I've seen some practical deployments that are based on LTE(-A) technology, which uses TTIs of 1 ms.
In those systems, assuming that the coherence time is greater than 1 ms, where for normal CP we have 14 symbols and therefore, 14 sub-carriers per frequency-bin per sub-frame, how can we have more than 14 different pilot sequences?
Thanks and Kind Regards,
Felipe
Check out the paper "Performance of Conjugate and Zero-Forcing Beamforming in Large-Scale Antenna Systems" by Yang and Marzetta. They treat 14 subcarriers and 14 OFDM systems as the coherence block, leading to 196 symbols, so there is "room for at most 196 orthogonal pilot sequences." In practice, you don't want to spend more than half of that on pilots, so that there is space for pilots.
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