  Intelligent reflecting surface (IRS) has emerged as a cost-effective solution
to enhance wireless communication performance via passive signal reflection.
Existing works on IRS have mainly focused on investigating IRS's passive
beamforming/reflection design to boost the communication rate for users
assuming that their channel state information (CSI) is fully or partially
known. However, how to exploit IRS to improve the wireless transmission
reliability without any CSI, which is typical in high-mobility/delay-sensitive
communication scenarios, remains largely open. In this paper, we study a new
IRS-aided communication system with the IRS integrated to its aided access
point (AP) to achieve both functions of transmit diversity and passive
beamforming simultaneously. Specifically, we first show an interesting result
that the IRS's passive beamforming gain in any direction is invariant to the
common phase-shift applied to all of its reflecting elements. Accordingly, we
design the common phase-shift of IRS elements to achieve transmit diversity at
the AP side without the need of any CSI of the users. In addition, we propose a
practical method for the users to estimate the CSI at the receiver side for
information decoding. Meanwhile, we show that the conventional passive
beamforming gain of IRS can be retained for the other users with their CSI
known at the AP. Furthermore, we derive the asymptotic performance of both
IRS-aided transmit diversity and passive beamforming in closed-form, by
considering the large-scale IRS with an infinite number of elements. Numerical
results validate our analysis and show the performance gains of the proposed
IRS-aided simultaneous transmit diversity and passive beamforming scheme over
other benchmark schemes.
