Wireless Networking Fundamentals

08

Link budget

At the preliminary stage of the wireless link organization, it is recommended to perform the link budget calculation, it shows the radio signal energy during propagation from the transmitter to the receiver. It allows to estimate the potential link parameters and to select the appropriate equipment and its configuration. In line-of-sight propagation conditions, seven sections where the signal energy changes can be dedicated, these sections are shown in Figure 1.

Figure 1 Energy conversion scheme of a radio signal during propagation

The following parameters are used to calculate the link budget:

Let's look at the example of link budget calculation to explain the energy transformations process:

  • The transmitter generates a 20 dB signal and transmits it to the cable that connects the transmitter and antenna;
  • During the signal propagation in the cable, some of its energy is converted to heat, i.e. the antenna receives an attenuated signal. The signal loss in the cable is 2 dB;
  • The length and characteristics of the cable on the receiving and transmitting sides are equal;
  • After that, as it was shown in section 2. Radio signal propagation fundamentals, part of the signal energy is lost during propagation in free space. The distance between the receiving and transmitting antennas is 4.74 km, and the central frequency is 5 GHz;
  • The receiving antenna converts the radio signal energy into a current and transmits it through the cable to the receiver. Same antennas with 16 dB gain are used on the receiving and transmitting sides;
  • An important receiver parameter is the sensitivity - the possibility of receiving weak signals. I.e. if the received signal power is below the sensitivity level, the signal will not be recognized. The receiver sensitivity is -82 dB.

Let's calculate losses during propagation in free-space:

The signal power at the receiver input is:

Fade margin:

Thus, the signal power at the receiver input exceeds the sensitivity by 10 dB. When building a wireless link, it is necessary to make the energy reserve due to the quality of alignment, radio environment parameters, etc.

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