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Receive a signal below analog noise floor with adc, question

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Mar 16, 2006
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Suppose we have to receive in a wideband of 100Mhz (with noise floor computed of -94 dBm ) a digitalized rf signal which needs 10dB of CNR; so the minimum detectable signal is -84dBm. If signal has a BW of 1Mhz, can an ADC demodulate this in case it is below -90dBm?
In case my ADC has 16 bit (enob) and sample @10 Mhz with dBmFS=6dBm, this translate to 6-98+7=-85dB NSD, would it be theoretically enough?
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Your question is not clear. As far as I correctly undestood you have a receiver channel 100 MHz wide sampled at a sampling frequency of 10 MHz. This violate Nyquist so all noise exceeding the 5 MHz Nyquist frequency will be folded into your channel. You should filter at sampling frequency/2 before entering the ADC to be sure no folding will happen. Then, as far as all the receiving chain is linear, you can apply a digital filter (eg. a FIR filter) to remove the noise outside your 1MHz channel. In this case you'll have an equivalent noise floor at the input of about -114 dBm + Noise figure of the receiver.

A question.
Can Any signal below a Noise Floor be recoverable ? E.g. If signal strength drops down into Noise level, can someone recover this signal from Noise level ?? I think No.
Basically, a modulated signal can be received with S/N < 1, channel capacity will be respectively reduced. See–Hartley_theorem

If the prerequisites are fulfilled in the present case isn't clear. albbg has listed important points. For a practical communication problem, you have to specify also acceptable error rate.
"Signals that are below the noise floor can be detected by using different techniques of spread spectrum communications, where signal of a particular information bandwidth is deliberately spread in the frequency domain resulting in a signal with a wider occupied bandwidth."



Are we talking about the whole energy of the noise floor (over the whole bandwidth).
Or are we talking about the noise level at the according frequency?

Thanks to all!
I meant that I don't use FIR filters, my wideband channel is 100 mhz because of a bandpass filter so noise floor is -94dBm on the whole band. (Leaving noise figure at the moment).My receiving signal is digitally modulated with 1Mhz of BW and needs 10dB of CNR (SNRthreshold, with BER=10^-6)and could be anywhere in this 100Mhz of BW.
IF this signal is below -90dBm cannot be recovered, regardless any Processing gain of ADC?

As explained by albbg, noise power within 1 MHz signal band is -114 dBm, thus the SNR requirement can be probably achieved. Question is if you are able/willing to provide the necessary signal processing effort, e.g. anti-alias filtering in front of a 10 MBPS ADC. It's also not clear if the modulation method is optimal or involves additional SNR loss.

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