RF Link Budget and Friis Transmission Equation : A Complete Guide

In wireless communication, signal reliability depends on efficient link planning. RF link budgets and the Friis transmission equation are essential tools for engineers to evaluate system performance. This guide simplifies these concepts, demonstrating their significance in ensuring seamless wireless communication. Example of link budget for Ku band satellite system in star topology has also been mentioned.

Wireless system usually made of various subsystems which incorporate gains and losses as the signal travels from the transmit end to the receive end. For example amplifier will introduce gain to the signal,attenuator or cable or free space introduces different amount of losses. Antenna will provide gain.

Calculations of the various parameters of the RF link to make it function with optimum BER or Eb/N0 is often called as RF Link Budget. Mainly RF engineers determine HPA/SSPA power requirement at transmit end taking into consideration various system specifications before deployment.

In simple terms all in decibel,
Power at receiver= Power at transmitter + Gains + Losses

Friis transmission equation: The equation is outlined below .
Pr=Pt x Gt x Gr x (Lambda/4xpixR)² , in linear form
Where,
Pt= Transmit power
Pr= Receive power
Gt= Transmitter antenna gain
Gr= Receiver antenna gain
R= Distance between two antennas.
pi=22/7

Pr= Pt + Gt + Gr + 20 Log₁₀(Lambda/4xpixR), in decibel form

RF Link Budget calculation for Ku band satellite system

Let us understand RF link budget with example of star configuration based VSAT system utilizing Ku band satellite. Both VSATs are of 2.4 meters in diameter. Input parameters are information data(information rate,FEC rate,modulation, BER, Eb/N0), satellite data (latitude,longitude, azimuth, elevation), various attenuations,Earth station data for VSAT1 and VSAT2.Output parameters are (C/N)uplink, (C/N)downlink, (C/N)total and HPA power required.Following table lists all these in a typical RF link budget calculation.
➨Download excel sheet from RF wireless world web site's download section.

Conclusion

By mastering the RF link budget and Friis transmission equation, engineers can design robust wireless systems. These tools ensure optimal signal strength and system reliability.

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