  Symbiotic radio (SR) has emerged as a spectrum- and energy-efficient paradigm
to support massive Internet of Things (IoT) connections. Two multiple access
schemes are proposed in this paper to facilitate the massive IoT connections
using the cellular network based on the SR technique, namely, the simultaneous
access (SA) scheme and the selection diversity access (SDA) scheme. In the SA
scheme, the base station (BS) transmits information to the receiver while
multiple IoT devices transmit their information simultaneously by passively
backscattering the BS signal to the receiver, while in the SDA scheme, only the
IoT device with the strongest backscatter link transmits information to the
receiver. In both of the schemes, the receiver jointly decodes the information
from the BS and the IoT devices. To evaluate the above two schemes, in this
paper, we have derived the closed-form expressions of the ergodic rates and the
outage probabilities for the cellular and IoT transmissions. Finally, numerical
results are provided to verify the theoretical analysis and compare the two
proposed multiple access schemes. When the number of IoT devices is small, the
SDA scheme is more appealing since it can significantly reduce the
computational complexity while achieving equivalent performance to the SA
scheme. When the number of IoT devices is large, the SA scheme is preferable
since it guarantees a significantly better rate performance and a lower outage
probability.
