  Symbiotic radio (SR) is a promising solution to achieve high spectrum- and
energy-efficiency due to its spectrum sharing and low-power consumption
properties, in which the secondary system achieves data transmissions by
backscattering the signal originating from the primary system. In this paper,
we are interested in the pilot design and signal detection when the primary
transmission adopts orthogonal frequency division multiplexing (OFDM). In
particular, to preserve the channel orthogonality among the OFDM sub-carriers,
each secondary symbol is designed to span an entire OFDM symbol. The comb-type
pilot structure is employed by the primary transmission, while the preamble
pilot structure is used by the secondary transmission. With the designed pilot
structures, the primary signal can be detected via the conventional methods by
treating the secondary signal as a part of the composite channel, i.e., the
effective channel of the primary transmission. Furthermore, the secondary
signal can be extracted from the estimated composite channel with the help of
the detected primary signal. The bit error rate (BER) performance with both
perfect and estimated CSI, the diversity orders of the primary and secondary
transmissions, and the sensitivity to symbol synchronization error are
analyzed. Simulation results show that the performance of the primary
transmission is enhanced thanks to the backscatter link established by the
secondary transmission. More importantly, even without the direct link, the
primary and secondary transmissions can be supported via only the backscatter
link.
