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Jim and All, Re: Fourth discussion topic in the previous emails about coexistence scenarios with regard to TDD and FDD variants of 802.20 with each other, versus 802.20 with other systems. In general, there are four interference cases between two geographically or spectrally adjacent systems.
(In the above format of x-to-y, x belongs to one system and y belongs to another system.) If a licensed band were regulated as FDD-only, then potential interference cases would be cases 1 and 2. Interference due to cases 3 and 4 would be insignificant since the duplex gap would usually provide sufficient isolation between the uplink and the downlink. In FDD-only (by tradition or by regulation) bands, cases 1 and 2 are typically handled by controlling transmitter mask, maximum allowed signal strength at the service area boundary, and receiver filter response. The first two are set by the regulations and the third is typically adopted by industry. An example is the PCS band in the US. Coexistence work in such case would include examining existing regulations and industry adopted values for adequacy. So, it seems that as long as FDD-only is the case, it wouldn't matter if the two adjacent systems are two 802.20 FDD or one 802.20 FDD and one non-802.20 FDD system as long as service rules are followed. If a licensed band is regulated as TDD-only and the systems are synchronized, then we would have the same situation as above. Again case 3 and case 4 wouldn't be problems due to synchronization. In licensed bands that are regulated as "flexible use", meaning that, paired or unpaired, they are not FDD-only or TDD-only, it is possible to have 802.20 TDD and FDD variants operating in adjacent channels in the same area, or in co-channels in adjacent areas. In such cases, interference due to third and fourth cases could also occur. Interference due to cases 3 and 4 could be more severe than case 1 and case 2 since base stations could have LOS to each other and mobile terminals could get geographically very close to each other. The same happens with adjacent unsynchronized TDD systems. Many of the bands below 3.5 GHz are licensed as "flexible use". Therefore, cases 3 and 4 need to be given proper attention. The question I posed in discussion item 4 was whether 802.20 needs to focus on coexistence among 802.20 systems (inclusive of TDD and FDD variants) as its number one priority. I hope these explanations helped. Regards, Reza |