--- title: "Channel coding and types of LPC" subject: "Wireless Communication" chapter: 6 key: "semester_7/wireless_communication:channel-coding-lpc-types" source: https://www.bctengineer.com/q/semester_7/wireless_communication/channel-coding-lpc-types site: BCT Engineering --- # Channel coding and types of LPC Wireless Communication · Chapter 6: Speech and Channel Coding - Asked in: 2071 Magh, 2+6 marks - Video: [W 1.11 Parametric coding -- Channel Vocoder and LPC Vocoder](https://www.youtube.com/watch?v=uC2j6p-wySU) — Ramanikumar animations -Telecom and Wireless - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/channel-coding-lpc-types - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6 ## Question What is channel coding? Explain types of linear predictive coder. ## Short answer Channel coding adds controlled redundancy so the receiver can detect and correct errors; block, convolutional, turbo and LDPC codes are examples. LPC coders share an all-pole vocal-tract model; LPC-10 uses pulses/noise at 2.4 kbps, MPE-LPC uses a few pulses, CELP uses a codebook, RELP sends the residual. ## Answer #### Channel coding **Channel coding** is the process of adding controlled redundancy to the data before transmission so that the receiver can **detect and correct errors** caused by noise, fading and interference. It lowers the BER for a given SNR (coding gain) at the cost of extra bandwidth or lower data rate. Types: block codes (Hamming, BCH, Reed–Solomon), convolutional codes (Viterbi decoding) and turbo/LDPC codes. In mobile systems it is used with interleaving. #### Types of linear predictive coder All LPC coders model the vocal tract with an all-pole filter 1/A(z), whose ~10 coefficients are found by linear prediction, ŝ(n) = Σ aₖ s(n − k). They differ in how the **excitation** is produced. **1. Basic LPC vocoder (LPC-10)** - Excitation is a pulse train (voiced) or white noise (unvoiced) chosen by a V/UV decision and pitch detector. - 2.4 kbps; intelligible but synthetic speech. **2. Multi-pulse excited LPC (MPE-LPC)** - Excitation is a small number of pulses (e.g. 4–8 per 5 ms) whose **positions and amplitudes** are found by **analysis-by-synthesis**: the encoder tries pulses and keeps those that minimise the perceptually weighted error. - No V/UV decision needed; good quality at about 9.6 kbps. GSM RPE-LTP uses regularly spaced pulses (regular pulse excitation). **3. Code-excited LPC (CELP)** - Excitation is chosen from a **codebook** of stored random (Gaussian) sequences. For each sub-frame the encoder passes every codeword through the synthesis filter (with long-term pitch predictor) and picks the one with least weighted error; only its **index** and gain are sent. ```text [codebook]->(×gain)->[pitch filter]->[1/A(z)]-> s^(n) | s(n) ---------------------------------(-)<-------+ error -> [weighting filter] -> min? ``` - Good quality at 4.8–8 kbps; used in IS-95 (QCELP), FS-1016 and ACELP in GSM EFR. **4. Residual excited LPC (RELP)** - The **prediction residual** e(n) = s(n) − ŝ(n) is low-pass filtered, down-sampled and sent as excitation; the receiver regenerates the high-frequency part by spectral folding. - No pitch detection needed; good quality at about 9.6 kbps. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does channel coding do? A. Removes redundancy from the source B. Adds controlled redundancy so errors can be detected and corrected C. Compresses speech to a lower bit rate D. Spreads the signal over a wide band Answer: B. Controlled redundancy gives coding gain at the cost of bandwidth or data rate. (Key point: Purpose of channel coding) 2. All LPC coders share the all-pole vocal tract filter 1/A(z). What differs between them? A. The number of channels B. The way the excitation is produced C. The carrier frequency D. The modulation type Answer: B. They differ in excitation: pulse/noise, multipulse, codebook, or residual. (Key point: What differs among LPC coders) 3. In CELP, what is sent for the excitation? A. All pulse positions and amplitudes B. The low-pass filtered residual C. The index of the best codebook entry and its gain D. A voiced/unvoiced flag only Answer: C. The encoder tries every codeword and sends only the index of the min-weighted-error one plus gain. (Key point: CELP operation) 4. How does RELP get its excitation? A. From random Gaussian codebook B. From a pulse train or noise by V/UV decision C. From the low-pass filtered, down-sampled prediction residual, with the high band regenerated by spectral folding D. From regularly spaced pulses found by analysis-by-synthesis Answer: C. RELP sends the baseband residual and needs no pitch detection. (Key point: RELP) ## Related in Wireless Communication - [Speech characteristics and linear predictive coder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-lpc) — Wireless Communication, chapter 6, asked 5× ## Related in other subjects - [Linear block coding with example](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Alinear-block-code-explain) — Data Communication, chapter 8, asked 3× - [Source coding vs channel coding](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Asource-vs-channel-coding) — Data Communication, chapter 8 - [Need for channel coding](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Achannel-coding-need) — Data Communication, chapter 8 - [Source vs channel coding; Huffman for five symbols](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Asource-channel-huffman-five) — Communication Systems, chapter 6