--- title: "Spread spectrum and DSSS transmitter/receiver" subject: "Wireless Communication" chapter: 4 key: "semester_7/wireless_communication:spread-spectrum-dsss-tx-rx" source: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-dsss-tx-rx site: BCT Engineering --- # Spread spectrum and DSSS transmitter/receiver Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication - Asked in: 2082 Bhadra, 2+6 marks - Video: [Direct Sequence Spread Spectrum DSSS (Basics, Block Diagram, Working, Waveforms & Applications)](https://www.youtube.com/watch?v=MqnzaHsQ90U) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-dsss-tx-rx - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4 ## Question Define spread spectrum modulation. Explain direct sequence spread spectrum with the block diagrams of transmitter and receiver. ## Short answer Spread spectrum uses bandwidth much larger than needed via PN code; processing gain is Bss/Rb. DSSS multiplies data by fast PN code, then BPSK modulates. Receiver multiplies by synchronized code to despread signal and spread interference. ## Answer #### Spread spectrum modulation **Spread spectrum (SS)** modulation is a technique in which the transmitted signal occupies a bandwidth much larger than the minimum needed for the data. The spreading is done by a **pseudo-noise (PN) code** that is independent of the data, and the receiver uses the same synchronized code to despread it. The ratio of spread bandwidth to data rate is the **processing gain** PG = B_ss/R_b = T_b/T_c. SS gives resistance to jamming, interference and multipath, low probability of interception, and allows multiple access (CDMA). #### Direct sequence spread spectrum (DSSS) In DSSS, each data bit is multiplied directly by a fast PN sequence with chip period T_c ≪ T_b. The product is then BPSK modulated on the carrier. ```text s(t) = √(2E_s/T_s) m(t) p(t) cos(2π f_c t + θ) ``` where m(t) is the data and p(t) the PN code. **Transmitter** ```text data m(t) ---> (x) ------> (x) ------> s(t) Rb ^ ^ (wideband) | | [PN generator] [Oscillator] (chip Rc) cos ωct ``` 1. Data m(t) (±1, rate R_b) is multiplied by the PN code p(t) (±1, rate R_c = N R_b). 2. The product is a wideband chip stream at rate R_c. 3. This chip stream BPSK-modulates the carrier, giving an RF bandwidth of about 2R_c, and is transmitted. **Receiver** ```text r(t)->[BPF]->(x)--->(x)--->[Integrate]->[Decision]-> m^(t) ^ ^ [0..Tb ] | | [PN generator] [Carrier] [+ sync ] [recovery] ``` 1. The received signal r(t) = s(t) + interference + noise is filtered. 2. It is multiplied by a locally generated, **synchronized** copy of p(t). Because p(t)·p(t) = 1, the wanted signal collapses back to its narrow data bandwidth (**despreading**). 3. Any narrowband interference gets multiplied by p(t) and is **spread** over the wide band, so most of it is removed by the following filter. 4. Coherent demodulation (multiply by carrier, integrate over T_b, decide) recovers the data. ```text Spectrum at receiver: before despreading after despreading jammer data | /\ ___|___ __/ \__ jammer spread / data \___ ___________ (low level) ``` **Example:** IS-95 CDMA uses a chip rate of 1.2288 Mcps for 9.6 kbps data, giving PG = 128 (21 dB). IEEE 802.11b also uses DSSS. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Processing gain of spread spectrum is: A. B_ss/R_b = T_b/T_c B. R_b/B_ss C. T_c/T_b D. Number of users Answer: A. PG is the ratio of spread bandwidth to data rate, equal to bit period over chip period. (Key point: Processing gain) 2. In DSSS the data m(t) is multiplied by: A. A slower PN code B. A fast PN code with chip period T_c much less than T_b C. A sinusoid only D. A noise source Answer: B. The fast PN chips spread the data to bandwidth about 2R_c. (Key point: DSSS spreading) 3. The receiver despreads because: A. p(t) x p(t) = 1 with a synchronized PN copy B. The carrier cancels C. The filter spreads the signal D. Noise is zero Answer: A. Multiplying by the synchronized code restores the narrow data bandwidth. (Key point: Despreading) 4. What happens to narrowband interference at the receiver? A. It is despread B. It is spread over a wide band and mostly filtered out C. It is amplified D. It is unchanged Answer: B. Multiplying by p(t) spreads the interferer so little of it passes the narrow filter. (Key point: Interference rejection) ## Related in Wireless Communication - [Direct sequence spread spectrum (DSSS)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-block-diagram) — Wireless Communication, chapter 4 - [DPSK transmitter/receiver and PN sequence](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adpsk-pn-sequence) — Wireless Communication, chapter 4 - [MSK modulation and DSSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-dsss) — Wireless Communication, chapter 4 - [OFDM transmitter and receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter-receiver) — Wireless Communication, chapter 4, asked 5× - [OQPSK transmitter/receiver and π/4-QPSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aoqpsk-pi4-qpsk) — Wireless Communication, chapter 4 - [Direct sequence and frequency hopped SS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-fhss-techniques) — Wireless Communication, chapter 4 ## Related in other subjects - [Spread spectrum: FHSS and DSSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-fhss-dsss) — Data Communication, chapter 6, asked 4× - [Stereo FM transmitter and receiver; output of mono receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Astereo-fm-mono-receiver) — Communication Systems, chapter 3 - [DSSS bandwidth spreading](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adsss-bandwidth-spreading) — Data Communication, chapter 6 - [Why DPCM over PCM; DPCM transmitter and receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adpcm-preferred-transmitter-receiver) — Communication Systems, chapter 4, asked 2×