--- title: "ISI mitigation and space diversity techniques" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:isi-space-diversity" source: https://www.bctengineer.com/q/semester_7/wireless_communication/isi-space-diversity site: BCT Engineering --- # ISI mitigation and space diversity techniques Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2075 Bhadra, 2+6 marks - Video: [Diversity Techniques in Antennas / Wireless Communication | Antenna and Wave Propagation Module - 6](https://www.youtube.com/watch?v=jJhxMI3jvi0) — THE BACKBENCH ENGINEERING COMMUNITY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/isi-space-diversity - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question Describe a signal processing operation that minimizes the effects of ISI. Explain various space diversity techniques. ## Short answer Equalization minimizes ISI by applying an adaptive inverse-channel filter, trained with a known sequence then tracking with decisions. Space diversity methods: selection (highest SNR branch), feedback/scanning (first branch above threshold), maximal ratio combining (co-phase, weight by SNR, sum), and equal gain combining (co-phase, sum with equal weights). ## Answer #### Signal processing operation that minimizes ISI: equalization **Equalization** is the signal processing used at the receiver to reduce **inter-symbol interference (ISI)** caused by multipath delay spread and band-limited channels. When the delay spread is larger than the symbol period, each symbol spreads into the next. An equalizer is a filter whose response is roughly the inverse of the channel response, so that H_eq(f) · H_ch(f) ≈ constant (flat), i.e. h_ch(t) * h_eq(t) ≈ δ(t) Because the mobile channel changes with time, the equalizer is **adaptive**: it is first trained with a known training sequence, then tracks the channel in decision-directed mode using algorithms such as LMS or RLS. Common types are linear transversal (FIR) equalizers, decision feedback equalizers (DFE) and MLSE (Viterbi) equalizers. #### Space diversity techniques **Space (antenna) diversity** uses two or more antennas spaced far enough apart that their fading is uncorrelated: about λ/2 at the mobile and 10λ or more at the base station. The branch signals are then used in one of these ways: 1. **Selection diversity** – The receiver monitors the SNR of all *M* branches and connects the branch with the highest instantaneous SNR to the demodulator. It is simple and needs one receiver after the switch. 2. **Feedback (scanning) diversity** – The branches are scanned in a fixed order until one is found above a preset threshold; that branch is used until it falls below the threshold, then scanning restarts. Only one receiver is needed, but it performs a little worse than selection. 3. **Maximal ratio combining (MRC)** – Every branch is co-phased, weighted in proportion to its signal amplitude (gain ∝ signal voltage / noise power), and summed. The output SNR is the sum of branch SNRs (γ_M = Σ γᵢ), which is the best possible. It needs a receiver and phase correction on every branch. 4. **Equal gain combining (EGC)** – Branches are co-phased and added with equal weights. It performs only slightly worse than MRC and is easier to build because no amplitude estimation is needed. ```text Ant1 Ant2 ... AntM | | | [G1] [G2] ... [GM] gains / co-phasing | | | +------+-----+-----+ | Select / Sum (combiner) | Demodulator ``` Selection and feedback pick one branch; MRC and EGC add all branches. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which signal processing operation minimizes ISI? A. Interleaving B. Equalization C. Frequency hopping D. Power control Answer: B. An equalizer's response is approximately the inverse of the channel's. (Key point: ISI processing) 2. What antenna spacing gives uncorrelated fading? A. 10 lambda at the mobile, lambda/2 at the base station B. Under lambda/100 at both C. About lambda/2 at the mobile, 10 lambda or more at the base station D. Exactly one coherence time Answer: C. The base station needs larger spacing than the mobile. (Key point: Antenna spacing) 3. How does feedback diversity compare with selection diversity? A. Needs M receivers and performs better B. Uses all branches equally C. Is identical in performance D. Needs one receiver but performs slightly worse Answer: D. Feedback scans to the first branch above threshold, so it is simpler but poorer. (Key point: Feedback vs selection) 4. How does equal gain combining differ from MRC? A. Branches are co-phased and added with equal weights; no amplitude estimation B. Branches are weighted by SNR and not co-phased C. Only one branch is used D. Branches are subtracted Answer: A. EGC is slightly worse than MRC but easier to build. (Key point: EGC) ## Related in Wireless Communication - [ISI mitigation and M-branch RAKE receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aisi-rake-m-branch) — Wireless Communication, chapter 5 - [Diversity techniques and space diversity combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-space-combining) — 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 - [ISI, RAKE and linear transversal equalizer](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aisi-rake-linear-transversal-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 - [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — Wireless Communication, chapter 5 ## Related in other subjects - [Air pollution: causes, effects and mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Aair-pollution-causes-mitigation) — Technology, Environment and Society, chapter 5, asked 3× - [Floods: causes, effects and mitigation](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Afloods-causes-effects-mitigation) — Engineering Hydrology, chapter 6 - [Kathmandu air pollution: causes, effects, mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Akathmandu-air-pollution-causes) — Technology, Environment and Society, chapter 5 - [QPSK modulator, demodulator and signal space](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aqpsk-mod-demod-signal-space) — Communication Systems, chapter 7, asked 2×