# 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.

• 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.

• Is there a constant pattern of how many subcarriers are unused in the payload data? Jun 1, 2016 at 14:58
• @MarcusMüller Yes, I've edited the question to include the payload symbols too. It seems that every 18th subcarrier is silent. Jun 2, 2016 at 7:24
• @jhegedus: Note that the typical term for unused carriers in OFDM is virtual carriers. A Google search yields many mentions in the literature on methods for exploiting virtual carriers to aid in synchronization. That could be one justification for including them. Jun 2, 2016 at 13:04

• If you have to deal with OFDM it is best to have some background in signal theory/communication engineering. Basically you have to look at synchronization and equalization techniques. The spread of the energy loss over a large spectrum is an educated guess but could work this way. Also I guess you could develop some kind of tracking of the CFO based on the virtual subcarrier positions. When searching for wifi virtual subcarriers Google suggests: MIMO-OFDM for LTE, WiFi and WiMAX. Jun 14, 2016 at 22:59