GATE EC 2026 Set 1 — Question 44
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Communications → Digital Modulation & Detection → Digital Modulation (ASK, PSK, FSK, QAM)
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Question
A QPSK modulated signal from an additive white Gaussian noise (AWGN) channel is received with an dB at the input of a coherent QPSK demodulator. A maximum-likelihood reception method is used in the demodulator. Assume the complimentary error functionWhich is the nearest bit error rate (BER) at the output of the demodulator?
Correct answer
(B) 10⁻⁴
Solution
Given dB.
First, convert this to a linear scale:
.For a coherent QPSK demodulator, the bit error rate (BER) is given by:
.Let .
Using the provided approximation for :
.Then, .
The nearest bit error rate is .
First, convert this to a linear scale:
.For a coherent QPSK demodulator, the bit error rate (BER) is given by:
.Let .
Using the provided approximation for :
.Then, .
The nearest bit error rate is .
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