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RF Link Budget

This tutorial on VSAT covers following.

• RF Terminology
• Satellite Terminology
• VSAT system overview
• FDMA versus TDMA versus CDMA
• RF Transceiver basics
• C band RF Transceiver Design and Development
• Microstrip basics and types
• Design of RF frequency converter
• RF Power Amplifier
• what is modulation
• satellite Modem design
• RF Link Budget

This page describes RF link budget,Friis transmission equation and explain RF link budget with Ku band satellite system example in star mode.

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

rf link budget

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

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.


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