  Channel estimation is the main hurdle to reaping the benefits promised by the
intelligent reflecting surface (IRS), due to its absence of ability to
transmit/receive pilot signals as well as the huge number of channel
coefficients associated with its reflecting elements. Recently, a breakthrough
was made in reducing the channel estimation overhead by revealing that the
IRS-BS (base station) channels are common in the cascaded user-IRS-BS channels
of all the users, and if the cascaded channel of one typical user is estimated,
the other users' cascaded channels can be estimated very quickly based on their
correlation with the typical user's channel \cite{b5}. One limitation of this
strategy, however, is the waste of user energy, because many users need to keep
silent when the typical user's channel is estimated. In this paper, we reveal
another correlation hidden in the cascaded user-IRS-BS channels by observing
that the user-IRS channel is common in all the cascaded channels from users to
each BS antenna as well. Building upon this finding, we propose a novel
two-phase channel estimation protocol in the uplink communication.
Specifically, in Phase I, the correlation coefficients between the channels of
a typical BS antenna and those of the other antennas are estimated; while in
Phase II, the cascaded channel of the typical antenna is estimated. In
particular, all the users can transmit throughput Phase I and Phase II. Under
this strategy, it is theoretically shown that the minimum number of time
instants required for perfect channel estimation is the same as that of the
aforementioned strategy in the ideal case without BS noise. Then, in the case
with BS noise, we show by simulation that the channel estimation error of our
proposed scheme is significantly reduced thanks to the full exploitation of the
user energy.
