  In this paper, a new approach for multiple access (MA) in fifth generation
(5G) of cellular networks called power domain sparse code multiple access
(PSMA) is proposed. In PSMA, we adopt both the power domain and the code domain
to transmit multiple users' signals over a subcarrier simultaneously. In such a
model, the same SCMA codebook can be used by multiple users where, for these
users, power domain non-orthogonal multiple access (PD-NOMA) technique is used
to send signals non-orthogonally. Although different SCMA codebooks are
orthogonal and produce no interference over each other, the same codebook used
by multiple users produces interference over these users. We investigate the
signal model as well as the receiver and transmitter of the PSMA method. To
evaluate the performance of PSMA, we consider a heterogeneous cellular network
(HetNet). In this case our design objective is to maximize the system sum rate
of the network subject to some system level and QoS constraints such as
transmit power constraints. We formulate the proposed resource allocation
problem as an optimization problem and solve it by successive convex
approximation (SCA) techniques. Moreover, we compare PSMA with sparse code
multiple access (SCMA) and PD-NOMA from the performance and computational
complexity perspective. Finally, the effectiveness of the proposed approach is
investigated using numerical results. We show that by a reasonable increase in
complexity, PSMA can improve the spectral efficiency about 50\% compared to
SCMA and PD-NOMA.
