Power delay profile doubt

On 6/30/2009 8:52 AM, commengr wrote:
>> On Jun 29, 11:23=A0am, "shekar1989"<yogicyog...@gmail.com>  wrote:
>>>> shekar1989 wrote:
>>>>> Is the power delay profile dependent on the carrier frequency?
>>>> The power delay profile depends on the propagation conditions.
> The=3D20
>>>> conditions depend on the frequency band (in the large scale),
> the=3D20
>>>> antennae position, the distance and the typical obstacles and
>>> reflectors.=3D
>>>
>>>>> I have read in many books that the delay spread for outdoor
>>> environment=3D
>>>> is
>>>>> in the order of 1 us. But that is for carrier frequency of around
> 900
>>> M=3D
>>>> hz.
>>>>> (cellular communication).=3D20
>>>> That's about right for the quazi optical propagation, near zone and
>>> the=3D20
>>>> hand held terminals.
>>>>> I need the power delay profile for FM
>>>>> transmission (88 =3DF2=3DC0=3DD3 108 Mhz. range).
>>>> You need the delay profile to do what?
>>>>> Is the delay spread the same ?
>>>> It is quite different from 900MHz cellular.
>>>> Vladimir Vassilevsky
>>>> DSP and Mixed Signal Design Consultant
>>>> http://www.abvolt.com
>>>> Thanks for the reply,
>>> =A0Now I understand what the power delay profile depends on.
>>> =A0It is independent of carrier frequency, since it is only the measure
> o=
>> f =A0
>>> the channel characteristics and the surrounding environment. I wanted
> to
>>> study the effect of fading for FM transmission (whether it is flat and
> sl=
>> ow
>>> fading ... and so on ). Nearly all websites mention for 802.11 or
> cellula=
>> r
>>> but not for FM, (digital). Can anyone provide a link/paper which deals
> wi=
>> th
>>> fading for Digital FM transmission (88-108 Mhz range) ? =A0
>>> Thanks.
>> You are too quick to dismiss carrier frequency, see
>>
>> title=3D{A propagation measurement-based analysis of frequency hopping
>> systems on VHF mobile fading channels},
>> author=3D{Pugh, J.A. and Vigneron, P.J. and Bultitude, R.J.C.   },
>> booktitle=3D{IEEE Canadian Conference on Electrical and Computer
>> Engineering},
>> year=3D{2005},
>> month=3D{May.},
>>
>> title=3D{Propagation models for mobile terrestrial VHF communications},
>> author=3D{Vigneron, P.J. and Pugh, J.A.},
>> booktitle=3D{Military Communications Conference, 2008. MILCOM 2008.
>> IEEE},
>> year=3D{2008},
>> month=3D{Nov.},
>>
>>
>>
>>
> ===================================================================
>
>
> PDP is the distribution of the multipath power over time, so it can't
> depend on frequency.

In a general sense this is mostly true, but material penetration and 
reflection properties are very frequency dependent, so in a practical 
sense it can make a difference.

> It must be said however, that it may include frequency only for better
> analysis of the channel, as suggested by Vladimir and the papers he put to
> support his argument, but we only consider the effect of fast fading due to
> doppler spread caused by doppler shift which does depend on frequency (I
> haven't gone through the papers but I can think of why they included
> frequency)
>
> Again we'd just be forcing the matter to include frequency, usually we use
> a worst case doppler shift is chosen along with the PDP

Don't confuse the dynamics of the channel with the average PDP.  All 
other things being the same (e.g., Tx power, antenna directivity and 
gain, reflectivity, absorption, etc., which they really can't all be the 
same) the *average* PDPs would be the same but the specific channel 
instances will differ with frequency.   In other words, averaged over 
many channels in similar environments the PDPs of multipath wavefronts 
would be very comparable, but each specific channel instance would 
differ due to the differing interference due to wavelength.

This kind of further illustrates the point, I think, that you really 
can't keep the antenna characteristics, environment reflectivity and 
absorption, etc., etc., constant with frequency.   So there will be an 
unavoidable dependence on frequency.   How big that difference compares 
with the practical considerations of antenna changes, PA and cable 
changes that lead to differing Tx powers, different AFEs with difference 
sensitivities and noise figures, etc., etc., is difficult to assess.

Terrain matters, too.   The reflectivity and absorption properties of 
rocks and sand doesn't change as much with frequency (IIRC) as foliage 
penetration.  A desert environment may produce different results than a 
jungle environment.

>On 6/30/2009 8:52 AM, commengr wrote:
>>> On Jun 29, 11:23=A0am, "shekar1989"<yogicyog...@gmail.com>  wrote:
>>>>> shekar1989 wrote:
>>>>>> Is the power delay profile dependent on the carrier frequency?
>>>>> The power delay profile depends on the propagation conditions.
>> The=3D20
>>>>> conditions depend on the frequency band (in the large scale),
>> the=3D20
>>>>> antennae position, the distance and the typical obstacles and
>>>> reflectors.=3D
>>>>
>>>>>> I have read in many books that the delay spread for outdoor
>>>> environment=3D
>>>>> is
>>>>>> in the order of 1 us. But that is for carrier frequency of around
>> 900
>>>> M=3D
>>>>> hz.
>>>>>> (cellular communication).=3D20
>>>>> That's about right for the quazi optical propagation, near zone and
>>>> the=3D20
>>>>> hand held terminals.
>>>>>> I need the power delay profile for FM
>>>>>> transmission (88 =3DF2=3DC0=3DD3 108 Mhz. range).
>>>>> You need the delay profile to do what?
>>>>>> Is the delay spread the same ?
>>>>> It is quite different from 900MHz cellular.
>>>>> Vladimir Vassilevsky
>>>>> DSP and Mixed Signal Design Consultant
>>>>> http://www.abvolt.com
>>>>> Thanks for the reply,
>>>> =A0Now I understand what the power delay profile depends on.
>>>> =A0It is independent of carrier frequency, since it is only the
measure
>> o=
>>> f =A0
>>>> the channel characteristics and the surrounding environment. I
wanted
>> to
>>>> study the effect of fading for FM transmission (whether it is flat
and
>> sl=
>>> ow
>>>> fading ... and so on ). Nearly all websites mention for 802.11 or
>> cellula=
>>> r
>>>> but not for FM, (digital). Can anyone provide a link/paper which
deals
>> wi=
>>> th
>>>> fading for Digital FM transmission (88-108 Mhz range) ? =A0
>>>> Thanks.
>>> You are too quick to dismiss carrier frequency, see
>>>
>>> title=3D{A propagation measurement-based analysis of frequency
hopping
>>> systems on VHF mobile fading channels},
>>> author=3D{Pugh, J.A. and Vigneron, P.J. and Bultitude, R.J.C.   },
>>> booktitle=3D{IEEE Canadian Conference on Electrical and Computer
>>> Engineering},
>>> year=3D{2005},
>>> month=3D{May.},
>>>
>>> title=3D{Propagation models for mobile terrestrial VHF
communications},
>>> author=3D{Vigneron, P.J. and Pugh, J.A.},
>>> booktitle=3D{Military Communications Conference, 2008. MILCOM 2008.
>>> IEEE},
>>> year=3D{2008},
>>> month=3D{Nov.},
>>>
>>>
>>>
>>>
>> ===================================================================
>>
>>
>> PDP is the distribution of the multipath power over time, so it can't
>> depend on frequency.
>
>In a general sense this is mostly true, but material penetration and 
>reflection properties are very frequency dependent, so in a practical 
>sense it can make a difference.
>
>> It must be said however, that it may include frequency only for better
>> analysis of the channel, as suggested by Vladimir and the papers he put
to
>> support his argument, but we only consider the effect of fast fading
due to
>> doppler spread caused by doppler shift which does depend on frequency
(I
>> haven't gone through the papers but I can think of why they included
>> frequency)
>>
>> Again we'd just be forcing the matter to include frequency, usually we
use
>> a worst case doppler shift is chosen along with the PDP
>
>Don't confuse the dynamics of the channel with the average PDP.  All 
>other things being the same (e.g., Tx power, antenna directivity and 
>gain, reflectivity, absorption, etc., which they really can't all be the

>same) the *average* PDPs would be the same but the specific channel 
>instances will differ with frequency.   In other words, averaged over 
>many channels in similar environments the PDPs of multipath wavefronts 
>would be very comparable, but each specific channel instance would 
>differ due to the differing interference due to wavelength.
>
>This kind of further illustrates the point, I think, that you really 
>can't keep the antenna characteristics, environment reflectivity and 
>absorption, etc., etc., constant with frequency.   So there will be an 
>unavoidable dependence on frequency.   How big that difference compares 
>with the practical considerations of antenna changes, PA and cable 
>changes that lead to differing Tx powers, different AFEs with difference

>sensitivities and noise figures, etc., etc., is difficult to assess.
>
>Terrain matters, too.   The reflectivity and absorption properties of 
>rocks and sand doesn't change as much with frequency (IIRC) as foliage 
>penetration.  A desert environment may produce different results than a 
>jungle environment.
>
>


Yeah I completely agree you, there are so many factors to the *power* in
the PDP that we just consider the average PDP. In fact thats the reason why
we choose a PDP that is a Worst Case Scenerio for a channel (urban, dense
urban) and make sure that a communication system works well under those
conditions.

> BTW, it is fun to simulate the FM quality in the presense of multipath 
> and interferrence; the effects are very audible.

and they should be, due to the multipath the frequency content of the FM
signal would change... meaning the information transmitted using FM would
also change. 

-----------------------------------------
*groups.yahoo.com/group/telecom_research*
-----------------------------------------