On 2006-11-12, SMS <scharf.steven@geemail.com> wrote:
> Perhaps, but this is demonstrably untrue in suburban areas, where the
> tower placement is to cover a geographic area, and the capacity is not
> the issue.
>
> T-Mobile is great how their web site lets you go down to a specific
> address, and you can clearly see the gaps caused by insufficient towers
> in many areas.

I wish Verizon had a map like that. I'm not sure it would have fewer
gaps in suburban areas (like where I live), though there is no way
to tell currently.

> I don't think that anyone argues that 1900 MHz has as much range or as
> much penetration as 800 MHz. Not even Navas would claim something like
> that. The rule of thumb has always been 2x the distance, mathematically
> it's more than 2x, but their are other factors (geologic features,
> buildings, etc.) that make the increase in range less than ideal.

I think this is a bit bogus. I believe the "math" you are considering
is the path loss alone which, as another post here pointed out, is about
7 db greater at 1900 MHz than at 850 MHz. A 6 db difference in the
overall system budget would indeed be expected to double your
line-of-sight distance, so the decrease in frequency might double your
distance if nothing else in the system changed with frequency. It is
the latter assumption which is highly dubious; other things change
with frequency too.

As an example of what, suppose we were talking not about cell phones
but rather a point-to-point microwave circuit that we were picking a
frequency for. Suppose the transmitter power and parabolic dish sizes
were fixed. In this case, I think a 1900 MHz link would give you twice
the distance of an 850 MHz link. While 1900 MHz would increase the
path *loss* by 7 db, it would also increase the antenna *gain* (for
same size antennas) by 7 db each, and since there are two antennas
involved the net improvement at 1900 MHz would be 7 db. Path loss
is not the only thing which changes with frequency, and some of the
other changes can be to the benefit of the higher frequency. In
particular, while path loss increases with increasing frequency,
antenna performance, given realistic constraints, tends to improve.

The same situation exists with cell phones, it is just that since
I don't know the particular numbers (I don't think you do either)
I can only guess at how this might come out. Since the output
power on either frequency is constrained to be the same (by the
handset) we're down to a comparison of likely antenna performance.
One thing we know about this for sure is that if the antennas at
the two frequencies are the same physical size, the 1900 MHz
antenna is very likely to have higher gain. This is exactly
the situation of a dual band handset where the same antenna is used
for both frequencies. Worse, a quarter wavelength at 850 MHz is
a highly unstylish 3.5 inches, but at 1900 MHz is is a svelt 1.5
inches, so 850 MHz performance on a modern dual band handset is
increasingly likely to be compromised with an electrically short
antenna. I hence wouldn't be surprised if the handset antenna
gain gave 1900 MHz a 2-5 db advantage (look at 802.11a/g dual band
access points, which sometimes also share antennas at two frequencies;
if they list antenna gains for each frequency then you'll generally find
802.11a has an advantage of several db, even without the antenna
size constraints that are important for a handset).

At the base station the same issue applies, though slightly differently.
Since it is easier to build higher gain antennas at 1900 MHz than at
850 MHz, you can always compensate for any remaining path loss advantage
at 850 MHz by deploying higher gain 1900 MHz antennas. The problem
with this at a base station, however, is that with the highly efficient
antennas in use there (unlike the handset), higher gain implies greater
directionality, so the only way to use higher gain antennas is to
increase the sectorization, which seems certain to increase cost if
you don't need the added capacity as well. So, while I would agree
that building a 1900 MHz network in less densely populated areas
is likely to be more expensive than 850 MHz given equivalent coverage,
I don't agree at all that this extra expense necessarily needs to be
incurred by building more towers; it can also be incurred by spending
more money at the towers you've got.

And while the above necessarily includes some speculation about how
it works, what I can tell you for certain that it doesn't seem
to work your way, with Verizon having this incredible coverage advantage,
where I live because I did that experiment. I live in a coverage seam
which shows up at 1 bar on the T-Mobile map. I also know for
certain that my service comes from a cell site on top of a building
downtown where all 5 carriers have equipment (I know this because there
have been articles in the local paper about attempts to construct more
towers here which mentioned where the existing cell sites were). I
was a Sprint PCS customer several years ago when I moved into that house.
My phone worked okay on the second floor of my house, was marginal on the
ground floor, and wouldn't work at all in the basement except near the
windows. I eventually got fed up and, even though I was still under
contract with Sprint, I went to Verizon and bought a phone. I bought the
same model phone from Verizon (a Samsung, I think) that I had with Sprint
and took it home. It helped nothing. The maintenance display which
showed the received signal strength in db(m?) showed virtually no
difference; I think Sprint was in fact usually a db or so better, for
whatever that's worth. The phones were equally good upstairs, equally
marginal on the main floor and got service (or not) in pretty much the
same locations in the basement. There being no detectable improvement
with Verizon, I took that phone back (and no one in the shop seemed
surprised the phone didn't work well in my neighbourhood). I've since
had T-Mobile, and now Cingular service, and I've replaced Sprint with
Verizon for other reasons, but there's really little to choose between
them for coverage at my house. There is certainly no 850 MHz advantage
in the suburbs where I live.

In any case, if you want to persist in arguing that 850 MHz provides
a 2x distance advantage over 1900 MHz, you can't just quote the path loss
and stop. You've also got to explain why the antenna gains at
both frequencies would need to be identical, and my understanding of
the problem leads me to believe you'll have some trouble doing this.

>> Obviously in rural areas the 800MHz carriers have an advantage where
>> capacity isn't an issue, and distance is the limiting factor.

I believe there is a a cost advantage at 800 MHz, I just don't believe
the extra money for 1900 MHz necessarily needs to be spent on more towers.
It can also be spent on improving the coverage of the towers you've
got.

>> I remember in the late 80's a rural Nebraska cellular carrier (aptly
>> named "Nebraska Cellular") managed to provide excellent cellular service
>> along I-80 through almost the entire state with a minimal number of
>> towers thanks to 800MHz propagation and some VERY flat terrain!
>
> Yes, this is the big advantage of AMPS, at 800 MHz. The hope is that if
> AMPS ever gets turned off in those rural areas, that something will take
> its place, maybe something like Australia did with CDMA.

If there was an advantage for AMPS it was entirely due to the use of
3 Watt car phones (i.e. an 8-12 db advantage in power output over
a modern handset) with an antenna outside the car, as was common in
the late 80's. If you are power limited, as modern handsets are,
either CDMA or GSM will do better than AMPS under pretty much any
common set of circumstances.

Dennis Ferguson