Topic: Satellite Communication
The satellite communication link between two point is established with uplink carrier-to-noise ratio of 20 dB and downlink carrier to noise ratio of 14 dB. The overall C/N is close to:
- A. 34 dB
- B. 6 dB
- C. 13 dB
- D. 13.5 dB
Answer: C. 13 dB
Explanation: The overall C/N ratio is calculated using the reciprocal formula: $\frac{1}{(C/N)_{overall}} = \frac{1}{(C/N)_{up}} + \frac{1}{(C/N)_{down}}$. Converting dB to linear ratios: $(C/N)_{up} = 10^{(20/10)} = 100$, $(C/N)_{down} = 10^{(14/10)} = 25.12$. $\frac{1}{(C/N)_{overall}} = \frac{1}{100} + \frac{1}{25.12} = 0.01 + 0.0398 = 0.0498$. So, $(C/N)_{overall} = \frac{1}{0.0498} \approx 20.08$. Converting back to dB gives $10\log_{10}(20.08) \approx 13$ dB.