Suppose the FCC allocates 300 kHz channels for a high quality video conference service. A service provider decides that this service requires 720 kbps (not including overhead and error control) for each channel.
Suppose that the target error rate required is 10-5. Estimate the Signal-to-Noise Ratio (SNR) required at the receiver by using the following procedure:
(i) Determine the SNR needed for BPSK from the error rate curves. It turns out that the probability of error for QPSK is the same as that for BPSK. (This is because the distance between nearest QPSK constellation points is the same as the distance between the BPSK points.)
(ii) Doubling the number of points in a PSK constellation with more than 2 points requires approximately 6 dB more transmit power to maintain
the same error rate. (That is, 8-PSK requires about 6 dB more than QPSK. This is due to the decrease in distance between nearest neighbor points.) Doubling the number of points in a QAM (rectangular) constellation requires approximately 3 dB more transmit power. (That is, 16 QAM requires approximately 6 dB more than QPSK, 32 QAM requires approximately 3 dB more than 16 QAM, and so forth.)