  The deployment of unmanned aerial vehicle (UAV) for surveillance and
monitoring gives rise to the confidential information leakage challenge in both
civilian and military environments. The security and covert communication
problems for a pair of terrestrial nodes against UAV surveillance are
considered in this paper. To overcome the information leakage and increase the
transmission reliability, a multi-hop relaying strategy is deployed. We aim to
optimize the throughput by carefully designing the parameters of the multi-hop
network, including the coding rates, transmit power, and required number of
hops. In the secure transmission scenario, the expressions of the connection
probability and secrecy outage probability of an end-to-end path are derived
and the closed-form expressions of the optimal transmit power, transmission and
secrecy rates under a fixed number of hops are obtained. In the covert
communication problem, under the constraints of the detection error rate and
aggregate power, the sub-problem of transmit power allocation is a convex
problem and can be solved numerically. Simulation shows the impact of network
settings on the transmission performance. The trade-off between
secrecy/covertness and efficiency of the multi-hop transmission is discussed
which leads to the existence of the optimal number of hops.
