What is RSSI

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turboscrew
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What is RSSI

Post by turboscrew » Wed Jul 19, 2017 11:06 pm

What does RSSI typically measure? Energy within a band or average energy density or what?

I was trying to find out the radio parameters of a 433 MHz USB dongle.
I wrote some kind of scanner for a blue pill with CC1101 radio module and I got weird results:
When the base frequency is the same, I got smaller RSSI with wider receive filter bandwidth.
I expected the wider bandwidth to give at least the same RSSI as the narrower gave - tha narrower was included into the wider.

Or do the USB dongles have software frequency hopping or something?

Without the dongle I got some frequencies with RSSI about -60 dBm. With the dongle I suddenly got about 60 frequencies with RSSI between -49 and -60 dBm. The dongle was about 6.5 meters away from the CC1101 radio module, but there were no obstacles in between.

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Pito
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Re: What is RSSI

Post by Pito » Thu Jul 20, 2017 9:48 am

I expected the wider bandwidth to give at least the same RSSI as the narrower gave - tha narrower was included into the wider.
That would work provided you have got a $xxK spectrum analyzer and you are watching peaks of your signal with cursors on it, but those RSSIs inside the chips are fairly simple circuits giving you a rough indication of the signal power inside a channel.

My current understanding:
Those RSSIs usually have got the same structure as the popular AD8307 (a broadband logarithmic amplifier).

At the logarithmic amplifier's output there is a usually a low pass filter with fairly long time constant (T) working as an "signal integrator" (therefore it takes some time to get max RSSI).

Thus the RSSI integrates log values of the input voltage (usually out of IF, or baseband filter) over a period T (an integral from 0 to T of log(u_input) ).

With narrow channels bandwidths (here we talking the bw of the filters at the logamp's input) you are "closer" to the useful signal and your useful signal fills in the entire channel bandwidth (because the I/Q mixer places your useful signal close around the IF freq), therefore you will integrate bigger values (you get bigger RSSI readings), while with wider bandwidths your useful signal is proportionally smaller (in freq domain it occupies only a part of the channel filter bandwidth, the reminder is the noise which is usually much lower -> while the "peak amplitude" of your signal is still the same your signal's envelope "is smaller in relation to set filter bandwidth"), and you will therefore integrate proportionally smaller signal with a smaller RSSI reading.

Mind the RSSI output depends on many factors, like antenna gain, LNA gain, I/Q mixer gain, IF gain, SNR, etc.
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turboscrew
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Re: What is RSSI

Post by turboscrew » Thu Jul 20, 2017 11:31 am

I could understand a RSSI 3dBm smaller with double bandwidth, but I got RSSI = -49 dBm with 464kHz RXBW, but RSSI -94 with RXBW=541kHz.

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Pito
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Re: What is RSSI

Post by Pito » Thu Jul 20, 2017 6:24 pm

Your RSSI reading is just a number. I doubt you may get dBm, dBuV or something like that.
You may use the readings for relative comparison, like: at this specific chip's radio setup, modulation scheme, BWs, channel n., antennas positions/radiation diagrams, gains, distances, etc., the transmitter T1 is -30 and the transmitter T2 is -20.
So, the signal from T2 is much stronger than from the T1. That is all..
For precise measurements you would need quite expensive equipment to calibrate your RSSI. The calibration must be done for each specific setup of the chip, frequencies, antenna positions, etc, btw.. A lot of work :)
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RogerClark
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Re: What is RSSI

Post by RogerClark » Fri Jul 21, 2017 3:34 am

Can I suggest you raise this topic on a TI forum about this device, as the would have specific knowledge on TI products

e.g.
http://e2e.ti.com/search?q=cc1101

turboscrew
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Re: What is RSSI

Post by turboscrew » Tue Jul 25, 2017 1:22 pm

RogerClark wrote:
Fri Jul 21, 2017 3:34 am
Can I suggest you raise this topic on a TI forum about this device, as the would have specific knowledge on TI products

e.g.
http://e2e.ti.com/search?q=cc1101
I did, but I didn't, really, get an answer.

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Squonk42
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Re: What is RSSI

Post by Squonk42 » Wed Jul 26, 2017 10:00 am


turboscrew
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Re: What is RSSI

Post by turboscrew » Thu Jul 27, 2017 9:38 am

Yep, but they don't explain the RSSI mesurement: what is measured an how.
What I'm wondering is that very often when I get some data, the RSSI is pretty small. I get better RSSI with smaller RX filter bandwidth, but no data.
What I'd like to know, if there is the same transmission that fits within two different RX filter bandwidths, what happens to the RSSI if you switch from the narrower to the wider bandwidth. If the wider bandwidth is twice the narrower, should the RSSI stay the same or drop by about 3 dBm, or maybe they are totally not comparable?

turboscrew
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Re: What is RSSI

Post by turboscrew » Fri Jul 28, 2017 6:14 pm

I was wondering...
If RSSI gives a measure of energy within a band, then widening the receive filter should produce higher RSSI,
because the band now contains the same energy as before, but also energy from background noise that didn't fit within the narrower band.

So would it work if I divided the RSSI by the RXBW? It would then give me some kind of average energy density within the band? One would think that the transmission has higher energy than the background noise (at least in general) and a band closer to the actual transmission frequency band would give higher energy density.

Or maybe dBm is not very suitable measure for that. Maybe (a conversion to) plain (approximate) milliWatts needs to be used?

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Pito
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Re: What is RSSI

Post by Pito » Fri Jul 28, 2017 7:33 pm

If RSSI gives a measure of energy within a band, then widening the receive filter should produce higher RSSI,
because the band now contains the same energy as before, but also energy from background noise that didn't fit within the narrower band.
That would work when your RSSI gives a measure of Energy. But it does not.
RSSI measures voltage (like an oscilloscope) at its input. There is some voltage related to the signal, and some voltage related to the noise.

Imagine you have got an oscilloscope. Time base is set to 1ms. Your signal is 1kHz square wave, duty 50%, amplitude 5V (0..5V) There is some noise on the signal say 10mV. You do not sync, so the signals moves across the screen.

1) what do you mostly see with the 1ms time base? (==1kHz bandwidth). You see a square wave moving around, 1x5V and 1x 0V, the average on the screen is 2.5V. Your signal fully fits into your BW.

2) switch to 1usec time base (==bandwidth 1MHz). What do you see? Noise. Sometimes a small piece of something like a piece of a square wave flies around. The average of noise is zero. You get noise and a little bit of your signal sometimes and from that you see on the screen do make an average.. It will be almost zero.

3) switch to 100ms time base (==bandwidth 10Hz). What do you see? A continual box full of slim square waves (100x5V and 100x0V). The average from the screen is 2.5V.

When messing with radio-electronics you always have to think in both time-domain and frequency-domain and switch in between such it gives you the required result :) :)
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