1. For QAM modulations how to obtain the bitrates with the used channel spacing, i have been searching but mathematically couldn`t find an equation that gives me the exact bitrate for example how to find the following bitrates for CS 28MHZ: ???
  • 4QAM : 44MBps
  • 16QAM : 87MBps
  • 32QAM : 109Mbps

these bitrates are for standard modulation not strong. If someone can give me a numerical application of how to find the bit rates please

  1. In order to generate many symbols we change phase and frequency of signal in order to obtain more symbols (16QAM, 32QAM, 64QAM, …), how is that possible while we have only have one carrier signal?
  • $\begingroup$ channel spacing? Which channel spacing? Channel spacing, aside from limiting the maximum bandwidth, which might loosely translate to a maximum symbol rate, has no direct relationship to bit rate. $\endgroup$ Jul 12, 2022 at 12:53
  • $\begingroup$ 2. we change the carrier. This is really "explain complex baseband to me", and that alone is a question that's a bit too broad, so let's have not two question in one question. $\endgroup$ Jul 12, 2022 at 12:54
  • $\begingroup$ I am a microwave planner, in microwave transmission links QAM is used, when i increase the CS the datarate is increased for same QAM modulation how can this be explained ? I guess that CS can be seen as a pipe the wider it is the more data is transmitted, but using which equation the datarate cen be calculated ? i Know that Datarate= baud rate*number of transmitted symbols per second $\endgroup$
    – Farah
    Jul 12, 2022 at 13:29
  • $\begingroup$ no, when you increase the channel spacing, you do not inherently increase data rate. increasing the channel spacing might allow you to increase symbol rate. $\endgroup$ Jul 12, 2022 at 15:33

1 Answer 1


It's not a hard-edged relationship.

Once you've established the channel spacing and modulation method, there's still a huge cost vs. bit-rate tradeoff.

Mostly this is in the filters -- filters with sharp cutoffs just cost more. To some extent it's in the required stability of the carrier frequencies of the devices -- again, more precise oscillators cost more. This isn't much of a consideration for the services I know of, but it used to be, and an unthinking system designer could make it so again.

So there's system design to be done outside of just considering the modulation method and the channel spacing.


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