  Pilot contamination in the uplink (UL) can severely degrade the channel
estimation quality at the base station (BS) in a massive multi-input
multi-output (MIMO) system. Thus, it is critical to explore all possible
avenues to enable more orthogonal resources for the users to transmit
non-interfering UL pilots. In conventional designs, pilot orthogonality
typically assumes constant channel gains over time, which limits the amount of
orthogonal resources in the case of time-selective channels. To circumvent this
constraint, in this paper, we show how to enable orthogonal multiplexing of
pilots in the case of Doppler fading by aligning the power spectrum densities
(PSD) of different users. From the derived PSD aligning rules, we can see
multiple users can be sounded simultaneously without creating/suffering pilot
contamination even when these users are experiencing time-varying channels.
Furthermore, we provide analytical formulas characterizing the channel
estimation mean square error (MSE) performance. Computer simulations further
confirm us the PSD alignment can serve as one important decontamination
mechanism for the UL pilots in massive MIMO.
