Discussion
In order to transmit at 4·8 kbps over a channel with 3·2 kHz bandwidth, which would be most suitable ?
Answer: B
The most suitable choice is QPSK.
With Nyquist signalling, a channel of bandwidth B can carry at most 2B symbols per second in theory, and in practice, with filtering, about B symbols per second. The 3.2 kHz channel therefore supports about 3.2 kbaud. Each scheme needs:
- BPSK, 1 bit per symbol: 4.8 kbaud, more than the channel carries;
- QPSK, 2 bits per symbol: 2.4 kbaud, which fits with margin;
- 8-PSK, 3 bits: 1.6 kbaud; 16-PSK, 4 bits: 1.2 kbaud.
Higher-order PSK would also fit, but its constellation points are closer together and it needs a much higher signal to noise ratio for the same error rate. QPSK is the simplest scheme whose symbol rate fits the band, which makes it the most suitable.
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