Spectrum issues of NATO narrowband waveform: On the spectral efficiency of CPM-Modulation with small modulation indices

In order to fulfill most of the military operational requirements, two kinds of waveforms are needed, a wideband networking waveform, e.g. the Coalition Wideband Networking waveform (COALWNW) with high data rates, enabling network centric warfare and a Narrow Band Waveform (NBWF) covering long ranges and enabling the parallel transmission of data and voice. As a complement to COALWNW, the NATO Line of Sight Communications Capability Team is developing a new NBWF. Legacy versions of the NBWF exist as national waveforms only, operating historically in the VHF tactical communications band ranging from 30 MHz to 88 MHz occupying a channel bandwidth of 25 kHz. The new NBWF is expected to operate in the same frequency range, employing the same channelization; furthermore, the NBWF should allow operation in the tactical UHF band as well. The current waveform proposal is based on Continuous Phase Modulation (CPM), which is widely used in mobile communications, due to the constant envelope property of the modulation scheme. In this paper, we first introduce principles of the newly defined NBWF. Furthermore, we want to point out some unapparent spectrum usage issues of the current proposal.