I searched for my answer. I can see that he is a master in our profession. I would like to ask him some questions about communication principles.

This is the first time someone has directly asked a question, thank you for watching. The previous answers were also based on a communication attitude, and I dare not say that I am absolutely correct. I can't be called a great person at all, I'm afraid of being laughed at. I am also taking the postgraduate entrance examination this year, so we can communicate with each other.

1. Yes, the code rate Rb after time division multiplexing has nothing to do with the duty cycle of the symbol, but is related to the time width between each bit

2. That’s right, When there is no correlation between symbols, the first zero bandwidth is the reciprocal of the pulse width

3.1? The question asks about the minimum system bandwidth required without ISI,? It can be understood that the system is a For an ideal low-pass band-limited system, this has nothing to do with the first zero-crossing bandwidth of the signal. It should be B?=?Rb/2. At this time, the highest frequency band utilization rate is?η=Rb/B=2?Baud/Hz. (This is binary, Rs=Rb, 1Baud=1bps).

I probably understand what you mean. You probably want to say that since it is an equivalent low pass, it is enough to pass all the frequency bands of the signal, but the first zero-crossing bandwidth 2Rb is greater than the Nyquist bandwidth Rb /2, right? But this is how I understand it, because this is a digital signal, and the original information can be restored from the sampled value at the sampling moment. It is still very different from the analog signal that requires the waveform to be undistorted or the complex envelope to be undistorted; in addition, from This can also be seen in the derivation process of the Nyquist criterion for ISI-free baseband transmission, as long as there are no bit errors in the sampled values. (Moreover, the so-called ISI-free transmission originally considers ISI-free transmission under limited band conditions)

3.2? First of all, I think the "ISI-free" and "minimum" mentioned in the question are the key. That's right. It depends on the transmission method. If it says it is a raised cosine roll-off system, then the transmission bandwidth is different, but it is not mentioned here. I think it should be understood as an ideal low-pass system, because It's asking for the minimum, an ideal situation. Band utilization has nothing to do with duty cycle. You must understand how band utilization is defined. Assume that the code rate is Rs and the system bandwidth is W, then the band utilization is eta = Rs/W, and if the roll-off coefficient is α (0≤α≤1), then the highest frequency band utilization (baseband) without ISI is η=2/(1 α)?Baud/Hz. When α=0, it is equivalent to the system being an ideal low pass. I think this You know, it's all in the books.

3.3? Based on what I said above, I think you should understand the previous question. In fact, these three terms may all refer to the same thing. I personally think that asking "minimum system bandwidth" and "minimum transmission bandwidth" in the absence of ISI should be the same question, while asking "system bandwidth" depends on the specific situation. I think There are many questions about "What is the frequency bandwidth of the signal?"

I haven’t learned much about the frame design problem you mentioned. I feel it should be a multiplexing problem between low-order groups and high-order groups. You said f1, 2f1, 3f1 (after sampling, they are still 1 times The relationship between 2 times and 3 times) can be regarded as the fundamental group, the quadratic group, and the cubic group. Take a look at this icon and the content of 6.3.4 on this webpage/jp2007/15/wangluokecheng/text/6.3.htmamp;nbsp; to see if it is what you want to know.