  In this work, we study a full-duplex (FD) cloud radio access network (C-RAN)
from the aspects of infrastructure sharing and information secrecy, where the
central unit utilizes FD remote radio units (RU)s belonging to the same
operator, i.e., the trusted RUs, as well as the RUs belonging to other
operators or private owners, i.e., the untrusted RUs. Furthermore, the
communication takes place in the presence of untrusted external receivers,
i.e., eavesdropper nodes. The communicated uplink (UL) and downlink (DL)
waveforms are quantized in order to comply with the limited capacity of the
fronthaul links. In order to provide information secrecy, we propose a novel
utilization of the quantization noise shaping in the DL, such that it is
simultaneously used to comply with the limited capacity of the fronthaul links,
as well as to degrade decoding capability of the individual eavesdropper and
the untrusted RUs for both the UL and DL communications. In this regard,
expressions describing the achievable secrecy rates are obtained. An
optimization problem for jointly designing the DL and UL quantization and
precoding strategies are then formulated, with the purpose of maximizing the
overall system weighted sum secrecy rate. Due to the intractability of the
formulated problem, an iterative solution is proposed, following the successive
inner approximation and semi-definite relaxation frameworks, with convergence
to a stationary point. Numerical evaluations indicate a promising gain of the
proposed approaches for providing information secrecy against the untrusted
infrastructure nodes and/or external eavesdroppers in the context of FD C-RAN
communications.
