Date: 9 Sep 1996 16:52:22 -0700
From: kleing@qualcomm.com (Klein Gilhousen)
Newsgroups: comp.dcom.telecom
Subject: Re: "Roaming" in Home Territory

In article <telecom16.473.6@massis.lcs.mit.edu>, Rupa Schomaker
<rupa@rupa.com> wrote:

> I have had an interesting experience a couple of times while driving
> up the Pacific Coast Highway.  There are some sections of the road
> where my cellular provider (LA Cellular) has horrible coverage --
> which perhaps is understandable.  The place where this happens most
> often is just north of Laguna Beach by the state park (moro canyon?)
> south of Crystal Cove.

> The odd this is that during those times my cellphone will go into ROAM
> mode.  If I attempt to dial I get an intercept welcoming me to (and
> I'm paraphrasing from memory) the cell company of Baja California.  It
> then gives me instructions on how to call the United States.  If I
> then turn my phone off and then back on the phone goes out of ROAM mode. 

This is caused by the fact that UHF signals propagate over water much
better than they do over land.  Normally, cellular systems can be
assumed to experience propagation loss proportional to the 4th power
of the distance between the mobile telephone and the base station.
This is substantially greater propagation loss then occurs in free
space which is proportional to the second power of distance.  In fact,
4th power propagation is necessary for high capacity cellular systems
to work at all and is one of the reasons why cellular phones are
prohibited from being used in airplanes where the 2nd power
propagation law would apply.

Over water propagation is nearly as good as free space and sometimes
can be even better than in free space when temperature inversions over
the water cause "ducting" effects.  Hams have exploited these freak
conditions to occasionally succeed in communicating from Hawaii to
California on VHF frequencies.  There are probably areas in Laguna
Beach where a good over water path exists to Tijuana and the path to
the nearby cell is effectively blocked by terrain (i.e., much worse
propagation than 4th power of distance.)


Klein Gilhousen
QUALCOMM Incorporated