Phase

• In the signal set above, the amplitude and frequency are the same for both signals.
• However, the signals have different signs.
• Another interpretation is that the two signals have different phases.
• The phase of a sinusoid indicates how much it is delayed.
• Phase values must be in the range $0-2\pi$.
• In the passband signal set above, the two sinusoids differ in phase by $\pi$.
• For this reason, the above signal set is sometimes referred to as binary phase shift keying or BPSK.

  

Figure: Two sinusoids: One has phase 0 and the other has phase $\frac{\pi}{ 4}$.