  The combination of non-orthogonal multiple access (NOMA) using power-domain
with backscatter sensor communication (BSC) is expected to connect a
large-scale Internet of things (IoT) devices in future sixth-generation (6G)
era. In this paper, we introduce a BSC in multi-cell IoT network, where a
source in each cell transmits superimposed signal to its associated IoT devices
using NOMA. The backscatter sensor tag (BST) also transmit data towards IoT
devices by reflecting and modulating the superimposed signal of the source. A
new optimization framework is provided that simultaneously optimizes the total
power of each source, power allocation coefficient of IoT devices and
reflection coefficient of BST under imperfect successive interference
cancellation decoding. The objective of this work is to maximize the total
energy efficiency of IoT network subject to quality of services of each IoT
device. The problem is first transformed using the Dinkelbach method and then
decoupled into two subproblems. The Karush-Kuhn-Tucker conditions and
Lagrangian dual method are employed to obtain the efficient solutions. In
addition, we also present the conventional NOMA network without BSC as a
benchmark framework. Simulation results unveil the advantage of our considered
NOMA BSC networks over the conventional NOMA network.
