Dear all, Consider a wireless channel where we have large scale fading (path- loss + shadowing) and small scale fading (multipath + doppler). Consider that given a certain distance between TX and RX we observe how the channel evolves in time and frequency. My question is if we can observe the frequency response of the channel for a long time, and then we average the observed path loss across frequency and time, would the small scale fading effects cancel out ? Then considering that shadowing is log-normal with average the only thing we should observe is the path-loss fading. Would there be any difference if in addition we consider a MIMO channel, and we also average across all the TX-RX paths ? Regards Dani
Does multipath effect average out in time ?
Started by ●May 14, 2008
Reply by ●May 14, 20082008-05-14
Dani Camps wrote:> Dear all, > > Consider a wireless channel where we have large scale fading (path- > loss + shadowing) and small scale fading (multipath + doppler). > Consider that given a certain distance between TX and RX we observe > how the channel evolves in time and frequency. > > My question is if we can observe the frequency response of the channel > for a long time, and then we average the observed path loss across > frequency and time, would the small scale fading effects cancel out ? > > Then considering that shadowing is log-normal with average the only > thing we should observe is the path-loss fading. > > Would there be any difference if in addition we consider a MIMO > channel, and we also average across all the TX-RX paths ?Suppose that the fading is so severe at some instants that there is no signal to recover. Might consideration of the channel properties at other times (averaging, for instance) be helpful? I don't think so. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●May 14, 20082008-05-14
On Wed, 14 May 2008 01:24:21 -0700 (PDT), Dani Camps <danicamps81@gmail.com> wrote:>Dear all, > >Consider a wireless channel where we have large scale fading (path- >loss + shadowing) and small scale fading (multipath + doppler). >Consider that given a certain distance between TX and RX we observe >how the channel evolves in time and frequency. > >My question is if we can observe the frequency response of the channel >for a long time, and then we average the observed path loss across >frequency and time, would the small scale fading effects cancel out ? > >Then considering that shadowing is log-normal with average the only >thing we should observe is the path-loss fading. > >Would there be any difference if in addition we consider a MIMO >channel, and we also average across all the TX-RX paths ? > >Regards > >DaniOver how long of a time do you want to average? FWIW, if the Tx and Rx sites are fixed, the channel may be very stable and have an extremely long coherence time. If nothing in the environment changes, the channel won't change, so you may never get an "unfaded" channel. If one wanted to average over a very large number of possible channels for a fixed link distance, that does tend to average pretty well. This means that both the Tx and Rx relocate often. For the MIMO case, and pretty much any diversity combining, the channel does flatten. Even just combining over two receive antennas provides a significant amount of channel flattening. I wouldn't expect, though, to reliably get a fully averaged channel over only 4-6 antennas, though. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org
Reply by ●May 15, 20082008-05-15
On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote:> Dear all, > > Consider a wireless channel where we have large scale fading (path- > loss + shadowing) and small scale fading (multipath + doppler). > Consider that given a certain distance between TX and RX we observe > how the channel evolves in time and frequency. > > My question is if we can observe the frequency response of the channel > for a long time, and then we average the observed path loss across > frequency and time, would the small scale fading effects cancel out ?If you have an 'evolving path' you necesarily have a time/varying scenario, either because the envronment changes, the source and/or reciever moves, or both. In that sort of scenario one might say that 'small scale effects average out' but it wouldn't be very helpful. The big picture remains the same, multi-path effects still mess up the signal. Once the path goes dynamic you have lost some of the keys you might have used to tune an adaptive system to compensate for multpath propagation. The key with multipath propagation is that you get destructive interference. It does not suffice to look at a frequency response go see how it works. If you have negative interference at some frequency, you get a zero in the frequency response, and it is fairly easy to understand why the signal is messed up. The key is to see that the same goes for 'non-cancelling' intereference, i.e. where the interefence does not cancel the 'business signal'. If you add an interfering signal which has a random amplitude and phase, the detection task does not become very easy, even if the frequency response might indicate there is plenty of signal energy present. If everything is stationary and the system is well designed, you might get enough time to detect and compensate for these effects. If anything starts moving, everything changes all the time. You might want to play around with some simple scenarios, like Lloyd's mirror. Rune
Reply by ●May 15, 20082008-05-15
On May 15, 9:12�am, Rune Allnor <all...@tele.ntnu.no> wrote:> On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote: > > > Dear all, > > > Consider a wireless channel where we have large scale fading (path- > > loss + shadowing) and small scale fading (multipath + doppler). > > Consider that given a certain distance between TX and RX we observe > > how the channel evolves in time and frequency. > > > My question is if we can observe the frequency response of the channel > > for a long time, and then we average the observed path loss across > > frequency and time, would the small scale fading effects cancel out ? > > If you have an 'evolving path' you necesarily have a time/varying > scenario, either because the envronment changes, the source and/or > reciever moves, or both. > > In that sort of scenario one might say that 'small scale effects > average out' but it wouldn't be very helpful. > > The big picture remains the same, multi-path effects still mess up > the signal. Once the path goes dynamic you have lost some of the > keys you might have used to tune an adaptive system to compensate > for multpath propagation. > > The key with multipath propagation is that you get destructive > interference. It does not suffice to look at a frequency > response go see how it works. If you have negative interference > at some frequency, you get a zero in the frequency response, > and it is fairly easy to understand why the signal is messed up. > > The key is to see that the same goes for 'non-cancelling' > intereference, i.e. where the interefence does not cancel > the 'business signal'. If you add an interfering signal > which has a random amplitude and phase, the detection task > does not become very easy, even if the frequency response > might indicate there is plenty of signal energy present. > > If everything is stationary and the system is well designed, > you might get enough time to detect and compensate for these > effects. If anything starts moving, everything changes all > the time. > > You might want to play around with some simple scenarios, > like Lloyd's mirror. > > RuneDear all, Thanks for your answers. It is now clear to me that I should not expect small scale fading effects to average out in time or frequency. Maybe another way to look at it is to consider the small scale fading modeled by a Rayleigh distribution, then if the expected value of this rayleigh distribution is not zero small scale fading should not average out. Regards Daniel
Reply by ●May 15, 20082008-05-15
>On May 15, 9:12=A0am, Rune Allnor <all...@tele.ntnu.no> wrote: >> On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote: >> >> > Dear all, >> >> > Consider a wireless channel where we have large scale fading (path- >> > loss + shadowing) and small scale fading (multipath + doppler). >> > Consider that given a certain distance between TX and RX we observe >> > how the channel evolves in time and frequency. >> >> > My question is if we can observe the frequency response of thechannel>> > for a long time, and then we average the observed path loss across >> > frequency and time, would the small scale fading effects cancel out?>> >> If you have an 'evolving path' you necesarily have a time/varying >> scenario, either because the envronment changes, the source and/or >> reciever moves, or both. >> >> In that sort of scenario one might say that 'small scale effects >> average out' but it wouldn't be very helpful. >> >> The big picture remains the same, multi-path effects still mess up >> the signal. Once the path goes dynamic you have lost some of the >> keys you might have used to tune an adaptive system to compensate >> for multpath propagation. >> >> The key with multipath propagation is that you get destructive >> interference. It does not suffice to look at a frequency >> response go see how it works. If you have negative interference >> at some frequency, you get a zero in the frequency response, >> and it is fairly easy to understand why the signal is messed up. >> >> The key is to see that the same goes for 'non-cancelling' >> intereference, i.e. where the interefence does not cancel >> the 'business signal'. If you add an interfering signal >> which has a random amplitude and phase, the detection task >> does not become very easy, even if the frequency response >> might indicate there is plenty of signal energy present. >> >> If everything is stationary and the system is well designed, >> you might get enough time to detect and compensate for these >> effects. If anything starts moving, everything changes all >> the time. >> >> You might want to play around with some simple scenarios, >> like Lloyd's mirror. >> >> Rune > >Dear all, > >Thanks for your answers. It is now clear to me that I should not >expect small scale fading effects to average out in time or frequency. >Maybe another way to look at it is to consider the small scale fading >modeled by a Rayleigh distribution, then if the expected value of this >rayleigh distribution is not zero small scale fading should not >average out. > >Regards > >Daniel >%%%% Dear All I have got one question related to this. If you want to simulate doppler, how do u do that? i.e. I have designed my transmitter in matlab, and i want to see if my receiver corrects the doppler and ISI. I do not know how to introduce doppler into system? Thanks Chintan
Reply by ●May 15, 20082008-05-15
On May 15, 4:28�am, "cpshah99" <cpsha...@rediffmail.com> wrote:> >On May 15, 9:12=A0am, Rune Allnor <all...@tele.ntnu.no> wrote: > >> On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote: > > >> > Dear all, > > >> > Consider a wireless channel where we have large scale fading (path- > >> > loss + shadowing) and small scale fading (multipath + doppler). > >> > Consider that given a certain distance between TX and RX we observe > >> > how the channel evolves in time and frequency. > > >> > My question is if we can observe the frequency response of the > channel > >> > for a long time, and then we average the observed path loss across > >> > frequency and time, would the small scale fading effects cancel out > ? > > >> If you have an 'evolving path' you necesarily have a time/varying > >> scenario, either because the envronment changes, the source and/or > >> reciever moves, or both. > > >> In that sort of scenario one might say that 'small scale effects > >> average out' but it wouldn't be very helpful. > > >> The big picture remains the same, multi-path effects still mess up > >> the signal. Once the path goes dynamic you have lost some of the > >> keys you might have used to tune an adaptive system to compensate > >> for multpath propagation. > > >> The key with multipath propagation is that you get destructive > >> interference. It does not suffice to look at a frequency > >> response go see how it works. If you have negative interference > >> at some frequency, you get a zero in the frequency response, > >> and it is fairly easy to understand why the signal is messed up. > > >> The key is to see that the same goes for 'non-cancelling' > >> intereference, i.e. where the interefence does not cancel > >> the 'business signal'. If you add an interfering signal > >> which has a random amplitude and phase, the detection task > >> does not become very easy, even if the frequency response > >> might indicate there is plenty of signal energy present. > > >> If everything is stationary and the system is well designed, > >> you might get enough time to detect and compensate for these > >> effects. If anything starts moving, everything changes all > >> the time. > > >> You might want to play around with some simple scenarios, > >> like Lloyd's mirror. > > >> Rune > > >Dear all, > > >Thanks for your answers. It is now clear to me that I should not > >expect small scale fading effects to average out in time or frequency. > >Maybe another way to look at it is to consider the small scale fading > >modeled by a Rayleigh distribution, then if the expected value of this > >rayleigh distribution is not zero small scale fading should not > >average out. > > >Regards > > >Daniel > > %%%% > > Dear All > > I have got one question related to this. > > If you want to simulate doppler, how do u do that? i.e. I have designed my > transmitter in matlab, and i want to see if my receiver corrects the > doppler and ISI. I do not know how to introduce doppler into system? > > Thanks > > ChintanEither use "rescale" or write an interpolator (such as a Farrow interpolator).
Reply by ●May 15, 20082008-05-15
>On May 15, 4:28=A0am, "cpshah99" <cpsha...@rediffmail.com> wrote: >> >On May 15, 9:12=3DA0am, Rune Allnor <all...@tele.ntnu.no> wrote: >> >> On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote: >> >> >> > Dear all, >> >> >> > Consider a wireless channel where we have large scale fading(path->> >> > loss + shadowing) and small scale fading (multipath + doppler). >> >> > Consider that given a certain distance between TX and RX weobserve>> >> > how the channel evolves in time and frequency. >> >> >> > My question is if we can observe the frequency response of the >> channel >> >> > for a long time, and then we average the observed path lossacross>> >> > frequency and time, would the small scale fading effects cancelout>> ? >> >> >> If you have an 'evolving path' you necesarily have a time/varying >> >> scenario, either because the envronment changes, the source and/or >> >> reciever moves, or both. >> >> >> In that sort of scenario one might say that 'small scale effects >> >> average out' but it wouldn't be very helpful. >> >> >> The big picture remains the same, multi-path effects still mess up >> >> the signal. Once the path goes dynamic you have lost some of the >> >> keys you might have used to tune an adaptive system to compensate >> >> for multpath propagation. >> >> >> The key with multipath propagation is that you get destructive >> >> interference. It does not suffice to look at a frequency >> >> response go see how it works. If you have negative interference >> >> at some frequency, you get a zero in the frequency response, >> >> and it is fairly easy to understand why the signal is messed up. >> >> >> The key is to see that the same goes for 'non-cancelling' >> >> intereference, i.e. where the interefence does not cancel >> >> the 'business signal'. If you add an interfering signal >> >> which has a random amplitude and phase, the detection task >> >> does not become very easy, even if the frequency response >> >> might indicate there is plenty of signal energy present. >> >> >> If everything is stationary and the system is well designed, >> >> you might get enough time to detect and compensate for these >> >> effects. If anything starts moving, everything changes all >> >> the time. >> >> >> You might want to play around with some simple scenarios, >> >> like Lloyd's mirror. >> >> >> Rune >> >> >Dear all, >> >> >Thanks for your answers. It is now clear to me that I should not >> >expect small scale fading effects to average out in time orfrequency.>> >Maybe another way to look at it is to consider the small scale fading >> >modeled by a Rayleigh distribution, then if the expected value ofthis>> >rayleigh distribution is not zero small scale fading should not >> >average out. >> >> >Regards >> >> >Daniel >> >> %%%% >> >> Dear All >> >> I have got one question related to this. >> >> If you want to simulate doppler, how do u do that? i.e. I have designedmy=> >> transmitter in matlab, and i want to see if my receiver corrects the >> doppler and ISI. I do not know how to introduce doppler into system? >> >> Thanks >> >> Chintan > >Either use "rescale" or write an interpolator (such as a Farrow >interpolator). >%%%% HI, But the meaning of doppler is that it is frequency modulation, then it should not affect the amplitude of the signal, and the change in the aplitude will depend on the path. And can u please explain, the use of 'rescale', I am also planning to add noise and 3-tap channel. Your guidance will be greatly appreciated. Thanks. Chintan
Reply by ●May 15, 20082008-05-15
On May 15, 8:10 am, "cpshah99" <cpsha...@rediffmail.com> wrote:> >On May 15, 4:28=A0am, "cpshah99" <cpsha...@rediffmail.com> wrote: > >> >On May 15, 9:12=3DA0am, Rune Allnor <all...@tele.ntnu.no> wrote: > >> >> On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote: > > >> >> > Dear all, > > >> >> > Consider a wireless channel where we have large scale fading > (path- > >> >> > loss + shadowing) and small scale fading (multipath + doppler). > >> >> > Consider that given a certain distance between TX and RX we > observe > >> >> > how the channel evolves in time and frequency. > > >> >> > My question is if we can observe the frequency response of the > >> channel > >> >> > for a long time, and then we average the observed path loss > across > >> >> > frequency and time, would the small scale fading effects cancel > out > >> ? > > >> >> If you have an 'evolving path' you necesarily have a time/varying > >> >> scenario, either because the envronment changes, the source and/or > >> >> reciever moves, or both. > > >> >> In that sort of scenario one might say that 'small scale effects > >> >> average out' but it wouldn't be very helpful. > > >> >> The big picture remains the same, multi-path effects still mess up > >> >> the signal. Once the path goes dynamic you have lost some of the > >> >> keys you might have used to tune an adaptive system to compensate > >> >> for multpath propagation. > > >> >> The key with multipath propagation is that you get destructive > >> >> interference. It does not suffice to look at a frequency > >> >> response go see how it works. If you have negative interference > >> >> at some frequency, you get a zero in the frequency response, > >> >> and it is fairly easy to understand why the signal is messed up. > > >> >> The key is to see that the same goes for 'non-cancelling' > >> >> intereference, i.e. where the interefence does not cancel > >> >> the 'business signal'. If you add an interfering signal > >> >> which has a random amplitude and phase, the detection task > >> >> does not become very easy, even if the frequency response > >> >> might indicate there is plenty of signal energy present. > > >> >> If everything is stationary and the system is well designed, > >> >> you might get enough time to detect and compensate for these > >> >> effects. If anything starts moving, everything changes all > >> >> the time. > > >> >> You might want to play around with some simple scenarios, > >> >> like Lloyd's mirror. > > >> >> Rune > > >> >Dear all, > > >> >Thanks for your answers. It is now clear to me that I should not > >> >expect small scale fading effects to average out in time or > frequency. > >> >Maybe another way to look at it is to consider the small scale fading > >> >modeled by a Rayleigh distribution, then if the expected value of > this > >> >rayleigh distribution is not zero small scale fading should not > >> >average out. > > >> >Regards > > >> >Daniel > > >> %%%% > > >> Dear All > > >> I have got one question related to this. > > >> If you want to simulate doppler, how do u do that? i.e. I have designed > my= > > >> transmitter in matlab, and i want to see if my receiver corrects the > >> doppler and ISI. I do not know how to introduce doppler into system? > > >> Thanks > > >> Chintan > > >Either use "rescale" or write an interpolator (such as a Farrow > >interpolator). > > %%%% > > HI, > > But the meaning of doppler is that it is frequency modulation, then it > should not affect the amplitude of the signal, and the change in the > aplitude will depend on the path. > > And can u please explain, the use of 'rescale', I am also planning to add > noise and 3-tap channel. > > Your guidance will be greatly appreciated. > > Thanks. > > ChintanI meant "resample", sorry. You have to do a time re-scaling, basically. I don't know what you really mean by "frequency modulation". To me, Doppler means that the time reference at the transmitter is "t", and at the receiver it is "t(1+\delta)", so you are compressing (and expanding) time (and freq) respectively. Modeling the above in discrete-time amounts to sampling rate conversion.
Reply by ●May 15, 20082008-05-15
>On May 15, 8:10 am, "cpshah99" <cpsha...@rediffmail.com> wrote: >> >On May 15, 4:28=A0am, "cpshah99" <cpsha...@rediffmail.com> wrote: >> >> >On May 15, 9:12=3DA0am, Rune Allnor <all...@tele.ntnu.no> wrote: >> >> >> On 14 Mai, 10:24, Dani Camps <danicamp...@gmail.com> wrote: >> >> >> >> > Dear all, >> >> >> >> > Consider a wireless channel where we have large scale fading >> (path- >> >> >> > loss + shadowing) and small scale fading (multipath +doppler).>> >> >> > Consider that given a certain distance between TX and RX we >> observe >> >> >> > how the channel evolves in time and frequency. >> >> >> >> > My question is if we can observe the frequency response of the >> >> channel >> >> >> > for a long time, and then we average the observed path loss >> across >> >> >> > frequency and time, would the small scale fading effectscancel>> out >> >> ? >> >> >> >> If you have an 'evolving path' you necesarily have atime/varying>> >> >> scenario, either because the envronment changes, the sourceand/or>> >> >> reciever moves, or both. >> >> >> >> In that sort of scenario one might say that 'small scale effects >> >> >> average out' but it wouldn't be very helpful. >> >> >> >> The big picture remains the same, multi-path effects still messup>> >> >> the signal. Once the path goes dynamic you have lost some of the >> >> >> keys you might have used to tune an adaptive system tocompensate>> >> >> for multpath propagation. >> >> >> >> The key with multipath propagation is that you get destructive >> >> >> interference. It does not suffice to look at a frequency >> >> >> response go see how it works. If you have negative interference >> >> >> at some frequency, you get a zero in the frequency response, >> >> >> and it is fairly easy to understand why the signal is messed up. >> >> >> >> The key is to see that the same goes for 'non-cancelling' >> >> >> intereference, i.e. where the interefence does not cancel >> >> >> the 'business signal'. If you add an interfering signal >> >> >> which has a random amplitude and phase, the detection task >> >> >> does not become very easy, even if the frequency response >> >> >> might indicate there is plenty of signal energy present. >> >> >> >> If everything is stationary and the system is well designed, >> >> >> you might get enough time to detect and compensate for these >> >> >> effects. If anything starts moving, everything changes all >> >> >> the time. >> >> >> >> You might want to play around with some simple scenarios, >> >> >> like Lloyd's mirror. >> >> >> >> Rune >> >> >> >Dear all, >> >> >> >Thanks for your answers. It is now clear to me that I should not >> >> >expect small scale fading effects to average out in time or >> frequency. >> >> >Maybe another way to look at it is to consider the small scalefading>> >> >modeled by a Rayleigh distribution, then if the expected value of >> this >> >> >rayleigh distribution is not zero small scale fading should not >> >> >average out. >> >> >> >Regards >> >> >> >Daniel >> >> >> %%%% >> >> >> Dear All >> >> >> I have got one question related to this. >> >> >> If you want to simulate doppler, how do u do that? i.e. I havedesigned>> my= >> >> >> transmitter in matlab, and i want to see if my receiver correctsthe>> >> doppler and ISI. I do not know how to introduce doppler intosystem?>> >> >> Thanks >> >> >> Chintan >> >> >Either use "rescale" or write an interpolator (such as a Farrow >> >interpolator). >> >> %%%% >> >> HI, >> >> But the meaning of doppler is that it is frequency modulation, then it >> should not affect the amplitude of the signal, and the change in the >> aplitude will depend on the path. >> >> And can u please explain, the use of 'rescale', I am also planning toadd>> noise and 3-tap channel. >> >> Your guidance will be greatly appreciated. >> >> Thanks. >> >> Chintan > >I meant "resample", sorry. You have to do a time re-scaling, >basically. >I don't know what you really mean by "frequency modulation". To me, >Doppler means that the time reference at the transmitter is "t", and >at >the receiver it is "t(1+\delta)", so you are compressing (and >expanding) >time (and freq) respectively. > >Modeling the above in discrete-time amounts to sampling rate >conversion. >HI.. You are correct....But I have already used this function, but the problem is my system works fine for more expantion then less. My original vector size is 30720; And I am using resample(x,10010,10000) -> works fine But resample(x,10001,10000) -> does not work. Now to correct the doppler I am using linear interpolation, a tested algorithm, which is adaptive. What if I set the initial Interpolation factor to some random value instead of setting it to 1. And see if my algorithm corrects it or not? I have tried this method and it gives me expected result. Thanks Chintan
