  In this paper, spectrum-sharing technology is integrated into cellular
systems to improve spectrum efficiency. Macrocell users are primary users (PUs)
while those within local cells, e.g., femtocell users, or desiring
cost-effective services, e.g., roamers, are identified as secondary users
(SUs). The SUs share the spectrum resources of the PUs in a underlay way, thus
the transmit power of a secondary is strictly limited by the primary's
tolerable interference power. Given such constraints, a cooperative relaying
transmission between a SU and the macrocell base station (BS) is necessary. In
order to guarantee the success of dual-hop relaying and avoid multi-hop
relaying, a new cooperative paradigm is proposed, where an idle PU (instead of
a secondary as assumed in general) in the vicinity of a target SU is chosen to
serve as a relaying node, thanks to the fact that any PU can always transmit to
the macrocell BS directly. Moreover, two-way relaying strategy is applied at
the chosen relaying node so as to further improve the spectral efficiency. Our
results demonstrate that the proposed system is particularly suitable for
delay-tolerant wireless services with asymmetric downlink/uplink traffics, such
as e-mail checking, web browsing, social networking and data streaming, which
are the most popular applications for SUs in spectrum-sharing cellular
networks.
