DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 396

10.13.2 Phase Shift Keying

Phase shift keying (PSK) is a method of transmitting and receiving digital signals
in which the phase of a transmitted signal is varied to convey information. Several
schemes can be used to accomplish PSK, the simplest one being binary PSK (BPSK),
using only two signal phases: 0° and 180°. If the phase of the wave is 0°, then the
signal state is low, and if the phase of the wave is 180° (if phase reverses), the signal
state is high ( biphase modulation ). More complex forms of PSK employ four- or
eight-wave phases, allowing binary data to be transmitted at a faster rate per phase
change. In four-phase modulation, the possible phase angles are 0°,

90°, and
180°; each phase shift can represent two bits per symbol. In eight-phase modulation,
the possible phase angles are 0°,

+ 90°, -

+ 45°, - 45°, + 90°, - 90°, + 135°, - 135°, and 180°; each

phase shift can represent 4 bits per symbol.

Binary Phase Shift Keying
A single data channel modulates the carrier. A single bit transition, 1 to 0 or 0 to 1,
causes a 180° phase shift in the carrier. Thus, the carrier is modulated by the data.
Detection of a BPSK signal uses the following: (1) a squarer that yields a DC com-
ponent and a component at 2 f c ; (2) a bandpass filter to extract the f c component; (3)
a frequency divider, the output of which is multiplied by the input. The result is
lowpass filtered to yield a PCM signal.

Quadrature Phase Shift Keying
Quadrature phase shift keying (QPSK) is a modulation scheme in which the phase
is modulated while the frequency and the amplitude are kept fixed. There are four
phases, each of which is separated by 90°. These phases are sometimes referred to
as states and are represented by a pair of bits. Each pair is represented by a partic-
ular waveform, called a symbol , to be sent across the channel after modulating the
carrier. The receiver demodulates the signal and look at the recovered symbol to
determine which pair of bits was sent. This requires a unique symbol for each pos-
sible combination of data bits in a pair. Because there are four possible combina-
tions of data bits in a pair, QPSK creates four different symbols, one for each pair,
by changing an in-phase (I) gain and a quadrature (Q) gain.
The QPSK transmitter system uses both sine and cosine at the carrier frequency
to transmit two separate message signals, sI[n] and sQ[n], referred to as the in-phase
and quadrature signals, respectively. Both the in-phase and quadrature signals can
be recovered, allowing transmission with twice the amount of signal information at
the same carrier frequency.

Transmitter/Receiver Algorithm
An input sample is obtained and stored in a memory location, which contains 16
bits. Depending on the type of PSK (two-level or four-level), appropriate masking