QAM - garden

The Limitation of QPSK

In QPSK, we used 4 phases to get 4 symbols = 2 bits per symbol.

But notice: all 4 symbols are the same distance from the center. We’re only changing phase, not amplitude.

All points sit on a circle. We’re leaving a lot of space unused.

What if we used both phase AND amplitude?

Instead of just a circle, we fill the 2D space with a grid of symbols.

More symbols = more bits per symbol.

16-QAM arranges symbols in a 4×4 grid:

That’s 16 symbols.

$\text{bits per symbol} = \log_2(16) = 4$

Each symbol carries 4 bits. Double what QPSK can do!

We use two carrier waves that are 90° apart:

By setting the amplitude of each wave independently, we can place the signal at any point on the grid.

For 16-QAM, each wave can have 4 amplitude levels: -3, -1, +1, +3

That gives us 4 × 4 = 16 possible combinations = 16 grid points.

Each unique combination of I and Q amplitudes represents a different symbol.

Want even more efficiency? Use an 8×8 grid:

That’s 64 symbols.

$\text{bits per symbol} = \log_2(64) = 6$

Each symbol carries 6 bits.

More symbols = more bits per symbol = higher data rate in the same bandwidth.

With more symbols packed into the same space, they get closer together.

If noise shifts a received signal slightly, it might land on the wrong symbol.

The more efficient the modulation, the cleaner the signal needs to be.

This is why WiFi and LTE adapt:

Your phone is constantly switching modulation based on signal quality.

Everywhere that needs high data rates: