The demand for higher signaling speed
has grown in recent times. These systems require synchronous
operation to obtain acceptable level of sampling accuracy.
Phase shift keying provides a solution to this.
In PSK, the mark and space signals are represented by
180º phase shift and zero phase shift respectively,
as shown in the illustration below. The system is sometimes
referred to as two phase PSK and is not much different
from ASK. However, it requires a smaller bandwidth.
As such, signal speeds upto 2400 bauds can be accommodated
in the speech channel.
Under noisy conditions, two phase PSK
may fail to provide satisfactory reception. In such
cases, PSK with more than two phases can be employed.
In a four phase PSK, phase shifts may be 0º, 90º,
180º and 270º. Noise immunity in this system
increases by three dbs, as compared to a two phase PSK.
More sophisticated PSK systems may employ an eight phase
PSK of four phases four amplitude or four level quartered
systems. The last provides the best noise immunity.
Instabilities in oscillator frequencies may cause phase
jitters in PSK system.
In short, PSK proves to be better than FSK and ASK for
working under noisy conditions and are capable of operating
at higher signaling speeds, but it requires complicated
detection process for recovery of the modulating signal
at the receiver. Under normal working conditions at
moderate signaling speeds, FSK may emply with performance
better than ASK under noisy environments. Lastly, to
overcome synchronizing difficulties, sometimes a form
of PSK which uses the phase information of the bit previously
transmitted as reference is employed. This is termed
as differential phase shift keying (DPSK).
Generation of Binary PSK Waves
In phase shifting, the differences in
phase of the two carriers used for representing logic
0 and logic 1 is 180º. Mathematically binary PSK wave
will be as follows:
Generation of Binary PSK Waves
Binary PSK wave is generated using a
product modulator whose inputs are the sinusoidal carrier
and the binary bit stream in the polar form. Generation
of binary PSK wave is shown in above illustration The
binary PSK wave is somewhat similar to DSB-SC wave.
The power spectral density of BPSK waveform is similar
to ASK waveform. The power spectral density of the BPSK
waveform, assuming random data is shown in the below
illustration,
