  Directional modulation (DM) is a physical layer security technique based on
antenna arrays and so far the polarisation information has not been considered
in its designs. To increase the channel capacity, we consider exploiting the
polarisation information and send two different signals simultaneously at the
same direction, same frequency, but with different polarisations. These two
signals can also be considered as one composite signal using the four
dimensional (4-D) modulation scheme across the two polarisation diversity
channels. In this paper, based on cross-dipole arrays, we formulate the design
to find a set of common weight coefficients to achieve directional modulation
for such a composite signal and examples are provided to verify the
effectiveness of the proposed method.
