Well not much I suppose since we use it for Wi-Fi - but what about commercial broadcast radio propagation? The UK has had digital radio for some time but it has not really caught on. I heard it had problems in moving vehicles though it is supposed to be immune to multipath. Hardy
So whats wrong with OFDM?
Started by ●January 15, 2009
Reply by ●January 16, 20092009-01-16
On Thu, 15 Jan 2009 20:41:08 -0600, Vladimir Vassilevsky <antispam_bogus@hotmail.com> wrote:> > >Eric Jacobsen wrote: >> On Thu, 15 Jan 2009 13:54:10 -0800 (PST), HardySpicer >> <gyansorova@gmail.com> wrote: >> >> >>>Well not much I suppose since we use it for Wi-Fi - but what about >>>commercial broadcast radio propagation? The UK has had digital radio >>>for some time but it has not really caught on. I heard it had problems >>>in moving vehicles though it is supposed to be immune to multipath. >>> >> >> There's more to a system than just the modulation method, so it's hard >> to pin down where the problem was if something fails. >> >> That being said, multipath and mobility are two different things. >> Often a system that can handle severe multipath can't do it well if >> the channel changes rapidly, which it does in a moving vehicle. >> Handling vehicular speeds in multipath is a pretty difficult >> proposition for wideband signals. > >A mobile receiver has a kind of spatial diversity because it is moving. >So, if the channel can handle error bursts of up to 10% of the total >traffic, then the multipath effects are efficiently averaged out. The >modulation scheme of GSM is designed keeping those effects in mind; >their parameters are actually optimal or near optimal for urban environment.GSM has a relatively narrow bandwidth compared to what a lot of folks want to do (and compared to how OFDM is typically applied). Just having a narrow bandwidth helps quite a bit in surviving mobility. Take a GSM signal and crank the symbol rate up to 5-15MHz and it doesn't work so well any more. 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 17, 20092009-01-17
Eric Jacobsen wrote:> On Thu, 15 Jan 2009 20:41:08 -0600, Vladimir Vassilevsky > <antispam_bogus@hotmail.com> wrote: > > >> >>Eric Jacobsen wrote: >> >>>On Thu, 15 Jan 2009 13:54:10 -0800 (PST), HardySpicer >>><gyansorova@gmail.com> wrote: >>> >>> >>> >>>>Well not much I suppose since we use it for Wi-Fi - but what about >>>>commercial broadcast radio propagation? The UK has had digital radio >>>>for some time but it has not really caught on. I heard it had problems >>>>in moving vehicles though it is supposed to be immune to multipath. >>>> >>> >>>There's more to a system than just the modulation method, so it's hard >>>to pin down where the problem was if something fails. >>> >>>That being said, multipath and mobility are two different things. >>>Often a system that can handle severe multipath can't do it well if >>>the channel changes rapidly, which it does in a moving vehicle. >>>Handling vehicular speeds in multipath is a pretty difficult >>>proposition for wideband signals. >> >>A mobile receiver has a kind of spatial diversity because it is moving. >>So, if the channel can handle error bursts of up to 10% of the total >>traffic, then the multipath effects are efficiently averaged out. The >>modulation scheme of GSM is designed keeping those effects in mind; >>their parameters are actually optimal or near optimal for urban environment. > > > GSM has a relatively narrow bandwidth compared to what a lot of folks > want to do (and compared to how OFDM is typically applied). Just > having a narrow bandwidth helps quite a bit in surviving mobility. > > Take a GSM signal and crank the symbol rate up to 5-15MHz and it > doesn't work so well any more.The DAB physical layer doesn't make much sense. First, the OFDM will require the power backoff. Simulcast is not a good idea either. Wideband processing in the receiver is another bad one. Mobility is the huge problem. Looks like the system designed by graduate students and their professors. Why couldn't they chose a narrowband scheme similar to that of GSM? That would allow for the gain ~ several dB over analog FM, frequency reuse 1:3 pattern, simple and efficient receivers, keeping the existent frequency allocation plans. Coverage of the large areas better achieved by automatic handover rather then by multicasting. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●January 17, 20092009-01-17
On Sat, 17 Jan 2009 11:14:41 -0600, Vladimir Vassilevsky <antispam_bogus@hotmail.com> wrote:> > >Eric Jacobsen wrote: > >> On Thu, 15 Jan 2009 20:41:08 -0600, Vladimir Vassilevsky >> <antispam_bogus@hotmail.com> wrote: >> >> >>> >>>Eric Jacobsen wrote: >>> >>>>On Thu, 15 Jan 2009 13:54:10 -0800 (PST), HardySpicer >>>><gyansorova@gmail.com> wrote: >>>> >>>> >>>> >>>>>Well not much I suppose since we use it for Wi-Fi - but what about >>>>>commercial broadcast radio propagation? The UK has had digital radio >>>>>for some time but it has not really caught on. I heard it had problems >>>>>in moving vehicles though it is supposed to be immune to multipath. >>>>> >>>> >>>>There's more to a system than just the modulation method, so it's hard >>>>to pin down where the problem was if something fails. >>>> >>>>That being said, multipath and mobility are two different things. >>>>Often a system that can handle severe multipath can't do it well if >>>>the channel changes rapidly, which it does in a moving vehicle. >>>>Handling vehicular speeds in multipath is a pretty difficult >>>>proposition for wideband signals. >>> >>>A mobile receiver has a kind of spatial diversity because it is moving. >>>So, if the channel can handle error bursts of up to 10% of the total >>>traffic, then the multipath effects are efficiently averaged out. The >>>modulation scheme of GSM is designed keeping those effects in mind; >>>their parameters are actually optimal or near optimal for urban environment. >> >> >> GSM has a relatively narrow bandwidth compared to what a lot of folks >> want to do (and compared to how OFDM is typically applied). Just >> having a narrow bandwidth helps quite a bit in surviving mobility. >> >> Take a GSM signal and crank the symbol rate up to 5-15MHz and it >> doesn't work so well any more. > > >The DAB physical layer doesn't make much sense. First, the OFDM will >require the power backoff. Simulcast is not a good idea either. Wideband > processing in the receiver is another bad one. Mobility is the huge >problem. Looks like the system designed by graduate students and their >professors. > >Why couldn't they chose a narrowband scheme similar to that of GSM? That >would allow for the gain ~ several dB over analog FM, frequency reuse >1:3 pattern, simple and efficient receivers, keeping the existent >frequency allocation plans. Coverage of the large areas better achieved >by automatic handover rather then by multicasting. > > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.comPersonally I think audio broadcast radio should stay analog. You're not going to beat it for simplicity and low power consumption in the receiver, which is something that's desirable for critical infrastructure. 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 17, 20092009-01-17
On Jan 18, 8:30�am, Eric Jacobsen <eric.jacob...@ieee.org> wrote:> On Sat, 17 Jan 2009 11:14:41 -0600, Vladimir Vassilevsky > > > > <antispam_bo...@hotmail.com> wrote: > > >Eric Jacobsen wrote: > > >> On Thu, 15 Jan 2009 20:41:08 -0600, Vladimir Vassilevsky > >> <antispam_bo...@hotmail.com> wrote: > > >>>Eric Jacobsen wrote: > > >>>>On Thu, 15 Jan 2009 13:54:10 -0800 (PST), HardySpicer > >>>><gyansor...@gmail.com> wrote: > > >>>>>Well not much I suppose since we use it for Wi-Fi - but what about > >>>>>commercial broadcast radio propagation? The UK has had digital radio > >>>>>for some time but it has not really caught on. I heard it had problems > >>>>>in moving vehicles though it is supposed to be immune to multipath. > > >>>>There's more to a system than just the modulation method, so it's hard > >>>>to pin down where the problem was if something fails. > > >>>>That being said, multipath and mobility are two different things. > >>>>Often a system that can handle severe multipath can't do it well if > >>>>the channel changes rapidly, which it does in a moving vehicle. > >>>>Handling vehicular speeds in multipath is a pretty difficult > >>>>proposition for wideband signals. > > >>>A mobile receiver has a kind of spatial diversity because it is moving. > >>>So, if the channel can handle error bursts of up to 10% of the total > >>>traffic, then the multipath effects are efficiently averaged out. The > >>>modulation scheme of GSM is designed keeping those effects in mind; > >>>their parameters are actually optimal or near optimal for urban environment. > > >> GSM has a relatively narrow bandwidth compared to what a lot of folks > >> want to do (and compared to how OFDM is typically applied). � �Just > >> having a narrow bandwidth helps quite a bit in surviving mobility. > > >> Take a GSM signal and crank the symbol rate up to 5-15MHz and it > >> doesn't work so well any more. > > >The DAB physical layer doesn't make much sense. First, the OFDM will > >require the power backoff. Simulcast is not a good idea either. Wideband > > �processing in the receiver is another bad one. Mobility is the huge > >problem. Looks like the system designed by graduate students and their > >professors. > > >Why couldn't they chose a narrowband scheme similar to that of GSM? That > >would allow for the gain ~ several dB over analog FM, frequency reuse > >1:3 pattern, simple and efficient receivers, keeping the existent > >frequency allocation plans. Coverage of the large areas better achieved > >by automatic handover rather then by multicasting. > > >Vladimir Vassilevsky > >DSP and Mixed Signal Design Consultant > >http://www.abvolt.com > > Personally I think audio broadcast radio should stay analog. � You're > not going to beat it for simplicity and low power consumption in the > receiver, which is something that's desirable for critical > infrastructure.Cellular benefited greatly from going digital. TV benefited greatly from going digital. Why is audio broadcast radio such a special case? I suspect that while the EU people who devised the TV standard were pretty smart about it, the people doing DAB just messed up. Though I've never looked at its details, it has some blatantly dumb decisions. The codec was a stupid choice - choosing something that already looked old and clunky at the launch date was hardly setting it up for a long and happy future. It imposes a higher than necessary bit rate on the radio channel. The way they do error handling must have seemed clever and tricky to someone, but they obviously hadn't though it through. It actually produces the artefacts people complain most heavily about. Regards, Steve
Reply by ●January 18, 20092009-01-18
Eric Jacobsen wrote:> Personally I think audio broadcast radio should stay analog. You're > not going to beat it for simplicity and low power consumption in the > receiver, which is something that's desirable for critical > infrastructure.With analog, you can't sell the content. That's the main push for going digital :-) Besides, there is no easy way to achieve the wide coverage. Also, the incredible complication of the system is one of the ways for keeping the small players away from the table. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●January 18, 20092009-01-18
On Sat, 17 Jan 2009 19:42:31 -0800 (PST), steveu@coppice.org wrote:>On Jan 18, 8:30�am, Eric Jacobsen <eric.jacob...@ieee.org> wrote: >> On Sat, 17 Jan 2009 11:14:41 -0600, Vladimir Vassilevsky >> >> >> >> <antispam_bo...@hotmail.com> wrote: >> >> >Eric Jacobsen wrote: >> >> >> On Thu, 15 Jan 2009 20:41:08 -0600, Vladimir Vassilevsky >> >> <antispam_bo...@hotmail.com> wrote: >> >> >>>Eric Jacobsen wrote: >> >> >>>>On Thu, 15 Jan 2009 13:54:10 -0800 (PST), HardySpicer >> >>>><gyansor...@gmail.com> wrote: >> >> >>>>>Well not much I suppose since we use it for Wi-Fi - but what about >> >>>>>commercial broadcast radio propagation? The UK has had digital radio >> >>>>>for some time but it has not really caught on. I heard it had problems >> >>>>>in moving vehicles though it is supposed to be immune to multipath. >> >> >>>>There's more to a system than just the modulation method, so it's hard >> >>>>to pin down where the problem was if something fails. >> >> >>>>That being said, multipath and mobility are two different things. >> >>>>Often a system that can handle severe multipath can't do it well if >> >>>>the channel changes rapidly, which it does in a moving vehicle. >> >>>>Handling vehicular speeds in multipath is a pretty difficult >> >>>>proposition for wideband signals. >> >> >>>A mobile receiver has a kind of spatial diversity because it is moving. >> >>>So, if the channel can handle error bursts of up to 10% of the total >> >>>traffic, then the multipath effects are efficiently averaged out. The >> >>>modulation scheme of GSM is designed keeping those effects in mind; >> >>>their parameters are actually optimal or near optimal for urban environment. >> >> >> GSM has a relatively narrow bandwidth compared to what a lot of folks >> >> want to do (and compared to how OFDM is typically applied). � �Just >> >> having a narrow bandwidth helps quite a bit in surviving mobility. >> >> >> Take a GSM signal and crank the symbol rate up to 5-15MHz and it >> >> doesn't work so well any more. >> >> >The DAB physical layer doesn't make much sense. First, the OFDM will >> >require the power backoff. Simulcast is not a good idea either. Wideband >> > �processing in the receiver is another bad one. Mobility is the huge >> >problem. Looks like the system designed by graduate students and their >> >professors. >> >> >Why couldn't they chose a narrowband scheme similar to that of GSM? That >> >would allow for the gain ~ several dB over analog FM, frequency reuse >> >1:3 pattern, simple and efficient receivers, keeping the existent >> >frequency allocation plans. Coverage of the large areas better achieved >> >by automatic handover rather then by multicasting. >> >> >Vladimir Vassilevsky >> >DSP and Mixed Signal Design Consultant >> >http://www.abvolt.com >> >> Personally I think audio broadcast radio should stay analog. � You're >> not going to beat it for simplicity and low power consumption in the >> receiver, which is something that's desirable for critical >> infrastructure. > >Cellular benefited greatly from going digital. >TV benefited greatly from going digital. >Why is audio broadcast radio such a special case?Because the usage model is different than the others, as well as the function. A lot of radio listeners are using very small, battery-powered devices for which extreme battery life is very useful. Keeping contact with the outside world while on a long backpack trip or while hunkered down in a shelter (without power) waiting for big storm cells (or tornados or a flood or whatever) to pass can be a life-critical function. That's just a lot harder to do at low cost with a long battery life with a digital receiver than it is with an analog equivalent. The effects of the full transition to digital for TV isn't yet known, so I'm not sure just yet that I'd conclude it's benefitted from the transition to digital. I suspect a lot of rural receivers are going to lose coverage (I'm just about willing to bet our cabin will have no TV after the transition, with the exception of the remaining analog translators). The same issue holds true with radio when considering the safety infrastructure function: range will likely be reduce with digital as compared to analog for similar transmit power. I think Vladimir's right that the real motivation is the additional services that can be done with digital, e.g., station ID, song ID, etc., etc. There'll be the possibility of adding services with additional revenue streams (small display ads, whatever) that are probably the main motivation. The tradeoff of reduced range and reducing the scope of users (via requiring more expensive, shorter-lived receivers) is yet to be experienced. I hope I'm wrong and everything comes up roses, but I suspect not. 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 18, 20092009-01-18
On Sun, 18 Jan 2009 09:37:48 -0600, Vladimir Vassilevsky <antispam_bogus@hotmail.com> wrote:> > >Eric Jacobsen wrote: > >> Personally I think audio broadcast radio should stay analog. You're >> not going to beat it for simplicity and low power consumption in the >> receiver, which is something that's desirable for critical >> infrastructure.Modern digital circuitry for decoding the signal takes less power than an audio amplifier to drive an earbud, so I don't see that part as a valid argument, unless you're comparing digital to an (AM only) crystal receiver. On the other hand, the digital compression (psychoacoustic, just like mp3) is lossy and has artifacts that are audible and annoying to a significant number of people. Going digital just trades off some types of classic analog distortion for Bold, New Distortions No Man Has Ever Heard Before. At least CD's use a straight, lossless PCM format, and that can be losslessly compressed by about 2 to 1 as is done on DVD (or it CAN be done, DVD has many audio and video formats, and some, probably the best quality, are rarely used).> >With analog, you can't sell the content. That's the main push for going >digital :-) Besides, there is no easy way to achieve the wide coverage.What do you mean by not achieving wide coverage?>Also, the incredible complication of the system is one of the ways for >keeping the small players away from the table.And of course those are advantages for large manufacturers, not consumers.> > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com
Reply by ●January 18, 20092009-01-18
Ben Bradley wrote:> Modern digital circuitry for decoding the signal takes less power > than an audio amplifier to drive an earbud,:)))))) Even the simple MP3 decoder on the specialised chip drains about 10 times more power then the headphone amplifier. The OFDM receiver for DAB is likely to drain more.> On the other hand, the digital compression (psychoacoustic, just > like mp3) is lossy and has artifacts that are audible and annoying to > a significant number of people.This is the matter of bit rate. MP3 at 192kb sounds very decent, and I doubt if one can distinguish the original CD from MP3 at 256k.> Going digital just trades off some > types of classic analog distortion for Bold, New Distortions No Man > Has Ever Heard Before.Aha. People were quite happy about Edison's phonograph.>>With analog, you can't sell the content. That's the main push for going >>digital :-) Besides, there is no easy way to achieve the wide coverage. > > What do you mean by not achieving wide coverage?I meant moving from one zone of reception to another. With the analog radio, you have to switch the channels. With digital, it can be done seemlessly. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●January 18, 20092009-01-18
On 18 Jan, 19:18, Vladimir Vassilevsky <antispam_bo...@hotmail.com> wrote:> Ben Bradley wrote:> > � �What do you mean by not achieving wide coverage? > > I meant moving from one zone of reception to another. With the analog > radio, you have to switch the channels. With digital, it can be done > seemlessly.I've used RDS FM radio for more than ten years. Very useful in cars. Once you select a station the reciever sticks with that station, seamlessly tuning on to the best transmitter available, until you turn it off, select a different station, or leave the area covered by the local station (*not* transmitter). If you choose one of the nation-wide stations you can drive all around the country listening to it without ever touching the reciever. Rune
