--- title: "RAKE receiver and M-branch RAKE" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:rake-m-branch" source: https://www.bctengineer.com/q/semester_7/wireless_communication/rake-m-branch site: BCT Engineering --- # RAKE receiver and M-branch RAKE Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2077 Chaitra, 3+5 marks; 2072 Asoj, 8 marks (asked 2 times) - 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/rake-m-branch - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question What do you understand by RAKE receiver? Explain the working of M branch RAKE receiver. ## Short answer A RAKE receiver captures multipath energy using M correlator fingers locked to delayed copies of the PN-coded signal. Each finger despreads and weights its path by strength, then the weighted outputs are summed and decided, providing multipath diversity to combat fading. ## Answer #### RAKE receiver A **RAKE receiver** is a CDMA receiver that collects the energy of several **multipath components** of the same signal and combines them, instead of treating them as interference. It has several correlators, called **fingers**, like the teeth of a garden rake. Each finger is locked to one multipath copy that is delayed by at least one chip, so the copies are nearly uncorrelated. Combining them gives **multipath (time) diversity**, which improves the SNR and reduces fading. #### Working of an 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] [PN code + path searcher] | v m'(t) ``` 1. **Path searching:** a searcher finds the M strongest multipath delays τ₁ … τ_M from the channel's power delay profile using the PN code's sharp autocorrelation. 2. **Correlation:** correlator m multiplies r(t) by the PN code delayed by τ_m and integrates over the bit. Its output Z_m is the despread data from path m. Other paths, being more than one chip apart, appear as low-level noise. 3. **Weighting:** each output is multiplied by a weight α_m proportional to the signal strength (SNR) of that path, so strong paths count more: ```text α_m = Z_m² / Σ_(m=1..M) Z_m² ``` 4. **Combining:** the weighted outputs are co-phased and summed: ```text Z' = Σ_(m=1..M) α_m Z_m ``` 5. **Decision:** the combined output is integrated and compared with a threshold to give the data bit. #### Why it works If one path fades, others are likely strong, because fading on paths separated by more than one chip is independent. With maximal ratio weights the output SNR approaches the sum of the finger SNRs. IS-95 uses 3 fingers in the mobile and 4 in the base station; WCDMA also uses RAKE receivers. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a RAKE receiver do with multipath components? A. Rejects them all as interference B. Removes them with a cyclic prefix C. Converts them into hops D. Combines them using separate correlators (fingers) to gain multipath diversity Answer: D. Each finger locks onto one path delayed by at least one chip, and the outputs are combined. (Key point: RAKE principle) 2. Why can fingers separate paths? A. PN autocorrelation is very small beyond one chip delay B. The paths use different carriers C. The paths have different data D. The fingers use different modulation Answer: A. Paths separated by more than a chip appear nearly uncorrelated. (Key point: Finger separation) 3. How are finger outputs combined? A. Equally averaged regardless of strength B. Weighted by alpha_m proportional to path strength, co-phased and summed: Z' = sum alpha_m Z_m C. Only the weakest is used D. Multiplied together Answer: B. Stronger paths count more, as in maximal ratio combining. (Key point: Combining) 4. Why does the RAKE receiver improve reliability? A. All paths fade together B. It removes the need for a PN code C. If one path fades the others, fading independently, are likely strong D. It increases the transmit power Answer: C. Fading on paths more than a chip apart is independent. (Key point: Why diversity helps) ## 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 - [Short note: RAKE receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Arake-receiver-note) — Wireless Communication, chapter 5 - [Diversity and RAKE receiver operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-rake-operation) — 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 - [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