  In this paper, transmission over the additive white Gaussian noise (AWGN)
channel, and coherent and noncoherent fading channels using M-ary orthogonal
frequency-shift keying (FSK) or on-off frequency-shift keying (OOFSK) is
considered. The receiver is assumed to perform hard-decision detection. In this
setting, energy required to reliably send one bit of information is
investigated. It is shown that for fixed M and duty cycle, bit energy
requirements grow without bound as the signal-to-noise ratio (SNR) vanishes.
The minimum bit energy values are numerically obtained for different values of
M and the duty cycle. The impact of fading on the energy efficiency is
identified. Requirements to approach the minimum bit energy of -1.59 dB are
determined.
