Dear Mr.Klein:

It is not the control channel that is the problem. It may be sent from
only one transmitter (and thus suffer only the old-fashion multipath type of
distortion). In other words, it appears that the control channel is decoded
successfully. This could be due to the existence of a robust type of coding
that combats old-fashion multipath type of distortion.

However, when mobile stations are sent by the control channel off to one
of the allocated frequencies, problems seem to occur with the digitally
encoded voice signals. Since systems that use a total of one transmitting
site do not appear to suffer and systems that use multiple transmitting
sites do suffer, one hypothecates reasons.

A form of multipath distortion exists that is due to the use of
multiple, well spaced transmitters that are transmitting at the same time.
That is the type of distortion I called "multiple-transmitter-multipath."

One might guess that if one is close to one of the transmitters (of a
multiple transmitter system) then one would not have a problems because the
near transmitter's emissions would dominate. I have not tested this
hypothesis.

However, when one is receiving multiple transmissions of approximately
equal amplitude but transmissions that have encountered different delays,
then one expects to see what looks like multipath. An enhanced receiver can
remove the problem at a cost that includes slight delay. A "low cost"
receiver without enhanced processing may have a hard time decoding signals
that have been contaminated by multipath of any kind.

When listening to a one transmitter site (Owosso), my BCD396T receives
almost all voice signals well most of the time. When listening to a system
(APCO25) that uses multiple transmitting sites (Genesee), my BCD396T
produces distorted audio much of the time. At my location, the signal from
the control channel of the multiple transmitter system is somewhat stronger
than the signal from the one transmitter system.

Even back in the RTTY days with each "bit" being 20 to 22 mS long,
multipath was capable of distorting signals. The much higher data rates of
the digital system in question makes the
decoding-in-the-presence-of-one-type-or-another-multipath difficult.

In short: multipath can be caused by means other than signal
reflections and the present "low cost" receivers seem not to contain
multipath resistance processing.

Regards, Mac



--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Al Klein" wrote in message
...
On Mon, 9 Jan 2006 08:47:35 -0500, "J. Mc Laughlin"
wrote:

The Owosso system uses only one
transmitter thus avoiding the multiple-transmitter-multipath of the
Genesee
system.

If you're talking about the MPSCS, it's a Motorola system, so there's
only 1 control channel on the air at a time from any site. Multipath
isn't caused by multiple transmitters, it's caused by signal
reflections.