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 <[email protected] <mailto:[email protected]>> 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
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