[aprssig] [APRS] LightSquared GPS Interference Issue Flares Again in Senators' Letter

Fri Apr 15 17:27:06 EDT 2011

On 4/15/2011 11:22 AM, Bob Bruninga wrote:
> The key to this is simple SPECTRUM PLANNING!
>
>
> DUH!  It is not the receiver's fault.  That is why we have SPECTRUM
> PLANNING, so that one does not place high power transmitters in bands
> ADJACENT to weak signal receivers.
>
> Aren't there any RF engineers left at the FCC?
>
> Bob, WB4APR
>

-------   NO!!! -------------     THEY ARE TERMINALLY CLUELESS

Two issues of this type nearly identical (in that they involve disrupting 
existing networks with high-power intruders) I have dealt with in the past two 
decades:

1)     "Coordinating"  half-KW ERP or higher one-way paging transmitters on 
frequencies directly adjacent to the 9 medical telemetry channels in the UHF band.

These "med" channels are used for EKG telemetry by paramedics at accident 
scenes to send data directly to hospitals.  The in-the-field end of these 
systems were devices such as the Motorola Apcor radio, which are 5 watt 
"porta-luggies" based on handhelds built into a box with the EKG gear.  This 
box with a so-so antenna typically sits on the ground next to victims.  These 
systems operate duplex like a classic "car phone" (the box actually has a 
traditional telephone handset on it), but no repeaters are used.    In other 
words, rather low TX ERP on a direct path back to the base station at a hospital.

The UHF channels for classic IMTS "car phones", located directly above the 9 
med channels, were repurposed for paging in the late 1980s, after cellular took 
off.   The 50-100 watt ERP base stations on distant mountain tops or buildings 
used for IMTS phones, were replaced with dozens of 500-1KW ERP paging 
transmitters on every tall building in the metro area, in order to reach cheap 
pagers with no real antenna inside buildings -- in other words total RF 
overload.     When these paging systems started turning up, I received numerous 
complaints from the county health system and paramedics in Los Angeles about 
interference and communications failures in the UHF "med band".    The usable 
range had dropped from 5-10 miles from the base to as little as 1-2 miles.

I ran RF coverage surveys and signal level measurements  with a spectrum 
analyzer in my car, and discovered these paging systems were laying down 
minimum RF levels of -70 dBm and typicall far higher virtually everywhere in 
the greater Los Angeles area. Some of these signals were as close as 50 KHz to 
MED 9 where the typical RX level at the hospital from a field unit was running 
at -100 to -110 dBm.  The modest med radio front ends were being totally 
overloaded by the massive RF levels of the paging networks.

I improvised some front end selectivity at some installations with strings of 
repeater-style cavities, but of course this limited the base to a single 
channel, and of course did nothing for the so-so handheld receivers in the field.

The problem has now sort of "gone away" now as paging has declined in the face 
of universal cellular usage, but for nearly two decades this was a major 
problem in L.A.

2)      The similar fiasco in the 800 MHz spectrum where NexTel was allowed to 
saturate urban areas with hundreds of essentially cellular transmitters on 
channels interleaved with conventional and trunked analog public safety systems.

The problem was that public safety systems are relatively thin networks with a 
handful of base stations at remote locations communicating with hand-helds in 
town; i.e. a rather low RF level delivered to users on the ground.  The Nextel 
systems involved hundreds of relatively high-power base stations "close-in" 
every half mile or a mile or so, delivering several orders of magnitude more RF 
on the ground "in-town".    Public safety hand-helds were desensed by the RF 
onslaught on adjacent channels.  Since the Nextel channels were mainly digital, 
users didn't perceive interference in the sense of "strange voices" on their 
channels.  The white-noise-like unwanted signals just held the noise squelch of 
the analog radios closed, causing large areas of their previous coverage area 
to just disappear and "not work".

In a frantic effort at damage control, the FCC essentially bribed Nextel by 
offering them additional spectrum in the 900 MHz band, if they would spend tons 
of money "rebanding"  800 MHz; i.e. segregating existing Nextel systems and 
public safety systems into separate parts of the 800 band. Tens of thousands of 
800 MHz public safety radios had to be replaced in order to make the move.    
This mess has been going on now for over a decade, with hundreds of millions of 
dollars spent on studies with no end in sight, as Nextel and it's public safety 
victims haggle endlessly on how much Nextel needs to pay for "equivalent systems".

Somehow the FCC has managed to overlook the fact that Nextel had massively 
overreached and violated the original charter of their license in the first 
place. Nextel was supposed to be a wide-area shared trunked community repeater 
system for business users, not a wanna-be cellular network.  Wireline 
interconnect was supposed to be a minor part of a system primarily for 
land-mobile communications for fleets of trucks.  Such a network, targeted at 
mobiles rather than handsets, would normally use a handful of relatively 
remotely-located base stations, instead of the massive saturation build-out of 
close-in cellular-style transmitters required to reach wimpy handsets indoors 
in town.

About the only justice to come out of this mess is that Nextel has been nearly 
bankrupted by the rebanding effort, and is about to take it's currrent owner, 
Sprint, down with it.......

-------------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
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Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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