Wireless 101
M05 · L04
Module 5 · Lesson 4

Amplitude and Phase

PSK rotates a point; ASK slides it in and out. Do both at once and you can place points anywhere on the plane, not just a ring. That is QAM — the scheme carrying most of the world’s wireless bits.

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Wireless 101
M05 · L04
The idea

Two Streams, One Carrier

QAM Signal
s(t) = I\cos(2\pi f_c t) - Q\sin(2\pi f_c t),\ \ I,Q \in \{\pm1,\pm3,\dots\}

Let I and Q each take several amplitude levels, independently. Two multi-level streams on the orthogonal cosine and sine carriers — amplitude and phase keyed together.

02 / 11
Wireless 101
M05 · L04
The first grid

16-QAM — a 4×4 Grid

Four levels on each axis make a 16-point grid: 2 bits on I, 2 on Q → 4 bits per symbol. Each point is a distinct (amplitude, phase) pair. Twice the bits of QPSK, in the same symbol slot.

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Wireless 101
M05 · L04
Climbing the grid

Up to 4096-QAM

Each doubling of levels per axis adds two bits: b = log₂M. WiFi 7 reaches 4096-QAM — 12 bits in one symbol.

256-QAM
8 bits
1024-QAM
10 bits
4096-QAM
12 bits
04 / 11
Wireless 101
M05 · L04
Cheap mistakes

Gray Coding

Label the axes so adjacent points differ by one bit. Since I and Q are independent, you Gray-code each axis on its own. Then the common error — a slip to the next cell — costs a single bit, even when the symbol carries twelve.

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Wireless 101
M05 · L04
Why not bigger PSK?

Fill the Plane

PSK is stuck on a circle — add points and they crowd around the ring, collapsing dmin. QAM spends the same power filling two dimensions, so its points sit farther apart. That is why PSK stops at 8 and everything denser is QAM.

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Wireless 101
M05 · L04
Try it

QAM vs PSK

Same order, same power, side by side. Watch the minimum distance — the gap that noise must cross — stay wide for QAM while PSK crowds its ring.

Order 16 points
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Wireless 101
M05 · L04
The bill

Not Free

QAM is not constant-envelope — its power swings symbol to symbol. That high peak-to-average ratio forces a linear amplifier with backoff (ASK’s old penalty) and makes it touchy about phase noise. And it still needs ~6 dB more SNR per extra 2 bits.

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Wireless 101
M05 · L04
In the wild

Ride the Ladder

Adaptive modulation picks the densest QAM the channel allows, updated every few ms: 4096-QAM beside the WiFi 7 router, dropping to 64-QAM, QPSK, BPSK as you walk away.

  • WiFi — 256-QAM (ac), 1024-QAM (WiFi 6), 4096-QAM (WiFi 7)
  • LTE & 5G NR — up to 256-QAM
  • Cable / DOCSIS 3.1 — up to 4096-QAM
09 / 11
Wireless 101
Knowledge Check

Check what stuck

Four questions from this lesson. Answer to see why — the explanation appears whether you were right or wrong. Nothing is scored or saved.

Question 1 of 0
Score 0/0

10 / 11
Wireless 101
M05 · L04
Recap

What you learned

  • QAM = independent multi-level amplitudes on the I and Q carriers — amplitude and phase at once
  • Square M-QAM is a √M grid: 16-QAM = 4 bits up to 4096-QAM = 12
  • Gray-code each axis so a neighbour slip costs one bit; QAM beats PSK by filling 2-D
  • Cost: high PAPR, a linear amplifier, ~6 dB per +2 bits — ridden with adaptive modulation
Module 5 complete — up next
Module 6: Shannon →
All lessons Read article
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