What is the link margin in a system with a transmit power level of 10 W (+40 dBm), a system antenna gain of 10 dBi, a cable loss of 3 dB, a path loss of 136 dB, a receiver minimum discernable signal of -103 dBm, and a required signal-to-noise ratio of 6 dB?
Why Link margin is how many dB the received signal exceeds the minimum usable level. Start with the transmitter: +40 dBm plus 10 dBi of system antenna gain minus 3 dB of cable loss gives +47 dBm of effective radiated power, and subtracting the 136 dB path loss leaves -89 dBm at the receiver. The receiver needs its MDS of -103 dBm plus the 6 dB required SNR, or -97 dBm, to be usable. The difference, -89 minus -97, is 8 dB of margin, and a positive number means the link works.
Watch out The choice of +14 dB comes from comparing the received -89 dBm directly to the -103 dBm MDS and forgetting to add the 6 dB signal-to-noise requirement; negative answers would mean the link fails, which is not the case here.
Add gains, subtract losses to get received power, then subtract (MDS + required SNR). Positive margin = link closes.