  We propose a new continuous phase frequency shift keying that is particularly
suited for multi-antenna communications when the link budget is critical and
beam alignment is problematic. It combines the constant envelope of frequency
modulation with low-rate repetition coding in order to compensate for the
absence of transmit beamforming. Although it is a frequency modulation, its
transmit signal shows close to rectangular spectral shape. Similar to GSM's
Gaussian minimum shift keying, it can be well approximated by linear
modulation, when combined with differential precoding. This allows for easy
coherent demodulation by means of a windowed fast Fourier transform.
