  This paper investigates the physical layer security issue of a
device-to-device (D2D) underlaid cellular system with a multi-antenna base
station (BS) and a multi-antenna eavesdropper. To investigate the potential of
D2D communication in improving network security, the conventional network
without D2D users (DUs) is first considered. It is shown that the problem of
maximizing the sum secrecy rate (SR) of cellular users (CUs) for this special
case can be transformed to an assignment problem and optimally solved. Then, a
D2D underlaid network is considered. Since the joint optimization of resource
block (RB) allocation, CU-DU matching and power control is a mixed integer
programming, the problem is difficult to handle. Hence, the RB assignment
process is first conducted by ignoring D2D communication, and an iterative
algorithm is then proposed to solve the remaining problem. Simulation results
show that the sum SR of CUs can be greatly increased by D2D communication, and
compared with the existing schemes, a better secrecy performance can be
obtained by the proposed algorithms.
