Link budget
An accounting of gains and losses in a telecommunication system.
A link budget is a complete tally of all power gains and losses a communication signal encounters as it travels from a transmitter, through a medium like radio waves, cables, waveguides, or optical fibers, to a receiver. It takes the form of an equation that calculates the received power based on the transmitter's output power, the attenuation from propagation, antenna gains, feedline and other losses, and any amplification from the receiver or repeaters along the path. Engineers use a link budget as a design tool when planning a communication system to verify that the received power is sufficient for the signal to be understood with an adequate signal-to-noise ratio.
In practice, many losses must be estimated and can vary, so a link margin is included as a safety buffer between the expected received power and the minimum power the receiver needs to detect the signal accurately. The size of this margin depends on how severe a communication dropout is expected to be, and it can be reduced by techniques like antenna diversity or multiple-input and multiple-output (MIMO).
A basic link budget equation is: Received power (dBm) = transmitted power (dBm) + gains (dB) − losses (dB). Power levels are in dBm, while gains and losses are in decibels (dB), a logarithmic scale where adding decibels multiplies the actual power ratios.
For a wireless radio system, a more detailed link budget equation might include: received power equals transmitter output power, plus transmitter antenna gain, minus transmitter losses (such as from coax and connectors), minus path loss, minus miscellaneous losses (like fading margin, body loss, or polarization mismatch), plus receiver antenna gain, minus receiver losses. Path loss, the signal reduction due to propagation between antennas, is usually the largest loss and the biggest unknown. In free space, it can be expressed by normalizing distance to wavelength, and when substituted into the link budget, this gives the logarithmic form of the Friis transmission equation. Sometimes it is convenient to treat distance and wavelength losses separately, but the units matter because each choice has a different constant offset.
- Path loss (free space) constant for mhz
- 36.6 dB
- Indoor wall loss at 2.4 ghz (2x4 stud wi
- 6 dB per wall
- Indoor los range in dense office
- about 3 meters
- Indoor propagation loss beyond 3 m
- up to 30 dB per 30 meters
- Voyager path loss (as of 2002)
- 308 dB
Lore & Background
The link budget equation expresses received power as transmitted power plus gains minus losses, with power levels in dBm and gains/losses in decibels. In radio systems, the path loss between transmitting and receiving antennas is usually the most significant contributor to losses and the largest unknown. For free-space transmission, path loss can be expressed in dimensionless form by normalizing distance to wavelength, and when substituted into the link budget equation yields the logarithmic form of the Friis transmission equation. Alternative forms of path loss exist depending on the units used for distance and frequency, each involving a differing constant offset.
In non-line-of-sight links, diffraction and reflection losses become important since the direct path is not available. Building obstructions such as walls and ceilings cause propagation losses indoors to be significantly higher, with a typical 2-by-4 wood stud wall with drywall on both sides resulting in about 6 dB loss per wall at 2.4 GHz. Experience has shown that in dense office environments, line-of-sight propagation holds only for about the first 3 meters, beyond which propagation losses indoors can increase at up to 30 dB per 30 meters.
A link margin is specified as a safety margin between the received power and the minimum power required by the receiver to accurately detect the signal. The link margin is chosen based on the anticipated severity of a communications dropout and can be reduced by the use of mitigating techniques such as antenna diversity or multiple-input and multiple-output (MIMO).
Reader's Guide
The link budget is a fundamental design tool for telecommunication systems, serving as the primary method for determining whether a received signal will have adequate strength for intelligible communication. Its significance lies in its ability to account for all gains and losses in a system, from transmitter output power through antenna gains, feedline losses, path loss, and miscellaneous losses such as fading margin, body loss, and polarization mismatch. The path loss component is typically the largest unknown and most significant contributor, making its accurate estimation critical for system design.
In practical applications, the link budget has enabled extreme communications links such as Earth–Moon–Earth communications, where the return distance of 770,000 kilometers and low lunar albedo (maximally 12% but usually closer to 7%) result in path losses around 250 to 310 dB, requiring high power and high-gain antennas. The Voyager program spacecraft have the highest known path loss at 308 dB as of 2002, with the Deep Space Network maintaining the link through improvements including increasing antenna size from 64m to 70m for a 1.2 dB gain and upgrading to low noise electronics for a 0.5 dB gain. The link budget concept also applies to guided media such as coaxial cable and optical fiber, where losses are exponential with distance and specified in dB per unit distance, with a crossover distance beyond which guided medium loss exceeds that of a line-of-sight path of the same length.
Did You Know?
- The Voyager program spacecraft have the highest known path loss of any telecommunications circuit, at 308 dB as of 2002.
- Earth–Moon–Earth communications have a path loss of around 250 to 310 dB depending on the VHF-UHF band used.
More in Radio Propagation 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced