Radio Link Budget: details & formula

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When designing a complete, i.e. end to end radio communications system, it is necessary to calculate what is termed the radio link budget.

The link budget is a summary of the transmitted power long with all the gains and losses in the system and this enables the strength of the received signal to be calculated.

Using this knowledge it is possible to determine whether power and gain levels are sufficient, too high, or too low and then apply corrective action to ensure the system will operate satisfactorily.

This ensures that once the system is installed and is ready for operation, there will be sufficient signal for it to operate correctly, or whether the signal is too even high and action can be taken to save costs..

Larger than required antennas, high transmitter power levels and the like can add considerably to the cost, so it is necessary to balance these to minimise the cost of the system while still maintaining performance.

Link budgets are used in many applications from satellite links to mobile phone systems, HF radio links and many more.

Link budget style calculations are also used within wireless survey tools. These wireless survey tools will not only look at the way radio signals propagate, but also the power levels, antennas and receiver sensitivity levels required to provide the required link quality.

Radio link budget – the basics

As the name implies, a radio link budget is a summary of all the gains and losses in a transmission system. The radio link budget sums the transmitted power along with the gains and loses to determine the signal strength arriving at the receiver input. The link budget may include the following items:

Where the losses may vary with time, e.g. fading, and allowance must be made within the link budget for this - often the worst case may be taken, or alternatively an acceptance of periods of increased bit error rate (for digital signals) or degraded signal to noise ratio for analogue systems.

In essence the link budget will take the form of the equation below:

The basic calculation to determine the link budget is quite straightforward. It is mainly a matter of accounting for all the different losses and gains between the transmitter and the receiver.

Once the link budget has been calculated, then it is possible to compare the calculated received level with the parameters for the receiver to discover whether it will be possible to meet the overall system performance requirements of signal to noise ratio, bit error rate, etc.

Radio link budget formula

In order to devise a radio link budget formula, it is necessary to investigate all the areas where gains and losses may occur between the transmitter and the receiver. Although guidelines and suggestions can be made regarding the possible areas for losses and gains, each link has to be analysed on its own merits.

A typical link budget equation for a radio communications system may look like the following:

Where:
    P_RX  = received power (dBm)
    P_TX  = transmitter output power (dBm)
    G_TX  = transmitter antenna gain (dBi)
    G_RX  = receiver antenna gain (dBi)
    L_TX  = transmit feeder and associated losses (feeder, connectors, etc.) (dB)
    L_FS  = free space loss or path loss (dB)
    L_P  = miscellaneous signal propagation losses (these include fading margin, polarization mismatch, losses associated with medium through which signal is travelling, other losses...) (dB)
    L_RX  = receiver feeder and associated losses (feeder, connectors, etc.) (d)B

NB for the sake of visibility, the losses in the link budget equation is shown with a negative sign e.g. LTX or LFS, etc. When entering the figures into the radio link budget formula, the figure should be entered as the modulus of the loss. In this way they will be subtracted and not added to the figure.

Antenna gain & radio link budget

The basic link budget equation where no levels of antenna gain are included assumes that the power spreads out equally in all directions from the source, i.e. from an isotropic source, an antenna that radiates equally in all directions.

This assumption is good for many theoretical calculations, but in reality all antennas radiate more in some directions than others. In addition to this it is often necessary to use antennas with gain to enable interference from other directions to be reduced at the receiver, and at the transmitter to focus the available transmitter power in the required direction.

In view of this it is necessary to accommodate these gains into the link budget equation as they have been in the equation above because they will affect the signal levels - increasing them by levels of the antenna gain, assuming the gain is in the direction of the required link.When quoting gain levels for antennas it is necessary to ensure they are gains when compared to an isotropic source, i.e. the basic type of antenna assumed in the equation when no gain levels are incorporated. The gain figures relative to an isotropic source are quoted as dBi, i.e. dB relative to an isotropic source. Often gain levels given for an antenna may be the gain relative to a dipole where the figures may be quoted as dBd, i.e. dB relative to a dipole. However a dipole has gain relative to an isotropic source, so the dipole gain of 2.1 dBi needs to be accommodated if figures relative to a dipole are quoted for an antenna gain.

Link budget calculations are an essential step in the design of a radio communications system. The link budget calculation enables the losses and gains to be seen, and devising a link budget enables the apportionment of losses, gains and power levels to be made if changes need to be made to enable the radio communications system to meet its operational requirements. Only by performing a link budget analysis is this possible.

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