  We explore the shock acceleration model for giant radio relics, in which
relativistic electrons are accelerated via diffusive shock acceleration (DSA)
by merger-driven shocks in the outskirts of galaxy clusters. In addition to
DSA, turbulent acceleration by compressive MHD mode downstream of the shock is
included as well as energy losses of postshock electrons by Coulomb scattering,
synchrotron emission, and inverse Compton scattering off the cosmic background
radiation. Considering that only a small fraction of merging clusters host
radio relics, we favor the reacceleration scenario in which radio relics are
generated preferentially when shocks encounter the regions containing
low-energy ($\gamma_{\rm e} \lesssim 300$) cosmic ray electrons (CRe). We
perform time-dependent DSA simulations of spherically expanding shocks with
physical parameters relevant for the Sausage radio relic, and calculate the
radio synchrotron emission from the accelerated CRe. We find that significant
level of postshock turbulent acceleration is required in order to reproduce
broad profiles of the observed radio flux densities of the Sausage relic.
Moreover, the spectral curvature in the observed integrated radio spectrum can
be explained, if the putative shock should have swept up and exited out of the
preshock region of fossil CRe about 10~Myr ago.
