In this letter, we consider a double-active-intelligent reflecting surface
(IRS) aided wireless communication system, where two active IRSs are properly
deployed to assist the communication from a base station (BS) to multiple users
located in a given zone via the double-reflection links. Under the assumption
of fixed per-element amplification power for each active-IRS element, we
formulate a rate maximization problem subject to practical constraints on the
reflection design, elements allocation, and placement of active IRSs. To solve
this non-convex problem, we first obtain the optimal active-IRS reflections and
BS beamforming, based on which we then jointly optimize the active-IRS elements
allocation and placement by using the alternating optimization (AO) method.
Moreover, we show that given the fixed per-element amplification power, the
received signal-to-noise ratio (SNR) at the user increases asymptotically with
the square of the number of reflecting elements; while given the fixed number
of reflecting elements, the SNR does not increase with the per-element
amplification power when it is asymptotically large. Last, numerical results
are presented to validate the effectiveness of the proposed AO-based algorithm
and compare the rate performance of the considered double-active-IRS aided
wireless system with various benchmark systems