  In multi-cast scenario, all desired users are divided into $K$ groups. Each
group receives its own individual confidential message stream. Eavesdropper
group aims to intercept $K$ confidential-message streams. To achieve a secure
transmission, two secure schemes are proposed: maximum group receive power plus
null-space (NS) projection (Max-GRP plus NSP) and leakage. The former obtains
its precoding vector per group by maximizing its own group receive power
subject to the orthogonal constraint, and its AN projection matrix consist of
all bases of NS of all desired steering vectors from all groups. The latter
attains its desired precoding vector per group by driving the current
confidential message power to its group steering space and reducing its power
leakage to eavesdropper group and other $K-1$ desired ones by maximizing signal
to leakage and noise ratio (Max-SLNR). And its AN projection matrix is designed
by forcing AN power into the eavesdropper steering space by viewing AN as a
useful signal for eavesdropper group and maximizing AN to leakage-and-noise
ratio (Max-ANLNR). Simulation results show that the proposed two methods are
better than conventional method in terms of both bit-error-rate (BER) and
secrecy sum-rate per group. Also, the leakage scheme performs better than
Max-GRP-NSP , especially in the presence of direction measurement errors.
However, the latter requires no channel statistical parameters and thus is
simpler compared to the former.
