Phase shift keying
Phase shift keying (PSK) is a digital modulation technique in which the phase of a carrier is modulated to convey digital information. The image to the right conveys the concept using a simple form of PSK known as binary phase shift keying (BPSK). In that scheme, one phase is chosen to represent a zero and another to represent a one. Specifically, the example shown depicts one bits being transmitted by shifting the phase 90 degrees from its current position, and zero bits by leaving the phase as is (i.e., a phase shift of zero degrees).
The sharp points in the wave at the phase shifts are detectable at the receiver as sudden high frequency components in the carrier wave. They thereby help the receiver maintain synchronization, making most PSK schemes self-clocking forms of digital signaling. This self-clocking ability is critical at higher speeds, and avoids the need to bit scrambling or lower efficiency mapping schemes like 4B5B. Note that the scheme just described, however, is only pseudo-self-clocking because a long series of zeroes will result in an unchanging carrier, causing synchronization problems (a problem similar to that encountered by a T-1 carrying data when the line code is AMI.
PSK schemes are typically depicted using a constellation diagram, like the one depicted to the left for a simple BPSK scheme. The basis for the constellation is a classic Cartesian coordinate system with an X and Y axis. The red dot represents a single point on the constellation. The distance of the red dot from the center of the graph represents the amplitude of the transmitted signal. The angle between the east axis (the one directly to the right) and the line between the constellation point and the center of the graph represents the desired phase shift. In the example depicted, a 45 degree phase shift is used to represent a zero and a 225 degree phase shift to represent a one. Notice that the use of two non-zero phase shifts eliminates the pseudo-self-clocking problem and makes this scheme fully self-clocking.
The constellation depicted to the right is for a more complex PSK scheme known as quadrature phase shift keying (QPSK). As the name implies, this scheme has four points on the constellation, making it possible to send two bits with each phase shift by assigning (as depicted) each of the four phase shifts one of the four possible two-bit codes.
Higher order PSK schemes are theoretically possible, but eight phase shift keying (8-PSK) is typically the maximum implemented. The reason is noise. Each increase in constellation density has to be a double of the previous level because of the need for twice as many signals to add just one more bit to the transmission (e.g., 1 bit per signal with 2 points, 2 bits per signal with 4 points, 3 bits per signal with 8 points, 4 bits per signal with 16 points, and so forth). Increasing the constellation density, however, reduces the difference between the phase shifts. Note that in the BPSK constellation depicted above, the difference in the phase shifts between the two points is 180 degrees - a wide and very discernible difference. With four points the phase shifts are only 90 degrees apart. With eight points they are 45 degrees apart. A sixteen-point constellation would have only 22.5 degrees between phase shifts, and at 32 points the shifts are only 11.25 degrees apart. This makes them very difficult to distinguish at the receiver if there is any distortion or noise on the medium.
Absolute vs. Differential PSK
Another important consideration for PSK schemes is the difference between an absolute and a differential PSK scheme. The former uses a base reference carrier and always sets the phase relative to that carrier. The problem with that approach is two-fold. First, anything that can disrupt the phase of the signal will make it very difficult to read at the receiver. Second, it creates synchronization problems if there is a series of bits that translates to the same phase shift over and over again, because that produces a continuous sine wave with no variation. In other words, it compromises their self-clocking nature.
Differential PSK schemes make the phase shift relative to the phase of the carrier after the previous shift. This makes them fully self-clocking (as long as the zero degree phase shift is not used), and less prone to misreading at the receiver.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/psk.mp3%7Cdownload</mp3> | Phase shift keying (PSK)|