My opinion - There are some unavoidable impacts, like reduced performance in channels near the edge of the band or losing those channels completely. Even if cost is no object, there are tradeoffs with “brick wall” filters. Depending on how the rules are interpreted, we might also go back to having to buy separate hardware for each sub-band rather than one radio that covers 5.1 to 5.8.
One thing about OFDM, you naturally get a decent amount of OOB attenuation especially if you have a lot of subcarriers. Stacking a bunch of narrow sinx/x shapes next to each other results in a spectral plot that has been described as looking like “Bart’s head”. So little or no additional analog filtering is needed to meet emissions masks. But look at the problems that TVWS manufacturers have had meeting those specs, and the guardbands that are required. I think a major problem would be how will manufacturers recover those costs? If they sell two models of CPE, one for low gain antennas and a more expensive one that is legal to use with a reflector dish or high gain antenna, how many of the expensive ones do you think they will sell? And if they take the high road and put the extra cost in every CPE, while a competitor sells two models, what do you think will happen? And would there be an international market for the expensive, filtered version? From: Jason McKemie via Af Sent: Sunday, October 26, 2014 2:03 AM To: af@afmug.com Subject: Re: [AFMUG] questions about filters Economies of scale, more like a "walk it off" tax at this point really. On Sunday, October 26, 2014, That One Guy via Af <af@afmug.com> wrote: with the changes in the 5ghz rules, it may force innovation in filtering technology to bring cost down, assuming the innovators arent stuck in a mindset of the only thing that would work is what there is. How do filters works? Are there electronically adjustable filters? Where does the cost come from on filters? It is not new technology, so recovery of R&D on a new tech has long since past, what is it that drives the cost up? Is it primarily a matter of it being something needed, so its more valuable, or is it something in the physical properties of the filters that drives up the cost? Can you filter electronically a transmitter using something along the same lines of noise cancelling headphones -- All parts should go together without forcing. You must remember that the parts you are reassembling were disassembled by you. Therefore, if you can't get them together again, there must be a reason. By all means, do not use a hammer. -- IBM maintenance manual, 1925
