  To cope with the explosive traffic growth of next-generation wireless
communications, it is necessary to design next-generation multiple access
techniques that can provide higher spectral efficiency as well as larger-scale
connectivity. As a promising candidate, power-domain non-orthogonal multiple
access (NOMA) has been widely studied. In conventional power-domain NOMA,
multiple users are multiplexed in the same time and frequency band by different
preset power levels, which, however, may limit the spectral efficiency under
practical finite alphabet inputs. Inspired by the concept of spatial
modulation, we propose to solve this problem by encoding extra information bits
into the power levels, and exploit different signal constellations to help the
receiver distinguish between them. To convey this idea, termed power selection
(PS)-NOMA, clearly, we consider a simple downlink two-user NOMA system with
finite input constellations. Assuming maximum-likelihood detection, we derive
closed-form approximate bit error ratio (BER) expressions for both users. The
achievable rates of both users are also derived in closed form. Simulation
results verify the analysis and show that the proposed PS-NOMA outperforms
conventional NOMA in terms of BER and achievable rate.
