Bandwidth of M-ary modulation schemes
The required bandwidth depends on the symbol rate (or baud rate). If you have a binary system (i.e. two symbols) then the bit rate and the symbol rate are equal. If you use $M&gt;2$ symbols, you can transmit more than 1 bit per symbol and your symbol rate, hence also the required bandwidth, is decreased. The problem with increasing the number of symbols is that you need more energy to keep the average distance between the symbols (i.e., the robustness against noise) the same. As pointed out by @vaz in a comment, the latter is only true for passband PAM systems, such as QAM or PSK.