Why are OFDM Pilot tones repeated?

[Weaver]

Homework Statement

Pilot tones can be used to detect the precise start of a symbol in OFDM.
Discuss why more than one pilot tone is needed to achieve this

Relevant Equations

N/A

So I understand the pilot tones are used for synchronisation, ensuring the phase of an OFDM signal is correct. It's for fine grained control after using the pre cylicguards to determine the rough start of a signal

My best guess for repeating pilot tones is redundancy incase there's noise that distorts a pilot tone(s)

I'm not sure what else there would be? As with my understanding, assuming the signal was received perfectly, one pilot tone would work? As you will know the signal is in phase, once the pilot tone amplitude is correct?

Or is it possible to somehow get the phrase for the pilot tone correct but the signal incorrect?

The question is worth 8 marks so I assume I am missing something else.

Any help or advice is appreciated

Thanks

 

[Joshy]

I'm curious to know the answer too. I'm having a hard time understanding, but something I see popping up often in casual conversation about OFDM with friends and my interpretation from Razavi's book is due to the spreading.

 

[Tom.G]

It is to characterize the communication channel and set the receiver gains for the individual frequencies. For details see Sec. 2.3 et seq of:
https://res.mdpi.com/d_attachment/applsci/applsci-10-02173/article_deploy/applsci-10-02173-v2.pdf

(found with:
https://www.google.com/search?&q=repeated+pilot+tones+in+OFDM)

Cheers,
Tom