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Link Budget Analysis

In the design of wireless communications links between two sites, issues of range, throughput, and received signal quality are of critical importance to the system engineer. Link budget analysis accounts for all gains and losses in the communication link. Some factors and design choices, such as propagation path length, signal polarization, and antenna feed cable, degrade signal quality, while others, such as the power amplifier and antenna size, can increase transmitted signal strength.

This example uses linkBudgetAnalyzer app to tabulate system parameters and compute gains and losses that impact system performance. Separate tabs specify settings for Uplink and Downlink. After specifying the uplink and downlink settings, select Analyze to update the gains and losses reported in the Results tab and the tabs with plots of free space path loss (FSPL) and G/T for uplink and downlink.

Settings

The Uplink and Downlink tabs contain these collapsible input parameter sections:

  • Link - Contains link level parameters, such as frequency, bandwidth, required Eb/N0

  • Transmitter - Contains transmitter specific parameters

  • Receiver - Contains the receiver specific parameters

  • Propagation - Contains parameters to specify various atmospheric elements that are used to compute losses in the signal propagation path.

Results

The Results tab contains Uplink and Downlink collapsible sections that provide the link budget results for uplink and downlink, respectively. The Appendix contains a list of functions used to compute the results.

  • Distance and Elevation are computed from the Latitude, Longitude and Altitude input parameters for the transmitter and receiver.

  • Tx Antenna gain and Rx Antenna gain are functions of the corresponding Antenna diameter, Antenna efficiency, and Frequency input parameters.

  • Tx EIRP is a function of the Amplifier power, Amplifier backoff loss, Feeder loss, Radome loss, Other losses, and Tx Antenna gain input parameters. Tx EIRP, which represents transmitted equivalent isotropically radiated power (EIRP), is the amount of power that would have to be radiated by an isotropic antenna to produce the equivalent power density observed from the actual antenna in a specified direction. Typically, EIRP is quoted for antenna boresight, which is defined as the axis of maximum radiation.

  • The transmitted signal power is diminished by geometric spreading of the wavefront. This loss is represented by Free space path loss which is computed using the fspl function, Distance, and Frequency.

  • Rain attenuation is computed by the rainpl function using Distance, Frequency, Rain rate, Elevation and Polarization tilt. The rainpl function applies the International Telecommunication Union (ITU) rainfall attenuation model which applies only for frequencies at 1-1000 GHz [ 1 ].

  • The fogpl function computes Fog/Cloud attenuation using Distance, Frequency, Fog/Cloud temperature and Fog/Cloud water density. The fogpl function applies the ITU cloud and fog attenuation model which is valid only for frequencies at 10-1000 GHz [ 2 ].

  • Atmospheric gas attenuation is a function of Distance, Frequency, Temperature, Atmospheric pressure and Water vapor density and is calculated using the gaspl function which applies ITU atmospheric gas attenuation model that is valid for frequencies at 1-1000 GHz [ 3 ].

  • Polarization loss is derived from Polarization tilt angle.

  • Total propagation losses consists of all the above-mentioned losses.

  • Tx EIRP is diminished by Total propagation losses to provide Received isotropic power at the receiver.

  • The receiver antenna amplifies the Received isotropic power by Rx Antenna gain to give Received signal power.

  • Rx G/T provides information on the performance of the receiver, as it increases with G/T and is computed from Rx Antenna gain and System temperature.

  • C/N represents SNR (Signal-to-Noise Ratio) and is a function of Received isotropic power, Rx G/T, Bandwidth and Boltzmann's constant.

  • C/No is computed from C/N and Bandwidth.

  • Received Eb/No indicates energy per bit and is a function of C/No and Bit rate.

  • Margin is computed from Received Eb/No, Required Eb/No, and Implementation loss. One goal when performing a link budget analysis is to have a satisfactory margin for the chosen data rate, bandwidth, EIRP and receiver figure of merit. Often some adjustment is needed to get the desired link margin.

Visualization

For path loss and receiver performance plots, see the uplink and downlink FSPL and G/T tabs. Free space path loss constitutes the largest component of propagation losses. It is proportional to distance and frequency. Receiver figure of merit increases with antenna gain, which is proportional to antenna diameter. The specified Frequency and receiver Antenna diameter are shown by the red * marker in the plots.

Appendix

Following functions are used to compute the various parameters and losses mentioned in this example:

References

  1. Radiocommunication Sector of International Telecommunication Union. Recommendation ITU-R P.838-3: Specific attenuation model for rain for use in prediction methods. 2005.

  2. Radiocommunication Sector of International Telecommunication Union. Recommendation ITU-R P.840-6: Attenuation due to clouds and fog. 2013.

  3. Radiocommunication Sector of International Telecommunication Union. Recommendation ITU-R P.676-10: Attenuation by atmospheric gases 2013.