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

packet_mod_X: 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. 'preamble' is fixed. 'access code' can be left as default (customisable preamble)

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)