DSSS-CDMA query
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From: ashu on 11 Aug 2010 04:11 On Aug 10, 10:30 pm, Tim Wescott <t...(a)seemywebsite.com> wrote: > On 08/10/2010 11:33 AM, dvsarwate wrote: > > > On Aug 10, 10:29 am, ashu<ashutosh.ghildi...(a)gmail.com> averred: > > >> Shannon says that the capacity of a channel is directly proportional > >> to bandwidth of signal. > > > I don't believe Shannon (or most of those who followed > > him) ever said that. The capacity of a channel is *not* > > ***directly proportional**** to the bandwidth of the signal; > > it is not even directly proportional to the bandwidth of the > > channel. The channel capacity increases with bandwidth > > but a law of diminishing returns sets in and the capacity > > approaches a finite limit (value depending on the ratio > > of signal power to noise density, which ratio is assumed > > to be fixed) as bandwidth goes to infinity. > > Actually the way he framed it was for a channel with a fixed SNR, in > which case the channel capacity _would_ be proportional to bandwidth. > > But the "fixed SNR" channel isn't a very representative model, as I > pointed out in my response. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" was written for you. > See details athttp://www.wescottdesign.com/actfes/actfes.html I thank all of you for your interesting replies. Yes I meant fixed SNR and in that case Capaciy would be directly proprtional to Bandwidth. What I really want to understand is.. suppose my pulse period is fixed(hence pulse bandwidth), if the noise characterstics of channel are such that at the receiver, pulse is deeply burried in noise. In this kind of scenario, if I transmit multiple pulses, can I achieve reliable communication, by some sort of avergaging at the recevier ? Will transmitting more pulses help ? regards and thanks ashu
From: dvsarwate on 11 Aug 2010 11:02 On Aug 10, 3:30 pm, Tim Wescott <t...(a)seemywebsite.com> wrote: > > Actually the way he framed it was for a channel with a fixed SNR, in > which case the channel capacity _would_ be proportional to bandwidth. > > But the "fixed SNR" channel isn't a very representative model, as I > pointed out in my response. > What the OP wrote was "Shannon says that the capacity of a channel is directly proportional to bandwidth of signal." without any mention of SNR, and that was what I was complaining about. While the OP mentioned SNR in the next sentence, as was pointed out in a different very recent thread with title "Tangled up with Shaonnon Bound", there are different definitions of what SNR means that are used in communication system studies. Thus, unless it is made clear what is meant by SNR, the OP's assertion "In other others for a given SNR if increasing B would give me more capacity." is not necessarily true. Even if we give credence to the OP's latest assertion that he was thinking of fixed SNR (presumably meaning the quantity P/N_0W in the capacity formula C = W log(1 + P/N_0W) being fixed as W varies, so that P is increasing with W and thus C is proportional to W), all along, the very next sentence in the original post (referring to direct-sequence spread-spectrum) suggests that the OP really was thinking of fixed SNR as meaning Eb/N0 being fixed. As both Tim and Vladimir pointed out, direct-sequence modulation does not change Eb/N0 and so the same BER performance is obtained regardless of whether the signal is spread or not. Moral: you can get all kinds of wonderful results by defining SNR appropriately, but you can't fake the BEND* ratio Eb/N0, and that's what really counts. --Dilip Sarwate *BEND = bit-energy-to-noise-density
From: Tim Wescott on 11 Aug 2010 11:30 On 08/11/2010 01:11 AM, ashu wrote: > On Aug 10, 10:30 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >> On 08/10/2010 11:33 AM, dvsarwate wrote: >> >>> On Aug 10, 10:29 am, ashu<ashutosh.ghildi...(a)gmail.com> averred: >> >>>> Shannon says that the capacity of a channel is directly proportional >>>> to bandwidth of signal. >> >>> I don't believe Shannon (or most of those who followed >>> him) ever said that. The capacity of a channel is *not* >>> ***directly proportional**** to the bandwidth of the signal; >>> it is not even directly proportional to the bandwidth of the >>> channel. The channel capacity increases with bandwidth >>> but a law of diminishing returns sets in and the capacity >>> approaches a finite limit (value depending on the ratio >>> of signal power to noise density, which ratio is assumed >>> to be fixed) as bandwidth goes to infinity. >> >> Actually the way he framed it was for a channel with a fixed SNR, in >> which case the channel capacity _would_ be proportional to bandwidth. >> >> But the "fixed SNR" channel isn't a very representative model, as I >> pointed out in my response. >> >> -- >> >> Tim Wescott >> Wescott Design Serviceshttp://www.wescottdesign.com >> >> Do you need to implement control loops in software? >> "Applied Control Theory for Embedded Systems" was written for you. >> See details athttp://www.wescottdesign.com/actfes/actfes.html > > I thank all of you for your interesting replies. > > Yes I meant fixed SNR and in that case Capaciy would be directly > proprtional to Bandwidth. > > What I really want to understand is.. suppose my pulse period is > fixed(hence pulse bandwidth), if the noise characterstics of channel > are such that at the receiver, pulse is deeply burried in noise. In > this kind of scenario, if I transmit multiple pulses, can I achieve > reliable communication, by some sort of avergaging at the recevier ? Yes. In a sense this is what you are doing when you increase your symbol length (consider a 1ms pulse as 'just' 1000 1us pulses). It is also what you are doing when you send long symbols over a spread-spectrum link, although you then have synchronization to deal with. > Will transmitting more pulses help ? Yes, but that may not be the best solution, or it may only be a small part of the best solution. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: Mark on 11 Aug 2010 13:07 > > Yes. In a sense this is what you are doing when you increase your > symbol length (consider a 1ms pulse as 'just' 1000 1us pulses). It is > also what you are doing when you send long symbols over a > spread-spectrum link, although you then have synchronization to deal with.. > > > Will transmitting more pulses help ? > > Yes, but that may not be the best solution, or it may only be a small > part of the best solution. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" was written for you. > See details athttp://www.wescottdesign.com/actfes/actfes.html- Hide quoted text - > > - Show quoted text - I think I'm getting an insight here... repeating the same symbol 1000 times and averaging the result to obtain 1 bit of info would be analagous to reducing the video BW on a spectrum analyzer i.e. it reduces the POST DETECTION BW, and while this does help it would be (much) more effective to reduce the resolution BW i.e. reduce the PRE DETECTION BW. To do this you must reduce the symbol rate. So instead of sending 1000 symbols that carry 1 bit, it is better to send 1 symbol that is 1000 time longer and hence requires 1000 less pre dection BW... is that it? Mark
From: Tim Wescott on 11 Aug 2010 13:27 On 08/11/2010 10:07 AM, Mark wrote: > >> >> Yes. In a sense this is what you are doing when you increase your >> symbol length (consider a 1ms pulse as 'just' 1000 1us pulses). It is >> also what you are doing when you send long symbols over a >> spread-spectrum link, although you then have synchronization to deal with. >> >>> Will transmitting more pulses help ? >> >> Yes, but that may not be the best solution, or it may only be a small >> part of the best solution. >> >> -- >> >> Tim Wescott >> Wescott Design Serviceshttp://www.wescottdesign.com >> >> Do you need to implement control loops in software? >> "Applied Control Theory for Embedded Systems" was written for you. >> See details athttp://www.wescottdesign.com/actfes/actfes.html- Hide quoted text - >> >> - Show quoted text - > > I think I'm getting an insight here... > > repeating the same symbol 1000 times and averaging the result to > obtain 1 bit of info would be analagous to reducing the video BW on a > spectrum analyzer i.e. it reduces the POST DETECTION BW, and while > this does help it would be (much) more effective to reduce the > resolution BW i.e. reduce the PRE DETECTION BW. To do this you must > reduce the symbol rate. > > So instead of sending 1000 symbols that carry 1 bit, it is better to > send 1 symbol that is 1000 time longer and hence requires 1000 less > pre dection BW... is that it? Well... If you knew the transmitter were repeating each bit 1000 times, you wouldn't detect each bit and average after the fact -- you'd just average over those 1000 'bits' as one symbol. This is your "pre detection" bandwidth reduction. Actually doing a hard detection on each bit, then doing a majority vote on the result is going to have worse performance than averaging the whole 1000 bits as a lump, but better than any individual bit detection. How much better depends on Eb/No. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
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