16-01-2013, 04:27 PM
Digital Communications Lab (CE-343L)
Digital Communications.pdf (Size: 452.19 KB / Downloads: 23)
Objective:
The objective of this lab is to implement the modulator and demodulator of M-ary Phase shift keying
(PSK) and to study the effect of noisy channel (AWGN).
Background and Preparation:
M-ary Pulse Amplitude Modulation
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating,
the phase of a reference signal (the carrier wave).
Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses
a finite number of phases; each assigned a unique pattern of binary bits. Usually, each phase encodes an
equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase.
The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the
phase of the received signal and maps it back to the symbol it represents, thus recovering the original
data. This requires the receiver to be able to compare the phase of the received signal to a reference signal
— such a system is termed coherent (and referred to as CPSK).
A convenient way to represent PSK schemes is on a constellation diagram. This shows the points in the
real and imaginary axes and are termed the in-phase and quadrature axes respectively due to their 90°
separation. Such a representation on perpendicular axes lends itself to straightforward implementation.
The amplitude of each point along the in-phase axis is used to modulate a cosine (or sine) wave and the
amplitude along the quadrature axis to modulate a sine (or cosine) wave.
In PSK, the constellation points chosen are usually positioned with uniform angular spacing around a
circle. This gives maximum phase-separation between adjacent points and thus the best immunity to
corruption. They are positioned on a circle so that they can all be transmitted with the same energy. In this
way, the module of the complex numbers they represent will be the same and thus so will the amplitudes
needed for the cosine and sine waves. Two common examples are "binary phase-shift keying" (BPSK)
which uses two phases and "quadrature phase-shift keying" (QPSK) which uses four phases, although any
number of phases may be used. Since the data to be conveyed are usually binary, the PSK scheme is
usually designed with the number of constellation points being a power of 2.
Binary Phase Shift keying (BPSK)
BPSK (also sometimes called PRK, Phase Reversal Keying) is the simplest form of PSK. It uses two
phases which are separated by 180° and so can also be termed 2-PSK. It does not particularly matter
exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at
0° and 180°. This modulation is the most robust of all the PSKs since it takes serious distortion to make
the demodulator reach an incorrect decision. It is, however, only able to modulate at 1 bit/symbol (as seen
in the figure) and so is unsuitable for high data-rate applications when bandwidth is limited.