  We develop a simple Ginsburg-Landau theory to study all the possible phases
and phase transitions in $^{4}He $, analyze the condition for the existence of
the supersolid (SS) and map out its global phase diagram from a unified
framework. If the condition favors the existence of the SS, we use the GL
theory to address several experimental facts and also make some predictions
that are amenable to experimental tests. A key prediction is that the X-ray
scattering intensity from the SS ought to have an additional modulation over
that of the NS. The modulation amplitude is proportional to the Non-Classical
Rotational-Inertial (NCRI) observed in the torsional oscillator experiments.
