Spectrally Efficient Multilevel CPM Waveforms for VHF Narrowband Communications

The paper addresses a comparison between binary and multilevel continuous phase modulation (CPM) with a serially concatenated convolutional code. In order to achieve a high spectral efficiency, the binary approach uses an increased baud rate and a small modulation index. Such signals are currently considered for standardization as narrowband tactical VHF waveforms. These waveforms provide user bit rates from 10 kbit/s to 82.285 kbit/s on a 25 kHz channel. As an alternative, multilevel CPM signals utilizing 4 up to 16 levels are proposed in this paper, allowing for baud rates approximately equivalent to the channel bandwidth. In order to take into account the harsh environment for tactical radio systems, resulting in increased phase noise and oscillator instabilities, noncoherent detection is used. From a power efficiency point of view, both binary and multilevel signals perform similar and close to coherent detection on a Gaussian channel. In presence of adjacent channel interference, the proposed multilevel signals perform clearly better than the binary signals. Especially the binary signals providing high bit rates are degraded drastically. We show that the receiver complexity of the proposed multilevel schemes is affordable with modern FPGAs.