  In the two-user Gaussian Strong Interference Channel (GSIC) with finite
constellation inputs, it is known that relative rotation between the
constellations of the two users enlarges the Constellation Constrained (CC)
capacity region. In this paper, a metric for finding the approximate angle of
rotation (with negligibly small error) to maximally enlarge the CC capacity for
the two-user GSIC is presented. In the case of Gaussian input alphabets with
equal powers for both the users and the modulus of both the cross-channel gains
being equal to unity, it is known that the FDMA rate curve touches the capacity
curve of the GSIC. It is shown that, with unequal powers for both the users
also, when the modulus of one of the cross-channel gains being equal to one and
the modulus of the other cross-channel gain being greater than or equal to one,
the FDMA rate curve touches the capacity curve of the GSIC. On the contrary, it
is shown that, under finite constellation inputs, with both the users using the
same constellation, the FDMA rate curve strictly lies within (never touches)
the enlarged CC capacity region throughout the strong-interference regime. This
means that using FDMA it is impossible to go close to the CC capacity. It is
well known that for the Gaussian input alphabets, the FDMA inner-bound, at the
optimum sum-rate point, is always better than the simultaneous-decoding
inner-bound throughout the weak-interference regime. For a portion of the weak
interference regime, it is shown that with identical finite constellation
inputs for both the users, the simultaneous-decoding inner-bound, enlarged by
relative rotation between the constellations, is strictly better than the FDMA
inner-bound.
