Hi Chuck-
Good to hear from you. We met at a CIC wireless meeting a year ago or so in Chicago? Here are my ramblings. I think you are right, except the system in the 902 band can put out 500 mw. That's pretty strong for that frequency, especially close to a very small filter. There still is a chance for front-end overload receiver interference. Anyhow, here are my ramblings:
1. Power from a 900MHz transmitter at or near 2.4GHz is very low.
a. Normally yes, except for a repeater
2. Receive sensitivity of the AP's radios at 900Mhz is very high (insensitive).
a. Not very high. It should be very low due to a band-pass filter that effectively notches the 902 band. This means that the AP should be very "selective" in the 2.4 and 5.8 bands only, and deaf at 902(plus now the DAS system frequencies)
3. If 1 & 2 are correct, it follows that the filter rejection at 900MHz really doesn't matter.
a. agreed
4. The cellular filtering is one or more band rejection filters, designed to have low rejection in the 2.4Ghz spectrum.
a. Agreed. Maybe that same filter works in the 902 band to some extent?
5. The relevant interference is not in the intended band of the interfering transmitter(s), but the 2nd order IMPs which will fall around 1800MHz.
a. Agreed.
6. To know if there will be enough interference to worry about, or if there will be enough filtering to deal with it, you need to know the characteristics of both the transmitting and receiving radios and the path loss between them in addition to the filter rejection.
a. Agreed. That's why I am an RF dork.
I don't think you are wrong about anything, and I think we are talking the same stuff here. To you it may seem pointless, but I need to know everything about radios because I am a radio/RF dork. It makes me happy. Now, I can test what the engineer claims and get real world results. It would be nice to see an image rejection sweep from when they confirmed the 902 band for the AP-225. But that would only satisfy my dorkness........... See you down the road!
Matthew L. Bonadies
Campus Network Operations
Wireless Networks
Indiana University - Bloomington
mbonadie@iu.edu