Topic: Communication Systems
A satellite system employs QPSK modulation with 40% excess bandwidth per carrier including guard band. The voice channels use 64 kbps PCM coding. The number of channels supported by 36 MHz bandwidth of the transponder in bandwidth limited case will be
- A. 1000
- B. 800
- C. 900
- D. 600
Answer: B. 800
Explanation: $(BW)_{QPSK}=R_{s}(1+\alpha)=1.4 R_{s}$. Since 40% excess bandwidth is used, $\alpha=0.4$.
$R_{s}=\frac{R_{b}}{\log_{2}M}=\frac{R_{b}}{\log_{2}4} = \frac{R_{b}}{2} = 32~\text{kbps}$.
So, BW for each QPSK channel $= 1.4 \times 32~\text{kHz} = 44.8~\text{kHz}$.
Total number of channels possible $= \lfloor \frac{36000}{44.8} \rfloor = 803$. Nearest matching option is 800.