These are not easy questions to answer - if they were, we would’ve had PMP in the 6 GHz band long ago! I should start out by pointing out that these are my personal insights based on 46 years in spectrum management in the US pt-to-pt microwave bands. I retired from Comsearch on October 13th and that’s why I was on the beach earlier today.
Faisal, your argument of ‘holes’ in part 101 allocations doesn’t agree with the facts when you look at this in the context of Pt-to-Pt, FDD licensing. I suppose one could argue that if everything used TDD and was nicely synched with one another that the spectrum could be used more efficiently. But, the only licensed pt-pt TDD systems are 11 GHz Mimosa B-11s, and these wreak havoc on co-located FDD systems that require a minimum frequency separation to avoid interference (what AT&T dubbed a high/low “buck” or “bump” many moons ago). 99.99998% of the current licenses in 6 GHz use FDD and 95% of these use paired high/low channels. Your example of a “channel” being available at one site while not at another is a fairly rare occurrence due the reciprocal nature of interference in highly directive pt-pt bands that use high/low channel “pairs”. The “Z” frequency may be available for transmit to “A” but if “A” is as congested as stated the interference into the “A” receiver would make the pair unusable. I think we already have a very good understanding of the SNR requirements for various modulations, and this isn’t really the issue. Most of the 6 GHz systems, licensed to Tier 1 and 2 carriers, State and Local Governments for Public Safety, Pipelines, Gas, Electric Utilities are designed for at least 99.9995% availability and some PS entities go even further than this (remember that this is computed on an annual basis so most fades in the 6 GHz band which is not rain limited will be extremely quick, but devastating nevertheless). How do they get this kind of availability? Primarily with larger antennas, space diversity, adaptive modulation and even higher power radios. Aviat and Nokia now offer radios that provide 5 watts at the highest modulations. We know that interference in digital systems primarily causes threshold degradation of the interfered receiver, and thus a reduction in the fade margin that was necessary to compute the required availability to begin with. So, the current work in this area is centering on how other systems (unlicensed, PTMP, etc. etc.) could be engineered and designed to work alongside the pt-to-pt systems - using this fade margin when the path does not need it, but somehow figuring out how to get off the channel when the licensed path goes into a fade. Some realtime communication between systems is going to be required and as you can bet, this is all extremely complex. It should be pointed out that a very large task force under TIA studied the 1.9 GHz band for pt-to-pt and PTMP coexistence in the early 90’s when PCS was coming into the 1.9 GHz band. After at least 2 years of exhaustive study and research, the conclusion was that these systems could not co-exist and the FCC require the new PCS carriers to pay for all licensed 1.9 GHz licensees to vacate the band. Chuck, I’m not sure if I understand your question concerning “lower portion of the band”. Also, it does not sound reasonable that you would have a 100 mile interfering path with the interferer pointed away from you. His signal would have to be a minimum of 55 dB down (required FCC Cat A front-to-back) to start off. I would seriously question whoever gave you that information. I’ll close with this thought - It’s really not difficult to see how devastating a 15, 30 or 45 degree or larger sector antenna can be to frequency re-use between pt-to-pt and PTMP systems. While the FCC antenna requirements in these bands are woefully behind the rest of the world, they do require highly directive antennas nonetheless. In point-to-point bands, the higher the directivity of the antenna, the better the frequency reuse and thus spectrum efficiency. Hope this helps - if not, it was free! Back to the beach or golf course.
