Symbiotic Ambient Backscatter Systems: Outage Behavior and Ergodic Capacity

This article investigates a symbiotic ambient backscatter communication (AmBC) system, where for the primary system, a source node T1 transmits information to a destination node T2. Whereas for the backscatter system, by riding on T1’s signal, the backscatter device passively conveys its own information
$c(n)$
to T1 and T2 via backscattering. For such, the coexistence outage probability (COP) and ergodic capacity (EC) of the AmBC system are characterized for three cases of coexistence constraints, i.e., 1) both T1 and T2 decode
$c(n)$
(Case I); 2) only T2 decodes
$c(n)$
(Case II); and 3) only T1 decodes
$c(n)$
(Case III). It is analytically shown that for sufficiently high transmit signal-to-noise ratio (SNR), the COP obeys the scaling law of
$({1}/{\sqrt {P_{s}}})$
(with
$P_{s}$
denoting T1’s transmit power) for Cases I and III, whereas its scaling law is determined by
$([{\mathrm {log}(P_{s})}]/{P_{s}})$
as well as
$({1}/{P_{s}})$
for Case II. In addition, it is shown that the restriction condition of decoding
$c(n)$
at T1 results in a dominating term
$({1}/{\sqrt {P_{s}}})$
for the COP at high SNR, whereas the restriction condition of decoding
$c(n)$
at T2 results in an infinitesimal relative to
$({1}/{\sqrt {P_{s}}})$
. It is also shown that for different cases, the effects of the T1–T2 channel statistics on the COP are significantly different. However, unlike the metric of COP, for the EC, the impacts of decoding constraints of
$c(n)$
gradually disappear at high SNR and the ECs of the backscatter channels for Cases II and III approach, respectively, toward the counterpart for Case I.