P25 signal strength

ButchGone · #1 (**permalink**) 08-07-2009, 08:29 PM

A new P25 700/800Mhz system is being installed in Chattanooga, TN, that will overlap the current mixed-mode SmartZone system. It's on the air now and a new master controller, or "brain," is actually contolling the mixed mode and P25 systems. There are four simulcast sites with additional stand alone sites to provide coverage in a large region.
Anyway. I have noticed that the P25 control channel signal strength varies, as if the transmit power level is wobbling up and down. It peaks full bars, then goes down to two or three. This is the four-site silmucast part. Is this the nature of P25, to see such fluctuations in signal stregth? The analog system shows a rock solid signal strength.
A new 700MHz stand alone site was just put up and it too has a fluctuating signal strength.
Could this be the system is not at full power yet and is undergoing fine tuning to synch times/signals? It's been less than a month on the air. My Uniden 396 does decode the audio very well at 0% error rate when the techs test this.
Should a P25 trunked system show a solid signal strength?
Curious,
BG..

N_Jay · #2 (**permalink**) 08-07-2009, 10:44 PM

Is tewh analog simulcast?

Seems like you are seeing the simulcast carries beating against each other.

KA1RBI · #3 (**permalink**) 08-08-2009, 06:08 PM

On a local P25 simulcast system here, I've seen the exact same phenomenon - the system is believed to be running LSM (Linear Simulcast Modulation). The CC (and in fact the discrete channels as well) seems to vary by as much as 20-40 dB (perhaps this is exaggerated, but not by a lot) several times per second, with no apparent pattern. This was verified by connecting an oscilloscope to the receiver AGC line.

So, your observation does fit the pattern ...

Do you have any data as to the type of system (manufacturer, modulation - C4FM vs. CQPSK, etc.) ?

Best Regards

Max

N_Jay · #4 (**permalink**) 08-08-2009, 06:22 PM

It is because it is SIMULCAST.

No mystery involved.

ButchGone · #5 (**permalink**) 08-09-2009, 03:02 PM

The new P25 system is Motorola and the four simulcast sites are 800MHz, while the stand alone sites covering the counties outside Chattanooga are 700MHz. Even the stand alone 700MHz P25 sites have a fluttering signal strength. The analog system is SmartZone Astro that has some digi talk groups as well. But that four-site simulcast system doesn't have the fluttering on the control channel signal strength, it's solid at either full bars or a few bars on the s-meter depending on where you are.
The P25 and analog system use the same towers, antennas, microwave links, etc and a new master controller handles both systems. Zone 1 is the SmartZone, while Zone 2 is the P25 stuff.
That's why I was wondering if the fluttering signal strength of the control channel is just the way P25 is. Otherwise the voice quality on the new P25 stuff is very good, better than on the SmartZone digi stuff according to what my 396 hears.
BG..

wlmr · #6 (**permalink**) 08-09-2009, 07:14 PM

It's normal for your scanner to see fluctuating levels when receiving P25 digital, more so with Linear Simulcast but definitely there even with C4FM.

Whenever technicians have to measure/adjust output power on any P25 digital transmitter they put the transmitter into a specific test pattern (V.52 I believe). This pattern has two things going for it, it's power output level is steadier for measurement purposes than the usual data coming out of the radios and it can be decoded and tested for received bit errors. (Noisy received signal has more bit errors - at about 5% errors a receiving radio would be badly broken/digital - much higher and no more decoded audio.)

System control channels, voice channels, and inbound mobiles/portables all act the same.