  Backscatter communications (BC) has emerged as a promising technology for
providing low-powered transmissions in nextG (i.e., beyond 5G) wireless
networks. The fundamental idea of BC is the possibility of communications among
wireless devices by using the existing ambient radio frequency signals.
Non-orthogonal multiple access (NOMA) has recently attracted significant
attention due to its high spectral efficiency and massive connectivity. This
paper proposes a new optimization framework to minimize total transmit power of
BC-NOMA cooperative vehicle-to-everything networks (V2XneT) while ensuring the
quality of services. More specifically, the base station (BS) transmits a
superimposed signal to its associated roadside units (RSUs) in the first time
slot. Then the RSUs transmit the superimposed signal to their serving vehicles
in the second time slot exploiting decode and forward protocol. A backscatter
device (BD) in the coverage area of RSU also receives the superimposed signal
and reflect it towards vehicles by modulating own information. Thus, the
objective is to simultaneously optimize the transmit power of BS and RSUs along
with reflection coefficient of BDs under perfect and imperfect channel state
information. The problem of energy efficiency is formulated as non-convex and
coupled on multiple optimization variables which makes it very complex and hard
to solve. Therefore, we first transform and decouple the original problem into
two sub-problems and then employ iterative sub-gradient method to obtain an
efficient solution. Simulation results demonstrate that the proposed BC-NOMA
V2XneT provides high energy efficiency than the conventional NOMA V2XneT
without BC.
