What can be an example of a pilot sequence for channel estimation in the case of OFDM or NOMA? Could it be any sequences, e.g., a sequence of 1's, or should it have some rules to follow? If so, what are these rules? And how can we set the number of required pilots for channel estimation?
A sequence of 1s would be fine, but one can also utilize the degrees-of-freedom in the sequence design to achieve some desired properties. The Zadoff-Chu sequences that are used by 3GPP has the feature that every pilot sequence is a cyclic switch of a another pilot sequence.
The number of pilots (= the length of the pilot sequence) must be equal to (or larger) number of unknowns that should be estimated at every receive antenna.
I write about pilot sequences in Chapter 3 of the book Massive MIMO networks, which you can download from https://massivemimobook.com
You can use Zadoff-Chu sequences or PN-sequences for channel estimation. As said by Emil Björnson Zadoff-Chu sequences are already in use for 3GPP LTE. Zadoff-Chu sequences exhibit excellent cross-correlation properties and therefore can be useful for channel estimation in MIMO-OFDM wireless communication system.
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