--- title: "OFDM principle and spread spectrum types" subject: "Wireless Communication" chapter: 4 key: "semester_7/wireless_communication:ofdm-spread-spectrum-types" source: https://www.bctengineer.com/q/semester_7/wireless_communication/ofdm-spread-spectrum-types site: BCT Engineering --- # OFDM principle and spread spectrum types Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication - Asked in: 2070 Bhadra, 4+4 marks - Video: [OFDM - Orthogonal Frequency Division Multiplexing](https://www.youtube.com/watch?v=KCHO7zlU25Q) — Sunny Classroom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/ofdm-spread-spectrum-types - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4 ## Question Discuss the principle of Orthogonal Frequency Division Multiplexing modulation scheme. Briefly explain different types of spread spectrum modulation techniques. ## Short answer OFDM splits a high-rate stream into N parallel low-rate streams on orthogonal subcarriers spaced by 1/Ts, implemented with IFFT/FFT and a cyclic prefix to remove ISI. Spread spectrum types are DSSS, FHSS, time hopping, and hybrid DS/FH systems. ## Answer #### Principle of OFDM **Orthogonal Frequency Division Multiplexing** splits a high-rate stream into N parallel low-rate streams, each on its own subcarrier. The subcarriers are spaced by Δf = 1/T_s, which makes them **orthogonal**: over one symbol period, the integral of the product of any two different subcarriers is zero. ```text ∫₀^Ts cos(2π f_i t) cos(2π f_j t) dt = 0, i ≠ j f_k = f_0 + k/T_s ``` So the spectra may overlap, yet each subcarrier peak falls at the nulls of all others, and no inter-carrier interference occurs. ```text |H| /\ /\ /\ /\ / \/ \/ \/ \ overlapping sinc spectra / /\ /\ /\ \ peak of one = null of others ---+---+---+---+---+---> f Δf=1/Ts ``` Key ideas: - **Long symbols:** each subcarrier's symbol is N times longer than the original, so the delay spread is small compared with T_s and each subcarrier sees flat fading. - **IFFT/FFT:** modulation and demodulation of all subcarriers is done by an N-point IFFT at the transmitter and FFT at the receiver. - **Cyclic prefix:** a copy of the end of each symbol is placed at its start; if it is longer than the delay spread, ISI is removed and a one-tap equalizer per subcarrier is enough. #### Types of spread spectrum modulation 1. **Direct sequence SS (DSSS):** data is multiplied by a fast PN chip sequence and BPSK modulated. The receiver multiplies by the same code to despread. Example: IS-95, 802.11b. 2. **Frequency hopping SS (FHSS):** the carrier of an FSK signal jumps among many frequencies in a PN-defined order; slow or fast hopping. Example: Bluetooth. 3. **Time hopping SS (THSS):** data is sent in short bursts in time slots chosen by a PN code within each frame. Example: impulse-radio UWB. 4. **Hybrid systems:** combinations such as DS/FH, which mix the advantages of both (e.g. military radios). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What makes OFDM subcarriers orthogonal? A. Spacing of 1/T_s, so the integral of the product of two different subcarriers over T_s is zero B. Spacing equal to the data rate C. Non-overlapping spectra with guard bands D. Using different PN codes Answer: A. Spectra overlap but each peak falls at the nulls of the others, giving no inter-carrier interference. (Key point: Orthogonality) 2. What does a cyclic prefix longer than the delay spread allow? A. Removal of the need for an FFT B. ISI removal and a one-tap equalizer per subcarrier C. Spreading with a PN code D. Constant envelope Answer: B. The CP absorbs ISI and turns the channel into flat sub-channels. (Key point: Cyclic prefix benefit) 3. Which is NOT one of the described spread spectrum types? A. Direct sequence SS B. Frequency hopping SS C. OFDM subcarrier division D. Time hopping SS Answer: C. The types are DSSS, FHSS, THSS and hybrid systems like DS/FH. (Key point: Types of SS) 4. How does time hopping spread spectrum send data? A. By hopping carrier frequency B. By multiplying with fast chips C. By using orthogonal subcarriers D. In short bursts in time slots chosen by a PN code in each frame Answer: D. THSS (e.g. impulse-radio UWB) varies transmission time instead of frequency. (Key point: Time hopping) ## Related in Wireless Communication - [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 and DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — Wireless Communication, chapter 4 - [FHMA principle and hybrid techniques for near-far](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-hybrid-near-far) — Wireless Communication, chapter 7 - [Hybrid spread spectrum and FHMA types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-ss-fhma-types) — Wireless Communication, chapter 7 - [CDMA features and hybrid spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-features-hybrid-ss) — Wireless Communication, chapter 7 - [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7 ## Related in other subjects - [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3× - [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2× - [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2× - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3×