  This paper addresses the four enabling technologies, namely multi-user sparse
code multiple access (SCMA), content caching, energy harvesting, and physical
layer security for proposing an energy and spectral efficient resource
allocation algorithm for the access and backhaul links in heterogeneous
cellular networks. Although each of the above mentioned issues could be a topic
of research, in a real situation, we would face a complicated scenario where
they should be considered jointly, and hence, our target is to consider these
technologies jointly in a unified framework. Moreover, we propose two novel
content delivery scenarios: 1) single frame content delivery (SFCD), and 2)
multiple frames content delivery (MFCD), where the time duration of serving
user requests is divided into several frames. In the first scenario, the
requested content by each user is served over one frame. However, in the second
scenario, the requested content by each user can be delivered over several
frames. We formulate the resource allocation for the proposed scenarios as
optimization problems where our main aim is to maximize the energy efficiency
of access links subject to the transmit power and rate constraints of access
and backhaul links, caching and energy harvesting constraints, and SCMA
codebook allocation limitations. Due to the practical limitations, we assume
that the channel state information values between eavesdroppers and base
stations are uncertain and design the network for the worst case scenario.
Since the corresponding optimization problems are mixed integer non-linear and
nonconvex programming, NP-hard, and intractable, we propose an iterative
algorithm based on the well-known alternate and successive convex approximation
methods.
