How to compute data rate I can reach with my design?

Question

I implemented a modulator for a transmitter, which sends baseband modulated signal.

The transmitter has the following parameters:

1. datarate : 520 Ksps (min), 64,44 Msps (max)
2. Bandwidth: BW = 56 MHz (max)
3. 47 MHz to 6 GHz

Of course, with my design probably I can not reach max data rate, that is why I want to compute the data rate which I can design with my design in this transmitter.

The modulator:

1. oversampling ratio \$V = 4\$
2. nonlinear modulation

I remember I can compute datarate \$R\$ by using the following equation $$ R= 2 \cdot BW \cdot \log_2 M, $$ where \$M\$ is modulation level. In my case, I don't know the modulation level.

Do I have another option to compute the data rate I can reach?

Answer 1

You need to define \$M\$. It's not something you can calculate from anything else that you're given.

Obviously, increasing it gives you more data per symbol, which gives you more data per second, but that only works until the differences between the symbols are small enough compared to noise that your error rate becomes higher than you can, including your channel coding, tolerate.

So, you will need to define:

1. Worst-case SNR you still need to work at
2. Maximum acceptable error rate at that SNR. This will depend on the channel code you choose and your application's needs. There's never a 0-error transmission, that's mathematically impossible in the presence of noise, so you will need to sit down and come up with a number like "It's OK if one in xxxxx bits is wrong"
3. A BER curve for each of your candidates for \$M\$, which relates error rate to SNR.
4. Pick an \$M\$ that still achieves the error rate at the given SNR.

Often, you have to try a few combinations between modulation methods, \$M\$ and channel codes.