Licensed or License-Free Radio For SCADA?

I get asked this question all the time, and opinions vary. Some folks are absolutely loyal to licensed radio for SCADA. Others won't touch it, and will only put in license-free, spread-spectrum systems. The bottom line is that both technologies have their benefits and drawbacks. So, what are they?

Licensed radio (VHF and UHF) simply 'talk' farther than spread spectrum radios. This fact (and yes, it is a fact) is due to two reasons: 1) Licensed radios can transmit at up to five watts of power and 2) VHF and UHF are less prone to attenuation by rain, fog, tree leaves, etc. Add those two basic facts of physics together, and you've got a long radio link. This is the case with all radio manufacturers; like I said, it's just physics and we've all got the same rules there.

Another benefit of licensed RF for SCADA is that you own the frequency. No one else can use it. If they do, the FCC tends to get rather annoyed.

So, licensed radio is better, right? Well, that depends. You DO have to get a license. It is a common misconception that you can't get licenses for SCADA applications. That is not true in many, if not most, cases. I have coordinated hundreds of licenses over the last five years, and I've only ever had three refused by the FCC. It really is a painless process, much less so than it was ten years ago or so. Just a few pages of information, and send it in. You get temporary approval within about 4 to 6 weeks. Why are they available now? Two reasons: 1) licenses are becoming available because users such as fire, EMT, police, etc. are switching over to (you guessed it) cellular technology and letting their licenses lapse. 2) The FCC is refarming the spectrum from 25 kHz channels to 12.5 and then 6.25 kHz. In other words, they are basically quadrupling the number of license available. So, when someone tells you, "You can't get an FCC license for SCADA," they are usually wrong.

Now, here's the rub: it IS still hard to get licenses in the metro areas of the United States. Not impossible, but the three I had refused were all within highly-populated areas.

Another drawback of licensed radio can be seen by those channels I mentioned above. Pretty narrow channels, eh? Well, you can't push a lot of data through narrow channels. Therefore, if your application REQUIRES high bandwidth, licensed radio might not be for you. Just think about how much data you NEED, and see if licensed RF would work. You get the benefits mentioned above.

Now, good old spread spectrum. Everyone loves spread spectrum (I know I do). But, just like licensed RF, it has its benefits and its drawbacks. The first is benefit is obvious: you don't need a license. So, in those areas where you can't get a license, it's a great fit. However, everyone and anyone can come in right next to your system and put in a system. Let's look at the 900 MHz spread spectrum (ISM) band. What other devices use that spectrum? Well, other SCADA radios for one. Baby monitors. Garage door openers. RC toys. You name it. Lots of noise. Also, the FCC limits transmit power to one watt at the antenna port and 36 dBm at the antenna (EIRP). Just make sure you get your gains right, and it's okay.

Now, line-of-sight is much more of an issue at 900 MHz. As you get higher in the spectrum, the wavelength gets shorter and becomes much more susceptible to attenuation by rain, fog and leaves. So, 900 MHz doesn't travel as far as licensed. But, the channels are much wider (sometimes 500 kHz or so), so you can pump more data through them. In other words, it's better for high-throughput requirements.

Okay, so what's the answer? Yes, you guessed it: It depends. Where is the system going in? What speeds do you require? If they tell me their system is going into the middle of Nebraska and they need 9600 baud, I recommend licensed radios. If they tell me they need 512 kbps and it's going to be deployed around Denver, I recommend spread-spectrum. BOTH technologies work and both are available. BOTH have their benefits AND their drawbacks. You just need to decide which works for your specific application.

Whew... now my fingers hurt.