---
title: "Training and tracking modes of adaptive equalizer"
subject: "Wireless Communication"
chapter: 5
key: "semester_7/wireless_communication:adaptive-equalizer-training-tracking"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/adaptive-equalizer-training-tracking
site: BCT Engineering
---

# Training and tracking modes of adaptive equalizer

Wireless Communication · Chapter 5: Equalization and Diversity

- Asked in: 2078 Chaitra, 4 marks
- Video: [Wireless Technology | Adaptive Equalizer](https://www.youtube.com/watch?v=04ltaflxFB8) — RANJI RAJ
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/adaptive-equalizer-training-tracking
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5

## Question

What is training and tracking mode in an adaptive equalizer?

## Short answer

Training mode uses a known sequence sent by the transmitter; the receiver computes the exact error and adjusts equalizer taps to near-optimum. Tracking mode follows, where the equalizer uses its own decisions as reference to continuously update taps, adapting to slow channel changes until the next training burst.

## Answer

An adaptive equalizer works in two modes.

#### Training mode
- The transmitter sends a **fixed-length, known training sequence** (e.g. a pseudo-random binary pattern or the 26-bit midamble in GSM) before or within each data burst.
- The receiver knows this sequence, so it computes the error e_k = x_k − d̂_k exactly and adjusts the tap weights (e.g. by LMS) towards their optimum.
- The training sequence is designed so that the equalizer can converge even in the worst channel; at the end of training the taps are near optimal for the current channel.

#### Tracking mode
- When user data follows, there is no known reference, so the equalizer uses its own **decisions** x̂_k (decision-directed adaptation): e_k = x̂_k − d̂_k.
- It continuously follows slow changes in the channel while data is being received.
- If the channel changes too much, decisions become wrong, so training is repeated periodically. In TDMA systems this happens in every time slot.

```text
 |Training|  User data (tracking)  |Training| Data ...
 <-known->                          <-known->
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In training mode, what does the equalizer use as reference?
   A. Its own previous decisions
   B. Random noise samples
   C. A known training sequence sent by the transmitter
   D. The pilot of another user
   Answer: C. Because the sequence is known, the error is computed exactly and the taps move toward optimum. (Key point: Training mode)

2. In tracking mode during user data, how is the error formed?
   A. From the known midamble
   B. From the transmitter feedback link
   C. It is set to zero
   D. From the equalizer's own decisions: e = x_hat - d_hat
   Answer: D. No reference is known, so decisions are used (decision-directed adaptation). (Key point: Tracking mode)

3. Why is training repeated periodically (e.g. each TDMA slot)?
   A. If the channel changes too much, decisions become wrong
   B. The training sequence expires
   C. Tap weights are lost after each slot
   D. The equalizer must save power
   Answer: A. Wrong decisions would corrupt adaptation, so a known sequence resets the taps. (Key point: Retraining)

## Related in Wireless Communication

- [Interleaving and adaptive equalization of ISI](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ainterleaving-adaptive-equalizer) — Wireless Communication, chapter 5

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