Extra exam question E4D12

WhyA system with 10 W transmit power, 10 dBi antenna gain, 3 dB cable loss, 136 dB path loss, a -103 dBm minimum discernible signal, and a 6 dB required signal-to-noise ratio has a link margin of +8 dB. Received signal level works out to transmit power plus antenna gain minus cable loss minus path loss: +40 dBm + 10 dB - 3 dB - 136 dB = -89 dBm. The signal level actually required for the specified 6 dB signal-to-noise ratio above the -103 dBm minimum discernible signal is -103 + 6 = -97 dBm, so the link margin, how far the received signal exceeds that requirement, is -89 minus -97, or +8 dB, answer C. Skipping the cable loss, mixing up a sign, or forgetting to add the required SNR margin to the MDS would each produce one of the other listed figures instead.