Ginu wrote:
On Dec 9, 6:42 pm, Jim Lux wrote:


So, let's look at a link budget for 10 meters
32.44 + 20log(2500)+20log(0.01) = 32.44+ 68 -40 -- about 60dB path loss
between isotropes 10m apart at 2.5GHz.

-60dBm receive power vs -100dBm sensitivity.. So, it should work ok at
250kbps and 10m (assuming no interference, multipath, etc.)

Now, bump to 100m.. That's a 20dB hit.
500m another 14dB.. now you're on the ragged edge. 6dB margin with a
-100dBm receiver and a 0dBm transmitter. And that's assuming isotropic
antennas, which may or may not be reasonable.

My path loss results verify this too. The issue is this: if the range
is between 10-100 metres, how do we use a minimum of -3 dBm of
transmit power?

To transmit at 250 kbps on a 5 MHz bandwidth channel centered at 2.5
GHz to a receiver say 10 metres away, you only require 5.8009e-010
watts. That's the problem. This is the maximum power required. It is
far less than the -3 dBm quoted.

A question you need to ask is what's the receiver bandwidth. The
information might only be 250 kHz wide, but if the receiver is 5MHz
wide, it's seeing 13dB more noise, and it might not be able to "acquire"
the narrow band signal.

60dB path loss, 250kHz BW is 54dBHz, so kTB noise floor is -120dBm, for
a "real" receiver and cabling, probably 5dB worse, call it -115dBm. Add
the 60dB, and you need an EIRP of -55dBm.. (You calculated 6E-10W,
-62dBm.. that's reasonably close)

But, if the receiver is seeing the full 5MHz (or more) BW, then you'll
need more.. 13dB at least (-100dBm at the receiver.. Obviously, most
receivers don't do this well, or they're counting some S/N margin, to
get a spec of -94dBm)

Then, you'd need -34dBm (with the same 60dB path loss)

But I'll bet you actually need more...

Non-ideal antennas
Losses in cabling
Mismatch (the Tx may put out 0dBm, but if the antenna presents a 2:1
mismatch, it's not radiating 0dBm, etc.

And, the "acquisition" threshold might be different than the
"communicate" threshold.