Forums

What is the importance of adding header and tail in OFDM system?

Started by runinrainy 2 years ago4 replieslatest reply 2 years ago161 views

Hello,

I am trying to understand the functionality of header and tail in #OFDM system. I know the question sounds like a general question but I have made some search on the web but couldn,t get a clear description for the header and tail parameters in OFDM sysem. I understand the Cyclic Prefix  and it's functionality but there are also 'tail and header' addition in OFDM. What is the purpose of adding these two in  OFDM? Are they just functioning for storing an additional information about the PHY layer of Tx-Rx system or at Receiver they can be used to obtain the signal from the multipath channel?

Many thanks!


[ - ]
Reply by napiermJuly 2, 2017

Hello,

I think what you mean is the Rolloff Period (RP).  Also the Rolloff Prefix or Rolloff Postfix depending on if it is at the front or back of the symbol.

It is there to add a time-domain window to each symbol for spectral control.  More samples are added in front of the Cyclic Prefix (CP) to allow for the roll off.  The same number of samples are added behind the data symbol.  The samples of the previous Rolloff Postfix overlap the current Rolloff Prefix and are added together.

The magnitude of the RP samples are typically shaped by the Raised Cosine (RC) window although others are used too.  There is a nice trick where the phase of the RC window is adjusted so that the sum of the two RPs add to 1 for every sample.  That way there is a very smooth transition from one symbol to the next.



[ - ]
Reply by artmezJuly 3, 2017

As a general observation for any kind of digital transmission: a header generally serves for synchronization (e.g. clock) and the tail often serves to prevent misreads of data mid-stream (similar to a floppy or hard-drive soft sectoring) or for half-duplex RF links, it simplifies the timing for make sure the transmitter is not turned off in the middle of an outbound data stream.

[ - ]
Reply by Y(J)SJuly 3, 2017

Not all OFDM systems use head and tail times in addition to the cyclic prefix.

Those systems that perform windowed overlap-and-add can benefit from lower out-of-band components if the increasing and decreasing parts of the window function overlap zero padding.


Y(J)S

[ - ]
Reply by SlartibartfastJuly 3, 2017

The terminology that you're using, "header" and "tail", are ambiguous in this context.   I think you need to clarify what you mean.   By "header" do you mean a preamble, like what is used for burst synchronization, or a physical-layer header (like the PLCP in 802.11) that tells the system information about the packet format, etc.?   Likewise "tail" may mean the rolloff period or it may mean a few bits added to the stream for tail-biting in the FEC.

I don't think you'll get focused answers until you clarify.