  Active intelligent reflecting surface (IRS) is a revolutionary technique for
the future 6G networks. The conventional far-field single-IRS-aided directional
modulation(DM) networks have only one (no direct path) or two (existing direct
path) degrees of freedom (DoFs). This means that there are only one or two
streams transmitted simultaneously from base station to user and will seriously
limit its rate gain achieved by IRS. How to create multiple DoFs more than two
for DM? In this paper, single large-scale IRS is divided to multiple small IRSs
and a novel multi-IRS-aided multi-stream DM network is proposed to achieve a
point-to-point multi-stream transmission by creating $K$ ($\geq3$) DoFs, where
multiple small IRSs are placed distributively via multiple unmanned aerial
vehicles (UAVs). The null-space projection, zero-forcing (ZF) and phase
alignment are adopted to design the transmit beamforming vector, receive
beamforming vector and phase shift matrix (PSM), respectively, called
NSP-ZF-PA. Here, $K$ PSMs and their corresponding beamforming vectors are
independently optimized. The weighted minimum mean-square error (WMMSE)
algorithm is involved in alternating iteration for the optimization variables
by introducing the power constraint on IRS, named WMMSE-PC, where the
majorization-minimization (MM) algorithm is used to solve the total PSM. To
achieve a lower computational complexity, a maximum trace method, called
Max-TR-SVD, is proposed by optimize the PSM of all IRSs. Numerical simulation
results has shown that the proposed NSP-ZF-PA performs much better than
Max-TR-SVD in terms of rate. In particular, the rate of NSP-ZF-PA with sixteen
small IRSs is about five times that of NSP-ZF-PA with combining all small IRSs
as a single large IRS. Thus, a dramatic rate enhancement may be achieved by
multiple distributed IRSs.
