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.
http://lists.gnu.org/archive/html/discuss-gnuradio/2010-09/msg00168.html;
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??
Thank You.
In case if u need to look at my transmitter model and parameters
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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