  In this paper, we investigate the impacts of transmitter and receiver windows
on orthogonal time-frequency space (OTFS) modulation and propose a window
design to improve the OTFS channel estimation performance. Assuming ideal pulse
shaping filters at the transceiver, we first identify the role of window in
effective channel and the reduced channel sparsity with conventional
rectangular window. Then, we characterize the impacts of windowing on the
effective channel estimation performance for OTFS modulation. Based on the
revealed insights, we propose to apply a Dolph-Chebyshev (DC) window at either
the transmitter or the receiver to effectively enhance the sparsity of the
effective channel. As such, the channel spread due to the fractional Doppler is
significantly reduced, which leads to a lower error floor in channel estimation
compared with that of the rectangular window. Simulation results verify the
accuracy of the obtained analytical results and confirm the superiority of the
proposed window designs in improving the channel estimation performance over
the conventional rectangular or Sine windows.
