Sirs, In case of frequency division multiplexing transmission, are different carriers combined before transmission. If so, what is the step known as. If not, are the carriers transmitted independently. Thanks a lot ...
FDM
Started by ●October 27, 2012
Reply by ●October 27, 20122012-10-27
On Sat, 27 Oct 2012 11:46:23 -0500, manishp wrote:> Sirs, > > In case of frequency division multiplexing transmission, are different > carriers combined before transmission. If so, what is the step known as. > If not, are the carriers transmitted independently. > > Thanks a lot ...Different _signals_, each riding on their own carrier, might be combined before final amplification. I don't think there's an industry-wide term. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Reply by ●October 27, 20122012-10-27
"manishp" <58525@dsprelated> writes:> Sirs, > > In case of frequency division multiplexing transmission, are different > carriers combined before transmission. If so, what is the step known > as.Multiplexing. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●October 27, 20122012-10-27
Tim Wescott <tim@seemywebsite.please> wrote:> On Sat, 27 Oct 2012 11:46:23 -0500, manishp wrote:>> In case of frequency division multiplexing transmission, are different >> carriers combined before transmission. If so, what is the step known as. >> If not, are the carriers transmitted independently.> Different _signals_, each riding on their own carrier, might be combined > before final amplification. I don't think there's an industry-wide term.In the olden days, and for things like telephone trunk lines, it would be a bunch of modulators, then combine the signals. Maybe one amplifier at the beginning, others along the way. In the DSP days, though, I think one could modulate and combine them digitally, such that the output of the DAC contains many signals. If there are still FDM cellular phones, how do the cell towers process the signals? -- glen
Reply by ●October 27, 20122012-10-27
On Sat, 27 Oct 2012 18:40:32 +0000, glen herrmannsfeldt wrote:> Tim Wescott <tim@seemywebsite.please> wrote: >> On Sat, 27 Oct 2012 11:46:23 -0500, manishp wrote: > >>> In case of frequency division multiplexing transmission, are different >>> carriers combined before transmission. If so, what is the step known >>> as. >>> If not, are the carriers transmitted independently. > >> Different _signals_, each riding on their own carrier, might be >> combined before final amplification. I don't think there's an >> industry-wide term. > > In the olden days, and for things like telephone trunk lines, it would > be a bunch of modulators, then combine the signals. Maybe one amplifier > at the beginning, others along the way. > > In the DSP days, though, I think one could modulate and combine them > digitally, such that the output of the DAC contains many signals. > > If there are still FDM cellular phones, how do the cell towers process > the signals?Any time you combine a bunch of carriers at low level and then amplify you challenge the following power amplifier with a high crest factor signal (crest factor = ratio of max instantaneous power to average power). So there's good reasons even now to amplify separately and then combine. This is one of the issues with OFDM; I'm not sure what, if anything, is done to alleviate it. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Reply by ●October 27, 20122012-10-27
>On Sat, 27 Oct 2012 18:40:32 +0000, glen herrmannsfeldt wrote: > >> Tim Wescott <tim@seemywebsite.please> wrote: >>> On Sat, 27 Oct 2012 11:46:23 -0500, manishp wrote: >> >>>> In case of frequency division multiplexing transmission, aredifferent>>>> carriers combined before transmission. If so, what is the step known >>>> as. >>>> If not, are the carriers transmitted independently. >> >>> Different _signals_, each riding on their own carrier, might be >>> combined before final amplification. I don't think there's an >>> industry-wide term. >> >> In the olden days, and for things like telephone trunk lines, it would >> be a bunch of modulators, then combine the signals. Maybe one amplifier >> at the beginning, others along the way. >> >> In the DSP days, though, I think one could modulate and combine them >> digitally, such that the output of the DAC contains many signals. >> >> If there are still FDM cellular phones, how do the cell towers process >> the signals? > >Any time you combine a bunch of carriers at low level and then amplify >you challenge the following power amplifier with a high crest factor >signal (crest factor = ratio of max instantaneous power to averagepower).> >So there's good reasons even now to amplify separately and then combine. > >This is one of the issues with OFDM; I'm not sure what, if anything, is >done to alleviate it. > >-- >Tim Wescott >Control system and signal processing consultingI work on cellular radios. We combine several cdma, lte carriers digitally then pass the composite signal to DAC. Yes it is challenge to power amplifiers but we lower the PAR(crest factor) to a prescribed level using a special peak limiting algorithm followed by digital predistortion. Kaz
Reply by ●October 27, 20122012-10-27
On Sat, 27 Oct 2012 13:14:38 -0400, Randy Yates <yates@digitalsignallabs.com> wrote:>"manishp" <58525@dsprelated> writes: > >> Sirs, >> >> In case of frequency division multiplexing transmission, are different >> carriers combined before transmission. If so, what is the step known >> as. > >Multiplexing. >-- >Randy Yates >Digital Signal Labs >http://www.digitalsignallabs.comI still prefer "addition". ;) Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●October 27, 20122012-10-27
On Sat, 27 Oct 2012 14:10:17 -0500, Tim Wescott <tim@seemywebsite.please> wrote:>On Sat, 27 Oct 2012 18:40:32 +0000, glen herrmannsfeldt wrote: > >> Tim Wescott <tim@seemywebsite.please> wrote: >>> On Sat, 27 Oct 2012 11:46:23 -0500, manishp wrote: >> >>>> In case of frequency division multiplexing transmission, are different >>>> carriers combined before transmission. If so, what is the step known >>>> as. >>>> If not, are the carriers transmitted independently. >> >>> Different _signals_, each riding on their own carrier, might be >>> combined before final amplification. I don't think there's an >>> industry-wide term. >> >> In the olden days, and for things like telephone trunk lines, it would >> be a bunch of modulators, then combine the signals. Maybe one amplifier >> at the beginning, others along the way. >> >> In the DSP days, though, I think one could modulate and combine them >> digitally, such that the output of the DAC contains many signals. >> >> If there are still FDM cellular phones, how do the cell towers process >> the signals? > >Any time you combine a bunch of carriers at low level and then amplify >you challenge the following power amplifier with a high crest factor >signal (crest factor = ratio of max instantaneous power to average power). > >So there's good reasons even now to amplify separately and then combine. > >This is one of the issues with OFDM; I'm not sure what, if anything, is >done to alleviate it. > >-- >Tim Wescott >Control system and signal processing consulting >www.wescottdesign.comThis is routinely done in many systems, perhaps a notable example being cellular base stations. And, yes, the PAPR goes up, but you can budget for it and/or do fancy signal processing or other tricks to minimize it. The "other tricks" have been driving state-of-the-art for rf PAs and AFEs for around ten years or so. There are a wide variety of techniques. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●October 27, 20122012-10-27
eric.jacobsen@ieee.org (Eric Jacobsen) writes:> On Sat, 27 Oct 2012 13:14:38 -0400, Randy Yates > <yates@digitalsignallabs.com> wrote: > >>"manishp" <58525@dsprelated> writes: >> >>> Sirs, >>> >>> In case of frequency division multiplexing transmission, are different >>> carriers combined before transmission. If so, what is the step known >>> as. >> >>Multiplexing. >>-- >>Randy Yates >>Digital Signal Labs >>http://www.digitalsignallabs.com > > I still prefer "addition". ;)Yes, that too. -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●October 27, 20122012-10-27
>I work on cellular radios. We combine several cdma, lte carriersdigitally>then pass the composite signal to DAC. Yes it is challenge to power >amplifiers but we lower the PAR(crest factor) to a prescribed level usinga>special peak limiting algorithm followed by digital predistortion. > >KazKaz, Can you tell me the typical processing used to combine these independent carrier in digital domain. Thanks ...
