  Symbiotic radio (SR) networks are possible solutions to the future low-power
wireless communications for massive Internet of Things devices. In this paper,
we investigate a multiple-input-single-output non-orthogonal multiple access
(NOMA) backscatter device (BD) aided SR network with a potential eavesdropper.
In the network, a base station (BS) broadcasts signals to a central user and a
cell-edge user using the NOMA protocol. With ambient backscatter modulation,
the BD transmits its own messages to the central user over incident signals
from the BS. We propose a constrained concave convex procedure-based algorithm
which maximizes the $\epsilon$-outage secrecy rate from the BD to the central
user under the achievable secrecy rate constraints from the BS to the central
and cell-edge users. Simulation results illustrate that our proposed network
achieves a much larger secrecy rate region than the orthogonal multiple access
(OMA) network.
