sudhi wrote:> Hello, > > I was looking at the DVB-H standard which has a serially concatenated > encoding with Reed Solomon code (RS(204,188, t=8) from RS(255,239, t=8) > ) as the outer code and Convolution code (K=7) as the inner code. At > the receiver, I can think of at least four decoding options, > 1. Viterbi followed by RS decoder. > 2. MAP (BCJR) followed by RS decoder. > 3. MAP followed by soft input RS decoder. > 4. Having an iterative decoding with MAP and SISO RS as the component > codes. > > I need to answer two questions before I select the decoder. > 1. Complexity (DVB-H will be sitting in a handheld device which makes > POWER a big factor.) > 2. Performance improvement coming along with the added complexity. > >>From the internet I was able to get the answer for MAP decoder. They > say that the MAP is 3-4 times as complex as compared to the standard > Viterbi decoder. > > I am yet to get answers for the following: > 1. Can any one tell me by how much factor does the complexity of soft > RS decoder and SISO RS decoder increase when compared to the hard input > hard output RS decoder.I have not seen any practical realization of SISO RS decoder except for the very simplest RS codes. The SI RS decoders are well known; there are multiple algorithms to do that. The problem with the soft decoding of RS is that the complexity grows exponentially with the size of the alphabet.> 2. Papers by Koetter-Vardy and Wicker show that soft decoding of RS > codes can give upto 6 db improvement for Rayleigh fading channels.I have not read this paper however it sounds over optimistic to me. In your case RS is concatenated with convolution code, and I would expect the gain of the SI RS decoder compared to hard RS to be at the order of several tenth of dB.> Does > any one implement soft RS decoders for commercial systems?AFAIR soft RS decoding was implemented for Voyager space mission. I haven't heard of any commercial applications.> 3. Knowing that MAP decoder performance is marginally better than > Viterbi that too at low Eb/No case, is it worth considering option 2 of > decoding.It depends. Does the gain of ~0.1dB really worth complexity?> 4. If complexity is not a factor, then it looks to me that Option 4 > (iterative decoding) should give the best results. Theoretically, for > the above coding scheme, what will be the coding gain.The coding scheme was not optimized for iterative decoding, therefore the expected gain is going to be low. Again, is 0.1 dB worth all of the complications? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com