P25 IMBE/AMBE+2 Interoperability

[devicelab]

Question for the P25 experts:

Recently, I've been having regular conversations on a P25 repeater. I'm mobile using a Kenwood TK-5720 VHF (factory new and latest firmware) and the other party is using a Motorola XTL 2500 VHF (base station) that has been professionally tuned. I'm using a properly tuned Larsen VHF 5/8 whip. Base station also has external VHF antenna.

Part of the problem is topography of the area. The repeater is on top of a mountain and my mobile station (we think) is likely under its shadow -- that is to say, I'm very close (< 2 miles) and the other party is farther away -- probably ~20 miles.

About 75% of transmissions are solid and there's no issue. The problem is that on receive, I hear a quick audio blurp but then no decode. On the 5720 I can see strong RF on the RSSI meter -- so it's not a repeater signal issue.

As a random test, I used an XTS 2500 (900 meg) and tuned NMO antenna and we were seeing very good decodes. We did hit issues where the signal dropped out but that was likely due to obstructions, etc. The other party was using a XTL 2500 900 meg radio. Thus, we had to (2) IBME radios. The funny thing is that my receive decodes seemed better between the Motorola radios. The audio quality was not nearly as good -- the IMBE seemed to struggle a bit -- but the audio still came through and was able to decode. The 900 repeater is on the same mountain and I was mobile in the same area. So even though it wasn't perfect, the XTS/XTL combo seemed more reliable.

This brought up an obvious question: Are there any interop issues between IBME and AMBE+2 radios? Does the AMBE+2 vocoder require more P25 frames for decoding purposes? In other words, could the Kenwood be actually making it worse..? I do realize that the repeater plays a role here too but I don't many details there. I'm presuming it's a Motorola repeater but I'm not 100% certain.

Anyway, I'm just curious if AMBE+2 radios require 'more' in order to decode properly than IMBE radios. I'm thinking the BER threshold is higher on the AMBE+2 and that might be actually hurting the decode performance a bit.

A side note: we've noticed that on weekends, our QSOs seem noticeably better and more consistent. Could the weekday RF around the region be causing issues..? Is it possible for an area to become RF saturated?

 

[slicerwizard]

Where does AMBE come into this? You didn't indicate that trunking was in use.

 

[devicelab]

Where does AMBE come into this? You didn't indicate that trunking was in use.

Correct I didn't. What does AMBE have to do with trunking?

The TK-5720 has AMBE+2 vocoder in its DSP and AFAIK (per the SM) the DSP does the encode and decode for all P25 operations.

 

[slicerwizard]

Non-trunked P25 would be IMBE, so its vocoder is also IMBE-capable. Questions like "Does the AMBE+2 vocoder require more P25 frames for decoding purposes?" make no sense when dealing with conventional comms.

 

[Project25_MASTR]

Second generation P25 radios (i.e. any P25 radio that uses AMBE+2) use an AMBE+2 vocoder instead of the IMBE vocoder regardless of it being Phase 1 (FDMA) or Phase 2 (TDMA). The AMBE+2 vocoder is backwards compatible with IMBE though...which simply means the AMBE+2. The data stream the vocoder sends and receives is identical to that of an IMBE vocoder (until TDMA becomes involved). The primary differences are with DSP advancements and higher resolution that the AMBE vocoder is capable of.

That being said, I would look at some other specifications of the radios. What you are likely running into is issues with the front end filtering on the radio not filtering some things adequately causing NULL decodes. I used to see something very similar with TK-5210's, TK-5220's, TK-5710's and TK-5720's. Hook them up to a service monitor and they would easily hang with a comparable Motorola XTS or XTL. Get them outside on a roving vehicle and they would begin having issues in noisy RF environments.

 

[speedway_navigator]

The data stream the vocoder sends and receives is identical to that of an IMBE vocoder (until TDMA becomes involved).

Can you expand on this? I don't have your expertise on P25 and don't understand why the data stream changes once it leaves the vocoder.
Thanks for you endorsement for teaching classes.

 

[Project25_MASTR]

Can you expand on this? I don't have your expertise on P25 and don't understand why the data stream changes once it leaves the vocoder.
Thanks for you endorsement for teaching classes.

Think of a P25 radio in parts. On transmit the vocoder samples analog audio from the microphone and converts it into a 4.4 kbps bitstream (essentially the sample rate). This is then "woven" into the serial data stream that makes up the CAI (Header Data Unit, Logical Link Data Unit 1, Logical Link Data Unit 2,...Terminator Data Unit) at 9600 bps. The radio then modulates these I/O's into the two deviation levels (600 Hz and 1800 Hz) much like a modem in a packet setup except these typically use an DAC to directly interface with the IF. On receive it's the other way, ADC on the IF to demodulate the serial data to feed the vocoder the CAI serial stream (and the rest of the process works in reverse). This process is the same regardless of whether or not the vocoder is IMBE AMBE2+ in Phase 1 operation (conventional and trunked).

 

[devicelab]

Second generation P25 radios (i.e. any P25 radio that uses AMBE+2) use an AMBE+2 vocoder instead of the IMBE vocoder regardless of it being Phase 1 (FDMA) or Phase 2 (TDMA). The AMBE+2 vocoder is backwards compatible with IMBE though...which simply means the AMBE+2. The data stream the vocoder sends and receives is identical to that of an IMBE vocoder (until TDMA becomes involved). The primary differences are with DSP advancements and higher resolution that the AMBE vocoder is capable of.

Okay, I knew I wasn't that crazy. I was tired when I wrote my first post so I was trying to make it as clear as possible. I did just read up on how the TDMA enters into the equation but admittedly it was getting pretty complicated.

What you are likely running into is issues with the front end filtering on the radio not filtering some things adequately causing NULL decodes. I used to see something very similar with TK-5210's, TK-5220's, TK-5710's and TK-5720's. Hook them up to a service monitor and they would easily hang with a comparable Motorola XTS or XTL. Get them outside on a roving vehicle and they would begin having issues in noisy RF environments.

Very cool thank you! This is likely what I'm experiencing then. This area is seriously RF saturated and your experience with the Kenwood seems to jive with what I'm seeing as well.

My buddy and I had a nice QSO yesterday. I was using XTS5k VHF with Larsen 5/8 NMO and him his usual XTL base. With the exception of a couple of dropouts, we had a pretty flawless conversation. Did it [audio quality] sound as good as the Kenwood -- not quite -- but it was pretty close.

I'm not knocking the Kenwood in any way -- they are awesome mobiles -- but yeah kind of shame they can't really address the front end problem.

PS. From your experience is there any advantage when it's the same radio brand? I remember the Mot/Harris debates with trunking but I wasn't sure if that was due to the trunking logic or the baseband logic from the radios. I'm 99% sure there's nothing wrong with Mot/Ken as my buddy and I have had really high-quality QSOs -- just not consistently.

 

[Project25_MASTR]

PS. From your experience is there any advantage when it's the same radio brand? I remember the Mot/Harris debates with trunking but I wasn't sure if that was due to the trunking logic or the baseband logic from the radios. I'm 99% sure there's nothing wrong with Mot/Ken as my buddy and I have had really high-quality QSOs -- just not consistently.

As long as you are not using proprietary features/options, no real advantage to using the same brand in terms of one thing or another.

In my experience, the debate with Motorola/Harris radios had more to do with the systems and how they were built leading to confusion between the two. For example in a multi-site Motorola system, Motorola uses the RFSS to signify the zone (or core controller). Essentially, sites talk to the zone core and zone cores talk to zone cores. So you can have a few hundred sites (for a wide area system) in a single zone. In Harris systems, when the site ID changes, so does the RFSS (my understanding is this id due to a limitation in the Vida core but I'm not a Harris tech) which also artificially limits Harris to 256 sites in a system (compared to Motorola which currently supports something like up to 7 zones of 150 sites each IIRC). On Motorola radios, when you hop an RFSS you need to have a feature on the radio called SmartZone with OmniLink which allows the radio to transition seamlessly between RFSS. So that can cause some issues with Motorola radios not working correctly on a Harris system.

Now...I've got quite a bit of experience VHF P25 trunking in a Motorola system. Even with Kenwood/EF Johnson subscribers on the system. It wasn't pretty for various reasons. At this moment in time, I'm going to chalk it up to I was never able to verify the Kenwoods were actually programmed correctly due to me not working for a Kenwood dealer and Kenwood not wanting to send an engineer to the area to see exactly why they couldn't get the radios to work correctly. They just stated "it's that Motorola system using proprietary features" but the Harris radios on the system worked just fine...