I have a question that troubles me a little( Maybe it is quite simple to answer though). Its about the noise bandwidth of a multichannel receiver.
Let's assume we have a multichannel receiver with e.g. 4 channels. Each channel has a BW of e.g. 20 MHz and the channels are next to each other.
Quesetion 1: If I design now the system such that I first have preselection filter that selects the 80 MHz Bw of interest (with subsequent power amplifier and, image rejection filter, etc...) and subsequentially a channel selection filter that selects e.g. one of the channels with 20 MHz that I want to receive/decode/process what will be the bandwidth to calculate the thermal noise power of the system before quantization?(aka: -174dBm/Hz + 10*log10(?Hz)) Would this then be 80 MHz or 20 MHz. It should be 20 MHz right, because the system works at 20 MHz after all. I know there are other possibilities to design such a receiver(direct conversion), but that question troubles me a little.
Question 2: Also, if I would digitize the signals with an ADC firstly (with a high enough resolution and sampling frequency double the BW of all channels => 160MHz) and do the channilization in digital domain (by an e.g. FIR filter) , what would be the bandwidth here? I.e, I quantize the 80 MHz signal and then I use channelization for 20MHz. Will the noise then also be calculated with 20MHz BW?
Maybe this question is rather easy to answer, but unfortunatelly I didn't find a discussion of this problem so far. Thanks so much steve
thank you very much for your answer! I still have a question though.
It is more concerning the sensitivity of the receiver now. So if I now think of a system with an overall noise figure of e.g. 10dB and i consider one 20 MHz channel of the 80 MHz, then the noise power can be calulated by -174dBm/Hz + 73(=10*log10(20000000))dBHz + 10dB = -91dBm; So if I want to decode a modulation scheme that requires e.g. 10 dB Eb/N0 for a certain (wanted) BER i would need an input power of at least -81dBm. So far so good.
And now I am still a little bit confused. What if now I would ADC all four channels (=> 80 MHz) and do then signal processing of a 20 MHz channel in digital domain after quantization, what is the sensitivity of the system then? Input noise power of the ADC would -174dBm/Hz + 79dBHz(80MHz) + 10dB = -79dBm. So this is the noise the ADC sees. If I assume, that the ADC does not add any quantization noise would this mean that in this case I would need an input power of at least -69dBm to be able to decode the 20 MHz channel with the same BER, or does it mean that, because i use a 20MHz filter in the digital domain, the noise bandwidth is reduced again to 20MHz and the sensitivity is still -81dBm?
I hope the question is clear so far.