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

# OFDM transmitter side, advantages and disadvantages

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

- Asked in: 2082 Bhadra, 4+4 marks
- 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-advantages
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4

## Question

Explain the block diagram of OFDM transmitting side. Also mention its advantage and disadvantage.

## Short answer

OFDM transmitter does coding, QAM mapping, S/P, N-point IFFT, adds cyclic prefix, then DAC and up-conversion. Advantages: multipath resistance, one-tap equalization, high spectral efficiency, FFT implementation. Disadvantages: high PAPR, sensitivity to frequency offset and Doppler.

## Answer

#### OFDM transmitting side
**OFDM** sends a high-rate data stream as many low-rate streams on closely spaced **orthogonal subcarriers** (spacing Δf = 1/T_s). The transmitter does this with an IFFT.
```text
bits->[FEC +     ]->[QAM/PSK]->[S/P]->[  IFFT  ]
      [interleave]  [mapper ]         [N-point ]
                                          |
RF <-[Up-conv]<-[DAC]<-[P/S]<-[Add CP]<---+
     [+ PA   ]
```
1. **Channel coding and interleaving:** adds redundancy (e.g. convolutional code) and spreads bits across subcarriers so one deep fade does not wipe out consecutive bits.
2. **Mapping:** bits are grouped and mapped to complex symbols (BPSK, QPSK, 16-QAM, 64-QAM).
3. **Serial-to-parallel:** N symbols are placed on N subcarriers; pilot tones and null (guard) carriers are added.
4. **IFFT:** computes x(n) = (1/N) Σ_{k=0}^{N−1} X_k e^(j2πkn/N), i.e. modulates every subcarrier at once and produces one time-domain OFDM symbol.
5. **Parallel-to-serial and cyclic prefix:** the last N_cp samples are copied to the front. If the CP is longer than the maximum delay spread, ISI is absorbed and orthogonality is kept.
6. **DAC, filter, up-converter and power amplifier:** send the signal at RF.

#### Advantages
- Strong resistance to multipath fading and ISI (long symbols + CP).
- Simple one-tap equalization per subcarrier instead of a long time-domain equalizer.
- High spectral efficiency because subcarrier spectra overlap.
- Efficient implementation with FFT/IFFT.
- Flexible: bit loading and different modulation per subcarrier; easy to extend to OFDMA and MIMO.
- Robust against narrowband interference (only a few subcarriers affected).

#### Disadvantages
- High **peak-to-average power ratio (PAPR)**, needing linear, less efficient power amplifiers.
- Very sensitive to **carrier frequency offset** and Doppler, which destroy orthogonality (inter-carrier interference).
- Cyclic prefix and pilots waste some bandwidth and power.
- Needs accurate time and frequency synchronization.

## Check yourself

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

1. Why is interleaving used in the OFDM transmitter?
   A. So one deep fade does not wipe out consecutive bits
   B. To add the cyclic prefix
   C. To increase PAPR
   D. To reduce subcarriers
   Answer: A. Interleaving spreads bits across subcarriers so burst errors become scattered. (Key point: Interleaving)

2. How is the cyclic prefix formed?
   A. Zeros are added
   B. The last N_cp samples are copied to the front
   C. The first samples are deleted
   D. A random sequence is added
   Answer: B. Copying the tail to the front absorbs ISI if CP exceeds the delay spread. (Key point: Cyclic prefix creation)

3. A key OFDM advantage is:
   A. Low PAPR
   B. One-tap equalization and strong resistance to multipath
   C. Insensitivity to frequency offset
   D. No synchronization
   Answer: B. Long symbols plus CP resist multipath, and each subcarrier needs only one-tap equalization. (Key point: Main advantage)

4. A main OFDM disadvantage is:
   A. High PAPR and sensitivity to carrier frequency offset and Doppler
   B. Low spectral efficiency
   C. Cannot use FFT
   D. Cannot use QAM
   Answer: A. High PAPR requires linear amplifiers, and frequency offset destroys orthogonality causing ICI. (Key point: Main disadvantage)

## 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](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter) — Wireless Communication, chapter 4
- [MSK, GMSK and OFDM modulator/demodulator](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-ofdm-modem) — 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

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