<div>
<div>
<div>
<p>In the last decade, multi-hop cooperation has evolved bringing several advantages including coverage
improvement, more reliability of wireless links, and power consumption reduction. Still, its application
has raised several challenges, such as the need for secure transmission at each hop, algorithms to
perform relay selection and the accurate models to facilitate performance analysis. This paper addresses
the problem of physical layer (PHY) security in a multi-hop wireless cooperative network, where
communication at each hop is assisted by multiple relays forming a cluster, each cluster being surrounded
by multiple eavesdroppers which together may tap transmissions from both the source and the relays. The
main focus of the study is on analyzing the benefits of various relay selection schemes for protecting the
source-destination transmission against the eavesdroppers, which can collude and combine information
via diversity combining techniques. To be specific, four relay selection schemes, which differ in the
way they employ available measures link quality, are considered to deliver the source information to
the destination via a decode-and-forward (DF) strategy. To evaluate the security performance of the
multi-hop cooperative link in the presence of colluding eavesdroppers, we derive novel closed-form
analytical expressions for the secrecy outage probability (SOP) with consideration of special cases of
practical interest. </p>
</div>
</div>
</div>