---
title: "OFDM transmitter"
subject: "Wireless Communication"
chapter: 4
key: "semester_7/wireless_communication:ofdm-transmitter"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/ofdm-transmitter
site: BCT Engineering
---

# OFDM transmitter

Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication

- Asked in: 2079 Chaitra, 4 marks
- Video: [OFDM Implementation Explained Visually!](https://www.youtube.com/watch?v=VwnUuBshXGw) — Wireless Explained
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/ofdm-transmitter
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4

## Question

Explain OFDM transmitter in brief.

## Short answer

The OFDM transmitter codes, maps bits to QAM symbols, and assigns N symbols to orthogonal subcarriers spaced by 1/Ts. An IFFT generates the time-domain symbol, a cyclic prefix longer than delay spread is added, then DAC and up-conversion produce the RF signal. This replaces many oscillators.

## Answer

The **OFDM transmitter** divides a high-rate data stream into N low-rate streams and places them on N **orthogonal subcarriers** (spacing Δf = 1/T_s) using an IFFT, then adds a cyclic prefix to fight multipath.

```text
bits->[FEC +]->[QAM  ]->[S/P]->[IFFT ]->[P/S]
      [intlv]  [map  ]          [N-pt ]    |
                                            v
RF <-[Up-conv + PA]<-[DAC]<-[Add cyclic prefix]
```
1. **Coding and interleaving** – add error-correction redundancy and spread bits across subcarriers.
2. **Symbol mapping** – map bits to QPSK/16-QAM/64-QAM symbols X_k.
3. **Serial-to-parallel** – N symbols assigned to N subcarriers; pilots inserted.
4. **IFFT** – x(n) = (1/N) Σ X_k e^(j2πkn/N) modulates all subcarriers at once into one time-domain OFDM symbol.
5. **Parallel-to-serial and cyclic prefix** – the last few samples are copied to the front; CP longer than the delay spread prevents ISI.
6. **DAC, up-conversion and power amplifier** – produce the RF signal.

The IFFT replaces a bank of N oscillators, making OFDM cheap to implement in Wi-Fi, LTE and DVB-T.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What subcarrier spacing makes OFDM subcarriers orthogonal?
   A. Delta f = 2 T_s
   B. Delta f = R_b/2 T_s
   C. Delta f = 1/T_s
   D. Any spacing larger than the bandwidth
   Answer: C. With spacing 1/T_s, subcarrier spectra overlap but each peak falls at the nulls of the others. (Key point: Orthogonal subcarriers)

2. What does the IFFT do in the OFDM transmitter?
   A. Spreads the symbols with a PN code
   B. Removes the cyclic prefix
   C. Estimates the channel from pilots
   D. Places all N symbols on orthogonal subcarriers to form one time-domain OFDM symbol
   Answer: D. The IFFT replaces a bank of N oscillators and modulates all subcarriers at once. (Key point: IFFT role)

3. What must be true of the cyclic prefix length to prevent ISI?
   A. It must be longer than the channel delay spread
   B. It must be shorter than one sample
   C. It must equal the symbol duration T_s
   D. It needs no relation to the delay spread
   Answer: A. The CP copies the last samples to the front; if longer than the delay spread, ISI falls inside the CP and is discarded. (Key point: Cyclic prefix)

## Related in Wireless Communication

- [OFDM transmitter and receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter-receiver) — Wireless Communication, chapter 4, asked 5×
- [OFDM transmitter side, advantages and disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter-advantages) — Wireless Communication, chapter 4
- [OFDM receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-receiver) — Wireless Communication, chapter 4