--- title: "ISI mitigation and M-branch RAKE receiver" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:isi-rake-m-branch" source: https://www.bctengineer.com/q/semester_7/wireless_communication/isi-rake-m-branch site: BCT Engineering --- # ISI mitigation and M-branch RAKE receiver Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2082 Baisakh, 2+2+4 marks - Video: [Rake Receiver (Principle, Construction and Working)- CDMA,WCDMA, Fading in Wireless Communication](https://www.youtube.com/watch?v=sC_Hx_SwMsk) — All About Education - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/isi-rake-m-branch - 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. What do you understand by RAKE receiver? Explain the working of a M branch RAKE receiver. ## Short answer Equalization minimizes ISI via an adaptive inverse filter. A RAKE receiver is a CDMA receiver with fingers locked to multipath components. In an M-branch RAKE, a searcher finds M strongest paths; each finger correlates with delayed PN code, outputs are weighted by path strength, co-phased, and summed for diversity gain. ## Answer #### Signal processing to minimize ISI **Equalization** minimizes ISI. An equalizer is a receiver filter with roughly the inverse frequency response of the channel, so that channel + equalizer gives a flat, linear-phase response. Since the mobile channel is random and time-varying, **adaptive equalizers** are used: a known **training sequence** first sets the tap weights, and then the equalizer **tracks** the channel using its own decisions. #### RAKE receiver A **RAKE receiver** is a CDMA receiver with several correlators (**fingers**), each locked to a different multipath copy of the signal. Since the PN code's autocorrelation is very small beyond one chip, copies delayed by more than one chip can be separated and then combined. This gives multipath diversity instead of ISI. #### Working of M-branch RAKE receiver ```text +->[Corr. 1, τ1]->Z1->(x)α1-+ | | r(t) --+->[Corr. 2, τ2]->Z2->(x)α2-(Σ)-> Z' | | | +->[Corr. M, τM]->ZM->(x)αM-+ v [∫ + decide]-> m' ``` 1. A **searcher** finds the delays of the M strongest paths. 2. Each **correlator** multiplies the input by the PN code shifted to its path delay τ_m and integrates, giving output Z_m. 3. Each output is **weighted** by α_m, proportional to that path's strength, e.g. α_m = Z_m² / Σ Z_m². 4. The outputs are **co-phased and summed**: Z' = Σ α_m Z_m. 5. Z' is integrated over the bit and compared with a threshold to decide the bit. If one path fades deeply, the others usually do not, so the combined output is much more reliable. IS-95 mobiles use 3 fingers; base stations use 4. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which operation minimizes ISI? A. Spreading with a PN code B. Frequency hopping C. Differential encoding D. Equalization, using a filter with roughly the inverse response of the channel Answer: D. Channel plus equalizer gives a flat linear-phase response. (Key point: ISI operation) 2. How does an adaptive equalizer operate? A. A training sequence sets the taps, then it tracks using its own decisions B. It never uses known symbols C. It is fixed after design D. It uses only the cyclic prefix Answer: A. This handles the random, time-varying mobile channel. (Key point: Training and tracking) 3. In an M-branch RAKE, what does each correlator do? A. Hops the carrier B. Multiplies r(t) by the PN code shifted to its path delay and integrates, giving Z_m C. Computes an FFT D. Subtracts the PN code Answer: B. A searcher first finds the M strongest delays. (Key point: Searcher and correlators) 4. How is the RAKE output formed? A. Z' = product of Z_m B. Z' is the largest Z_m with no weighting C. Z' = sum alpha_m Z_m with alpha_m proportional to path strength, then integrate and threshold D. Z' = Z_1 only Answer: C. Weighted co-phased summation then decision. (Key point: Combining) ## Related in Wireless Communication - [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 - [RAKE receiver and M-branch RAKE](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Arake-m-branch) — Wireless Communication, chapter 5, asked 2× - [Diversity and RAKE receiver operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-rake-operation) — 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 - [Short note: RAKE receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Arake-receiver-note) — 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 - [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