Control Channel Modulation

[chrismol1]

After reading the postings in a state forum on control channel modulation regarding trunking sites astro25 9600 control channel modulation C4FM vs CQPSK. I understand that simulcast sites are CQPSK(LSM) and ASR Astro Standalone Repeater fill in sites typically do not use simulcast? and are typically of C4FM modulation. How does the subscriber handle this? I've seen astro25 smartzone systems with sometimes neighbors as these standalone sites. Does it automatically switch modulation? Will a subscriber system programmed in CF4M see a decreased performance on a switch to a CQPSK neighbor site or a CQPSK programmed system see decreased performance switched to a C4FM neighbor site? Does the radio figure it out itself?

I've noticed in Astro25 XTS/XTL CPS it states

Digital Modulator Type (Trunking System, Digital)
C4FM: Compatible 4-Level Frequency Modulation for non-simulcast operation
CQPSK: Compatible Quadrature Phase Shift Keying for narrowband simulcast operation.
Note: This function is available to radio models equipped with CAI (Common Air Interface) Digital Operation.
WIDE: Wideband simulcast operation
Note: Cannot be chosen if a Channel Bandwidth of 12.5 kHz is selected.
The factory default is "CQPSK" forASTRO25 Trunking
The factory default is "C4FM" for Type II Trunking

But then APX CPS it is revised

Digital Modulator Type (Trunking System, Digital)

C4FM ( Factory Default for Type II Trunking )

Continuos 4-Level Frequency Modulation for non-simulcast operation.

CQPSK ( Factory Default for ASTRO 25 Trunking )

Compatible Quadrature Phase Shift Keying for narrowband simulcast operation and non-simulcast operation. This selection is only available to radio models equipped with CAI (Common Air Interface) Digital Operation.

 

[mrsvensven]

A subscriber set to C4FM will work very poorly when decoding CQPSK transmissions.

A subscriber set to CQPSK seems to receive either CQPSK or C4FM just fine.

On Motorola radios, the C4FM/CQPSK setting is per trunk system. So generally it will be set to CQPSK even when operating on ASR sites.

 

[GTR8000]

and ASR Astro Standalone Repeater fill in sites typically do not use simulcast?

ASR sites are not simulcast by their very nature of being standalone, so I'm a bit confused by this question.

The default Digital Modulator Type for P25 systems in both ASTRO 25 and APX CPS is CQPSK. That setting works just fine on ASR (C4FM) or IP Simulcast (LSM/QPSK) sites.

There are many examples of ASTRO 25 systems that feature a mixture of IP Simulcast cells and ASR sites, and so the subscribers are expected to perform just as well when roaming between them.

Also note that ASR are not exactly "fill in" sites as the old SmartZone IntelliRepeaters were. Many ASR sites on modern P25 systems are full fledged sites in areas where there is no need for a full simulcast cell. This is particularly true with wide-area systems, where it may not be possible to license the same frequencies over a wide enough area to create a simulcast cell.

 

[mrsvensven]

On conventional channels, you can change the setting per channel. I have spent quite a lot of effort trying to figure out if my neighboring systems are C4FM or CQPSK so I can set my codeplug to the appropriate value.

All this raises the question- Why is it even selectable? Why not just have the radio always in CQPSK mode if it works fine with either modulation type? I went as far as to ask this question to one of the engineers on the P25 standards body and the only response I got was "I guess nobody has thought about it before"

 

[KevinC]

On conventional channels, you can change the setting per channel. I have spent quite a lot of effort trying to figure out if my neighboring systems are C4FM or CQPSK so I can set my codeplug to the appropriate value.

All this raises the question- Why is it even selectable? Why not just have the radio always in CQPSK mode if it works fine with either modulation type? I went as far as to ask this question to one of the engineers on the P25 standards body and the only response I got was "I guess nobody has thought about it before"

You lose about 1.2 dB of outbound coverage when using LSM/CQPSK. But the recommendation is to set it to LSM/CQPSK unless you are severely outbound limited.

 

[BMDaug]

With a Harris radio, you can’t set the SU modulation… You can check the Linear Simulcast box (which does not determine a modulation as simulcasts can be either), and if you check that box, you can then optionally check the Adaptive Filter box, which according to the manual, should only be used if the simulcast CCs are CQPSK. Maybe a Harris SU simply always matches the CC modulation? I have no way to test this hypothesis…

-B

 

[chrismol1]

Okay, I have many not quite understandable questions. I use Motorola in NAS. I've use TX Inhibit and moved over to the trunked talkgroup to check RSSI, where I was met with no signal and Out of Range message in CQPSK which fixed it self when I re-programmed that systems site/(with neighbors on simulcast cells in db)...to C4FM where I got a signal meter on that single cell site and reasonable RSSI. I'm wondering if an actual subscriber will ignore the NAS results I received when roaming between that single site and actual simulcast site

 

[iowajm780]

Okay, I have many not quite understandable questions. I use Motorola in NAS. I've use TX Inhibit and moved over to the trunked talkgroup to check RSSI, where I was met with no signal and Out of Range message in CQPSK which fixed it self when I re-programmed that systems site/(with neighbors on simulcast cells in db)...to C4FM where I got a signal meter on that single cell site and reasonable RSSI. I'm wondering if an actual subscriber will ignore the NAS results I received when roaming between that single site and actual simulcast site

You did what?.