Binary Phase Shift Keying, or BiPhase Shift Keying, (BPSK) is a method of encoding and/or transmitting data on top of a carrier. These carriers can be baseband signals (as with Manchester encoding), free-space transmissions (as with amateur radio's PSK31 or G3RUH modes, or with optical Ethernet technology such as the RONJA project), or embedded as sub-carriers within the larger context of another signal (e.g., digital audio transmission in PAL-format TV signals).
The basic idea behind BPSK is to provide a carrier whose phase is alternated between 0 degrees and 180 degrees, as needed, to convey digital information. When transmission first starts, the carrier wave, typically a sine wave but it needn't be, might follow the pattern up, down, up, down, up, down, etc. When it's time to transmit something different, the pattern will change to down, up, down, up, down, up, etc. Note the "sense" of the carrier is opposite from what it used to be.
BPSK is trivially implemented in digital electronics. It has other interesting characteristics as well. For example, the bandwidth of a BPSK signal is precisely that of its data rate (although the generated bandwidth of a BPSK is much wider than its base datarate, analog filters can be employed to narrow the bandwidth down to its base datarate without loss of any data). In addition, its immunity to noise makes it the preferred modulation for many links demanding high reliability, such as long-haul, shortwave radio transmissions (e.g., amateur radio's PSK31), or satellite communications links.
See also: