Explains the differences between quadrature amplitude modulation (QAM) and amplitude and phase shift Keying (APSK) modulation.

In the video I should probably have also pointed out that the square QAM constellation has higher Peak-to-Average Power Ratio (PAPR) than APSK, due to its "corner constellation points". Since, (as explained in the video) it is these points that cause the problems with nonlinear threshold amplifiers, there are some advantages to using constellations that don't have them, and therefore have lower PAPR (such as APSK, or QAM without the corner points as is done for large constellation sizes). But there's a trade-off. If the PAPR is totally minimised (eg. M-ary PSK), then the constellation points will be much closer together and the BER will be higher, even with fully linear amplifiers.

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