Any PDK has at least 2 types of transistors: core and I/O , in this case 1.8 transistors are for core, and 3.3 for I/O ( transistors that are connected cu I/O pads ). Usually when you have to interconect thoes two, you use a level shifter.
obi1,level shifter,you mean that 3.3V mos should use under 3.3V.so I must make the 1.8 up to 3.3 through a level shifter,then ,I can use 3.3V under 3.3V.