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[Discuss-gnuradio] Problem in designing Coded OFDM Rxr (using trellis-vi

From: Venkat Vinod
Subject: [Discuss-gnuradio] Problem in designing Coded OFDM Rxr (using trellis-viterbi )
Date: Thu, 11 Nov 2010 23:34:44 -0600

Hello Everyone,
    I'm trying to implement a coded OFDM system by using the trellis blocks. I  used benchmark_ofdm  and trellis coded modulation(gr_trellis)  provided in the  GNU Radio.

I have achieved the transmission similar to previous post on VIterbi-OFDM ---by Tobias Schmid; but,I couldn't understand his design for the receiver.

For Reception
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. 

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??

Thank You.

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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