OFDM preamble

GNU Radio
1

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

2

On Fri, Jul 1, 2011 at 2:36 AM, John W. [email protected] wrote:

  1. 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