Why do the OFDM training (and payload) symbols have silent sub-carriers (aka virtual carriers)? I am examining an OFDM signal. 

Training symbol:


The first two OFDM symbols are training symbols, they are 2PSK modulated (within the subcarriers). 
In the first OFDM symbol the $(0+18N)^{\rm th}$ subcarriers are silent ($N$ is integer). 
In the second OFDM symbol the $(9+18N)^{\rm th}$ subcarriers are silent.


Payload:


In the first payload symbol, i.e. in the $3^{\rm rd}$ OFDM symbol, the $(6 + 18N)^{\rm th}$ subcarriers are silent.
In the second payload symbol, i.e. in the $4^{\rm th}$ OFDM symbol, the $(6 +9 + 18N)^{\rm th}$ subcarriers are silent.
In the third payload symbol, i.e. in the $5^{\rm th}$ OFDM symbol, the $(3 + 18N)^{\rm th}$ subcarriers are silent.
In the $4^{\rm th}$ payload symbol, i.e. in the $6^{\rm th}$ OFDM symbol, the $(3 +9 + 18N)^{\rm th}$ subcarriers are silent.
The payload symbols are QPSK modulated.


Then this pattern seems to repeat.

Pattern:


So to summarise, the offsets are : $0, 9, 6, 6 + 9, 3, 3 + 9, 0, 9, \ldots$ 


Questions:


How does this make sense ? 
What purpose does such a unique silencing of subcarriers serve ?
I suspect it has to do with equalizations , but why is silencing useful ? 
And why this odd way of silencing the subcarriers ? 
Is this (way of silencing subcarriers) somehow connected to the Schmidl and Cox method ? 


EDIT: Google gives this on "OFDM silent subcarriers"

EDIT2: The signal has been emitted by a MIMO system.

EDIT3: This is the Google search for "OFDM virtual carriers".

EDIT4: This is a 1024 carrier OFDM signal.
From the first to the sixth OFDM-Symbol you have spread virtual carriers equally across [0,..,51] with a distance of 3. With this way you have spread your 'energy' loss over a wide spectrum. In difference to having a single carrier serving as virtual carrier all the time you can transmit information over every 'data' carrier and maybe have a constant mean energy level across all subcarriers.

You could use this virtual carries to detect carrier frequency offset (CFO) since you know the pattern. In the worst case you have to store 6 OFDM-Symbols to find the beginning of your pattern. 

If you have an OFDM-Symbol with 64 subcarriers you can set 6 carriers at each end as subcarrier and use the other 52 with this virtual carrier pattern to transmit information with equalization.

I think this virtual carrier pattern is not connected to the Schmidl &amp; Cox synchronsiation pattern.