stds-80220-ch-models: CHAN: Re: Delay spread limitation
[NOTE: This posting is made as a contributor to, not as chairman
of, the modeling correspondence group. No special weight should
be attached to any opinions, technical observations, suggestions,
or other information that may be expressed in this posting.]
Channel Modelers,
I agree with Fred and Samir's reasoning that it is not appropriate to
simply drop the last two taps of the Vehicular B model.
To expand a bit on Fred's observation regarding the validity of
simply truncating Vehicular B at 10 usec, I would like to focus on
what appears to be the source of that proposal in the first place,
which is the discrepancy between two views regarding the frequency
of occurrence of such channels in the real world. If we can understand
why this discrepancy exists, it may be possible to develop a consensus
view that we should move in one direction or the other, i.e. either
(a) coming up with a vehicular channel model for MBWA which would
have a power delay profile narrower than UMTS Vehicular B, yet
still be justifiable based on real-world channel measurements,
or
(b) satisfying ourselves that there is a valid purpose to be served
by including channels with delay profiles like Vehicular B in the
MBWA requirement.
As one co-author of the contribution which suggested Vehicular B
(C802.20-03/48) I would be agreeable to move in the direction of
(a), provided that the group consensus is that we understand _why_
Vehicular B is not (or perhaps 'no longer') sufficiently realistic
and/or frequent to warrant its inclusion in the MBWA channel set.
Here are the two conflicting views, as best I understand them:
Marianna Goldhammer <marianna.goldhammer@alvarion.com> writes:
>
> Nevertheless, if Sprint found that apparition probability of this tap
> is very low, why to mess the standard development with delays that
> describe almost un-existing channels?!
>
> This channel model is a "selected test environments", not an absolute
> channel, and actually Sprint message was: Vehicular B is not a
> valid selection!
Here is the text from [1], Section B.1.4.2, giving an overview of the
channel impulse response models:
"For each terrestrial test environment, a channel impulse response
model based on a tapped-delay line model is given... A majority of
the time, rms delay spreads are relatively small, but occasionally,
there are 'worst case' multipath characteristics that lead to much
larger rms delay spreads. Measurements in outdoor environments show
that rms delay spread can vary over an order of magnitude, within the
same environment. Although large delay spreads occur relatively
infrequently, they can have a major impact on system performance.
To accurately evaluate the relative performance of candidate RTTs,
it is desirable to model the variability of delay spread as well as
the 'worst case' locations where delay spread is relatively large.
As this delay spread variability cannot be captured using a single tapped
delay line, up to two multipath channels are defined for each test
environment. Within one test environment channel A is the low delay
spread case that occurs frequently, channel B is the median delay spread
case that also occurs frequently. Each of these two channels is expected
to be encountered for some percentage of time in a given test environment."
There follows a table giving these relative percentages for each of the
three test environments (Indoor, Outdoor to Indoor and Pedestrian, and
Vehicular). For the Vehicular test environment, these relative percentages
are respectively 40% and 55% for the A and B sub-cases.
Thus, it seems like we are in need of a satisfactory explanation for
the discrepancy between the views that channels like Vehicular B are
"almost un-existent" vs. the view that they occur around "55% of the time".
Perhaps someone in the group has (or knows where to find) more
information regarding the measurement campaigns leading to these
widely differing views? If so, it may help us to understand the
discrepancy, so that we can feel comfortable moving towards either
(a) or (b) above.
Regards,
Glenn Golden
Flarion Technologies, Inc.
[1] ITU document TR 101 112 V3.1.0 (1997-11), UMTS 30.03 version 3.1.0.