Weird Situation, receiving some transmissions

[Maxx102]

Hello everyone, we experienced something weird yesterday that made me go "huh". Before I begin, let me give you the technical details of the system. We are running two repeaters in IPSC mode and all of our radios are XPR 3500's.


Yesterday four of us were at the same desk having a small team meeting when one of our fellow users, who was outside, transmitted asking for a radio check. We all heard the transmission, but it was only on two of our four radios, not all of them like I would have expected.

One of my co-workers who heard the transmission and gave a "loud and clear" response. Here's where it gets weird. I keyed up my radio and also requested a radio check, but the only radio that heard the transmission was the other radio that didn't receive previously. The two that did receive on the first radio check, now didn't hear anything.

I verified the channel selection on all four radios and it was the same (channel #2, our security channel) and no one had changed channels nor did they power cycle the radios (turn them off then back on).

It was very weird and this isn't the first time this situation has happened. It's like the radios go into a "sleep" mode or something.

Any thoughts on what this could be?

 

[33kracing]

Hello everyone, we experienced something weird yesterday that made me go "huh". Before I begin, let me give you the technical details of the system. We are running two repeaters in IPSC mode and all of our radios are XPR 3500's.


Yesterday four of us were at the same desk having a small team meeting when one of our fellow users, who was outside, transmitted asking for a radio check. We all heard the transmission, but it was only on two of our four radios, not all of them like I would have expected.

One of my co-workers who heard the transmission and gave a "loud and clear" response. Here's where it gets weird. I keyed up my radio and also requested a radio check, but the only radio that heard the transmission was the other radio that didn't receive previously. The two that did receive on the first radio check, now didn't hear anything.

I verified the channel selection on all four radios and it was the same (channel #2, our security channel) and no one had changed channels nor did they power cycle the radios (turn them off then back on).

It was very weird and this isn't the first time this situation has happened. It's like the radios go into a "sleep" mode or something.

Any thoughts on what this could be?

i have had something similar happen to me if the radios are too close to one another the furthest away radios will open speaker like normal but the closest radios will not work... something on the lines of signal overload protection. this was in nmy case, maybe yours?

 

[WA0CBW]

More likely de-sense. There is no receiver overload cutout protection.
BB

 

[twhitson]

It sounds like desensing to me; just being too close to the receiver. Weird things can cause the transmission to go through on some radios (sensitivity of the receiver, obstructions between the user antenna and receiving antenna) but not on others.

 

[Maxx102]

It seems that after we experience this, if we switch to a different channel and then back again, everything works fine. Transmission is clear. Transmission can be heard on all radios not just some.

This system was installed last month and these "symptoms" are rather interesting. Prior to the Mototrbo system being installed we used the PTT phones from a well known carrier and were very unhappy with the performance which is why we switched. I have used radios before and never experienced this before, but then again they were all analog systems not Mototrbo digital (GP300, HT750, HT1250).

 

[jeatock]

Question 1: is this simplex (my radio talks directly to your radio on frequency A) or through a repeater in duplex (my radio talks to the repeater on frequency B, and the repeater relays my voice to your radio on frequency A)?

Question 2: If duplex through a repeater is it your own repeater located on site, or part of a wide area system with the repeater located off-site? Is it a talkgroup on a trunking system? Does anyone share the same system with you?

Possible explanation:

In simplex, your radio hears me directly; I transmit on frequency A and your radio hears me on frequency A. If my signal is weak it may still work but be scratchy if it is analog; digital signals come and go in pieces as the signal level drops towards 'too low'. If I am very close to you my signal will appear to your radio to be very tall and wide, but it should still work. Trbo simplex uses the entire 12.5KHz frequency channel for a single conversation.

Duplex is a different can of worms. You may think you are on the same 'channel' but in reality my portable transmitter is emitting all of its RF energy on frequency B while your receivers are listening to a more distant repeater output on frequency A. Even if the repeater is your own and on-site, there will always be areas where the signal is less than other areas. Antenna selection and site installation is an art, and Murphy's Law holds that the weak coverage spots will always coincide with places you need radio coverage the most. Also depending on the setup Trbo may be splitting the repeater into two virtual channels by flipping the same 12.5 KHz bandwidth between 'Us' and 'Them' at an insanely high speed (TDMA). If things get out of time sync there will be issues.

Non-radio example: If I walk a block away on a dark night and signal you in Morse code with a small flashlight you will probably see the flashing. But if we do the same thing during the day with the sun behind me the sunlight will be too bright for you to see my flashing light.

The same applies to radio. All radios may be able to hear the repeater (the flashlight) just fine on frequency A. But if I am close to you and transmit into the repeater, the off-frequency energy from my radio on frequency B (full sunlight) will desense (overload) your radio to the point it can no longer see my voice being relayed by the repeater (the flashlight) on frequency A.

Analog is a bit more forgiving in that my voice may be garbled but something will be heard. Digital is finicky- depending on the format if 3 to 7% (BER or Bit Error Rate) of the digital ones and zeros can't be decoded into a recognizable sentence your radio will give up and be blissfully silent. BER can also be affected by the same signal being reflected and arriving at two slightly different times. Being out of phase will drive the BER out of range.

The off-frequency RF energy can also come from other sources. Computers, wireless network access points and even clock radios can be an issue just the same as an ambulance or taxi transmitting from just outside the building.

The simple test is to re-create the failure. Then do nothing but walk apart and try it again. Do the same thing with everyone the same distance apart but 50 feet down the hall. If far works and close doesn't, and your partner outside in the parking lot could hear everything, it's probably a desense issue. If 'back there' works but 50 feet different doesn't, it may be a signal strength or interference issue.

[Note: don't take my post in a negative manner. Simply trying to explain a technical issue in common terms.]

 

[Maxx102]

Jeatock - no offense taken.

So to answer your question, we have about 10 channels programmed on our radios, combination of digital and analog which I will explain.

As stated in my original post, the system is a Mototrbo in IP Site Connect mode and the radios are the XPR3500 model (we have 30 of them).

Channels 1 & 2 are both digital, goes through the repeater (duplex), and assigned to Talk Slot #1
Channels 3, 4, & 5 are digital, goes through the repeater (duplex) assigned to Talk Slot #2
Channels 6, 7, 8, 9 are analog simplex and doesn't go through the repeater....only radio to radio.
Channel 10 is digital, goes through the repeater (duplex), and is assigned to Talk Slot #2

Both repeaters are located on site (we are large, multiple story, multi-building business complex). The primary repeater (master unit) is located in the penthouse of our tallest building. When we did our walk test, it was determined there was too much interference from concrete, rebar, steel supports for the primary repeater to reach through to the lower level, so a secondary repeater was added to cover the lower level.

The channel we were on in my original post was Channel #2 (talk slot #1, digital, and through the repeater).

I hope the helps.

 

[jeatock]

Okay, complex system, multi-site repeated Trbo possibly with voting.

What you are describing is neither unusual nor rare. Multi-floor buildings are troublesome and valid solutions expensive.

Simply, desense keeps radio 1 & 2 from hearing each other 50 feet through the system, but #3 a distance away can hear everything just fine. I would do the stand-together-then-move-apart test for desense on repeater channels, suspecting that there are places where the cross channel RF overpowers the repeater output. It takes three people, #3 staying far away but within range, and #'s 1 & 2 within a few feet of each other. Start where you know there is a fail, then wander around looking for more.

Keep radio 3 on the same repeater 1 & 2 are associated with to eliminate inter-linking between repeater issues. Then do it again with #3 on a different site.

Probably when you are close (closer) to the repeater antenna there will be no problems - when you are in a bad spot desense will clobber you. Changing channels may in reality do a lot more and possibly causes the portable to re-associate and re-sync, effectively resetting it.

A just in case reminder - change only one thing at a time so you know what change caused a different effect, and keep good notes. That is common sense engineering which I cannot always convince my #1 son of. He will confess to moving, changing channels, AND changing six other things to save time between tests, and hour/days/weeks later i will find that 'nothing working anywhere' was really channel 1 working there but not working here and channel 2 the exact opposite.

Being digital, weird reflections and out-of-phase reception won't help and a bubble of failure may only be a few feet around and modified by something off the wall, like elevator position in the shaft. I found out a long time ago that RF is one third science, one third black art and the remainder plain dumb luck. Welcome to my world.

A final thought- key the mic, wait ten seconds, then talk. That extra-long time will eliminate and sleeping or voting issues.

For anything beyond that I would suggest going to the engineer that set it up. Either the design has problems, or the installers didn't put it in according to the plan. That size system was not inexpensive and the vendor should be the one to make it right.

 

[JRayfield]

What has the dealer, who put the system in, said about this?

John Rayfield, Jr.

Hello everyone, we experienced something weird yesterday that made me go "huh". Before I begin, let me give you the technical details of the system. We are running two repeaters in IPSC mode and all of our radios are XPR 3500's.


Yesterday four of us were at the same desk having a small team meeting when one of our fellow users, who was outside, transmitted asking for a radio check. We all heard the transmission, but it was only on two of our four radios, not all of them like I would have expected.

One of my co-workers who heard the transmission and gave a "loud and clear" response. Here's where it gets weird. I keyed up my radio and also requested a radio check, but the only radio that heard the transmission was the other radio that didn't receive previously. The two that did receive on the first radio check, now didn't hear anything.

I verified the channel selection on all four radios and it was the same (channel #2, our security channel) and no one had changed channels nor did they power cycle the radios (turn them off then back on).

It was very weird and this isn't the first time this situation has happened. It's like the radios go into a "sleep" mode or something.

Any thoughts on what this could be?

 

[Maxx102]

Fortunately for us this problem happened when the technician from deal was here so they could see what we were experiencing. The technician contacted Motorola and they first instructed them to update the firmware on the repeaters and the 3500's to see if that would solve the problem, it didn't.

They are working on trying to correct the problem, but I am tech junkie so I thought I would come here and see if there was any suggestions on here.

 

[WA0CBW]

If it is IP site connect are the radios programmed for scan or roam?
BB

 

[Maxx102]

Yes, but it is limited.

So if we are on Channel #2, we can also here Channel #1. However, when we are on Channel #1, that is the only channel we hear.

 

[jeatock]

Yes, but it is limited.

So if we are on Channel #2, we can also here Channel #1. However, when we are on Channel #1, that is the only channel we hear.

Are you sure Ch 2 goes through the repeater?

As I recall, Trbo puts two voice paths on a single 12.5 KHz channel by time-sharing (TDMA) two simultaneous conversations through a repeater. Simplex radios do not have the time-share ability (since they operate independently of a synchronizing repeater) and take up the entire 12.5 KHz for both time slots. If a radio is simplex it may hear traffic on either time slot. A radio transmitting in simplex will interfere with both time slots.

RF energy of a given bandwidth on a frequency is still RF energy of a given bandwidth on a frequency no matter how the signal is encoded. The only difference is how the radio is programmed to handle the signal it receives. Trbo radios use the entire 12.5 KHz for both simplex and repeater operation.

I believe Trbo uses 'colors' instead of RAN's or CTCSS/DCS squelch codes. Same thing only different. Codes still don't change the occupancy of a 12.5 KHz channel; they only tell a radio to ignore unwanted/unauthorized audio.


This is definitely a 'call the vendor' situation.

 

[jeatock]

Another question: how many physical frequency pairs are in use, and how close together are they?

 

[TampaTyron]

Sounds like one or more of the following:
-weird roam or scan configuration ( you can roam or scan, not both)
-poor RSSI roam threshold programming
-IPSC repeaters are set to a too high of Type 1 lockout threshold
-poor IP connectivity between repeaters
-stubby antennas on the edge of coverage area
-firmware issue (already updated)

Sounds like fun! TT

 

[Maxx102]

Are you sure Ch 2 goes through the repeater?

As I recall, Trbo puts two voice paths on a single 12.5 KHz channel by time-sharing (TDMA) two simultaneous conversations through a repeater. Simplex radios do not have the time-share ability (since they operate independently of a synchronizing repeater) and take up the entire 12.5 KHz for both time slots. If a radio is simplex it may hear traffic on either time slot. A radio transmitting in simplex will interfere with both time slots.

From what I have read, you are correct about the two voice paths and we've taken account. As I understand the programming, each channel had to be programmed for a specific voice path (Path #1 or Path #2).

Channels #1 & #2 share Path #1, while Channels #3 - #5 share Path #2.


As far as frequency pairs, I had to pull up our license and it says this.

Frequency
000461.55000000 FB2 1 40.000 40.000
000461.55000000 MO 100 25.000 25.000
000464.28750000 FB2 1 40.000 40.000
000464.28750000 MO 100 25.000 25.000
000464.63750000 MO 100 25.000 25.000
000466.55000000 MO 100 25.000 25.000
000469.28750000 MO 100 25.000 25.000

 

[jeatock]

Okay, two FB2 pairs with standard 5-MHz splits, and a simplex. The TDMA splits the FB2's into two slots each, giving you a total of five physical paths, plus possible squelch code separation (colors) for some channel sharing. Even though you said you have 10 channels listed there are only 5 physical paths. (I see 6 repeater 'channels' but only 4 physical repeater paths, and four simplex channels that hopefully share the single simplex path and do not go simplex on a TDMA channel and take up both time slots.) When you are on any given 'channel' the channel's path is busy for other users sharing it in a different 'color' configuration

Repeater channels are possibly simple TDMA divisions, possibly simulcast from both repeaters. I don't believe XPR3500's are trunking radios, so that is probably not an issue. Not sure how CH's 1-2-3-4-5-10 work into the four physical FB2 paths, but it is possible some are roaming or interconnected personalities. If that's the case there could be issues with the interconnection, possibly physical layer, network bandwidth, dropped packets or latency, or IT black arts.

All licenses are close enough in frequency that desense would probably be an issue if two portables are close and the FB2 downlink signal strength is only adequate for low noise floor operations, aggravated by the inherent weirdness of digital (any and all digital types). BER during your acceptance walk through was adequate talking to a distant radio, but possibly the walkthrough didn't include close proximity portable to portable testing where the noise floor was sky-high and the BER off the scale.

Within the TDMA architecture there may possibly be firmware issues. That could possibly be in the portables, or one or both repeaters. There may possibly be harmful interference from unrelated sources.

It is possible that your radios are programmed to roam between FB2's and there may be programing issues in the portables, or in the repeaters. It could be something as simple as an automatic roaming timer being too long or short, or the roaming start and stop RSSI's being wrong, or a failure in possible registration or authentication.

It could be an antenna system issue, possibly on one or both FB2's (they are a royal pain in multi-building, multi-story installs), or possibly in the portable antennas themselves. You never mentioned if you are taking the portable off your belt, or using a speaker mic. Speaker mic's are handy, but leaving the antenna under your arm can possibly reduce performance by 75%.


Possible, possible, possible... With that many possibilities your issues are looking more and more like something that can only be cured by boots on the ground there.

Repeating an earlier comment, this is a dealer/vendor/installer issue. Hopefully your organization bought a solution that works, not a bill of materials installed as directed.


----


EDIT: 461.550 shows over a dozen licenses in the Minneapolis Metro area. 464.2875 shows over two dozen. 464.6375 is a cluster from Best Buy on up. There are clearer CB channels. Is this right?

 

[hitechRadio]

Referring to his original post.

That sounds like some radio's where on one repeater and others on another. Which would lead me to believe it is a network issue between repeaters.

From your license, I doubt it would be a decense problem.

Keep it simple, don't over think things.

It's the job of your vendor to figure this out, it helps them that your keeping detailed notes to identify the issue. I would assume other departments may be having same issue.

Sounds like fun trouble shooting to me, I enjoy that part of the job.

Keep us posted.

 

[thassler]

TampaTyron probably nailed it. Set the RSSI Threshold to -40 on both repeaters. We had the same thing happen when we first set our IPSC system up.

 

[Maxx102]

Update - well it appears that our vendor has isolated the problem and updated our portables to correct it.

As it was explained to me, it was an issue with how the portables were originally programmed. This is how was explained to me.

We have an IPSC system and with each "channel" you have to tell it what talk slot you want it be on. Example

Ch #1 - Talk Slot #1
Ch #2 - Talk Slot #1
Ch #3 - Talk Slot #2
Ch #4 - Talk Slot #2

It appears that on the security channel, the portables were originally told to basically pick whatever talk slot was was available, hence some people could hear and others couldn't.

I am not sure if that is correct but that is what I understood, regardless, after updating all of our portables, we haven't had a single missed call or drop or anything they are working as I had expected them to be and I can start breathing a sigh of relief. I'm going to give this another two weeks before saying that I am happy.