Relation between Modulation, Capacity & S/N ratio in Microwave Links

  • As modulation order increasing, number of symbols increase too.

  • Tthen number of bits transmitted per symbol also increase.

  • But the probability of interference and errors increasing too.

  • To solve this issue we have to use adaptive modulation.

  • S/N ratio is an important paramters that represents the ratio between signal power to noise power.

  • It’s , measured at Rx side of the Link and it should satisfy the planned value to ensure the good performance.

  • As you increasing the modulation order of the Link, the required S/N ratio increasing too.

  • And also the Rx threshold ( signal strength of the power at Rx end ) also increase.

  • So , if you need to increase the link data rate / capacity you can increase the modulation order but you have to think also about the Rx threshold.

  • For example : at 4Qam , the Rx strength required is -85 dbm while at 1024 QAM we have to guarantee -60 dbm.

  • So we can conclude that, as modulation increase , capacity of Link increase , bit errors also increase , Rx threshold required increase.

  • At higher modulations , we got higher noise at Rx side so we have to guarantee enough Rx level to allow the receiver distinguish the signal from noise coupled with it.

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