Re: [Madwifi-users] RSSI vs. txpower
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From: Jim T. <ji...@ne...> - 2006-07-07 22:11:41
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On Jul 7, 2006, at 8:53 AM, Robert Owen wrote: > Hi all again! > > On 7/7/06, Jose Ildefonso Camargo Tolosa > <ild...@gm...> wrote: Hi! > > On 7/7/06, Jim Thompson <ji...@ne...> wrote: > > On Jul 7, 2006, at 6:42 AM, Robert Owen wrote: > > > Hi Jim, > > > > > > Thank you for the detailed explanation. > > > As I understand, these "attenuation pads" are used at the very > > > early stage of the incoming signal processing. Does that mean that > > > there is no way to get a "true" RSSI when the txpower (or received > > > signal strength) is relatively high? > > > > > > > > > To some degree of precision, yes. > > What would happend if he increase the distance? couldn't that help in > the measurement?, ie: make the signal fall into a range where the > attenuation isn't used. > > I did measure RSSIs at longer distances. The results were similar. > For example, between two nodes about 30~40ft apart (pure guess), > the RSSI were around 17~18 for all 4 power levels: 30,40,50,60mW. > At such distance, the received signal strength is not that high. > There are about 40~50% packet loss. > Is the attenuation pad still used here? You don't say what the EIRP is. Lets assume you're sending 32mW (16dBm) and that after coax/connector losses, your EIRP is also 16dBm. 30ft = ~10m. LOS "path loss" @ 10m = ~60dB. LOS "path loss" @ 20m = ~63dB. (20m = 65' or so) And lets assume that your receiver also has only enough antenna gain to overcome the coax losses. (unlikely, but...) So the signal arrives at the antenna port at 16 - 60, or -44dBm. This is actually still quite high. Its more than enough (by at least 20dBm! (100X)) to decode 54Mbps, assuming that the real killer for 48/54Mbps (EVM) isn't causing you trouble, and, of course, discounting that your card probably can't send 16dBm @ 54Mbps and stay in compliance (both regulatory as well as .. EVM.) Even if your card is pulling the power back by 4-5dBm (due to transmitting @ 48/54Mbps), the signal is still higher than 50dBm, and the IEEE minimum rx sensitivity for 54Mbps is -65dBm. Since the baseband has to be able to decode all the way down to -80 dBm, yeah, there probably is one attenuation pad in-use, but I >don't know for sure<. Atheros (or an ADC) might be able to tell you, or you could hang a scope on the pin(s) that control(s) the switch(es), and probe for signal level .vs attenuation pads. Its going to be card (design) specific behavior in any case. In any case, your "power control" might not be doing anything in the real world. The cal tables might have your power pulled back no matter what you set. Have you tried @ 11Mbps? Jim |