OFDM

OFDM Mod GRC block

packet_mod_X(ofdm_mod(<options>)) - argument is 'packet_source'

packet_mod:

grc/grc_gnuradio/blks2/packet.py

packet_utils:

<core>/python/gnuradio/packet_utils.py

Default payload: 512 bytes (if supplied length is default 0, must be multiple of input stream size)
Input signature: X
Output signature: inherited from 'packet_source' stream #0 (complex for ofdm)

Starts packet encoder thread that packages incoming data into payload size and submits to 'packet_source' via 'send_pkt' call.

packet_utils.make_packet: preamble, access code, length of payload and CRC, (optionally whitened) payload with CRC-32, end byte.

ofdm_mod:

<core>/python/gnuradio/blks2impl/ofdm.py

_pkt_input (ofdm_mapper_bcv) -> preambles (ofdm_insert_preamble) -> ifft (fft_vcc) -> cp_adder (ofdm_cyclic prefixer) -> scale (multiply_const_cc)

Padded preamble generation (for correlation in time domain):

zeros_on_left = ceil((FFT length - occupied tones) / 2.0)
ksfreq = take 'occupied symbols' length from known_symbols_4512_3 (randomly generated fixed list of 1/-1, 4512 items long (=4000+512 ?))
Each item (offset by zeros_on_left) that has an odd index is zeroed.
Padded preamble is FFT length, with zeros_on_left, then ksfreq hardcoded symbols, then remaining zeros on right.

ofdm_mapper_bcv: constellation based on modulation, message queue limit (default: 2), occupied tones, FFT length

ofdm_insert_preamble (insert 'pre-modulated' preamble symbols before each payload): FFT length, padded preambles

ofdm_cyclic prefixer (prefix output with CP length from end of input symbols): FFT length, symbol length (= FFT length + cyclic prefix length)

'multiply_const_cc: 1.0 / sqrt(FFT length)