  Even though the observed spectra for GRB prompt emission is well constrained,
no single radiation mechanism can robustly explain its distinct non-thermal
nature. Here we explore the radiation mechanism with the photospheric emission
model using our Monte Carlo Radiative Transfer (MCRaT) code. We study the
sub-photospheric Comptonization of fast cooled synchrotron photons while the
Maxwellian electrons and mono-energetic protons are accelerated to relativistic
energies by repeated dissipation events. Unlike previous simulations, we
implement a realistic photon to electron number ratio $N_{\gamma}/N_{e} \sim
10^5$ consistent with the observed radiative efficiency of a few percent. We
show that it is necessary to have a critical number of episodic energy
injection events $N_{rh,cr} \sim {\rm few}\ 10{\rm s}-100$ in the jet in
addition to the electron-proton Coulomb coupling in order to inject sufficient
energy $E_{inj,cr} \sim 2500-4000\ m_e c^2$ per electron and produce an output
photon spectrum consistent with observations. The observed GRB spectrum can be
generated when the electrons are repeatedly accelerated to highly relativistic
energies $\gamma_{e,in} \sim {\rm few}\ 10{\rm s}-100$ in a jet with bulk
Lorentz factor $\Gamma \sim 30-100$, starting out from moderate optical depths
$\tau_{in} \sim 20-40$. The shape of the photon spectrum is independent of the
initial photon energy distribution and baryonic energy content of the jet and
hence independent of the emission mechanism, as expected for photospheric
emission.
