Low Band Trunking (ASTRO)?

[PhillyPhoto]

Besides having 4 foot whips on your vehicle, are there any limitations to bringing about a low band trunking system? Possibly even an ASTRO system? Is this something that wouldn't pass FCC approval, or something Motorola doesn't look at as being profitable? With the range of low band frequencies, fewer sites would be required, therefore less equipment for mother /\/\ to sell. I don't know of any low band ASTRO radios, so it would obviously need to be developed.

And before people start flaming the post, I'm asking about the feasibility of such as system and regulatory/technical limitations of it. I'm not suggesting it or promoting the idea.

[thebigphish]

The lower frequency would serve as to limit the amount of information that you can effectively transmit across the carrier, limiting bandwidth...thusly limiting potential protocols and information carried...thusly making it either a) require more compression and a more lossy codec for encoding / decoding b) seriously degrade audio quality by downsampling c) limit the potential overhead for protocol used in transmission.

[nmfire10]

That and the skip from the other side of the country would probably wreak havoc,

[Wowbagger]

The lower frequency would serve as to limit the amount of information that you can effectively transmit across the carrier, limiting bandwidth...thusly limiting potential protocols and information carried...thusly making it either a) require more compression and a more lossy codec for encoding / decoding b) seriously degrade audio quality by downsampling c) limit the potential overhead for protocol used in transmission.

BOVINE SCAT.

Please, if you don't know what you are talking about, don't talk.

Information carrying capacity of a channel is determined by the signal to noise level only (Shannon's law), which in practice is determined by the bandwidth of the channel, the number of symbol states allowed, and a few other things.

Put simply, you can cram as much information across a 12.5 kHz bandwidth channel at 10 MHz as you can at 800 MHz - indeed, the first thing ALL radios do will be to take the incoming signal and mix it down to something around 10.7 MHz before processing it.

There is no reason you couldn't do APCO-25 signalling at 2.5 MHz just as well as at 800MHz - in fact I've DONE IT. The only issue is getting equipment that supports tuning to those frequencies.

[Victor Xray]

pwn3d

[Jim202]

In my long years in the radio field, the main issue that first comes to mind is the problem
with having repeaters. The FCC has been rather down on low band repeaters. They have
allowed it in the forestry service and for a few police agencies. It may be more of a lack
of available low band channels for a trunking system.

Second problem and maybe a more pressing issue is the size and cost of cavities. They are
huge at the low band frequencies. Trying to combine 5 or 6 transmitters onto a single
antenna will take up a large amount of floor space. It could be done, but I wouldn't
think many would tackle it. Cost of floor space in a shelter is high enough for the radio
equipment. To probably take up again at least double that space just for a combining
system may not fit too many radio budgets.

You did hit the nail when you said there is not much in the way of good low band base
station equipment being made these days. With everyone looking at 700 and 800 MHz,
why would any company even consider spending the time to develop a trunking low band
base station? There is no technology to prevent it, just the return on investment. The
only people that would look to a low band trunking system would be the plains states
that need to cover huge areas. Given the state of planning, the chances of a state
going to low band trunking today are about zero. Most of the states are already installing
their trunking system or are in the early stages of doing it. The large coverage area
systems have gone to the VHF band to keep the number of sites needed to a minimum.

Great idea, but probably about 3 years too late.

Jim



[quote="LuiePL"]Besides having 4 foot whips on your vehicle, are there any limitations to bringing about a low band trunking system?

[citylink_uk]

Hi,

In the late 1980's the French national military police setup a national TETRAPOL system called RUBIS installed that ran in the 80mhz region which is classed as lowband in Europe earning it the title of the first national digital network in the world.

Indeed it it still operating today and has just undergone an upgrade with new mobiles and a gateway to allow VHF > UHF portable operation.

Slightly off topic but thought it might be of interest.

Rich

[thebigphish]

Ok, so i was wrong. No biggie. IWMU that lower frequencies had inherently lower data transmissive capabilities & rates due to the longer wavelength / slower modulation of the carrier? Or is that true for long wavelength frequencies such as VLF and the like, as compared to your 800, 900 & gigahertz frequencies? Would not a slower modulation limit throughput?

[Wowbagger]

Ok, so i was wrong. No biggie. IWMU that lower frequencies had inherently lower data transmissive capabilities & rates due to the longer wavelength / slower modulation of the carrier? Or is that true for long wavelength frequencies such as VLF and the like, as compared to your 800, 900 & gigahertz frequencies? Would not a slower modulation limit throughput?

Again, the only thing that matters is signal to noise, and that in turn is effected by the number of states per symbol. Now, to transmit more bits per second, you either have to increase the number of bits per symbol, or the number of symbols per second. Increasing the number of bits per symbol eats into your signal to noise ratio, and increasing the number of symbols per second increases your occupied bandwidth.

Now, when you are dealing with frequencies like 25 kHz, then a 12.5 kHz bandwidth would be huge - that's why things like VLF for submarines run in the single bits per second range. But there is a three orders of magnitude difference between 25 kHz carrier frequency and 25 MHz carrier frequency. At 25 MHz, a 12.5 kHz bandwidth is negligible.

So to bring it down to simplest terms: yes, you aren't going to run APCO-25 to a submarine on a 25 kHz carrier. Running APCO-25 on a 5 MHz carrier would be no problem (from a technical perspective, obviously there are legal issues here).

[mr.syntrx]

In the late 1980's the French national military police setup a national TETRAPOL system called RUBIS installed that ran in the 80mhz region which is classed as lowband in Europe earning it the title of the first national digital network in the world.

I understand there used to be (and probably still are) a few MPT-1327 midband systems in the UK as well, used by gas boards etc.

[nmfire10]

I use that as a joke with our radio vendor. I made up a purchase order for a "Low Band LTR Astro Trunking System".

[bradlington]

An mpt1327 system can do single channel allocation -where the unit is a control channel as per normal signalling and data -when NPD data and voice is used it switches over to a voice channel and then back once the call is completed.Many of the radio units have some intelligence where if a "loss of service "is noted it can revert into a type of softtrunk mode where you can tell it to monitor a given frequency eg: simplex alloacation , then the radio will periodically look at the mpt1327 for any availible CC .If it decodes a valid CC then it returns to the trunking mode.

Depending on the type of controllers this could be on the same frequency or a different voice channel allocation .
Only the other users would lose service while you are using it for that voice call.

Nice thing about the open platforms is if one company gives you a hard time and you dont like the service-you can shop around in any other shop .

[mancow]

Isnt the IF in the radio proof that digital works at frequencies far below regular VHF and UHF?


BTW, when I type an apostraphe in this board it opens a quick find search box at the bottom of the page. Its damn annoying. I am using Vista. Does anyone else have this problem?

[nmfire10]

BTW, when I type an apostraphe in this board it opens a quick find search box at the bottom of the page. Its damn annoying. I am using Vista. Does anyone else have this problem?

That is a firefox thing. It should only happen if you aren't in a text box. But sometimes it has a meltdown and does it anyway.