Members of the ISP-Wireless list discuss dealing with interference. Some possible solutions are illegal, so it pays to know the FCC rules on spectrum use and frequency hopping. [December 13, 2000]

On the ISP-Wireless list in December, JD brushed the edge of the law when he asked,

"I've recently installed FHSS equipment, and there's a DSSS radio nearby; both are having problems as a result. Is it possible to eliminate the frequency of the radio from the hopping sequence on my equipment?"

A couple of respondents suggested an underhanded solution:

[DS suggested] "You can try changing the call sign from US FCC to Japan (higher frequencies) or France (lower frequencies)."

[SGF agreed] "You can change the hopping standard. You can choose which adapts best to your needs. I'm using France's here."

[AM enthused] "That would be great; then I could 'cherry pick' the best patterns out of the sequences available so that I spread things out better." A number of respondents contended that such a change would be illegal:

[KEM explained] "I understand that the FCC is very strict on how the hopping is done, and does not allow only subsets of the bandwidth to be used."

[KW added] "The whole object of FHSS is that it changes frequencies thirty times a second. In your scenario, your FHSS may step on the DSSS system, but that will only be temporary-unless you're very close; then the potential is that your FHSS may shut down the DSSS system."

JS agreed, but suggested some ways around the problem:

"What isn't allowed in the US is forward looking error correction, whereby the radio discovers what frequencies aren't usable and doesn't try them. Your problem can be helped to some degree, though, by not using an omni antenna.

If you use a directional antenna, some portion of your customer base will be free from the interference because your antenna will not be 'seeing' the DSSS antenna. Similarly, talk to the people running the DSSS radio about changing over to a directional antenna."

RB concluded with a quick lesson in FCC regulations:

"Here's a few snippets of FCC Part 15.247. 'The system shall hop to channel frequencies that are selected at the system hopping rate from a pseudo-randomly ordered list of hopping frequencies. Each frequency must be used equally on the average by each transmitter.' And later in the same section: 'The average time of occupancy on any frequency shall not be greater than 0.4 seconds within a 20 second period.' These are the rules. We can't do anything else unless the rules change."

—End

 

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