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AEiE0902 Equalization and diversity techniques

Core purpose

  • Equalization combats channel distortion, especially ISI.
  • Diversity combats fading by providing independently faded replicas of the signal.
  • Equalization and diversity are complementary, not identical.

Equalization basics

  • Equalization attempts to undo the channel effect at the receiver.
  • A simple equalizer approximates the inverse channel response over the signal band.
  • It is especially relevant when delay spread makes symbols smear into adjacent symbols.
Equalizer type Use case Cue
Linear equalizer Moderate ISI Simpler but can amplify noise
Decision-feedback equalizer Severe postcursor ISI Uses previous decisions
Adaptive equalizer Time-varying channel Coefficients update automatically

A decision-feedback equalizer uses previously detected symbols to cancel postcursor ISI caused by a dispersive or frequency-selective channel. It is not a generic remedy for flat or deep fading; diversity is the direct fading countermeasure.

Adaptive equalization

  • Adaptive equalizers adjust coefficients in real time.
  • A training sequence or decision-directed mode is commonly used.
  • LMS and RLS are common adaptive algorithms in broader communication study.
  • Adaptive equalization is important in mobile or changing channels.

Diversity principle

  • Diversity improves reliability by combining independently faded versions of the same information.
  • Independence or low correlation between branches is the key benefit.
  • Diversity does not require exact inversion of the channel like equalization does.

Diversity types in syllabus

Diversity type Distinguishing idea Typical cue
Space diversity Multiple separated antennas Antenna spacing
Polarization diversity Different wave polarizations Vertical versus horizontal or dual-pol
Frequency diversity Same information on separated frequencies Frequencies fade differently
Time diversity Repeat or code across time Interleaving or retransmission

Equalization versus diversity

Question Equalization Diversity
Main problem attacked ISI and channel distortion Deep fades
Core mechanism Channel compensation Multiple independently faded paths
Best cue Filter coefficients Multiple branches, copies, or carriers

Combining ideas

  • Equalization is common in dispersive channels.
  • Diversity is common in fading channels.
  • Real systems often use both equalization and diversity, plus coding.

Recognition cues for equalization and diversity

  • "Adaptive filter at receiver" suggests adaptive equalizer.
  • "Several antennas at receiver" suggests space diversity.
  • "Transmit same data on multiple frequencies" suggests frequency diversity.
  • "Delayed repeated copies with coding/interleaving" suggests time diversity.

Common traps in equalization and diversity

  • Diversity is not the same as power increase.
  • Polarization diversity uses differently polarized branches, not merely more gain.
  • Equalization may improve BER but its immediate target is channel distortion.
  • Frequency diversity requires frequency separation beyond highly correlated fading.

One-step examples for equalization and diversity

  • If a mobile channel changes with movement, adaptive equalization is more appropriate than a fixed equalizer.
  • Two antennas spaced so they do not fade together illustrate space diversity.
  • Repetition across time slots is time diversity, not frequency diversity.

Equalization-and-diversity revision box

Equalization repairs channel distortion; diversity supplies alternate versions of the signal. Adaptive equalizers track changing channels. Space, polarization, frequency, and time diversity are different ways to decorrelate fades.