Help with last piece of a RF link budget analysis

[MArnold]

I am trying to determine if an RF signal is strong enough to be successfully heard by a receiver. Any nudge in getting my calculations correct would be greatly appreciated! I have the following values available from receiver specs and measurements:

P: Signal power at the receiver (watts) - after all antenna gains, path loss, etc.
NF: Receiver Noise Figure (dB) - from receiver spec
B: Bandwidth - from receiver spec
T: Receiver temperature (K) - from measurement in similar experiment
S: Receiver sensitivity (dBm) - from receiver spec

So far, I think I have this much right...
Noise in watts = NF * T * B * Boltzmann
Signal/Noise = P/Noise

So, in essense, I have a unitless Signal/Noise ratio and a Sensitivity in dBm, but I'm not sure how to compare the two to see if that signal is strong enough. Do I have all the data I need to answer the question of if this signal is strong enough?

Thanks!

 

[chuckey]

"and a Sensitivity in dBm," I presume that is of the receiver delivering a signal that is satisfactory for your purpose. You don't have to re-invent the wheel as the receiver manufacturer has already incorporated the Rxs noise figure into the sensitivity figure.
Frank

 

[MArnold]

Thanks for the response -- I think I'm too ignorant to respond intelligently - but I'll try! Since the manufacturer did give me a sensitivity figure, are you saying that to determine if the signal is strong enough, I just convert S from dBm into watts -- if the number of watts in the signal is larger than S, then the signal is strong enough?

I should also clarify the reason for my question -- we don't know if the receiver is adequate for this purposse. I have the specs for this receiver, and used a simulation to determine the signal power I could acheive at a certain distance with certain antennae attached. That power is before receiver noise has been considered. I am trying to see if this receiver is indeed adequate, and using the simulation, determine how far the signal can be broadcast before it isn't heard.

 

[biff44]

There are two ways to do this.

If you have an actual receiver, the manufacturer (or you) can set up a bit error rate test (BER). In that you provide a known pattern of modulation bits to the receiver front end, and you lower the input power until a certain BER is acheived, such as -95 dBm at 10^-5 bit errors. Somethimes for low data rates, like a battery powered wireless sensor, the packet error rate is more important, since it more emulates the header recognition, etc, problems of bursty signals.

If you do not have a receiver to test, then you do it your way. You compute a signal to noise ration IN DB, not dBm. Then you go look at a table or graphs that tells you "for this type of modulation you need a S/N ratio of X dB to achieve Y BER), and you use that. This 2nd method is, of course, less accurate than an actual measurement.

In the real world, there are other factors other than S/N ratio that may cause additional bit errors, like a dispersive fade in the path loss, adjacent channel jamming, etc.

 

[chuckey]

What ever you choice of modulation, you have to consider fade margin and /or use height diversity aerial switching. For SHF a fade margin of 20 dB would not be unusual. You have to have a good look at the path.
Frank

 

[MArnold]

Thanks to all who are chiming in. The RF propogation path is modeled by our simulation. So any path losses are computed and for any test run we know how much signal is left when it reaces this receiver. The receiver is in development, so while we have some values from tests, many are still calculated. Our study is purely notional, to determine the theoretical signal power we need to see at the receiver for it to use the signal. The manufacturers of the receiver provided us with the Noise Figure and Sensitivity numbers. What I'm hoping for is some help in understanding the mathematics I need to perform to use those numbers and calculate the minimum Signal Power that is necessary.

I think biff44 answered the question -- that I need to calculate S/N in dB and the use fact that the receiver uses GMSK modulation to "look up" the dB needed for our BER. Is there an online reference source for these tables?

 

[tony_lth]

AWR can use simulate any modulation up to 256QAM at least.