Re: [Discuss-gnuradio] pilot symbols in ofdm

[Martin Braun]

On 11.06.2014 17:18, Karan Talasila wrote:
> Hi Martin,
>                  Thanks for the reply. I would like to just clarify two
> questions I have from your comment.
>
> 1.  In gnuradio packet structure , what is difference between frame and
> packet? I see that packet consists of preamble,header,payload and crc
> fields. But what does a frame mean. Is it  the entire data that is sent
> from the transmitter which is then broken into packets or is it
> something else.

I think they can be used interchangeably.

> 2. what is the  payload size in a packet. Like I think that every packet
> consists of a few ofdm symbols(maybe around 2 or 3 FFT length symbols)
> as a payload along with preamble, header and crc. Is the number  of ofdm
> symbols to be sent as payload fixed or variable for a  packet.

You set the length of the packet. The ofdm_tx hier block takes a tagged 
stream as entry, and every streamed packet will be one OFDM packet.

M
> On Wed, Jun 11, 2014 at 7:07 PM, Martin Braun <address@hidden
> <mailto:address@hidden>> wrote:
>
>     On 11.06.2014 14:43, Karan Talasila wrote:
>
>         Hi
>
>         Looking at the ofdm receiver at ofdm_rx grc, I see that schmidl
>         and cox
>         synchronisation algorithm uses sync1 and sync2 which are two ofdm
>         training symbols that help in frequency offset correction and also
>         timing estimate to detect the start of the packet.
>
>         Then there is channel estimator block in grc that again uses
>         preamble
>         ofdm symbol  to detect coarse frequency offset. In the entire
>         implementation I haven't seen the use of pilot symbols in each ofdm
>         symbol for either channel estimation or synchronisation. So does
>         that
>         mean that if you are using a packet structure with a preamble
>         there is
>         no need of pilots for any receiver processing.
>
>         I get this doubt because benchmark_rx.py which uses ofdm gives you a
>         terminal option saying occupied tones which can be set by user at
>         terminal. So if you can set or increase occupied tones to any number
>         less than fft length, then that means you can be flexible with
>         increasing or decreasing pilot symbols to a value that you want.
>         Am I right?
>
>
>     Yes, you can use zero pilot carriers. They are used in the
>     ofdm_frame_equalizer_vcvc, but our current implementations aren't
>     very good. For longer frames, I recommend using pilot symbols
>     (distributed in time, not frequency!), but for short packets, the
>     initial estimation is sometimes enough, if your setup is two USRPs
>     close to each other and you have something like 10 OFDM symbols total.
>
>     M
>
>
>
>
> --
> Regards
> Karan Talasila