Inherent weakness?

[PHOENIX_SCANNER]

Yesterday I was listening to a conversation on a nearby 800mhz p25 trs, and the dispatcher was having quite the hard time hearing the cop well. After a bit she asked, "are you standing under a tree?". The cops said "yeah, let me get away from it", and he did, and things worked fine.


It seems like the digital systems are more vulnerable to strange things like this.


Do things like trees have more of an impact on digital radio systems than they would on analog radio operations, or is this just due to the frequency range?

Is this related to the fact that many fire departments claim digital radio systems don't work as well for their applications?

[dan1son]

The signal itself at the same frequency will have the same effect from the tree. The problem is that digital signals are all or nothing. If the noise gets too high, or signal too low, the digital signal will drop completely off. With an analog system a noisy signal can still get picked up by the operator since they are far more able to decipher a voice from a noisy background than a digital signal is. That's not to say one is inherently better, just quite different. Digital has other advantages.

[SLWilson]

Generally, the FD's problems with digital are that they are on a "fringe" area of their radio systems designed peak operating area.

This causes problems in that like the poster above said, ALL or NOTHING with digital.

Or, their system wasn't designed for IN BUILDING operations. If not properly designed, for just that type of operation, as soon as the fireman leaves the "open line of sight area" to their tower, NO RADIO SIGNAL to his portable, hence to talking to anyone.

Steve/KB8FAR

[qt31005]

Wow that would suck to be inside and not be able to hear comand or someone needing help. We are going 800 with p25 some time soon hope they get all the bugs worked out by then

[SLWilson]

Originally Posted by qt31005

Wow that would suck to be inside and not be able to hear comand or someone needing help. We are going 800 with p25 some time soon hope they get all the bugs worked out by then

It isn't actually "bugs" in the trunked systems that have the issues.

It is IN THE DESIGN, DEPLOYMENT and APPLICATION where the problem begins.

If there are enough tower sites to cover evreywhere you go it'll be OK. Consider 800Mhz trunked radio just like your own little cell phone system. You KNOW where your cell phone will and won't work.

With trunked radio, you'll FIND OUT where it won't work just like you did with your cell phone....As you find out, you go back to your vendor and have them fix these dead spots (if your purchasing authority worked that into your equipment bid!)

Steve/KB8FAR

[Raccon]

Originally Posted by dan1son

The signal itself at the same frequency will have the same effect from the tree. The problem is that digital signals are all or nothing. If the noise gets too high, or signal too low, the digital signal will drop completely off. With an analog system a noisy signal can still get picked up by the operator since they are far more able to decipher a voice from a noisy background than a digital signal is. That's not to say one is inherently better, just quite different. Digital has other advantages.

1. It's not "all or nothing" in a digital system, read up on Bit Error Rate (BER)
2. Considering all things are the same (they usually are not when people make those comparisons) and a well-designed system, a digital signal will completely break off roughly where an analog signal has become so noisy that you can't understand anything anymore. In other words the range where a radio user can still communicate with the dispatcher would more or less be the same.
3. I have never heard about any "tree-problem" with digital systems before.

Originally Posted by SLWilson

It isn't actually "bugs" in the trunked systems that have the issues.

It is IN THE DESIGN, DEPLOYMENT and APPLICATION where the problem begins.

Well said.

With trunked radio, you'll FIND OUT where it won't work just like you did with your cell phone....As you find out, you go back to your vendor and have them fix these dead spots (if your purchasing authority worked that into your equipment bid!)

Usually the contracts define the coverage requirement. To ensure the same the vendor would do a coverage and performance measurement during and after installation to identify such spots before they are discovered by the end-user.
But of course, if it wasn't included in the bid you are out of luck - but please do not blame the type of signal (analog/digital), technology, manufacturer of the equipment or the company that implements the system (assuming they all complied with their contractual obligations).

[richardc63]

Originally Posted by PHOENIX_SCANNER

Yesterday I was listening to a conversation on a nearby 800mhz p25 trs, and the dispatcher was having quite the hard time hearing the cop well. After a bit she asked, "are you standing under a tree?". The cops said "yeah, let me get away from it", and he did, and things worked fine.


It seems like the digital systems are more vulnerable to strange things like this.


Do things like trees have more of an impact on digital radio systems than they would on analog radio operations, or is this just due to the frequency range?

Is this related to the fact that many fire departments claim digital radio systems don't work as well for their applications?

I can give you an example here in Australia where a certain type of tree (Murray Red Gum) can have a dramatic effect on coverage at 400MHz using analogue FM. The distance to our site is about 8 miles, flat, and the base antenna is a 6dBd omni at about 270 feet above ground. Signal strength drops about 20dB when behind the trees.

This is a propagation issue not related to whether it is FM or C4FM.

Regards,


Richard

[SLWilson]

Originally Posted by richardc63

I can give you an example here in Australia where a certain type of tree (Murray Red Gum) can have a dramatic effect on coverage at 400MHz using analogue FM. The distance to our site is about 8 miles, flat, and the base antenna is a 6dBd omni at about 270 feet above ground. Signal strength drops about 20dB when behind the trees.

This is a propagation issue not related to whether it is FM or C4FM.

Regards,


Richard

Those operating Amature radios well know about the effect foliage on radio signals. It isn't inherent to just to 400Mhz or 800Mhz....

Even on 2 meter (147.???) the tree leaves in the spring and summer have a big effect on radio signals.

Steve/KB8FAR

[dan1son]

Originally Posted by Raccon

1. It's not "all or nothing" in a digital system, read up on Bit Error Rate (BER)
2. Considering all things are the same (they usually are not when people make those comparisons) and a well-designed system, a digital signal will completely break off roughly where an analog signal has become so noisy that you can't understand anything anymore. In other words the range where a radio user can still communicate with the dispatcher would more or less be the same.
3. I have never heard about any "tree-problem" with digital systems before.

I was referring to the end information on a digital signal. It is all or nothing. If the bit error rate gets high enough that the errors can no longer be corrected the digital 'connection' drops resulting in a completely empty signal for a portion of time, even if that portion of signal is short. In an analog system if the noise gets to high for a very brief moment you can sometimes still get a pretty good indication of what was said since the complete signal doesn't drop off, it just gets fuzzy for a very brief moment.

I completely agree a digital system can be remarkably effective and has numerous advantages over a straight analog system if the digital system is setup correctly. Simply swapping out the system and attempting to use it in the same manor as the analog system can cause unexpected problems.

The "tree problem" wasn't something I meant to specifically mention. I was just trying to explain that the RF signal will be effected in the same way, if on the same frequency, when comparing the analog and digital systems. And yes... trees do effect RF. Some types of wood more than others, some frequencies more than others, some distances more than others, etc.

[richardc63]

Originally Posted by dan1son

I was referring to the end information on a digital signal. It is all or nothing. If the bit error rate gets high enough that the errors can no longer be corrected the digital 'connection' drops resulting in a completely empty signal for a portion of time, even if that portion of signal is short. In an analog system if the noise gets to high for a very brief moment you can sometimes still get a pretty good indication of what was said since the complete signal doesn't drop off, it just gets fuzzy for a very brief moment.

I completely agree a digital system can be remarkably effective and has numerous advantages over a straight analog system if the digital system is setup correctly. Simply swapping out the system and attempting to use it in the same manor as the analog system can cause unexpected problems.

The "tree problem" wasn't something I meant to specifically mention. I was just trying to explain that the RF signal will be effected in the same way, if on the same frequency, when comparing the analog and digital systems. And yes... trees do effect RF. Some types of wood more than others, some frequencies more than others, some distances more than others, etc.

Raccon,

I still think this "all or nothing" argument is way over played. For years we have had operators complaining about signals that are "so bl%$dy noisy we can't understand them" and now some seem to expect the unachievable... absolutely no noise down to infinity. There must be a point where the signal dies. There is a practical minimum for analog signals, and digital modulation is no different.

Your second paragraph nails the argument... lazy planners who don't properly design their networks when migrating.

Cheers,


Richard

[Raccon]

Originally Posted by dan1son

I was referring to the end information on a digital signal. It is all or nothing. If the bit error rate gets high enough that the errors can no longer be corrected the digital 'connection' drops resulting in a completely empty signal for a portion of time, even if that portion of signal is short. In an analog system if the noise gets to high for a very brief moment you can sometimes still get a pretty good indication of what was said since the complete signal doesn't drop off, it just gets fuzzy for a very brief moment.

A digital system can be distorted, too, without that the connection drops or without that I loose the signal completely, I have experienced that with GSM before. If you can still understand the other party depends on how high the BER is (>=4% is considered the limit). If the connection drops entirely then there is no signal and hence no BER, i.e. a total downlink failure.

I completely agree a digital system can be remarkably effective and has numerous advantages over a straight analog system if the digital system is setup correctly. Simply swapping out the system and attempting to use it in the same manor as the analog system can cause unexpected problems.

Agree, but it is unfortunately what most people expect or how they do it.

The "tree problem" wasn't something I meant to specifically mention. I was just trying to explain that the RF signal will be effected in the same way, if on the same frequency, when comparing the analog and digital systems. And yes... trees do effect RF. Some types of wood more than others, some frequencies more than others, some distances more than others, etc.

Yep. Wasn't actually disagreeing with you, I was just trying to say that I haven't heard of a tree problem specific to digital systems. My mistake for not being clear.