Subject: [NEXTEL-1] Foxcom Wireless

From: Marcel <[email-masked]>
Date: Mon, 18 Nov 2002 18:17:25 -0500

-------- Original Message --------
From: Jack Daniel <[email-masked]>
Subject: Re:Foxcom Wireless

Since this is getting some legs, those who want to use fiber optic for
RF distribution need to know some basics.

Almost every fiber-RF system was designed for cellular & PCS services,
not LMR. RS232 data ports are a common but seldom used option.

These systems are NOT the same as those used in data fiber networks,
they are almost always analog and directly modulate and demodulate at
the RF operating frequencies, so what comes out is what you put in... no
mixers or local oscillators. There has been attempts to use digitalized
RF but it has a tendency to increase noise and intermod if not
implemented very carefully. Analog modulation on the fiber is simply
better when you are dealing with low distortion 800 MHz signals and very
wide dynamic signal level swings.

Consider the fiber as an alternative to coax cable because it does not
radiate itself and must use FO (fiber optic) transmitters and FO
receivers at each end of every fiber to provide the conversions from RF
to optic and back.

But unlike coax, fiber doesn't pick up (or emit) RF along the way, the
fiber is very broadband (about 2000 MHz wide) and it cannot handle very
weak or very strong RF levels. It has very low loss. Typically, the RF
levels are between -70 and +10 and the output level is about the same as
the input level.

There's two major categories of products and applications:

a. Point-to-point: Fiber transceivers exchange RF over single mode fiber
optic cables up to about 12 miles with no net loss. This is available
form at least 4 sources already and the same as what Andrew apparently
announced. 

Some manufacturers like Foxcom INCLUDE the broad band low noise amp as
part of their standard unit. Bandwidths are 80 - 1000 MHz, 80 to 2200
MHz and 1700 to 2200 MHz. Naturally the first range is most used by LMR.

Filters are usually added to reduce the noise bandwidth and to improve
duplex frequency isolation. Preamps and power amps may be added.

Two fibers ( One for downlink, the other for uplink) are more popular
than single 'duplex' fibers because duplexing adds complexity, cost and
increases the test equipment needed while the second fiber is very
little more overall cost.

The second kind of product and application is in-building distribution
applications. The basic technical principles are the same, but the
product is optimized to have one 'head-end' or 'base' unit interconnect
to 4, 8 or more "remote" units or "remote hubs" via fiber optic cable.

The remote units usually include low power duplexers that accommodate 1
to 4 local antennas connected by common coax cable. Imagine a "hub" on
each floor that drives 4 antennas scattered around that floor. 

Since Cellular, PCS and the 800 MHz bands have predictable duplexer
requirements they are the only standard bands offered. UHF and VHF could
be accommodated by adding external cavity or combline filters.

The Foxcom Modulite system is a multiple user, multiple band type
product that has grown out of their many years of in-building common
carrier coverage experience.

Again, RF power is limited. The maximum inputs and outputs being about
+10 dBm for ALL the signals together (the composite signal).

The Andrew press release and web site does not indicate they have any of
the second type of product, so they probably use a lot of paralleled
point-to-point devices with filters, splitters and combiners custom
fitted to the specific application. This is a common practice when using
fiber instead of coax for complex tunnel and subway projects.

Last IMPORTANT point: Fiber optic links have high noise figures and I
personally would NEVER recommend the output of a fiber receiver be
passed direct to the outside antenna. (i.e. without very good filtering
or other clean-ups). This noise can be much hotter than even Nextel!

Visit http://www.rfsolutions.com/fo-rf1.htm to see my application note
for LMR usage.

Like everything else, fiber is another tool to put into your kit but it
is NOT the solution for all applications.

But I may be wrong............
Jack Daniel

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