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What is Link budget?

A calculation that adds up the gains and losses from transmit power to received power to estimate the power reaching the receiver and the margin over the power needed for reception (the sensitivity). It is done in dB and dBm by adding and subtracting terms such as transmit power + antenna gains − propagation loss − cable loss. The same approach is used for power budgets in optical communication as well as radio.

Japanese name: リンクバジェット

Link budget appears in 2 practice questions across 2 topics, including Silicon photonics, Wireless Communication Basics.

Field: Wireless Communication Basics

The calculation steps

First, add the transmit and receive antenna gains (dBi) to the transmit power (dBm) and subtract the propagation loss and cable and connector losses (dB) to get the received power. Next, find the sensitivity from the receiver's noise figure and bandwidth, and take the difference from the received power as the link margin. For example, with 20 dBm transmit power, 2 dBi of gain at each antenna, 100 dB of propagation loss and 4 dB of cable loss, the received power is −80 dBm, giving a 10 dB margin if the sensitivity is −90 dBm.

Why margin is included

Actual propagation loss is generally larger than free-space path loss, and there is also blockage by buildings and people, attenuation by rain and temporary drops due to fading. Designs therefore include margins such as a fading margin to meet a target probability of keeping the link up. If the margin is insufficient, countermeasures include raising transmit power or antenna gain, lowering the receiver's noise figure, or lowering the transmission rate to reduce the required S/N.

The advantage of working in dB

Using dB, which expresses power ratios logarithmically, and dBm, referenced to 1 mW, turns multiplication and division of powers spanning many orders of magnitude into addition and subtraction. Quantities differing by more than ten orders of magnitude, such as a transmitter of several watts and received power below 1 pW, fit in the same table, and it is easy to see at a glance where losses are large.

Try it: questions about Link budget

Laser light with an output of +3 dBm reaches a photodetector after a total loss of 10 dB (fiber coupling, waveguides, modulator, etc.). Which is closest to the received power?

  1. AAbout 0.2 mW (−7 dBm)
  2. BAbout 20 mW (+13 dBm)
  3. CAbout 0.05 mW (−13 dBm)
Show answer and explanation

Answer: A. About 0.2 mW (−7 dBm)

dBm is an absolute power unit referenced to 1 mW: 0 dBm is 1 mW, +3 dBm is about 2 mW, and −10 dBm is 0.1 mW. Losses can simply be subtracted in dB: 3 − 10 = −7 dBm, and 10^(−0.7) ≈ 0.2 mW. This calculation—subtracting losses from the transmit power and checking that the result exceeds the minimum power the photodetector needs (receiver sensitivity)—is called a link budget (power budget). +13 dBm is the result of adding the loss, and −13 dBm results from getting the sign of the transmit power wrong.

With a transmit power of 20 dBm, a transmit antenna gain of 2 dBi, a receive antenna gain of 2 dBi, a propagation loss of 100 dB and cable losses on the transmit and receive sides totaling 4 dB, which is the received power?

  1. A−88 dBm
  2. B−80 dBm
  3. C−76 dBm
Show answer and explanation

Answer: B. −80 dBm

Received power is found by adding the antenna gains (dBi) to the transmit power in dBm and subtracting the propagation and cable losses (dB). This calculation is called a link budget: 20 + 2 + 2 − 100 − 4 = −80 dBm. If the minimum power the receiver can receive correctly (its sensitivity) is, for example, −90 dBm, there is a 10 dB margin (link margin). In practice, designs include margin for drops in received power due to fading.

FAQ

How is the sensitivity determined?

Adding the bandwidth (10·log₁₀ B) and the receiver's noise figure to the thermal noise power density at 290 K, −174 dBm/Hz, gives the input-referred noise power. Adding the S/N needed for demodulation gives an estimate of the sensitivity. With a 1 MHz bandwidth, a 6 dB noise figure and a required S/N of 10 dB, it is −174 + 60 + 6 + 10 = −98 dBm.

Is it the same as a power budget in optical communication?

The idea is the same. In optical communication, the power reaching the photodetector is found by subtracting fiber, waveguide, connector and coupling losses (dB) from the laser output (dBm), and the margin over the detector's sensitivity is checked. Unlike radio there are no antenna gains, but the procedure of adding and subtracting in dB and dBm is shared.

Link budget in the explanations

Questions that mention Link budget (2)

  1. Laser light with an output of +3 dBm reaches a photodetector after a total loss of 10 dB (fiber coupling, waveguides, modulator, etc.). Which is closest to the received power?
  2. With a transmit power of 20 dBm, a transmit antenna gain of 2 dBi, a receive antenna gain of 2 dBi, a propagation loss of 100 dB and cable losses on the transmit and receive sides totaling 4 dB, which is the received power?

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Related terms

Fading Beamforming Bluetooth Low Energy (BLE) Direct-conversion receiver Free-space path loss (FSPL) Hidden node problem LPWA (low-power wide-area) MIMO (multiple-input multiple-output) NFC (near-field communication) Noise figure (NF)

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