  In this letter, we consider an intelligent reflecting surface (IRS)-aided
wireless communication system, where an active or passive IRS is employed to
assist the communication between an access point and a user. First, we consider
the downlink/uplink communication separately and optimize the IRS placement for
rate maximization with an active or passive IRS. We show that the active IRS
should be deployed closer to the receiver with the IRS's decreasing
amplification power; while in contrast, the passive IRS should be deployed near
either the transmitter or receiver. Moreover, with optimized IRS placement, the
passive IRS is shown to outperform its active counterpart when the number of
reflecting elements is sufficiently large and/or the active-IRS amplification
power is too small. Next, we optimize the IRS placement for both active and
passive IRSs to maximize the weighted sum-rate of uplink and downlink
communications. We show that in this case, the passive IRS is more likely to
achieve superior rate performance. This is because the optimal active-IRS
placement needs to balance the rate performance in the uplink and downlink,
while deploying the passive IRS near the transmitter or receiver is optimal
regardless of the uplink or downlink.
