What problems have people experienced with spreadspectrum?

Any recommended vendors?

 

Compared to FM radios, my experience is that (Rx/Tx) path distance and obstructions are a bigger problem. They are harder to predict, legal power levels are less and confident diagnosis is harder.

For instance, with some experience, many FM radio problems can be diagnosed simply through listening to the signals. This can't be done with SS. SS troubleshooting is mostly replacement with a known good radio. Of course, the trick is that "known good" part.

 

Spread spectrum radio technology is cheaper, easier to troubleshoot, less costly to install, and has wider attraction than any licensed radio system. Our experience has been very positive.

Kurt Raymond
IDAC, INc.

 

I have a few problems with the below statements which are almost but not completely true or accurate.
1 - Rx/Tx path distance are a bigger problem at higher data rate _because_ of the wider bandwidth . Regardless of spread spectrum or
narrowband, the system with higher modulation bandwidth will have higher receiver noise and less sensitivity (Vn2 =4 x k x T x Bw x Rg ;
where k=Boltzmann's constant, T= temperature , Bw=channel equivalent noise bandwidth , Rg=receiver input impedance usually 50 Ohm). A true statement would be that in general the faster system will have less range for the same antenna gain and the same quality receiver.
2 - FM radio problems can be diagnosed by listening only for very simple modulation methods and very slow signals . The human ear can't
diagnose a GMSK 9600bps system for example, which would be (and is) achievable by a narrowband system. Same human ear can't diagnose any form of digital packet data that happens in short bursts , regardless of narrowband or spread spectrum .
3 - All decent quality spread spectrum radios have software diagnosis tools built in . One can conduct a whole site survey remotely and
monitor parameters such as error rate, number of dropped packets etc per each radio node(and some other parameters as radio temperature or local voltage) . The drawback is this usually requires some form of rs232 terminal (pc or laptop) or the vendor supplied diagnosis/configuration software.
4 - the "known good" radio is not that tricky to identify in the lab in a bench test before the actual installation . Lab testing and setup
_first_ is always less time consuming and results in a lot more confidence in the field setup than a trial and error field setup.

Matt Tudor

RF Data Corp.
http://www.rfdatacorp.com

 

From a general, and theoretical, point of view you are right. However, when working with typical VHF/UHF FM radios used in industrial SCADA that "I
have experience with" (less than 9600baud, FSK modulation, messsages usually in the 2-10 second range in length), I stand by my earlier post
(even if I should have been more specific in the situations I meant to comment on).

In addition, I think the higher operating frequency, and lower operating power, of of typical SS telemetry radios have more to do with range and obstruction problems than the bandwidth (used). For equal bandwidth, I believe you will have more trouble with a wet pine tree in the path of a 900MHz, 1W SS radio than with a 250MHz, 5W FSK unit.

And, there are good quality radios without built-in diagnostics. Built-in diagnostics can be nice, but they also add to component cost.

And, while lab testing is good too, it does not eliminate field problems. I say being able to hear and/or see (on a scope) the actual signal can be pretty useful in the field. How certain are you of the "known good" spare radio when you plug it in to cure a problem 6 months after startup and it doesn't?

Finally, don't think I am against SS radios. I am not. Like everything, there are situations where they would be a good choice. Un-licensed
operation, for instance, can be a big plus. There are also still some advantages to using VHF/UHF, non-SS radios in telemetry/SCADA.

 

Kurt ,

You sound like a salesman....

I've fielded both types of systems. The real truth lies in the quality of the Radios that you select. I've seen some brands of SS that shine ... others that were dogs.

The same is true with licensed radios.

My experience with 900 MHz radios [both MAS & SS] :

If you have selected a high quality SS system -- you will find that it will compete nicely with a licsensed system under ideal conditions.

Under 'real world' conditions which you have no control over -

1. Sites located in high RF congested areas [like downtown].
2. Fringe area sites.
3. Radios that have exotic 'setup' parameters and your support staff does not have a clue...
4. Shared mountian top sites that contain -
1. Other SS systems [and more on the way].
2. High power Pager systems [931-932 MHz].
5. Diagnostics that indicate a 'good' radio [RF wise] but are useless in troubleshooting packet repeat problems...

Conclusion? SS systems are 'no free lunch' if your project contains several of the aboved named problems. Stick with licsensed radios if you have a choice.

Only my opinion...

Dave Gunderson Hoover Dam Computer Section
Web Page: http://www.river.f2s.com

 

Another thing to keep in mind is that if your application requires reliability and range as more important than very fast data rates, you
should select a frequency hopper rather than a direct sequence radio. The frequency hoppers don't have a problem in congested areas . The direct sequence radios have this problem though and what Dave says applies to them. Basically a frequency hopper is a 900MHz narrowband which changes channels relatively fast for us human beings but very slowly for a radio . Typically
they avoid the congested channels and retry on the interference free channels. It would take a major disaster like an EMP from an atomic blast to
completely jam all frequencies to such extent that nothing goes through. That's why frequency hoppers were developed by the military for secure
communications in the presence of electronic countermeasures . Direct sequence radios are most useful for short range in relatively benign
environments and usually come with a much faster data rate . Hoppers have an over the air bit rate of 19kb to 512kb (depending on model) , direct
sequence have a data rate from 512kb to multimegabit . Their range is inversely proportional to datarate .
Any radio that doesn't have a good input filter and a good front end (high IP3) will exhibit out of band interference with strong signals . A 450MHz radio next to a 100W 930MHz transmitter could have the same problems, assuming a front end filter with 50db out of band rejection (the overall receiver might claim more, like 100 dB but the rest would be accomplished in
the IF sections) that would still allow too much power (1mW) to reach its front end and possibly kill any other signal by saturating the front end . Even lower out of band interference could generate enough intermodulation distortion to kill your signal for a radio with a poor IP3 point . That's why paper specs are mostly meaningless unless they're very detailed and
accurate , and are read by someone who knows radios .