I'm having a hard time wrapping my head around doppler and how it affects a digital communications signal's baud rate. The examples that I've seen explain that the center frequency of a signal is shifted when either the receiver and/or transmitter are moving relative to each other. What I don't understand is the why doppler also affects baud rate? Can someone point me to some tutorials on this topic or explain? What I'd like to get understand clearly is: 1.) Intuitively understand why this happens? 2.) Mathematically understand why this happens? 3.) I'd like to be able to calculate when this is something that a communications receiver has to take into account? My guess is that based on the doppler shift that will be experienced by the receiver, in combination with the signal's baud rate, but I guess I need some hints or help to get me along with this.
Doppler effect on Baud Rate
Started by ●January 13, 2014
Reply by ●January 13, 20142014-01-13
On Monday, January 13, 2014 9:42:29 AM UTC-5, dsp student wrote:> I'm having a hard time wrapping my head around doppler and how it affects a digital communications signal's baud rate. The examples that I've seen explain that the center frequency of a signal is shifted when either the receiver and/or transmitter are moving relative to each other. What I don't understand is the why doppler also affects baud rate? Can someone point me to some tutorials on this topic or explain? What I'd like to get understand clearly is: > > > > 1.) Intuitively understand why this happens? > > 2.) Mathematically understand why this happens? > > 3.) I'd like to be able to calculate when this is something that a communications receiver has to take into account? My guess is that based on the doppler shift that will be experienced by the receiver, in combination with the signal's baud rate, but I guess I need some hints or help to get me along with this.Doppler dilates time. So the transmitter thinks it is sending symbols at a particular symbol rate, however the receiver's perception is subjected to time dilation. Simply put in the AWGN case, the transmitter sends x(t), where as the receiver perceives y(t) = x(t') + noise, where t' = t(1+doppler).
Reply by ●January 13, 20142014-01-13
On Monday, January 13, 2014 9:42:29 AM UTC-5, dsp student wrote:> I'm having a hard time wrapping my head around doppler and how it affects a digital communications signal's baud rate. The examples that I've seen explain that the center frequency of a signal is shifted when either the receiver and/or transmitter are moving relative to each other. What I don't understand is the why doppler also affects baud rate? Can someone point me to some tutorials on this topic or explain? What I'd like to get understand clearly is: > > > > 1.) Intuitively understand why this happens? > > 2.) Mathematically understand why this happens? > > 3.) I'd like to be able to calculate when this is something that a communications receiver has to take into account? My guess is that based on the doppler shift that will be experienced by the receiver, in combination with the signal's baud rate, but I guess I need some hints or help to get me along with this.The doppler effect affects all frequencies so that is why the baud rate changes too. Think about travelling waves on a lake. If you drive your boat towards them you hit them more often than if you drive away from them. This is true regardless of their shape (harmonic content). The formula is F = f*sqrt( (c+v)/(c-v) ) where F is the new freq., and f is the old frequency. "c" is the speed of light (3.0*10^8 m/s or for the pedantic 299792458 m/s) and v is the velocity of either the transmitter or the receiver. Be sure to use the same units for v and c. Positive v is for the two approaching each other. IHTH, Clay
Reply by ●January 13, 20142014-01-13
Forget all about the math for a second. imagine your transmitter as an old-fashioned endless paper printer. It has already printed a long strip of signal that's between you and the printer (which is the channel. It delays the signal a bit). You're reading at the same time as the printer is printing (a non-moving radio channel). Now you start to walk away from the printer, very slowly compared to the speed of the paper. You''ll have to read faster or slower to keep in sync (Doppler shift) _____________________________ Posted through www.DSPRelated.com
Reply by ●January 13, 20142014-01-13
and one more way to think about it Doppler doesn't SHIFT the RF frequency spectrum, it stretches or compresses it relative to DC, including all the sidebands. Mark
Reply by ●January 13, 20142014-01-13
Yep. However, in many applications, depending on the exact parameters, doppler is approximated as a frequency shift. It is valid in some regimes but not in all regimes. Of course, many authors of tutorials and textbooks are too sloppy to give the proper qualifiers when they jump straight into the approximation. On Monday, January 13, 2014 12:20:25 PM UTC-5, mako...@yahoo.com wrote:> and one more way to think about it > > Doppler doesn't SHIFT the RF frequency spectrum, it stretches or compresses it relative to DC, including all the sidebands. > > Mark
Reply by ●January 13, 20142014-01-13
On Mon, 13 Jan 2014 06:59:28 -0800, clay wrote:> On Monday, January 13, 2014 9:42:29 AM UTC-5, dsp student wrote: >> I'm having a hard time wrapping my head around doppler and how it >> affects a digital communications signal's baud rate. The examples that >> I've seen explain that the center frequency of a signal is shifted when >> either the receiver and/or transmitter are moving relative to each >> other. What I don't understand is the why doppler also affects baud >> rate? Can someone point me to some tutorials on this topic or explain? >> What I'd like to get understand clearly is: >> >> >> >> 1.) Intuitively understand why this happens? >> >> 2.) Mathematically understand why this happens? >> >> 3.) I'd like to be able to calculate when this is something that a >> communications receiver has to take into account? My guess is that >> based on the doppler shift that will be experienced by the receiver, in >> combination with the signal's baud rate, but I guess I need some hints >> or help to get me along with this. > > The doppler effect affects all frequencies so that is why the baud rate > changes too. Think about travelling waves on a lake. If you drive your > boat towards them you hit them more often than if you drive away from > them. This is true regardless of their shape (harmonic content). > > > The formula is F = f*sqrt( (c+v)/(c-v) ) > > where F is the new freq., and f is the old frequency. "c" is the speed > of light (3.0*10^8 m/s or for the pedantic 299792458 m/s) and v is the > velocity of either the transmitter or the receiver. Be sure to use the > same units for v and c. Positive v is for the two approaching each > other.And note that Clay's formula above isn't some super-zoot relativistic thing: you can derive it with simple geometry and a bit of algebra. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 13, 20142014-01-13
On Monday, January 13, 2014 12:23:31 PM UTC-5, julius wrote:> Yep. However, in many applications, depending on the exact parameters, doppler is approximated as a frequency shift. It is valid in some regimes but not in all regimes. Of course, many authors of tutorials and textbooks are too sloppy to give the proper qualifiers when they jump straight into the approximation. > > > > On Monday, January 13, 2014 12:20:25 PM UTC-5, mako...@yahoo.com wrote: > > > and one more way to think about it > > > > > > Doppler doesn't SHIFT the RF frequency spectrum, it stretches or compresses it relative to DC, including all the sidebands. > > > > > > MarkThat's why I gave the relativistic formula. It is simple, correct and doesn't use approximations. Clay
Reply by ●January 13, 20142014-01-13
On Mon, 13 Jan 2014 06:42:29 -0800, dsp student wrote:> I'm having a hard time wrapping my head around doppler and how it > affects a digital communications signal's baud rate. The examples that > I've seen explain that the center frequency of a signal is shifted when > either the receiver and/or transmitter are moving relative to each > other. What I don't understand is the why doppler also affects baud > rate? Can someone point me to some tutorials on this topic or explain? > What I'd like to get understand clearly is: > > 1.) Intuitively understand why this happens? > 2.) Mathematically understand why this happens? > 3.) I'd like to be able to calculate when this is something that a > communications receiver has to take into account? My guess is that based > on the doppler shift that will be experienced by the receiver, in > combination with the signal's baud rate, but I guess I need some hints > or help to get me along with this.1: You've gotten some good explanations. Basically, though, it's because the signal is both strung out between you and the transmitter, and it's coming toward you at a finite speed. So as you move toward or away from the signal, you change the frequency with which events arrive. Another way to think of it from the ways given is as cars on a highway. Imagine that you're standing by a highway, with cars whizzing by. Now get in a car yourself and go driving. When you get up to the same speed as traffic, the cars in your lane aren't moving at all relative to you, so the passing frequency has dropped to zero. Meanwhile, the cars in the opposing lane are coming at you twice as often. 2: You can work it out geometrically. I'll bet that Wikipedia has a good page on Doppler shift -- their math and physics stuff is usually pretty good, if shallow. 3: Get the math, and you'll know. Unless the relative speeds are really fast the Doppler shift doesn't have much to do with the baud rate -- the change due to Doppler shift goes as a percentage of whatever frequency is being transmitted. So I'm surprised it came up at all, unless you're looking at some scheme that really needs to hold the bit clock unusually steady. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 13, 20142014-01-13
On Monday, January 13, 2014 9:59:28 AM UTC-5, cl...@claysturner.com wrote:> The doppler effect affects all frequencies so that is why the baud rate changes too. Think about travelling waves on a lake. If you drive your boat towards them you hit them more often than if you drive away from them. This is true regardless of their shape (harmonic content). > > The formula is F = f*sqrt( (c+v)/(c-v) ) > > > where F is the new freq., and f is the old frequency. "c" is the speed of light (3.0*10^8 m/s or for the pedantic 299792458 m/s) and v is the velocity of either the transmitter or the receiver. Be sure to use the same units for v and c. Positive v is for the two approaching each other.Couple of things 1 - There is no square root in the Doppler Shift expression. 2 - The Doppler shift effect is different depending on whether the transmitter or receiver is moving. F=f(c+vr)/(c-vs) vr= radial velocity of receiver vs= radial velocity of transmitter Cheers, David
