--- title: "Need for equalization and basic technique" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:equalization-need-basic" source: https://www.bctengineer.com/q/semester_7/wireless_communication/equalization-need-basic site: BCT Engineering --- # Need for equalization and basic technique Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2072 Magh, 2+5 marks - Video: [Equalization and diversity techniques Chapter-5 part-1](https://www.youtube.com/watch?v=g6-NXBSGXRY) — IOE Electronics and communication engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/equalization-need-basic - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question Why do we need equalization technique in communication? Explain the basic equalization technique. ## Short answer Equalization is needed to mitigate inter-symbol interference from multipath delay spread, which causes irreducible errors and limits data rate. The basic technique is a transversal filter trained with a known sequence to approximate the channel inverse, then adapted by tracking decisions to maintain a flat overall response. ## Answer #### Need for equalization In mobile channels the transmitted signal arrives over many paths with different delays. When the **delay spread** is comparable to or larger than the symbol period (or the signal bandwidth exceeds the coherence bandwidth), each symbol overlaps the following symbols. This is **inter-symbol interference (ISI)**. ISI causes: - High, irreducible bit error rate that cannot be cured by raising power. - A limit on the maximum data rate of the link. **Equalization** is needed to compensate for this ISI (and for band-limited, frequency-selective channels) so that high data rates can be sent reliably. #### Basic equalization technique The equalizer is a filter at the receiver whose response is the inverse of the channel. If f(t) is the combined impulse response of transmitter, channel and receiver RF/IF, and h_eq(t) is the equalizer's, the target is f(t) * h_eq(t) = δ(t), i.e. F(f) · H_eq(f) = 1 so the overall response is flat and ISI-free. ```text x(t)->[Modulator]->[Channel f(t)]->(+)->[Rx RF/IF] ^ | n(t) v y(t) +-----------------+ +-------------+ d~(k) <----| Decision device |<------| Equalizer | | +-----------------+ d^(k) | h_eq(t) | | +-------------+ +--> e(k) = d(k) - d^(k) --> adjust taps --^ ``` **Steps** 1. **Training:** a known training sequence is sent first. The equalizer compares its output with the known symbols and adjusts its taps (e.g. by LMS) to minimise the error. 2. **Tracking:** user data follows; the equalizer uses its own decisions to keep adjusting as the channel changes. 3. The training is repeated periodically, e.g. each TDMA time slot. **Structure:** the most basic form is a **linear transversal (FIR) equalizer**: a tapped delay line with delays T and tap weights wₖ: d̂(k) = Σₙ wₙ · y(k − n) Non-linear types (decision feedback equalizer, MLSE) are used when the channel has deep spectral nulls. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When does ISI occur? A. Delay spread much smaller than symbol period B. Delay spread comparable to or larger than symbol period C. Only with zero noise D. Only in analog systems Answer: B. Symbols then overlap following symbols. (Key point: ISI cause) 2. Why can raising power not cure ISI? A. ISI vanishes at high power B. Power is limited by regulation C. It causes irreducible BER since ISI scales with signal D. ISI is only noise Answer: C. ISI is an error floor independent of transmit power. (Key point: ISI effect) 3. The target of the basic equalizer is... A. H_eq(f) = F(f) B. H_eq(f) = 0 C. H_eq(f) = noise D. F(f) H_eq(f) = 1 Answer: D. So the overall response is flat and ISI-free. (Key point: Equalizer target) 4. The basic equalizer form is... A. Linear transversal (FIR) tapped delay line B. MLSE trellis C. RAKE D. Selection switch Answer: A. d_hat(k) = sum w_n y(k-n). (Key point: Basic structure) ## 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 - [Equalization and diversity; space diversity methods](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aequalization-diversity-space-methods) — Wireless Communication, chapter 5 - [Short note: Adaptive equalization](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aadaptive-equalization-note) — Wireless Communication, chapter 5 - [Adaptive equalization algorithms (any two)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aadaptive-equalization-algorithms) — Wireless Communication, chapter 5 - [MLSE equalization](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amlse-equalization) — Wireless Communication, chapter 5 - [Equalization fundamentals and RAKE receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aequalization-fundamentals-rake) — Wireless Communication, chapter 5 ## Related in other subjects - [Why do we need DFT](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-need-dft) — Digital Signal Analysis and Processing, chapter 7, asked 6× - [Why do we need FFT](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-need-fft) — Digital Signal Analysis and Processing, chapter 7, asked 2× - [Need of energy audit and technique](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aenergy-audit-need-technique) — Industrial Power Distribution and Illumination, chapter 10, asked 3× - [Function of basic components of a digital communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adcs-basic-components) — Communication Systems, chapter 1