  Movable antenna (MA) has emerged as a promising technology to enhance
wireless communication performance by enabling the local movement of antennas
at the transmitter (Tx) and/or receiver (Rx) for achieving more favorable
channel conditions. As the existing studies on MA-aided wireless communications
have mainly considered narrow-band transmission in flat fading channels, we
investigate in this paper the MA-aided wideband communications employing
orthogonal frequency division multiplexing (OFDM) in frequency-selective fading
channels. Under the general multi-tap field-response channel model, the
wireless channel variations in both space and frequency are characterized with
different positions of the MAs. Unlike the narrow-band transmission where the
optimal MA position at the Tx/Rx simply maximizes the single-tap channel
amplitude, the MA position in the wideband case needs to balance the amplitudes
and phases over multiple channel taps in order to maximize the OFDM
transmission rate over multiple frequency subcarriers. First, we derive an
upper bound on the OFDM achievable rate in closed form when the size of the
Tx/Rx region for antenna movement is arbitrarily large. Next, we develop a
parallel greedy ascent (PGA) algorithm to obtain locally optimal solutions to
the MAs' positions for OFDM rate maximization subject to finite-size Tx/Rx
regions. To reduce computational complexity, a simplified PGA algorithm is also
provided to optimize the MAs' positions more efficiently. Simulation results
demonstrate that the proposed PGA algorithms can approach the OFDM rate upper
bound closely with the increase of Tx/Rx region sizes and outperform
conventional systems with fixed-position antennas (FPAs) under the wideband
channel setup.
