Subject: Re: [manet] draft-ramrekha-manet-cml-00.txt

From: "Stephan Bohacek" <[email-masked]>
Date: Thu, 4 Mar 2010 10:22:33 -0500

> Do you have any public results that can show that "large-scale fading is
more significant than fast-fading even at moderate displacements of a few
meters of the effects".?

There is a draft of a Master's thesis on the UDelModels website,
http://udelmodels.eecis.udel.edu/publications1.php. Briefly, the student
placed the transmitter antenna on linear actuator and several receivers in
various places. Experiments were performance in various locations. The
linear actuator moved about 1.5 cm at each step. At each location, a large
number of packets were broadcast in order to estimate the received signal
strength. The measurements were performed at night to ensure that the
scenario was static.

Using common models of multipath channels, we modeled the change in the
signal strength when the antenna moved only a few centimeters. At this
range, fast fading models explain the variations in the channel. However,
the fast fading model that worked for small distances does not provide a
good fit for larger displacement. However, a good fit is provided by adding
a lognormal fading.

Figure 1.1 in the student's thesis explains the conclusion (I posted the
figure at http://www.eecis.udel.edu/~bohacek/ChangeInSSVsDisplacement.ppt).
The observation is the solid blue curve and the model is the dashed blue
curve. Narrow band fading is the green curve. Clearly, narrow band only over
estimates the change in the signal strength. However, wide bandwidth
fast-fading (yellow curve) does provide a good fit up to 60 cm of
displacement. For larger distances, the fast fading plus lognormal fading
provides a good fit. At 2 meters, we see that the yellow curve (fast-fading
only) is well below the observations (and model). Therefore, we conclude
that at moderate displacements, fast-fading has a significantly smaller
impact than lognormal fading.

The idea that wide bandwidth mitigates fast fading is a not new. Rappoport's
textbook Wireless Communication is a widely cited reference on wireless
communication. Page 330 of Wireless Communication states:

"Resistance to multipath fading is another reason for considering spread
spectrum systems for wireless communication. Chapter 5 showed that wideband
signals are frequency selective. Since spread spectrum signals have a
uniform energy over a very large bandwidth, at any given time only a small
portion of the spectrum will undergo fading."

While this quote focuses on spread spectrum, the same ideas apply to wide
bandwidth OFDM.

I hope this helps,

Stephan

-----Original Message-----

From: [email-masked] [mailto:[email-masked]] On Behalf Of
Ramrekha, Arvind

Sent: Thursday, March 04, 2010 9:16 AM

To: Stephan Bohacek; [email-masked]

Subject: Re: [manet] draft-ramrekha-manet-cml-00.txt

Does anyone know if there is much difference in Qulanet and ns-2 propagation
models? 

Do you have any public results that can show that  "large-scale fading is
more significant than fast-fading even at moderate displacements of a few
meters of the effects".?

Arvind.

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