---
title: "PAM versus QAM"
subject: "Data Communication"
chapter: 5
key: "semester_5/data_communication:pam-vs-qam"
source: https://www.bctengineer.com/q/semester_5/data_communication/pam-vs-qam
site: BCT Engineering
---

# PAM versus QAM

Data Communication · Chapter 5: Data Encoding and Modulation

- Asked in: 2069 Asar, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/pam-vs-qam
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-5

## Question

Differentiate between PAM and QAM.

## Short answer

PAM varies only amplitude of baseband pulses (1-D). QAM modulates both amplitude and phase of a carrier by two quadrature components (2-D). QAM achieves higher spectral efficiency than PAM but needs better SNR and complex receiver.

## Answer

**PAM (Pulse Amplitude Modulation)** varies the amplitude of baseband pulses according to the message (or, in digital form, uses several amplitude levels to carry multiple bits per symbol). **QAM (Quadrature Amplitude Modulation)** varies both the amplitude and the phase of a carrier by modulating two quadrature carriers (cos and sin) with two PAM signals.

| Point | PAM | QAM |
|---|---|---|
| Parameter varied | Pulse amplitude only | Amplitude and phase (two carriers 90° apart) |
| Signal type | Baseband pulses | Bandpass (carrier modulated) |
| Dimensions | One (I only) | Two (I and Q) |
| Constellation | Points on a line | Points on a 2-D grid (e.g. 16-QAM) |
| Bits per symbol | log₂M for M levels | log₂M for M points (twice the data of PAM with the same bandwidth) |
| Bandwidth efficiency | Lower | Higher |
| Complexity | Simple | Complex (carrier recovery, two mixers) |
| Noise | Sensitive to amplitude noise | Dense constellations (e.g. 64-QAM) have closely spaced points and need a good SNR |
| Use | Step in PCM, Ethernet (PAM-4/5) | Modems, cable, DSL, Wi-Fi, LTE |

## Check yourself

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

1. What does QAM do?
   A. Varies pulse width only
   B. Varies both amplitude and phase using two quadrature carriers
   C. Varies carrier frequency only
   D. Varies pulse position
   Answer: B. QAM modulates cos and sin carriers with two PAM signals. (Key point: QAM definition)

2. How does the constellation differ?
   A. PAM is a 2-D grid, QAM is on a line
   B. Both are on a line
   C. PAM points lie on a line, QAM on a 2-D grid
   D. Both are 2-D
   Answer: C. PAM has one dimension (I), QAM two (I and Q). (Key point: Dimensions)

3. What is the cost of QAM's higher efficiency?
   A. Lower data rate
   B. Needs a good SNR and complex carrier recovery
   C. Needs no carrier
   D. Larger pulses
   Answer: B. Points are closer, so QAM needs good SNR and more complex hardware. (Key point: Noise trade-off)

## Related in Data Communication

- [PAM versus PWM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apam-vs-pwm) — Data Communication, chapter 5

## Related in other subjects

- [PAM vs PPM; generation of PWM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apam-ppm-pwm-generation) — Communication Systems, chapter 4