  We study the physical-layer security of a multiple source-destination (SD)
pairs coexisting wireless network in the face of an eavesdropper, where an
eavesdropper intends to wiretap the signal transmitted by the SD pairs. In
order to protect the wireless transmission against eavesdropping, we propose a
cooperation framework relying on two stages. Specifically, an SD pair is
selected to access the total allocated spectrum using an appropriately designed
scheme at the beginning of the first stage. The other source nodes (SNs)
simultaneously transmit their data to the SN of the above-mentioned SD pair
relying on an orthogonal way during the first stage. Then, the SN of the chosen
SD pair transmits the data packets containing its own messages and the other
SNs' messages to its dedicated destination node (DN) in the second stage, which
in turn will forward all the other DNs' data to the application center via the
core network. We conceive a specific SD pair selection scheme, termed as the
transmit antenna selection aided source-destination pair selection (TAS-SDPS).
We derive the secrecy outage probability (SOP) expressions for the TAS-SDPS, as
well as for the conventional round-robin source-destination pair selection
(RSDPS) and non-cooperative (Non-coop) schemes for comparison purposes.
Furthermore, we carry out the secrecy diversity gain analysis in the high
main-to-eavesdropper ratio (MER) region, showing that the TAS-SDPS scheme is
capable of achieving the maximum attainable secrecy diversity order.
