  Coherent radio emission from pulsars originates from excited plasma waves in
an ultra-relativistic and strongly magnetized electron-positron pair plasma
streaming along the open magnetic field lines of the pulsar. Traditional
coherent radio emission models have relied on instabilities in this pair
plasma. Recently alternative models have been suggested. These models appeal to
direct coupling of the external electromagnetic field to the superluminal
O-mode ($lt_2$ mode) during the time-dependent pair cascade process at the
polar gap. The objective of this work is to provide generic constraints on
plasma models based on $lt_2$ mode using realistic pulsar parameters. We find
that the very short timescale associated with pair cascades does not allow
$lt_{2}$ mode to be excited at radio frequencies and the impulsive energy
transfer can only increase the kinetic spread ("temperature") of the pair
plasma particles. Moreover, under homogeneous plasma conditions, plasma waves
on both branches of O-mode (i.e. superluminal $lt_2$ and subluminal $lt_1$)
cannot escape the plasma. In the strongly magnetized pair plasma, only the
extraordinary mode ($t$ mode) can escape freely. We show that any generic
fictitious mechanisms does not result in the wave electric field of $t$ mode to
have predominant orientation either parallel or perpendicular to the magnetic
field plane as observed. Such fictitious mechanisms will inevitably lead to
depolarization of signals and cannot account for the highly polarized single
pulses observed in pulsars. We suggest coherent curvature radiation as a
promising candidate for pulsar radio emission mechanism.
