--- title: "Equalization and diversity; space diversity methods" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:equalization-diversity-space-methods" source: https://www.bctengineer.com/q/semester_7/wireless_communication/equalization-diversity-space-methods site: BCT Engineering --- # Equalization and diversity; space diversity methods Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2080 Baisakh, 3+3+2 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-diversity-space-methods - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question Why are equalization and diversity needed in wireless communication systems? List out different diversity reception methods used for space diversity. Explain any one of them. ## Short answer Equalization removes ISI from frequency-selective multipath; diversity guards against deep flat fades. Space diversity methods include selection, feedback, maximal ratio, and equal gain combining. Selection diversity picks the branch with highest instantaneous SNR; for M=2, average SNR improves by 1.5 times (1.76 dB). ## Answer #### Need for equalization and diversity - **Equalization** is needed to remove **inter-symbol interference (ISI)** caused by time-dispersive (frequency-selective) multipath. When the delay spread is a large part of the symbol period, earlier symbols overlap with later ones; an adaptive equalizer reverses the channel's effect. - **Diversity** is needed to fight **deep fading** in flat-fading channels. Because the received signal randomly falls 20–40 dB, receiving multiple independently fading copies lets the receiver avoid or combine around deep fades, reducing BER without more transmit power. - Both together raise link quality, allow higher data rates and lower outage probability. #### Space diversity reception methods 1. Selection diversity 2. Feedback (scanning) diversity 3. Maximal ratio combining 4. Equal gain combining #### Selection diversity (explained) M antennas, spaced at least about λ/2 (at the mobile) apart, feed M receivers. The receiver continuously selects the branch with the **highest instantaneous SNR** and connects it to the demodulator. ```text Ant1 ->[Rx1]-SNR1-+ Ant2 ->[Rx2]-SNR2-+->[Select max]-> demod AntM ->[RxM]-SNRM-+ ``` Probability that all M branches are below threshold γ (Rayleigh, mean SNR Γ): ```text P[γ₁..γ_M ≤ γ] = (1 − e^(−γ/Γ))^M ``` Mean output SNR improvement: ```text γ̄/Γ = Σ_(k=1)^(M) 1/k ``` Example: 2 branches give 1.5 times (≈ 1.76 dB) average SNR improvement, and much larger gain at low outage levels. Simple to build, but only one branch is used at a time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Equalization is mainly needed to counter which problem? A. Deep flat fading B. Co-channel interference from other cells C. Inter-symbol interference from time-dispersive multipath D. Thermal noise at the antenna Answer: C. Delay spread makes symbols overlap, causing ISI which an equalizer removes. (Key point: Equalization need) 2. Diversity is mainly used to combat what? A. ISI by inverting the channel B. Doppler shift by tuning the oscillator C. Path loss by amplifying the signal D. Deep fading, by using independently fading copies Answer: D. Independent copies mean the receiver can avoid or combine around deep fades. (Key point: Diversity need) 3. Which list gives the space diversity reception methods? A. Selection, feedback (scanning), maximal ratio and equal gain combining B. Selection, polarization, frequency and time C. LMS, RLS, zero forcing, MLSE D. FDMA, TDMA, CDMA, OFDMA Answer: A. These four are the combining methods for space diversity. (Key point: Space diversity methods) 4. Selection diversity connects which branch to the demodulator? A. The first one above a threshold B. The one with the highest instantaneous SNR C. All branches with equal weights D. The one with the lowest SNR Answer: B. It continuously picks the branch with the best instantaneous SNR. (Key point: Selection diversity rule) ## Related in Wireless Communication - [Diversity techniques and space diversity combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-space-combining) — Wireless Communication, chapter 5 - [ISI mitigation and space diversity techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aisi-space-diversity) — Wireless Communication, chapter 5 - [Space diversity techniques with block diagrams](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspace-diversity-block-diagrams) — Wireless Communication, chapter 5 - [Regulatory issues in wireless communication](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aregulatory-issues) — Wireless Communication, chapter 1, asked 6× - [Trends in wireless communication beyond 3G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atrends-beyond-3g) — Wireless Communication, chapter 1 - [Benefits of wireless communication and 4G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Awireless-benefits-4g) — Wireless Communication, chapter 1 ## Related in other subjects - [Wired and wireless communication systems](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awired-wireless-systems) — Basic Electronics Engineering, chapter 5, asked 2× - [Differentiate wired and wireless systems](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awired-vs-wireless-diff) — Basic Electronics Engineering, chapter 5, asked 2× - [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — Telecommunications, chapter 4 - [Compare AM systems; one method of DSB-FC generation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-types-compare-dsbfc-method) — Communication Systems, chapter 2