For Reception Problem: The channel decoding should be performed on the output of ofdm_demod block(ofdm_frame_sink ; the last block that produces demodulated OFDM symbols). But, the combined_ viterbi block performs the demodulation of BPSK or QPSK based on the signal constellation and provides appropriate metrics to viterbi_algorithm.
Approach: I tried to avoid ofdm_frame sink that performs the demodulation of OFDM symbols and passed the output of synchronized OFDM symbols(i.e., the output of ofdm_frame_acquistion) onto a viterbi_combined block and then packed into a message to check the status of crc.(All packets received are False).
Flowgraph Model usrp ------> channel filter ------> symbol synchronization ----------> FFT -------> ofdm_frame_acquistion --------> vector to stream ------->viterbi_combined ---------> unpack to pack -------> packets to a message sink
I have few Questions !!!!!!!! 1.How can I append viterbi_ combined block to the OFDM demodulator blocks?? 2 .Do I need to pass the trellis modulated symbols to ofdm_mod block instead of encoded byte stream??( I think that allows to use the demodulated ofdm symbols through viterbi_combined ).If so,How can I achieve it??
Hope you guys respond to my email. As, I have seen lot of people working on them.Correct me if my analysis or understanding went wrong??
In case if u need to look at my transmitter model and parameters
trellis finite state machine of rate 1/2 and 4 state (example of awgn102_4.fsm)
Modulation(QPSK)--since the rate of convolution encoder is 1/2
Default ofdm parameters (FFT lenngth-512 , occupied tones-200)
For Transmission : I have achieved the transmitter blocks by passing the packet (containing payload and CRC) into a message queue that counts the incoming items, when reached certain limit passes the message to a trellis encoder that is bypassed from a unpack to pack module(for proper input stream to the encoder).The output of trellis encoder is packed into a message and passed through the ofdm_mod block.
Flow Graph Model packets from message source --------> pack to unpack---------->trellis encoder --------> unpack to pack------->packets to a message sink ofdm_mod(packets from message sink) -------> ampl--------> usrp.
-- Venkat Vinod Patcha Tel: +1 225 328 7356
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[Discuss-gnuradio] Problem in designing Coded OFDM Rxr (using trellis-viterbi ),