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What is Fading?

In radio communication, large variation of received power with location, time and frequency. In multipath fading, waves reflected by buildings and other objects arrive over paths of different lengths and interfere, and received power can drop by several tens of dB. The variation becomes faster when the terminal moves. Countermeasures include diversity, OFDM and error-correcting codes.

Japanese name: フェージング

Fading appears in 2 practice questions across 1 topic, including Wireless Communication Basics.

Field: Wireless Communication Basics

Try it: questions about Fading

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.

In radio communication, which is the phenomenon in which waves reflected by buildings and other objects arrive late because of differing path lengths, interfere with the directly arriving wave, and cause the received power to drop sharply at certain locations or frequencies?

  1. ADoppler effect
  2. BMultipath fading
  3. CDiffraction
Show answer and explanation

Answer: B. Multipath fading

Radio waves are reflected and diffracted by buildings and the ground, arriving over multiple paths of different lengths (multipath). Because the phase differs from path to path, there are places where the waves reinforce each other and places where they cancel, and the received power can drop by several tens of dB. This is called multipath fading, and when a terminal moves, the received power changes rapidly over time. Countermeasures include diversity reception with multiple antennas, schemes such as OFDM that split transmission across many frequencies, and error-correcting codes.

Fading in the explanations

Questions that mention Fading (2)

  1. 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?
  2. In radio communication, which is the phenomenon in which waves reflected by buildings and other objects arrive late because of differing path lengths, interfere with the directly arriving wave, and cause the received power to drop sharply at certain locations or frequencies?

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Related terms

Link budget OFDM (orthogonal frequency-division multiplexing) 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)

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