- 2069 Asar · 4 marks
PAM versus 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.
Differentiate between PAM and QAM.
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 |
✓ Answer checked by Claude Sonnet 5.5
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