What is a Baseband Equivalent Signal in Communications?
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- เผยแพร่เมื่อ 11 ก.ย. 2021
- Explains how passband and baseband representations of signals are related in digital communications. Shows how QAM passband signals are generated from lowpass (baseband) constellation points (effectively "up converting" them to the passband), and then shows how the demodulation process at the receiver "cancels out" the up conversion and recovers the baseband signals. This shows that the baseband representation is enough to capture (mathematically) the input-output relationship that results from the modulation/demodulation process.
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For a full list of Videos and Summary Sheets, goto: www.iaincollings.com
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Sir, you deserve many many more views than what you actually do ! Not one word is out of place, which makes me respect it much more !! Great Job !
Thanks so much for your very nice comment. I'm so glad you like the videos.
Great explanation as always. Low pass equivalent was given by many sites but you connected it with Nyquist and gave the reason. Very thankful.
Glad it was helpful!
Your videos are the best, thanks for making these!
Thanks. I'm so glad you like them!
Thank you very much, professor! Your teaching video is really nice and inspiring. I am a junior student majoring in EE, and you make me much closer to the nature of these conceptions.
That's great to hear. I'm so glad you found the video helpful. You might like to check out the other videos on my channel too. You can find them in categorised order at iaincollings.com
Always thanks for your appreciation.
All of your efforts makes me the real engineer.
Thanks John, that's so nice to hear. I'm glad you are finding the videos helpful.
@@iain_explains I am a fresh ph. D. but I don’t have insight as much as you, so I am so surprise when I watch your video. Really thanks again : )
Super great explanation
Glad it was helpful!
Thank you Iain for all these efforts and great explainations. So in practice are all sampling is done on the baseband equivalent signal or actually on the baseband signal before modulation? I mean in normal comm. system we do sampling first then modulation and upconversion and at the Rx downconversion. Complex equivalent is more about facilitating the analysis rather than the sampling itself as I think we do not need to do sampling on the passband signal in practice even if we do not consider the complex baseband equivalant. Is this right?
Hi Azzam, yes, the baseband equivalent helps with analysis, because it means we don't need to keep all the sin(.) and cos(.) terms in the equations, and it is much easier to calculate things like BER etc. I'm not sure what you mean by "sampling first then modulation" though. The sampling I was talking about happens at the receiver. The reason I mentioned the sampling issue was to point out that even though the actual (passband) signal is at high frequencies, the information contained in the signal is not changing at that rate. It only changes at the rate of the equivalent baseband signal. To put it another way, when sampling the signal at the receiver, it is not necessary to sample at a rate that would mean you could perfectly reconstruct the complete passband signal (which is what you could do if you sampled at the Nyquist rate). You only care about reconstructing the equivalent low pass signal, which only requires a much lower sampling rate.
@@iain_explains Thank you. It is clear now.
Thank you so much!!!!!!!
My pleasure. I'm glad you found the video helpful.
is the multiplication by 2 on the lowpass signal there to match the 1/2 scale factor on the fourier transform of cosine to fully recreate the original signal?
Yes. But that actually they are both there because of the same underlying reason. It's because half the signal is represented with positive frequencies and half is represented with negative frequencies. So when I drew the X_+(f+f_c) function, I only kept half of the original function. So when I write a mathematical formula for the full function, in terms of the "half function", I need to multiply by 2. More information on this can be found here: "What is Negative Frequency?" th-cam.com/video/gz6AKW-R69s/w-d-xo.html
Sir can you make a video that related to wideband IQ Signal generation at receiver Side and how can we monitor spectrum using this wideband IQ
Sorry, I'm not sure what you mean by "IQ Signal generation at receiver Side". Are you referring to the superhet receiver structure?
This video might be what you are looking for: "Sampling Bandlimited Signals: Why are the Samples "Complex"?" th-cam.com/video/JglRGRizqGM/w-d-xo.html
Great, Thanks
You are welcome!
hi sometimes we have a rf signal that it's not repeatitive like a sound wave how can we downconvert this signals to sampling with low sample rate analog to digital converters? is there any way?
I think perhaps you haven't understood that this whole video relates to signals that are not repetitive (like sound waves from music or speech). These are low-frequency baseband signals. Your question about down converting doesn't apply to signals that are already at baseband. I suspect you may be asking more about sampling, right? Perhaps this video might help: "Sampling Signals" th-cam.com/video/AcuQnIXiZ2A/w-d-xo.html
hey professor thanks for amazing video.
in digital domain message can be complex and real like BPSK is real and QPSK is complex data.
doubt - can analog message signal be complex because in analog domain what ever we transmit is real only like the voice transmission radio FM etc?
I'm not sure you're understanding it fully. QPSK is not "complex data". The "data" is not complex. "Data" is 1's and 0's. They are real. If you want to send multiple bits of data at the same time (eg. 2 bits, in the case of QPSK), then you need to have multiple waveforms (one to represent each possible combination of bits). If you choose to design the waveforms to be phase-offsets of each other, and you want to have more than two phases (ie. more than just 0 and 180 deg) then you will need to have a sin() component as well as a cos() component. Then, if you want to, you can represent those components mathematically as "real" and "imaginary". .... but they are both real signals! Hopefully this video will help: "Is the Imaginary Part of QAM Real?" th-cam.com/video/6asDtzaVjbQ/w-d-xo.html
@@iain_explains thanks dear professor fir clearing my doubt that means even if in case of analog communication complex signal transmission means transmission of signal on two orthogonal set of sub carriers , may be thats why we use complex exponential for purpose of modulation.
and my be complex data is possible because there is possibility of only two orthogonal subcarrier at a particular instant of time .
now all thing make sense you are just amazing professor 👍
Yes, that's correct. For more details on orthogonal waveforms, see: "Orthogonal Basis Functions in the Fourier Transform" th-cam.com/video/n2kesLcPY7o/w-d-xo.html
Superheterodyne receiver downconverts the incoming RF signal to IF band and samples the IF signal only one ADC. After sampling NCO generate digital oscillator to multiply sampled signal. İ dont understand how can we obtain IQ data at here. Can you help me?
Ah, OK, now I think I understand your question. It's probably best to think of the ADC samples of the IF signal (over a time period equal to the symbol period) being put into a vector. Another vector can be generated with elements that are sinusoidal at a digital frequency that corresponds to the analog IF frequency. Then you multiply (element-by-element) the two vectors together. And then sum up the elements of the resulting vector. Then you need to repeat that for a second sinusoidal vector that is phase-offset by 90 degrees. Then you'll have two numbers - one is the "real" value and the other is the "imaginary" value of the complex IQ digital symbol.
This video is also related: "Sampling Bandlimited Signals: Why are the Samples "Complex"?" th-cam.com/video/JglRGRizqGM/w-d-xo.html
✔️💐💐💐