Hello, I am honing my skills on the Physical layer of communication
systems. I request the knowledgeable members of this group to help me
with the following topics. I'd appreciate that, and I believe this may
help other people who would want similar knowledge.
1) On what criterion do we select the original data bits (like it is
244 bits for UMTS 12.2 Kbps)
2) What would be the optimal length for CRC for any 'x' bits like x =
244 or 500
3) How should we determine the length of the interleaver and the
interleaving permutation for output
of the interleaver?
4) How can we know how many bits will our convolutional encoder
correct (for block it has been
explained here, I guess)
5) In the models given with Matlab for CDMA, spreading is done after
modulation, however in
standards, it is done after modulation
If anyone has more information like this, It'd be really appreciated
as these are the basic question one asks when self studying...
(I'm a student by the way not a pro comm engr)
Working on the PHYSICAL LAYER (coding, interleaving,modulation)
Started by ●December 29, 2008
Reply by ●December 29, 20082008-12-29
On Dec 29, 10:44�am, Communications_engineer <communications_engin...@yahoo.com> wrote:> Hello, I am honing my skills on the Physical layer of communication > systems. I request the knowledgeable members of this group to help me > with the following topics. I'd appreciate that, and I believe this may > help other people who would want similar knowledge. > > 1) On what criterion do we select the original data bits (like it is > 244 bits for UMTS 12.2 Kbps)There are three criteria: cost, schedule, and performance. You get to dictate any two.> > 2) What would be the optimal length for CRC for any 'x' bits like x = > 244 or 500The optimal length is that which meets your requirements.> > 3) How should we determine the length of the interleaver and the > interleaving permutation for output > � � of the interleaver?In a manner consistent with governing procedures and doctrine.> > 4) How can we know how many bits will our convolutional encoder > correct (for block it has been > � � explained here, I guess)By reading a book.> > 5) In the models given with Matlab for CDMA, spreading is done after > modulation, however in > � � standards, it is done after modulation >And your question is? John
Reply by ●December 31, 20082008-12-31
On Dec 29, 10:59�pm, John <sampson...@gmail.com> wrote:> On Dec 29, 10:44�am, Communications_engineer > > <communications_engin...@yahoo.com> wrote: > > Hello, I am honing my skills on the Physical layer of communication > > systems. I request the knowledgeable members of this group to help me > > with the following topics. I'd appreciate that, and I believe this may > > help other people who would want similar knowledge. > > > 1) On what criterion do we select the original data bits (like it is > > 244 bits for UMTS 12.2 Kbps) > > There are three criteria: cost, schedule, and performance. You get to > dictate any two. >Ok, so whats wrong with having 245 bits or 200 bits what would it effect the cost, schedule, and performance.> > > 2) What would be the optimal length for CRC for any 'x' bits like x = > > 244 or 500 > > The optimal length is that which meets your requirements. >For what kind of requirements would an 8-bit CRC or 16 or 32-bit CRC suffice> > 3) How should we determine the length of the interleaver and the > > interleaving permutation for output > > � � of the interleaver? > > In a manner consistent with governing procedures and doctrine. >I understand that the interleaving period is really based on the coherence time & the channel conditions, right? So coherence time we can find but what about channel conditions> > > > 4) How can we know how many bits will our convolutional encoder > > correct (for block it has been > > � � explained here, I guess) > > By reading a book. >Would you suggest a good book, something that gets to the point rather than having to read 500 pages before getting the answer> > > 5) In the models given with Matlab for CDMA, spreading is done after > > modulation, however in > > � � standards, it is done after modulation > > And your question is? >sorry, I'll repeat "5) In the models given with Matlab (examples) for PN sequences, spreading is done after modulation, however in 3GPP standards (25.212), spreading is done BEFORE modulation" If there is a good book regarding these questions, please let me know> JohnSee you, John
Reply by ●December 31, 20082008-12-31
On Wed, 31 Dec 2008 00:42:05 -0800 (PST), Communications_engineer <communications_engineer@yahoo.com> wrote:>On Dec 29, 10:59�pm, John <sampson...@gmail.com> wrote: >> On Dec 29, 10:44�am, Communications_engineer >> >> <communications_engin...@yahoo.com> wrote: >> > Hello, I am honing my skills on the Physical layer of communication >> > systems. I request the knowledgeable members of this group to help me >> > with the following topics. I'd appreciate that, and I believe this may >> > help other people who would want similar knowledge. >> >> > 1) On what criterion do we select the original data bits (like it is >> > 244 bits for UMTS 12.2 Kbps) >> >> There are three criteria: cost, schedule, and performance. You get to >> dictate any two. >> > >Ok, so whats wrong with having 245 bits or 200 bits what would it >effect the cost, schedule, and performance.That depends entirely on your system requirements and how the work is done.>> > 2) What would be the optimal length for CRC for any 'x' bits like x = >> > 244 or 500 >> >> The optimal length is that which meets your requirements. >> > >For what kind of requirements would an 8-bit CRC or 16 or 32-bit CRC >sufficeThere's no need to use a CRC larger than what is needed to provide the reliability you desire.>> > 3) How should we determine the length of the interleaver and the >> > interleaving permutation for output >> > � � of the interleaver? >> >> In a manner consistent with governing procedures and doctrine. >> > >I understand that the interleaving period is really based on the >coherence time & the channel conditions, right? So coherence time we >can find but what about channel conditionsThere are models available for different applications, but they're just models. You can go out and measure propogation in a number of environments that you think will be representative of your application, or you can find a model that's close. If you know the coherence time you must already know something about the channel.>> > 4) How can we know how many bits will our convolutional encoder >> > correct (for block it has been >> > � � explained here, I guess) >> >> By reading a book. >> > >Would you suggest a good book, something that gets to the point rather >than having to read 500 pages before getting the answerYes, I would suggest using a good book rather than one that's too obtuse. The sorts of details you're asking about don't have answers that are easily packaged or spoon-fed. Some system understanding is required, so it may take more than looking up a page or a paragraph in book or two. Or three. Or more.> >> >> > 5) In the models given with Matlab for CDMA, spreading is done after >> > modulation, however in >> > � � standards, it is done after modulation >> >> And your question is? >> > >sorry, I'll repeat "5) In the models given with Matlab (examples) for >PN sequences, spreading is done after modulation, however in 3GPP >standards (25.212), spreading is done BEFORE modulation"I still don't see a question there. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Reply by ●January 4, 20092009-01-04
Communications_engineer <communications_engineer@yahoo.com> wrote:>I understand that the interleaving period is really based on the >coherence time & the channel conditions, right? So coherence time we >can find but what about channel conditionsActually there are a ton of other factors that might influence choice of interleaving block size. Off the top of my head: (1) even on a AWGN channel you may need interleaving to spread errors among FEC codewords, or spread them around within a convolutional codeword; (2) latency needs by the application dictate how long an interleaver you can use. The interleaver length may or may not be short relative to the coherence time. I've seen systems designed either way. Steve
Reply by ●January 11, 20092009-01-11
On Jan 5, 3:28�am, spop...@speedymail.org (Steve Pope) wrote:> Communications_engineer�<communications_engin...@yahoo.com> wrote: > >I understand that the interleaving period is really based on the > >coherence time & the channel conditions, right? So coherence time we > >can find but what about channel conditions > > Actually there are a ton of other factors that might influence > choice of interleaving block size. �Off the top of my head: > (1) even on a AWGN channel you may need interleaving to spread > errors among FEC codewords, or spread them around within a > convolutional codeword; (2) latency needs by the application > dictate how long an interleaver you can use. � > > The interleaver length may or may not be short relative to > the coherence time. �I've seen systems designed either way. > > SteveI actually looked up regarding my questions. We design interleavers (the size of the interleaver is chosen so the interleaver attains its required depth, and the depth is in turn found by the Average Fade Time Duration, an analysis of the time the signal is below a power threshold required for a BER constraint) and FEC correction capability is dictated by the same i.e. calculate how many bits are in error in that given Fade Time Duration Now this fade that we create is based on the channel model we use. Rayleigh, Rician, Nakagami etc However steve I'm not too sure about using interleaver with AWGN?
Reply by ●January 12, 20092009-01-12
Communications_engineer <communications_engineer@yahoo.com> wrote:>However steve I'm not too sure about using interleaver with AWGN?For a classic example of an interleaver being necessary on an AWGN channel, look at the decades-old Deep Space Network concatenated Reed-Solomon / Convolutional coding standards. You will see a depth 5 interleaver is needed between the two codes. The channel is AWGN (being deep space), but the performance definitely suffers if the interleaver is not there. Steve
Reply by ●January 13, 20092009-01-13
On Jan 13, 12:56�am, spop...@speedymail.org (Steve Pope) wrote:> Communications_engineer�<communications_engin...@yahoo.com> wrote: > >However steve I'm not too sure about using interleaver with AWGN? > > For a classic example of an interleaver being necessary on > an AWGN channel, look at the decades-old Deep Space Network > concatenated Reed-Solomon / Convolutional coding standards. > You will see a depth 5 interleaver is needed between the > two codes. �The channel is AWGN (being deep space), but the > performance definitely suffers if the interleaver is not there. > > SteveHmm.. well you are right I suppose. But my view was that since White noise has almost equal noise power for entire bandwidth of the channel and more so what would be use of interleaver as it protects against contiguous bursts of noise, and really there aren't any bursts in AWGN. Also Steve can you tell me what should be the optimal length of a CRC? To a beginner, because I find it easier to learn if I already have a direction to go into rather than going through a lot of stuff that isn't important (atleast as far as CRC is concerned) An example for the use of 8bit, 16-bit CRC would be highly appreciated, or some refernce of book tutorial Thanks
Reply by ●January 13, 20092009-01-13
Communications_engineer <communications_engineer@yahoo.com> wrote:>On Jan 13, 12:56�am, spop...@speedymail.org (Steve Pope) wrote:>> Communications_engineer�<communications_engin...@yahoo.com> wrote:>> >However steve I'm not too sure about using interleaver with AWGN?>> For a classic example of an interleaver being necessary on >> an AWGN channel, look at the decades-old Deep Space Network >> concatenated Reed-Solomon / Convolutional coding standards. >> You will see a depth 5 interleaver is needed between the >> two codes. �The channel is AWGN (being deep space), but the >> performance definitely suffers if the interleaver is not there.>Hmm.. well you are right I suppose. But my view was that since White >noise has almost equal noise power for entire bandwidth of the channel >and more so what would be use of interleaver as it protects against >contiguous bursts of noise, and really there aren't any bursts in >AWGN.Right, in the example I gave the channel has no bursts of noise, but there are still bursts of errors at the convolutional decoder output, requiring interleaving. Somewhat similarly, symbol interleaving in an OFDM system isn't protecting agaist burts of errors either.>Also Steve can you tell me what should be the optimal length of a CRC?Too general a question to answer. But if you know the block length and the acceptable probability of a failure to detect errors, that would allow you to calculate the minimum CRC field length, based purely on combinatorics. Steve
Reply by ●January 13, 20092009-01-13
On Tue, 13 Jan 2009 17:57:38 +0000 (UTC), spope33@speedymail.org (Steve Pope) wrote:>Communications_engineer <communications_engineer@yahoo.com> wrote: > >>On Jan 13, 12:56�am, spop...@speedymail.org (Steve Pope) wrote: > >>> Communications_engineer�<communications_engin...@yahoo.com> wrote: > >>> >However steve I'm not too sure about using interleaver with AWGN? > >>> For a classic example of an interleaver being necessary on >>> an AWGN channel, look at the decades-old Deep Space Network >>> concatenated Reed-Solomon / Convolutional coding standards. >>> You will see a depth 5 interleaver is needed between the >>> two codes. �The channel is AWGN (being deep space), but the >>> performance definitely suffers if the interleaver is not there. > >>Hmm.. well you are right I suppose. But my view was that since White >>noise has almost equal noise power for entire bandwidth of the channel >>and more so what would be use of interleaver as it protects against >>contiguous bursts of noise, and really there aren't any bursts in >>AWGN. > >Right, in the example I gave the channel has no bursts of >noise, but there are still bursts of errors at the convolutional >decoder output, requiring interleaving.Exactly. Even with distributed input errors a Viterbi decoder tends to produce clumpy output errors at low SNR. The interleaver helps the outer RS clean those up.>Somewhat similarly, symbol interleaving in an OFDM system isn't >protecting agaist burts of errors either.Hmmm...I think it is, but the mechanism is a bit different. i.e., at the input of the inner-most FEC decoder the errors will be less clumpy when a symbol interleaver is used than without. Kind of a roundabout way of saying BICM is better than no interleaving with OFDM from that perspective.>>Also Steve can you tell me what should be the optimal length of a CRC? > >Too general a question to answer. But if you know the block >length and the acceptable probability of a failure to detect >errors, that would allow you to calculate the minimum CRC >field length, based purely on combinatorics. > >SteveEric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
