Which statement best describes the effect of increasing modulation order on spectral efficiency when comparing QPSK to BPSK?

Which statement best describes the effect of increasing modulation order on spectral efficiency when comparing QPSK to BPSK?

• A. QPSK has lower spectral efficiency than BPSK.
• B. QPSK has higher spectral efficiency due to more phase states.
• C. QPSK uses more bandwidth than BPSK.
• D. QPSK has the same spectral efficiency as BPSK.

Answer: (B)

The main idea is how many bits you can pack into each symbol, which directly affects spectral efficiency. In quadrature phase-shift keying, there are four distinct phase states, so each symbol carries 2 bits. In binary phase-shift keying, there are only two phase states, so each symbol carries 1 bit. Spectral efficiency is data rate per unit bandwidth. If you keep the symbol rate the same, QPSK delivers twice as many bits per second as BPSK, because it carries 2 bits per symbol instead of 1. That means higher spectral efficiency. Conversely, to transmit the same data rate with BPSK you’d need a higher symbol rate, which would require more bandwidth, while with QPSK you can achieve the same data rate with less bandwidth, or deliver more data within the same bandwidth. So the statement that QPSK has higher spectral efficiency due to having more phase states is the best description.

The main idea is how many bits you can pack into each symbol, which directly affects spectral efficiency. In quadrature phase-shift keying, there are four distinct phase states, so each symbol carries 2 bits. In binary phase-shift keying, there are only two phase states, so each symbol carries 1 bit.

Spectral efficiency is data rate per unit bandwidth. If you keep the symbol rate the same, QPSK delivers twice as many bits per second as BPSK, because it carries 2 bits per symbol instead of 1. That means higher spectral efficiency. Conversely, to transmit the same data rate with BPSK you’d need a higher symbol rate, which would require more bandwidth, while with QPSK you can achieve the same data rate with less bandwidth, or deliver more data within the same bandwidth.

So the statement that QPSK has higher spectral efficiency due to having more phase states is the best description.