How are Complex Baseband Digital Signals Transmitted?

Explains how complex baseband digital signals are transmitted, from both a a time domain and frequency domain perspective.

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[AM-turc]

Always wondered about this, finally makes sense to me. Thank you Professor Iain!

[mauricio]

Alright so i am still a bit confused about this subject. Would it be correct to say that we want to send a complex signal with a certain amplitude and phase (i.e. a point on the constellation diagram) but this is not possible with only one signal (because we can only create real signals). So to circumvent this problem we send two signals, one sine and one cosine. Each of which has a certain amplitude to create the desired point on the constellation diagram. We combine them and send them through the antenna. Then at the receiver we decombine them again into the seperate sine and cosine, read of their amplitudes and then use those two amplitudes to create the point on the constellation diagram?

[MrFab]

Very good step by step explanation !
So clear for a French people. Every tricky technics becomes easy with you ☺️.
I would have liked to have you as a teacher. Thanks 🙏

[logandihel]

Nice video! It's always nice to come back here and review the basics after a long day of studying for my graduate level data communication classes

[Astu]

for complex we have too mutiply with sine term and real cos term is it so?

[elijahsmith]

This is the perfect mix of mathematical formulas and graphic intuition. Thanks

[yanivnet]

Thank you. I like to watch your videos. This time I think a key principal has been overlooked. The statment that sending 2 signal at the same time over the same baseband do not interfere each other may be missleading. The fact that one has been multplied with a sine and the other with a cosine, means that the signals are transmitted with a phase shift of pi/2.

[killua_]

14:07 I'm sorry I don't understand why you approximate the integral of cos to 0. It's going to be 0 only if T is a multiple of 1/fc, am I wrong?

[kimjohn-dx]

Thank you. Such a nice video! Can I ask why complex baseband is used rather than real baseband? What are the advantages?

[maazawan]

Wonderul explanation sir!
Why are we integrating the product of alpha cos squared over a time period T?

[muhammadahmedtariq]

Sir you are a world class researcher. You may explain modern access techniques like RSMA. Also you may explain fractional programming for communication systems to maximize data rate. As these techniques are state of the art as is your level.

[eswnl]

could you do a talk on the costas loop? I think its related but this performs carrier recovery.

[rudrasingh]

very helpful sir
i have a request video on topic OQPSK offset quadrature phase shift keying
thanks

[lucidasser]

A short question, please. From the IF we convert down with a Digital Down Converter (DDC) to get to the baseband. Do we always digitize on that way? It is often said "the digital baseband" - can we also be in the analog baseband?
And within the bb, can we represent it in the time and frequency domain?
Thanks a lot.

[yinliu]

Thank you for your video and your careful lecture! May I ask another question? This question has puzzled me for a long time. I completely agree with what you said, that a complex signal is actually composed of two real signals. In the frequency domain, the baseband signals and the passband signals shifted to the carrier point are conjugate symmetric. But why is the spectrum not symmetric when they are added together to form the so-called complex signal? How can we visually explain all of this in the frequency domain?

[mohamedtalha]

One question here for this example, the symbol which is represented by four bits is multiplied by cos and sine ? or there’s 2 bits (four levels ) for cos signal and the other 2 bits for sine

[alexnyquist]

Hi Iain, first a lot of thanks, the videos really helping and good. I have a question, where did the Integral (in the receiving end) came from? when I learned this topic, it was similar but after the multiply by cos or sin there was LPF that killed the other component, so I little confused, thanks!

[teerawatmagkeethum]

Could you present a carrier recovery of high order QAM? that would be great! Thank you.

[pitmaler]

Thank You. I have 2 short questions. In the second line, there is the complex signal in the frequency domain in the bb. In the time domain that signal would be with a complex part (z=x+jy), and in the first line, the real signal is without a complex part in the equation, is that correct?
To my mind that is the difference of a real and complex signal.
2. The equation that you show on the bottom (integral), is the equation of the orthogonal criterion of signals - is that correct?

[nanjiang]

Hi Iain, thanks for your video! I found some books making a complex signal by multiplexing the quadrature component with -sin wave, which is Q(t)*(-sin(2pf_ct)), but what you did is Q(t)*sin(2pf_ct). May I ask what is the difference between them?