  This work considers load-balance control among the relays under the secure
transmission protocol via relay cooperation in two-hop wireless networks
without the information of both eavesdropper channels and locations. The
available two-hop secure transmission protocols in physical layer secrecy
framework cannot provide a flexible load-balance control, which may
significantly limit their application scopes. This paper proposes a secure
transmission protocol in case that the path-loss is identical between all pairs
of nodes, in which the relay is randomly selected from the first $k$ preferable
assistant relays. This protocol enables load-balance among relays to be
flexibly controlled by a proper setting of the parameter $k$, and covers the
available works as special cases, like ones with the optimal relay selection
($k=1$) and ones with the random relay selection ($k = n$, i.e. the number of
system nodes). The theoretic analysis is further provided to determine the
maximum number of eavesdroppers one network can tolerate by applying the
proposed protocol to ensure a desired performance in terms of the secrecy
outage probability and transmission outage probability.
