Dear All, Okay, probably this sound a bit silly, but what happens when repetition coding is used and when soft demodulation is used ? Soft QAM demodulation produces Log Likelihood Ratio (LLR), and should I take average value of the repeated symbols ? Here's the simple view of the receiver Channel -->Soft QAM demodulator-->Repetition Decoding-->De Interleaver--> FEC decoder Any impact if I take the average ? Or should I take the maximum (or minimum) value ? Thanks. Hear from you guys. Regards, Arthur.

# Repetition Coding and Soft demodulation

Started by ●April 13, 2006

Reply by ●April 14, 20062006-04-14

Dear All, Okay, probably this sound a bit silly, but what happens when repetition decoding is done along with soft demodulation ? Soft QAM demodulation produces Log Likelihood Ratio (LLR) of repeated bits, and how LLR of repeated bits decoded with a repetition decoder ? Here's the simple view of the receiver Channel -->OFDM Demodulator-->Soft QAM demodulator-->Repetition Decoding-->De Interleaver--> FEC decoder Usually majority logic decoding is used with hard demodulation, but if hard demodulation is used, does it have any impact on soft decoder like LDPC, turbo or virtebi decoder ? Regards, Arthur.

Reply by ●April 14, 20062006-04-14

Arthur wrote:> Dear All, > > Okay, probably this sound a bit silly, but what happens when repetition > coding is used and when soft demodulation is used ? Soft QAM > demodulation produces Log Likelihood Ratio (LLR), and should I take > average value of the repeated symbols ? Here's the simple view of the > receiver > > Channel -->Soft QAM demodulator-->Repetition Decoding-->De > Interleaver--> FEC decoder > > Any impact if I take the average ? Or should I take the maximum (or > minimum) value ? Thanks. Hear from you guys. > > Regards, > Arthur. >Soft demodulation + repetition coding = longer symbols. If your soft decisions reflect the a posteriori probability of a given bit being received and if your noise is steady Gaussian then the mathematically correct answer is _exactly_ the same one as just using longer symbols to start. So if you're always sending symbols out in groups of (say) three, you have no gain over just using symbols that are three times longer. I suppose that if you scattered your symbols around you'd get some gain in a bursty noise environment, but only if your soft demodulation is no longer linear. You get better coding gain from real codes -- I've seen this proven it at least one book on coding theory, and I'd be surprised if it wasn't there in every elementary coding theory book. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/

Reply by ●April 14, 20062006-04-14

But in real life scenerio where channel experience multipath fading, doesn't repetition codes provide some kind of delay diversity gain ? Actually what kind of decoding logic optimal for this kind of concatenated code ? It is in fact in wimax, where repetition code is use as inner code to outter code LDPC.

Reply by ●April 14, 20062006-04-14

Arthur wrote:> But in real life scenerio where channel experience multipath fading, > doesn't repetition codes provide some kind of delay diversity gain ? > > Actually what kind of decoding logic optimal for this kind of > concatenated code ? It is in fact in wimax, where repetition code is > use as inner code to outter code LDPC. >Are they repeating each bit several times in a row, or are they sending sequences where the bits are separated like 1, 2, 3, 4, 3, 2, 1, 4? In a non-Gaussian channel, particularly one with bursty noise, a straight repetitive code may give better gain. If it's predictably non-Gaussian you have a better idea of which bits have been hammered, and can therefore be neglected. If it's bursty then something that takes out several bits in a row may only take out a fraction of each bit in a repetition code. I suppose that if your bit rate is very high compared to your multipath then the intersymbol interference may look like bursty noise. As for why they used it in Wi-Fi -- I dunno. I'll put finding out on my 'to do' list. Unfortunately it'll be toward the bottom, though. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/

Reply by ●April 15, 20062006-04-15

I think they repeat it several time immediately after each bit, like, 1,1,1,2,2,2,3,3,3,4,4,4, where there repetition factor is three. WIMAX include it to increase reliability of control signals like FCH, DL-MAP and UL-MAP. I am implementing it, just that I am not too sure how the decoder should be when log likelihood ratio (LLR) is used. In hard decision, I can easily use major logic decoding, but for soft value like LLR, not too sure about that......

Reply by ●April 15, 20062006-04-15

Arthur wrote:> I think they repeat it several time immediately after each bit, like, > 1,1,1,2,2,2,3,3,3,4,4,4, where there repetition factor is three. WIMAX > include it to increase reliability of control signals like FCH, DL-MAP > and UL-MAP. > > I am implementing it, just that I am not too sure how the decoder > should be when log likelihood ratio (LLR) is used. In hard decision, I > can easily use major logic decoding, but for soft value like LLR, not > too sure about that...... >If I'm not completely off base, the LLR is just the amplitude out of the demodulator, before it's made into a 1 or a 0. So just adding the three bits' LLRs together should do it. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/