  Sparse code multiple access (SCMA) has been recently proposed for the future
wireless networks, which allows non-orthogonal spectrum resource sharing and
enables system overloading. In this paper, we apply SCMA into device-to-device
(D2D) communication and cellular hybrid network, targeting at using the
overload feature of SCMA to support massive device connectivity and expand
network capacity. Particularly, we develop a stochastic geometry based
framework to model and analyze SCMA, considering underlaid and overlaid mode.
Based on the results, we analytically compare SCMA with orthogonal
frequency-division multiple access (OFDMA) using area spectral efficiency (ASE)
and quantify closed-form ASE gain of SCMA over OFDMA. Notably, it is shown that
system ASE can be significantly improved using SCMA and the ASE gain scales
linearly with the SCMA codeword dimension. Besides, we endow D2D users with an
activated probability to balance cross-tier interference in the underlaid mode
and derive the optimal activated probability. Meanwhile, we study resource
allocation in the overlaid mode and obtain the optimal codebook allocation
rule. It is interestingly found that the optimal SCMA codebook allocation rule
is independent of cellular network parameters when cellular users are densely
deployed. The results are helpful in the implementation of SCMA in the hybrid
system.
