I have finished a basic OFDM simulation with BPSK modulation on it. Now I want to use the convolutional code in transmitter and a multipath channel with AWGN. At receiver , a viterbi decoder would be used. But I have some question in it.First,when the data have done FFT at the receiver.There would be still imaginary part in signal,should I discard the imaginary part or not? Second,because of the IFFT block in OFDM,I don't know how to creat the trellis diagram. The IFFT seems disordered the states after convolutional coding.Third,if i couldn't use the viterbi to find the best path and reduce the multipath effect.Should I estimate the channel and equalize it? Sorry,I have many problems in this system.But i truely want to learn more about OFDM. Thanks for your advice, Spaz Bai

# OFDM + Convolutional code

Started by ●October 16, 2006

Reply by ●October 16, 20062006-10-16

spazspaz wrote:> I have finished a basic OFDM simulation with BPSK modulation on it. Now > I want to use the convolutional code in transmitter and a multipath > channel with AWGN. At receiver , a viterbi decoder would be used. > But I have some question in it.First,when the data have done FFT at the > receiver.There would be still imaginary part in signal,should I discard the > imaginary part or not?Yes, throw away.> Second,because of the IFFT block in OFDM,I don't > know how to creat the trellis diagram.These two statements are not dependent on one another, you need to elaborate.> The IFFT seems disordered the > states after convolutional coding.hmm...again, these two operations are usually treated independently, you need to explain exactly what you mean here.> Third,if i couldn't use the viterbi to > find the best path and reduce the multipath effect.Are you using the OFDM cyclic prefix to remove the multipath? i.e. is your prefix guard time greater than your max multipath delay?> Should I estimate the > channel and equalize it?I think this is where you should start *before* you include the convolutional coding. The big advantage with OFDM is the ease with which the channel frequency selectivity can be equalised. A conventional OFDM system usually does the following (assuming sync has been achieved) discard prefix -> FFT -> estimate chnl -> equalise -> demodulate the symbols When you get to this stage, you have a sequence of bits which can then be deinterleaved and fed through your FEC decoder.> Sorry,I have many problems in this system.But i truely want to learn > more about OFDM.It sounds to me like you've probably broken the back of the problem so far, it's just the tweaking that's the issue. col

Reply by ●October 16, 20062006-10-16

cb135@hotmail.com wrote:> spazspaz wrote: > > I have finished a basic OFDM simulation with BPSK modulation on it. Now > > I want to use the convolutional code in transmitter and a multipath > > channel with AWGN. At receiver , a viterbi decoder would be used. > > But I have some question in it.First,when the data have done FFT at the > > receiver.There would be still imaginary part in signal,should I discard the > > imaginary part or not? > > Yes, throw away.Not if you intend to equalise. -- Oli