  Millimeter-wave (mmWave) communications are highly promising to improve the
capacity of modern wireless networks, while the physical layer security (PLS)
techniques hold great potential to enhance the critical secrecy performance
therein. By carefully exploiting the significant signal difference between the
Non-Light-of-Sight (NLoS) and Line-of-Sight (LoS) mmWave links, this paper
proposes a Sight-based Cooperative Jamming (SCJ) scheme to improve the PLS
performance of mmWave ad hoc communications. In this scheme, each potential
jammer that has no LoS link to its nearest receiver but may have LoS links to
eavesdroppers is selected with a certain probability to generate artificial
noise such that channel advantages at legitimate receivers can be achieved. For
performance modeling of the new jamming scheme, novel and efficient theoretical
approximation approaches are firstly developed to enable the challenging issue
of interference distribution modeling to be tackled, and then a theoretical
framework based on stochastic geometry is proposed to capture the secrecy
transmission capacity behavior under the SCJ scheme. Finally, extensive
numerical results are provided to illustrate the SCJ scheme under various
network scenarios.
