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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.

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  • $\begingroup$ Is there a constant pattern of how many subcarriers are unused in the payload data? $\endgroup$ – Marcus Müller Jun 1 '16 at 14:58
  • $\begingroup$ @MarcusMüller Yes, I've edited the question to include the payload symbols too. It seems that every 18th subcarrier is silent. $\endgroup$ – jhegedus Jun 2 '16 at 7:24
  • $\begingroup$ @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. $\endgroup$ – Jason R Jun 2 '16 at 13:04
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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 & Cox synchronsiation pattern.

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  • $\begingroup$ Thanks Andrej, is there any book/article you could recommend on this topic ? Where can I read more about the background behind your answer ? Something that would help me to understand your answer. Perhaps I need to read up on this topic before asking more technical questions. $\endgroup$ – jhegedus Jun 3 '16 at 8:01
  • $\begingroup$ 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. $\endgroup$ – Andrej Rode Jun 14 '16 at 22:59
  • $\begingroup$ Thanks Andrej, So the spreading of the energy loss is good for diversity, I assume, so interference can be reduced ? So in some sense, this is a way to do frequency hopping spread spectrum within OFDM ? $\endgroup$ – jhegedus Jun 15 '16 at 7:29
  • $\begingroup$ I assume you can mitigate interference which is on a specific frequency band and otherwise reducing your ability to synchronize in time and frequency. I don't think this is frequency hopping spread spectrum since you are still using the same spectrum for all of your OFDM-Symbols. Only the position of the virtual carriers differs. Maybe there are some more advantages with usage of MIMO. But I can't make a guess. $\endgroup$ – Andrej Rode Jun 23 '16 at 14:44

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