  Ion cyclotron emission (ICE) holds great potential as a diagnostic tool for
fast ions in fusion devices. The theory of magnetoacoustic cyclotron
instability (MCI), as an emission mechanism for ICE, states that MCI is driven
by a velocity distribution of fast ions that approximates a drift ring beam.
The influence of key parameters on the linear MCI is systematically
investigated using the linear kinetic dispersion relation solver BO (Xie H.
2019 Comput. Phys. Comm. 244 343). The computational spectra region considered
extends up to 40 times the ion cyclotron frequency. By examining the influence
of these key parameters on MCI, several novel results have been obtained. In
the case of MCI excited by super-Alfv\'enic fast ions, the parallel velocity
spread significantly affects the bandwidth of harmonics and the continuous
spectrum, while the perpendicular velocity spread has a decisive effect on the
MCI growth rate. As the velocity spread increases, the linear relationship
between the MCI growth rate and the square root of the number density ratio
transitions to a linear relationship between the MCI growth rate and the number
density ratio. This finding provides a linear perspective explanation for the
observed linear relation between fast ion number density and ICE intensity in
JET. Furthermore, high harmonics are more sensitive to changes in propagation
angle than low harmonics because a decrease in the propagation angle alters the
dispersion relation of the fast Alfv\'en wave. In the case of MCI excited by
sub-Alfv\'enic fast ions, a significant growth rate increase occurs at high
harmonics due to the transition of sub-Alfv\'enic fast ions to super-Alfv\'enic
fast ions.
