Spread-spectrum radios

[jtester]

I have a client, a small municipality, who is looking at SCADA system. One manufacturer is offering spread-spectrum radios which, among other things, require no licensing.

I am not familiar with spread-spectrum radios. Does anyone have experience with these systems?

Thanks in advance

Jim T

 

[hbiss]

Re: Spread-spectrum radios

You probably already do. Many cordless phones, among other wireless devices, use this technology. Instead of transmitting and receiving on single frequencies with spread spectrum the frequencies are constantly changing. This minimizes interference as well as enhances security.

-Hal

 

[dereckbc]

Re: Spread-spectrum radios

Worked with it for years. Used to be the domain of the malitary and telephone companies microwave radio networks. But now days just about everybody has a spread-spectrum radio, it is called your CDMA cell phone.

Unlike conventional FM or AM transmission that use channelized methods, spread spectrum is a wideband transmission. Users are not assinged a specific frequency channel or time slot. Instead the transmission is spread out over a wide band of frequencies which make it almost impossible to eavesdrop or decode the signal.

 

[tom baker]

Re: Spread-spectrum radios

I have installed both licensed and license free -spread spectrum (SS) SCADA systems. SS is limited to 1 watt out of the antenna, which is not much power, and they run 902-928 mhz, more loss at this frequency. My licensed systems run 2 watts at 450, lower frequency is less loss.
SS may work but you have no control over the spectrum. I am installing a SS cross band repeater to extend my licensed system into a bad reception area.
Any one considering a SCADA radio system needs to:
1. have a terrain model done to predict the field strength
2. Verify transmission paths with actual data.

Licensing is not hard and a UHF license can be gotten in about 2 months. I had a SS system and gave up on it. There is a licensed IP ethernet system I am looking at, most control systems are IP based now.

 

[jtester]

Re: Spread-spectrum radios

I have heard that spread-spectrum was a technology in my cordless phone. That concerns me a lot.

This area is in the desert, and there are no mountains, hills, or other obstructions higher than a 2 story building in the area. I have asked for terrain models with licensed radios, and apreciate the suggestion here.

I was curious about reliability, interference with existing users, etc. One manufacturer hints that these systems are problems. Another touts them. I am used to salesmanship, and that's why I posted for the advice.

Thanks all

Jim T

 

[hbiss]

Re: Spread-spectrum radios

Well, might be a good idea to ask the rep for some customers where their equipment has be in service for awhile. Call them and get their opinion.

-Hal

 

[tom baker]

Re: Spread-spectrum radios

This area is in the desert, and there are no mountains, hills, or other obstructions higher than a 2 story building in the area. I have asked for terrain models with licensed radios, and apreciate the suggestion here.

Then they would work pretty well.
Around Seattle, we get considerable signal loss due to pine needles, trees aren't too bad.
Also the antenna make is important. Spend more and get a quality antenna. I was using an $80 antenna and was having problems, but now use a $220 antenna. Both have the same specs, but the $220 is far better.

 

[jlmiller]

Re: Spread-spectrum radios

Unlicensed spread spectrum is quick. However, your customer may want to look at a licensed band (900mhz,2.4ghz, etc. etc)just for the sake of protection.

I do telecomm/electrical work in Gulfport,Ms (hurricane Katrina, Ground zero)and since most of the communications infrastructure was down (literally) everybody put up unlicensed spread sprectrum stuff (scada, mas, m/w, etc) to get back on the air. It created havoc for those systems that interfered with each other, even though you can select different algorithms for the spread spectrum modulation.

Spread spectrum operates darn near the noise floor.

Kinda different perspective to look at it from....

 

[justdavemamm]

Re: Spread-spectrum radios

Surf over to the plcs.net Q&A forum (http://www.plctalk.net/qanda/) and ask about the particular unit there as many people have used those types of units and have experience.

 

[RCinFLA]

Re: Spread-spectrum radios

There are two types of spread spectrum systems, slow frequency hopping and direct sequence.

CDMA cell phones use direct sequence, many cordless telephones, particulary cheaper ones, use frequency hopping.

One thing of potential consideration is if your client intends to also have a WLAN (wireless local area network) computer network. If so you should stay away from freq hopping system as they tend to interfere more with WLAN network.