Subject: [NEXTEL-1] RE: 800 MHz interference - Testing the performance ofdigital technologies

From: [email-masked]
Date: Mon, 19 Apr 2004 06:00:08 -0400

-------- Original Message --------
   From: "Buchanan, David" <[email-masked]>
Subject:  RE: 800 MHz interference - Testing the performance ofdigital
         technologies

So what does all that have to do with receiver inter-modulation from
cellular transmitters to an analog receiver? The interference is in the
front end of the receiver.Or the interference is from OOBE which is an
on channel interferer.Either way you can measure the interference all
day long however you want to but that doesn’t do anything to fix the
problem.The officer still can’t hear dispatch and worse if the
interference is on the control channel of a trunked system the radio
won’t TX a message back to dispatch.

So how long to we piece meal trying to fix every new location the pops
up with interference.What is wrong with wanting to separate public
safety channels from cellular channels?

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]
Sent: Wednesday, April 14, 2004 12:49 AM
To: [email-masked]
Subject: Re: [WFHSG] RE: 800 MHz interference - Testing the performance
of digital technologies

Example #1

Testing the performance of digital technologies

By Doug Mach, M.S.E.E.

<SNIP>

The new digital systems are changing
the way radio performance is specified
and measured. Ensuring the best
system operation and user satisfaction
requires that new tests and
measurements be made to properly
maintain the equipment. Here are some
of the similarities and differences in
performance measurement between
existing FM radios and some of the new
digital systems.

Digital radio standards differences
Anyone who has serviced two-way
radios is familiar with the tests and
measurements used to confirm proper
operation of analog FM radios. The most
basic measurements commonly made
in the field are likely to be power output
and receiver sensitivity. Simple
talk-and-listen tests verify that the radio
is ready to be put back into service.

The new digital standards have
changed things quite a bit. Different
digital standards result in different test
methods and specifications, which
further complicates the issue. The
method of specifying and measuring the
two basic parameters of power output
and receiver sensitivity varies from
system to system.

A digital radio or system is one in which
the desired intelligence (either voice or
data) is transported by digital, rather
than analog, means. Systems using
digital signaling_such as trunking
control channels or those transmitting
low-speed digital data_have been
around for years. Voice systems using a
12kbps data rate have been in operation
on the land mobile bands since at least
1976. In this case, some audio quality is
sacrificed for security and simplicity.

Two trends have resulted in the
development of new radio technology:

* The increasing ability to pack higher
and higher data rates into existing radio
channels using improved modulation
techniques. * New voice compression
technology allowing increasingly lower
data rates without sacrificing good audio
quality.

Low-power-consumption
microprocessors make it possible to
use both of these computation-intensive
trends.

Understanding the basic layers of a
digital system helps in understanding
the issues raised in measuring
performance. The most basic layer is
the over-the-air connection. This layer
establishes a stream of data between
two radios: The type of RF modulation is
specified, and the means for
establishing synchronization and data
framing is determined. (See Figure 1
below.) At this point, the traditional radio
begins to evolve into a digital device.
Once the radio-to-radio data connection
is established, data are then passed on
to be processed for signaling
information, user verification, error
correction and other radio control
functions. Channels of data, including
voice data, are prepared to be passed
on to their appropriate applications. In
the case of voice operation, data pass
on to the vocoder.

For voice transmissions, the vocoder
converts analog voice information into a
stream of digital data that can then be
transmitted on the established data
channels. On the receiving end, the
reverse process is carried out. Many
types of vocoders are available today,
each with good and bad attributes. For
two-way land mobile communications,
some of the desirable characteristics
are:

* Good audio quality under noisy
conditions. * Good data compression. *
Immunity to degradation of voice quality
caused by received bit errors.

On a voice channel, the resulting
intelligibility is of key importance.

As in analog systems, power output and
receiver sensitivity are important
measurements of digital radio
performance. How power output is
measured depends on the type of digital
system being measured. For an FM
system such as an APCO Project
25-compliant system using C4FM
modulation, the measurement is
identical to that for an analog FM
system. In the case of TDMA systems
such as iDEN*, where the transmitter is
pulsed on and off, a more complicated
method that takes into account the duty
cycle must be used.

Measuring receiver sensitivity is more
difficult. A common method of
measuring sensitivity in a digital data
system is the bit-error rate (BER). This
measurement compares a known
transmitted pattern with the received
data stream and expresses the result
as a percentage of the bits in error.

The problem with using BER to indicate
receiver sensitivity is that its effect on
radio voice performance depends, to a
great extent, on where in the radio layers
the measurement was taken. For
instance, a single bit error that results in
the loss of synchronization could result
in a series of incorrect bits being sent to
the vocoder. How well the vocoder
reacts to this series of incorrect bits
determines the effect of this single bit
error on intelligibility. On the other hand,
various levels of error correction may be
used before data are sent to the
vocoder. This would correct for a single
over-the-air bit error and, therefore,
would have absolutely no effect on voice
quality. Measuring only the over-the-air
BER before framing, error correction and
vocoding may not always be the best
indication of actual voice performance.

"Buchanan, David" wrote:

As to this question of iDEN expertise being a prerequisite to
determining a solution, that a smokescreen and you know it. The problem
is basic physics.