  In this paper, multiple reconfigurable intelligent surfaces (RIS) aided
secure precise wireless transmission (SPWT) schemes are proposed in the
three-dimensional (3D) wireless communication scenario. Unavailable direct path
channels from transmitter to receivers are considered when the direct paths are
obstructed by obstacles. Then, multiple RISs are utilized to achieve SPWT
through the reflection path among transmitter, RISs and receivers in order to
enhance the communication performance and energy efficiency simultaneously.
First, a maximum-signal-to-interference-and-noise ratio (MSINR) scheme is
proposed in a single user scenario. Then, the multi-user scenario is considered
where the illegitimate users are regarded as eavesdroppers. A
maximum-secrecy-rate (MSR) scheme and a maximum-signal-to-leakage-and-noise
ratio (MSLNR) are proposed. The former achieves a better secrecy rate (SR)
performance but incurs a higher complexity. The latter has a lower complexity
than the MSR scheme with an SR performance loss. Simulation results show that
both single-user scheme and multi-user scheme can achieve SPWT which transmits
confidential message precisely to location of desired users. Moreover, MSLNR
scheme has a lower complexity than the MSR scheme, while the SR performance is
close to that of the MSR scheme.
