What cause the power difference among different wireless mode / rate?

[sam1275]

Hi everyone.
I noticed that in some high power wireless network devices, the higher speed it working, the less tx power / rx sensitivity it will be, a sample from a data sheet:

Transmitter Power:
802.11b: 28 ± 1.5 dBm
802.11g: 23 ± 1.5 dBm
802.11n: 23 ± 1.5 dBm
Receive Sensitivity:
-73 dBm @ 802.11n
-76 dBm @ 54Mbps
-91 dBm @ 11Mbps
-95 dBm @ 1 Mbps

Why is this?
Thank you.

[ConKbot]

IIRC, 802.11b has a lower peak to average power ratio (PAPR) than a/g/n because it uses a different encoding. Since the amplifiers peak power is fixed, that means the average TX power ( the spec shown) can be higher for 802.11b.

As far as the recieve side I cant give an authoritative response either, but generally, as your bandwidth of your receiver increases, so does the noise power, which decreases sensitivity. Also, with a encoding that does more bits per symbol, your SNR requirements increase. With an equivalent receiver, your "energy per bit" stays the same, so higher bit rate, higher energy put into the RX required.  http://en.wikipedia.org/wiki/Eb/N0 

[sam1275]

IIRC, 802.11b has a lower peak to average power ratio (PAPR) than a/g/n because it uses a different encoding. Since the amplifiers peak power is fixed, that means the average TX power ( the spec shown) can be higher for 802.11b.

As far as the recieve side I cant give an authoritative response either, but generally, as your bandwidth of your receiver increases, so does the noise power, which decreases sensitivity. Also, with a encoding that does more bits per symbol, your SNR requirements increase. With an equivalent receiver, your "energy per bit" stays the same, so higher bit rate, higher energy put into the RX required.  http://en.wikipedia.org/wiki/Eb/N0

Wow, great explanation, I think that make sense, thank you very much.