  Reconfigurable intelligent surface (RIS) is a promising technology for future
wireless communication systems. Channel estimation (CE) of RIS device is a
critical but also challenging issue for its development. The mainstream of
existing CE methods is confined to the so-called cascaded channel (CscdChn)
estimation scheme, which treats the multiplicative two-hop RIS channels as an
effective one and measures it as a whole. This CscdChn training method suffers
from severe double-fading attenuation loss, which significantly degrades the CE
accuracy. In this paper, we propose a novel RIS-transmitting (RIS-TX) based CE
scheme, which has lower pilot overhead than CscdChn scheme and effectively
overcomes the double-fading curse via incorporating only one single transmit
radio frequency (RF)-chain into RIS. We develop highly efficient gradient
descent (GD) and penalty duality decomposition (PDD)-based solutions to resolve
the pilot design task for the RIS-TX CE scheme, which is a difficult quartic
optimization problem. Our designed pilot signal outperforms the discrete
Fourier transform (DFT) sequence, which is reported to be optimal for CscdChn
scheme. Besides, both theoretical analysis and numerical results demonstrate
that our proposed RIS-TX scheme exhibits distinct performance characteristics
as opposed to its CscdChn counterpart and yields superior accuracy when RIS
device is not extremely large.
