  The physical layer security (PLS) is investigated for reconfigurable
intelligent surface (RIS) assisted wireless networks, where a source transmits
its confidential information to a legitimate destination with the aid of a
single small RIS in the presence of a malicious eavesdropper. A new joint RIS
location and passive beamforming (J-LPB) optimization scheme is proposed for
the sake of maximizing the secrecy rate under the RIS location constraint and
the constraint that the modulus of the reflecting coefficient at each RIS's
unit is not larger than 1. Specifically, we analyze the optimal location of the
RIS, and conclude that the product involving the source-RIS distance and the
RIS-destination distance should be minimized. Since the product minimization
problem is nonconvex, we then propose a two-tier optimization algorithm for
solving it. Based on the near-optimal RIS 3D location obtained, we further
formulate the passive beamforming optimization problem, and then propose to
apply the Charnes-Cooper transformation along with the sequential rank-one
constraint relaxation (SROCR) algorithm to solve it. Our numerical results show
that the secrecy rate of the proposed J-LPB optimization scheme is higher than
that of the benchmarks. Explicitly, we use the following benchmarks: the
near-source-based RIS location and passive beamforming (NSB-LPB) optimization
scheme, the near-destination-based RIS location and passive beamforming
(NDB-LPB) optimization scheme, and the random RIS location and passive
beamforming (R-LPB) optimization scheme. Finally, the benefits of our J-LPB
scheme are further increased with the number of RIS units.
