  Non-orthogonal access techniques have recently gained renewed interest in the
context of next generation wireless networks. As the relative gain, with
respect to traditionally employed orthogonal-access techniques depends on many
factors, it is of interest to obtain insights by considering the simplest
scenario, the two-user downlink (broadcast) channel where all nodes are
equipped with a single antenna. Further, we focus on rate pairs that are in the
vicinity of sum-rate optimalilty with respect to the capacity region of the
broadcast channel. A simple and explicit characterization of the relative gain
of non-orthogonal transmission with respect to orthogonal transmission is
obtained under these conditions as an immediate consequence of the capacity
regions of the two.
