QPSK
参考答案：
即四相移相键控。可采用相干解调。相邻码元相位变化为四种，取值±π/4和±3π/4。
--- Did you notice him come into the hall?
--- Sorry, sir. I _______ with my friends online.
A．was chatting
B．chatted
C．would chat
D．am chatting
在框架式结构的装配中，常用挡铁定位进行装配。( )
