  A fundamental objective of the forthcoming sixth-generation wireless networks
is to concurrently serve a vast array of devices many of which, such as
Internet-of-Things (IoT) sensors, are projected to have low power requirements
or even operate in a battery-free manner. To achieve this goal, non-orthogonal
multiple access (NOMA) and ambient backscatter communications (AmBC) are
regarded as two pivotal and promising technologies. In this work, we present a
novel analytical framework for studying the reliability and security of uplink
NOMA-based AmBC systems. Specifically, closed-form analytical expressions for
both NOMA-users' and IoT backscatter device's (BD's) outage probabilities (OPs)
are derived for both cases of perfect and imperfect successive interference
cancellation (SIC). In addition, assuming that one NOMA-user transmits an
artificial noise in order to enhance system's security, the physical layer
security (PLS) of the system is investigated by extracting analytical
expressions for NOMA-users' and BD's intercept probabilities (IPs). To gain
insightful understandings, an asymptotic analysis is carried out by focusing on
the high signal-to-noise (SNR) regime, which reveals that NOMA-users and BDs
face outage floors in the high SNR regime as well as that IPs reach constant
values at high SNR. Additionally, practical insights regarding how different
system parameters affect these OP floors and IP constant values are extracted.
Numerical results verify the accuracy of othe developed theoretical framework,
offer performance comparisons between the presented NOMA-based AmBC system and
a conventional orthogonal multiple access-based AmBC system, and reveal the
impact of different system parameters on the reliability and security of
NOMA-based AmBC networks.
