64 QAM versus lower order modulation This question is regarding digital modulation.

If 64 QAM always provides rates greater than BPSK or 16 QAM, then why do systems always use higher order modulation of QAM?

Thanks
There is a trade-off between modulation order, required energy and bit-error rate (BER). As you increase the modulation order, you need to increase the average energy per bit to keep the same BER.

As an easy example, consider BPSK vs 4-PAM. Say you have BPSK with distance between constellation points $d_{min}=1$; this works out to an average bit energy of 0.25 joules per bit.

Now, you want to double the bit rate by going to 4-PAM, but you want to keep roughly the same BER, which means having the same minimum distance $d_{min}$ between constellation neighbors. Now, your average symbol energy is $$\overline{E_s}=\frac{2\cdot1.5^2+2\cdot0.5^2}{4}=1.25$$ joules, or an average bit energy $\overline{E_b}=0.625$.

As you increase the modulation order, you'll see the energy required increase exponentially. Exactly the same thing happens when using QAM.