(E-Mail Removed) wrote:
> The relationship between receiver sensitivity and noise is given as
> Receiver Sensitivity = thermal noise+ station noise
> +10log(bandwith)+SNRmin.
> the bandwidth is the receiver's bandwidth.
> When I am working with 802.11b network standards should I consider my
> bandwidth as 2.4GHz or only the channel frequency allocated?
I don't think you have receiver sensitivity quite right.
Thermal noise level is expressed as K*Tsys*B where K is Boltzmann's
constant, Tsys is the effective system temperature (mainly a combination
of antenna temperature [electrical not physical], feedline and input
circuit losses and receiver noise), and B is the bandwidth in Hz. For
Tsys=300 Kelvin, more or less room temperature and a reasonable starting
guess for typical system temperatures, in a bandwidth of 1 Hz this is
about -174 dBm.
Center frequency is not an issue, what is of concern is the information or
channel bandwidth. For 802.11b,g,a with a channel bandwidth on the order
of 20 MHz, this number is 20 million times, or 73 dB, bigger and the noise
is about (-174 + 73 =) -101 dBm.
Receiver sensitivity usually relates to the minimum signal level for which
a receiver can properly function, in a digital system this is often
described in terms of some bit error or frame error rate. However, if that
were all that mattered, amplification could simply be added to a "poorer"
receiver to make the signal big enough to allow it to operate. Instead,
what is normally the case with decent receivers is that the limitation is
on input signal/noise ratio rather than signal level. Adding further
amplification to an already low noise receiver doesn't improve this ratio,
only the absolute levels, which doesn't improve the performance.
For 802.11b systems running at 11 Mbps, the input signal/noise ratio
(usually referred to as C/N for carrier/noise ratio) this ratio must be
greater than about 7 dB. For 11a systems at 54 Mbps it needs to be almost
25 dB. This means that the lowest signal a perfect 11b receiver could
receive would have to be about 7 dB above the noise (on the order of -100
dBm from above) or -93 dBm. This level is the limit. In actuality there
are cable losses, other noise sources and receiver imperfections that
probably make the reality ~10 dB worse than this so that one probably
doesn't find 11b hardware capable of going much below -83 dBm sensitivity
at full rate. If a manufacturer claims much more it is time to be skeptical.
For 802.11a systems, things are roughly the difference of the required C/N
worse, (25-7)=18 dB and levels in the vicinity of -65 to -70 dBm are
probably the best sensitivities that will be common.
With all three standards, 802.11b, 802.11a and 802.11g, the slower
fallback rates require less C/N and these can operate at somewhat lower
C/N. This is the reason for having them.
I guess this was the long answer to your question. It's channel bandwidth
that should be considered and usually C/N rather than sensitivity.
regards,
Glenn Elmore
n6gn
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