--- title: "Diversity and RAKE receiver operation" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:diversity-rake-operation" source: https://www.bctengineer.com/q/semester_7/wireless_communication/diversity-rake-operation site: BCT Engineering --- # Diversity and RAKE receiver operation Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2076 Bhadra, 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/diversity-rake-operation - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question Explain how diversity improves the quality of network. Explain the operation of RAKE receiver with appropriate block diagram. ## Short answer Diversity provides independent signal copies, greatly reducing the probability that all are in deep fade, thus improving average SNR and BER. A RAKE receiver deploys multiple correlator fingers, each despreading a distinct multipath component with appropriate code delay; the finger outputs are weighted by SNR and combined to exploit multipath diversity in CDMA. ## Answer #### How diversity improves network quality **Diversity** is a receiver technique that gives the receiver two or more independently faded copies of the same signal and then selects or combines them. Because deep fades on independent branches rarely happen at the same time, the chance that *all* copies are weak is very small. - If one branch is in a fade with probability *p*, the probability that all *M* independent branches fade together is about *p*ᴹ (e.g. 0.1 → 0.001 for *M* = 3). - The combined signal has a higher and more stable average SNR, so the **bit error rate falls sharply** and fewer calls drop. - Unlike equalization, diversity needs no training sequence and adds little delay. - Result: better coverage at cell edges, lower required transmit power, higher capacity and better voice/data quality. #### RAKE receiver A **RAKE receiver** is a diversity receiver used in CDMA (IS-95, WCDMA). In CDMA the chip duration is very short, so multipath copies that arrive more than one chip (Tc) apart are *resolvable*. Instead of treating these delayed copies as interference, the RAKE receiver collects them, like the fingers of a garden rake gathering leaves. ```text +------------+ +----+ w1 +--->| Correlator |-->|Z1 |--(x)--+ | | code c(t) | +----+ | | +------------+ | r(t) | +------------+ +----+ w2 v +-------+ ------>+--->| Correlator |-->|Z2 |--(x)-(+)->|Integr.| | | c(t - τ2) | +----+ ^ | & dec.| | +------------+ | +-------+ | +------------+ +----+ wM | | +--->| Correlator |-->|ZM |--(x)--+ v | c(t - τM) | +----+ data bits +------------+ (one "finger" per resolvable multipath) ``` **Operation** 1. The received signal r(t) is fed to *M* correlators, called **fingers**. 2. Each finger multiplies r(t) by the user's PN code delayed by the delay τₖ of one multipath component, and integrates over one bit. This despreads that one path; other paths look like noise to it because the PN code has low autocorrelation for shifts ≥ Tc. 3. A searcher (path finder) estimates the delays τₖ and strengths of the strongest paths and assigns them to the fingers. 4. Finger outputs Z₁ … Z_M are weighted by w₁ … w_M, proportional to the SNR (amplitude) of each path, so strong paths count more. This is maximal-ratio style combining: Z′ = Σ wₖ·Zₖ 5. The combined output Z′ is integrated and passed to a decision device to recover the bits. **Benefit:** multipath, which harms narrowband systems, becomes a source of **multipath (time) diversity**, improving SNR and reducing fading in CDMA systems. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. If a branch is in a fade with probability p, what is the probability that all M independent branches fade together? A. About M*p B. About p/M C. About p^M D. About 1-p Answer: C. Independent fades multiply, e.g. 0.1 with M=3 gives 0.001. (Key point: Diversity probability) 2. Why can a RAKE receiver treat delayed copies as useful in CDMA? A. All copies arrive at the same time B. The PN code has high autocorrelation at all shifts C. Chip duration is very long D. Copies more than one chip apart are resolvable Answer: D. With short chips, multipath separated by more than Tc is resolvable and can be collected. (Key point: Resolvable multipath) 3. What does each finger multiply the received signal r(t) by? A. The user's PN code delayed by that path's delay B. A random carrier C. The data bits of another user D. The inverse of the channel Answer: A. This despreads that one path while other paths look like noise. (Key point: Finger despreading) 4. What does the searcher (path finder) do? A. Generates the training sequence B. Estimates delays and strengths of the strongest paths and assigns them to fingers C. Selects the antenna D. Corrects frequency offset Answer: B. It finds the strongest paths so fingers are locked to them. 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