  In cognitive radio networks (CRNs), secondary users (SUs) can proactively
obtain spectrum access opportunities by helping with primary users' (PUs') data
transmissions. Currently, such kind of spectrum access is implemented via a
cooperative communications based link-level frame-based cooperative (LLC)
approach where individual SUs independently serve as relays for PUs in order to
gain spectrum access opportunities. Unfortunately, this LLC approach cannot
fully exploit spectrum access opportunities to enhance the throughput of CRNs
and fails to motivate PUs to join the spectrum sharing processes. To address
these challenges, we propose a network-level session-based cooperative (NLC)
approach where SUs are grouped together to cooperate with PUs session by
session, instead of frame by frame as what has been done in existing works, for
spectrum access opportunities of the corresponding group. Thanks to our
group-based session-by-session cooperating strategy, our NLC approach is able
to address all those challenges in the LLC approach. To articulate our NLC
approach, we further develop an NLC scheme under a cognitive capacity
harvesting network (CCHN) architecture. We formulate the cooperative mechanism
design as a cross-layer optimization problem with constraints on primary
session selection, flow routing and link scheduling. To search for solutions to
the optimization problem, we propose an augmented scheduling index ordering
based (SIO-based) algorithm to identify maximal independent sets. Through
extensive simulations, we demonstrate the effectiveness of the proposed NLC
approach and the superiority of the augmented SIO-based algorithm over the
traditional method.
