementation is made easier. In additive white Gaussian noise, both schemes are found practical.