Subject: [manet] Another question regarding channel model in highly mobile environment

From: Fan Bai <[email-masked]>
Date: Wed, 2 Nov 2005 11:10:07 -0500

Hi, Joe, and other colleagues,
 I am also looking at a very tricky question: what is the channel model for
highly mobile environment (i.e. the vehicle-to-vehicle communication on
freeway and local streets in the metropolitan area)?
 Obviously, the open space model does not provide a good approximation for
such environment. My data sets collected from the field test also suggested
that Rayleigh fading or two-ray model also do not work well in such a
complex environment. I am still looking at the possibility of using Rican
fading to explain my data.
 I am wondering whether there is some existing channel model (including
shadowing and fading, especially the fading caused by delay spread and
Doppler effect) for such environment? How wireless communication PHY layer
experts approach this problem? As a side word, I am not sure whether the
model used in cellular system is working here or not, because both wireless
transmitter and wireless receiver are highly mobile in vehicle-to-vehicle
communication while at least the Base Station is stationary in cellular
system.
 Any comments and suggestions, or links, books and papers are highly
appreciated. Thanks for your comments in advance!
 regards,
Fan
             Chris Yao wrote:

> hi,
>
> in wireless networks, based on the received Received Signal Strength
> Indicator (RSSI), a node can estimate its distance to the beacon node.
>
> could someone point out some related articles about how to estimate
> the distance? what popular theory/emperical RF models have been used?

Like so many things, this is an easy question to ask and a difficult one
to answer. In fact, you can't measure distance directly, but you can
measure path loss and estimate distance based on that. The problem is
that in practice there is typically a wide statistical variance on path
loss vs. distance.

I would recommend Ted Rappaport's book "Wireless Communications" for a
review of the effects typically encountered in the wireless environment
and some models that address them.

We begin by assuming you know the transmit power. Perhaps this is
obvious, but if you don't know this then it is impossible to estimate
the distance based on receive power. You must also know the gain of
both the transmit and receive antennas.

The most obvious model is the free space transmission model. It is only
accurate in free space (i.e. outer space), you can find it by searching
on "Friis Transmission formula". Beyond that, modeling indoors and
outdoors tends to be very different.

In typical terrestrial propagation environments the Friis formula
doesn't apply because of absorbtion, reflections, etc.. Cellular
networks typically use the "Hata-Okumura Model," in planning, which is
an empirical model.

NIST in Boulder has done extensive research on modeling the indoor
propagation environment. Here's a link to a bibliography they've put
online.
http://w3.antd.nist.gov/wctg/manet/wirelesspropagation_bibliog.html

good luck!
joe

> Best Regards,
> Chris
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From: Joe Carey <[email-masked]>
Date: Wed, 02 Nov 2005 11:23:22 -0700

Fan Bai wrote:

> I am also looking at a very tricky question: what is the channel model 
> for highly mobile environment (i.e. the vehicle-to-vehicle 
> communication on freeway and local streets in the metropolitan area)?
>  
> Obviously, the open space model does not provide a good 
> approximation for such environment. My data sets collected from the 
> field test also suggested that Rayleigh fading or two-ray model also 
> do not work well in such a complex environment. I am still looking at 
> the possibility of using Rican fading to explain my data.
>  
> Doppler effect) for such environment?

As far as I know, there is no general consensus on any model fitting the 
data especially well.  In fact, my impression is quite the opposite: no 
model is known to be good in all environments.

Your "highly mobile" environment could be different depending on several 
factors, including the reflections from buildings and absorbtion by 
foliage.  These will be different depending on urban vs. suburban 
environments.  Recent data taken by the MIT roofnet team showed very 
little correllation between distance and path loss for their 
environment, and they speculated the cause was that there was a wide 
variation in heights of the transceivers.  I would expect that hills 
would dramatically affect your propagation model.

With respect to doppler shift, the 802.11 cards typically used in ad-hoc 
networking experiments were not designed to track significant doppler 
shift, but some have been shown empirically to work well. 

> How wireless communication PHY layer experts approach this problem?

They continue to research it ;)  This is fertile ground.  Unfortunately, 
it's an unbounded problem - you can never be sure when you've reached a 
final conclusion.

> As a side word, I am not sure whether the model used in cellular 
> system is working here or not, because both wireless transmitter and 
> wireless receiver are highly mobile in vehicle-to-vehicle 
> communication while at least the Base Station is stationary in 
> cellular system. 
>  

The cellular model is a good starting point, but it can certainly be 
improved upon.  Besides the mobility aspect you mention, the cellular 
waveform is typically relatively narrow (~ 1.25 MHz for CDMA, despread 
to 8 kbps) compared with the 802.11 band (22 MHz waveform, despread to 1 
MBit/sec at the lowest data rate).  IS-95 CDMA, for example, explicitly 
uses a RAKE receiver and power control to manage the awful fading channel.

 I know several companies have done internal research that they guard as 
proprietary information.  Here is a paper the MIT team published that 
was for a static system:

http://pdos.lcs.mit.edu/~rtm/papers/p442-aguayo.pdf