  In this paper, we look into the issue of intra-cell uplink (UL) pilot
orthogonalization and schemes for mitigating the inter-cell pilot contamination
with a realistic massive multi-input multi-output (MIMO) orthogonal
frequency-division multiplexing (OFDM) system model. First, we show how to
align the power-delay profiles (PDP) of different users served by one BS so
that the pilots sent within one common OFDM symbol are orthogonal. From the
derived aligning rule, we see much more users can be sounded in the same OFDM
symbol as their channels are sparse in time. Second, in the case of massive
MIMO, we show how PDP alignment can help to alleviate the pilot contamination
due to inter-cell interference. We demonstrate that, by utilizing the fact that
different paths in time are associated with different angles of arrival (AoA),
the pilot contamination can be significantly reduced through aligning the PDPs
of the users served by different BSs appropriately. Computer simulations
further convince us PDP aligning can serve as the new baseline design
philosophy for the UL pilots in massive MIMO.
