---
title: "Short note: RAKE receiver"
subject: "Wireless Communication"
chapter: 5
key: "semester_7/wireless_communication:rake-receiver-note"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/rake-receiver-note
site: BCT Engineering
---

# Short note: RAKE receiver

Wireless Communication · Chapter 5: Equalization and Diversity

- Asked in: 2079 Bhadra, 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/rake-receiver-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5

## Question

Write a short note on RAKE receiver.

## Short answer

A RAKE receiver is a CDMA receiver that exploits multipath by assigning a correlator finger to each resolvable path. Each finger correlates with the appropriate delayed PN code, and the outputs are co-phased, weighted by path strength, and summed, converting destructive multipath into diversity gain. Used in IS-95 and WCDMA.

## Answer

A **RAKE receiver** is a receiver used in CDMA (DSSS) systems that collects and combines the energy of several **multipath components** of the same signal, providing multipath (time) diversity. It is named after a garden rake because it has several "fingers".

```text
       +->[Finger 1: corr, τ1]-Z1-(xα1)-+
r(t) --+->[Finger 2: corr, τ2]-Z2-(xα2)-(Σ)-> decide
       +->[Finger M: corr, τM]-ZM-(xαM)-+
```
- Each finger is a correlator using the PN code delayed to match one multipath component. Paths more than one chip apart are uncorrelated, so each finger isolates one path.
- Outputs are weighted by path strength, α_m = Z_m²/ΣZ_m², co-phased and summed: Z' = Σ α_m Z_m.
- If one path fades, others still contribute, so fading is greatly reduced.
- Turns multipath, normally harmful, into a gain. Used in IS-95 (3 fingers in the mobile, 4 at the base station) and WCDMA.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Where is a RAKE receiver used and what does it combine?
   A. CDMA/DSSS systems; resolvable multipath components
   B. FDMA systems; adjacent channels
   C. TDMA systems; training sequences
   D. Analog FM; harmonics
   Answer: A. RAKE collects the energy of multipath components in spread-spectrum systems. (Key point: RAKE use)

2. What does each RAKE finger do?
   A. Filters out all multipath components except the first
   B. Correlates with the PN code delayed to match one multipath component
   C. Generates the PN code for the transmitter
   D. Estimates the Doppler shift
   Answer: B. Each finger uses a delayed PN code to isolate one path, since paths more than one chip apart are uncorrelated. (Key point: Finger function)

3. How are the finger outputs combined?
   A. Subtracted from each other
   B. Only the strongest finger is used
   C. Weighted by path strength, co-phased and summed
   D. Averaged with equal weights without phase
   Answer: C. Z' = sum of alpha_m Z_m with weights by path strength. (Key point: Combining weights)

4. Why does a RAKE receiver reduce fading?
   A. It removes all multipath paths
   B. It raises the transmit power
   C. It shortens the chip period
   D. If one path fades, others still contribute, so multipath becomes a gain
   Answer: D. Combining several paths turns multipath into time diversity. (Key point: Benefit of RAKE)

## Related in Wireless Communication

- [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×
- [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 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