  Sparse code multiple access (SCMA) is the most concerning scheme among
non-orthogonal multiple access (NOMA) technologies for 5G wireless
communication new interface. Another efficient technique in 5G aimed to improve
spectral efficiency for local communications is device-to-device (D2D)
communications. Therefore, we utilize the SCMA cellular network coexisting with
D2D communications for the connection demand of the Internet of things (IOT),
and improve the system sum rate performance of the hybrid network. We first
derive the information-theoretic expression of the capacity for all users and
find the capacity bound of cellular users based on the mutual interference
between cellular users and D2D users. Then we consider the power optimization
problem for the cellular users and D2D users jointly to maximize the system sum
rate. To tackle the non-convex optimization problem, we propose a geometric
programming (GP) based iterative power allocation algorithm. Simulation results
demonstrate that the proposed algorithm converges fast and well improves the
sum rate performance.
