--- title: "Direct sequence spread spectrum (DSSS)" subject: "Wireless Communication" chapter: 4 key: "semester_7/wireless_communication:dsss-block-diagram" source: https://www.bctengineer.com/q/semester_7/wireless_communication/dsss-block-diagram site: BCT Engineering --- # Direct sequence spread spectrum (DSSS) Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication - Asked in: 2081 Baisakh, 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/dsss-block-diagram - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4 ## Question Explain the working of the Direct Sequence Spread Spectrum (DSSS) technique with an appropriate block diagram. ## Short answer DSSS transmitter multiplies data m(t) by PN code p(t), then BPSK modulates carrier. Receiver multiplies received signal by synchronized PN code to despread data and spread interference, then integrates and decides. Processing gain equals Tb/Tc. ## Answer **Direct Sequence Spread Spectrum (DSSS)** spreads a narrowband data signal over a wide bandwidth by multiplying each data bit with a fast **pseudo-noise (PN) code** whose chip period T_c is much smaller than the bit period T_b. The receiver multiplies again by the same synchronized code to recover the data. ```text s(t) = A m(t) p(t) cos(2π f_c t) Processing gain PG = T_b / T_c = R_c / R_b ``` #### Transmitter ```text m(t) ±1 --->(x)--- m(t)p(t) --->(x)---> s(t) (Rb) ^ ^ | | [PN code p(t)] [cos ωct] [ rate Rc ] ``` - Data m(t) (±1) is multiplied by PN code p(t) (±1). Each bit becomes N = T_b/T_c chips. - The chip stream BPSK-modulates the carrier. Bandwidth grows from about 2R_b to 2R_c. #### Receiver ```text r(t)->(x)------>(x)---->[∫ over Tb]->[Decision]->m^(t) ^ ^ [Local PN ] [cos ωct] [synced ] ``` - The received signal is multiplied by a locally generated PN code, aligned by an acquisition and tracking circuit. - Since p(t)² = 1, the wanted signal is **despread** to its original data bandwidth. - Narrowband interference or jamming is **spread** by the same multiplication and mostly filtered out. - Coherent BPSK demodulation (multiply, integrate over T_b, threshold) gives the data. #### Waveforms (one data bit = 7 chips) ```text m(t): ‾‾‾‾‾‾‾‾‾‾‾‾‾‾|______________ p(t): ‾|_|‾‾|_|‾|_‾ |‾|_|‾‾|_|‾|_ m·p : ‾|_|‾‾|_|‾|_‾ |_|‾|__|‾|_|‾ ``` #### Features - Interference suppression by a factor equal to PG. - Multipath rejection: delayed copies more than one chip late look like noise; a RAKE receiver can combine them. - Allows CDMA: users share the band using different codes. Used in IS-95, WCDMA and IEEE 802.11b. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a DSSS transmitter spread the data? A. It hops the carrier among many channels using a PN code B. It multiplies each data bit with a fast PN chip sequence with chip period T_c much smaller than T_b C. It sends the data in short bursts at PN-chosen time slots D. It adds a cyclic prefix to each data bit Answer: B. DSSS multiplies the +/-1 data by a faster +/-1 PN code, so each bit becomes N = T_b/T_c chips and the bandwidth grows. (Key point: Spreading operation) 2. Why does multiplying by the synchronized local PN code at the receiver recover the data? A. Because the PN code amplifies the signal above the noise B. Because the PN code shifts the carrier to baseband C. Because p(t)^2 = 1, so the code cancels and the wanted signal returns to its original bandwidth D. Because the PN code removes multipath delay Answer: C. The PN code is +/-1, so p(t) x p(t) = 1. The data is despread while interference gets spread by the same multiplication. (Key point: Despreading) 3. What is the processing gain of DSSS? A. PG = T_c / T_b B. PG = R_b / R_c C. PG = 2 R_b D. PG = T_b / T_c = R_c / R_b Answer: D. Processing gain is the ratio of bit period to chip period, equal to chip rate over bit rate, which is also the interference suppression factor. (Key point: Processing gain) ## Related in Wireless Communication - [Spread spectrum and DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — 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 - [MSK, GMSK and advantages of spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-spread-spectrum-advantages) — Wireless Communication, chapter 4 - [Spread spectrum disadvantages; CDMA implementation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-disadvantages-cdma) — Wireless Communication, chapter 7 - [Spread spectrum and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — Wireless Communication, chapter 4 - [Spread spectrum multiple access variants](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Assma-variants-applications) — Wireless Communication, chapter 7 ## 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× - [Spread spectrum security; pros and cons](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ass-security-jamming) — Data Communication, chapter 6 - [CRO block diagram and working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acro-block-diagram) — Basic Electronics Engineering, chapter 7 - [DSSS bandwidth spreading](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adsss-bandwidth-spreading) — Data Communication, chapter 6