Re: [Discuss-gnuradio] OFDM preamble

[Tom Rondeau]

On Fri, Jul 1, 2011 at 2:36 AM, John Waugh <address@hidden> wrote:

Hi.
I wanted to know about the preambles that are inserted in the ofdm packet. I know they are inserted for the synchronization and frequency offset purposes.
But I want to know where in the OFDM packet they are inserted. I have following questions.

1. How can I vary the length of preambles? Can I do that ?

2. On which parameters length of preamble depends?

3. What is the length of preamble sequence if I am using fft-length=512, occupied carriers=200 cp-length=128, size of ofdm packet=400 bytes, modulation='bpsk'.

4. Where are they inserted in the OFDM packet? I mean before Payload?

       --------------------------------------------
      preamble| payload|
       --------------------------------------------
5.  Both the preambles are inserted at the start ?

6. Which modulation is used for preambles?

Need your help

Regards

John,

Look in gnuradio-core/src/python/gnuradio/blks2impl/ofdm.py at the ofdm_mod class. This is where we construct and insert the preamble, using gr_ofdm_insert_preamble.

The preamble we use is based on the Schmidl and Cox model, where we take one OFDM symbol and, in time, repeat the same info twice. The length of the symbol is based on the occupied tones, it is then padded to fft-length to fill the entire OFDM symbol.

The repetition is created in the frequency domain by setting every other subcarrier to 0, which is the same as repeating the pattern in time. Likewise, if we set two subcarriers to 0, that is  X00X00X00..., you would get a triple redundancy in time. The receiver can then correlate against the repetition in time to recovery the frequency and timing offset.

Tom