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A satellite system employs QPSK modulation with 40% excess bandwidth per carrier including guard band. The voice channels use 64 kbps PCM coding. The no. of channels supported by 36 MHz bandwidth of the transponder in bandwidth limited case will be
1. 1000
2. 800
3. 900
4. 600

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Correct Answer - Option 2 : 800

Concept:

QPSK is a modulation scheme that allows one symbol to transfer two bits of data. There are four possible two-bit numbers (00, 01, 10, 11).

\(Bandwidth = \frac{{2{R_b}}}{{{{\log }_2}M}}\)

Analysis:

Transmission bandwidth

\(= \frac{{2{R_b}}}{{{{\log }_2}4}} = {R_b}\)

Maximum required bandwidth

\(= \frac{{{R_b}}}{2}\)

40% of \(\frac{{{R_b}}}{2} = 0.2\;{R_b}\)

Total = (0.2 + 0.5) Rb = 0.7 Rb

No. of channels required

\( = \frac{{36 \times {{10}^6}}}{{0.7 \times 64 \times {{10}^3}}} \approx 800\)

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