How do you calculate spectral efficiency?

1.Find the Fourier Transform of the pulse used to transmit data over the channel.

2.Determine the bitrate of the signal by the modulation format (QPSK for example has 2bits/symbol so 1 symbol per second would equate to 2 bits/s)

3.The first null in the Fourier transform is the required bandwidth (~0.75 x bitrate in optical communications, depends on channel)

4. Divide bits/s by the required bandwidth to find the spectral efficiency.