1. Connectivity of Cognitive Device-to-Device Communications
Underlying Cellular Networks
Abstract:
Providing direct communications among a rapidly growing number
of wireless devices within the coverage area of a cellular system is an
attractive way of exploiting the proximity among them to enhance
coverage and spectral and energy efficiency. However, such device-to-
device (D2D) communications create a new type of interference in cellular
systems, calling for rigorous system analysis and design to both protect
mobile users (MUs) and guarantee the connectivity of devices. Motivated
by the potential advantages of cognitive radio (CR) technology in detecting
and exploiting underutilized spectrum, we investigate CR-assisted D2D
communications in a cellular network as a viable solution for D2D
communications, in which devices access the network with mixed overlay-
underlay spectrum sharing. Our comprehensive analysis reveals several
engineering insights useful to system design. We first derive bounds of
pivotal performance metrics. For a given collision probability constraint, as
the prime spectrum-sharing criterion, we also derive the maximum

2. allowable density of devices. This captures the density of MUs and that of
active macro base stations. Limited in spatial density, devices may not have
connectivity among them. Nevertheless, it is shown that for the derived
maximum allowable density, one should judiciously push a portion of
devices into receiving mode in order to preserve the connectivity and to
keep the isolation probability low. Furthermore, upper bounds on the
cellular coverage probability are obtained incorporating load-based power
allocation for both path-loss and fading-based cell association mechanisms,
which are fairly accurate and consistent with our in-depth simulation
results. Finally, implementation issues are discussed.