Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H.

Majeed

PHASE SHIFT KEYING:

Phase-shift keying (PSK) is another form of angle-modulated, constant-amplitude
digital modulation.

In PSK the phase of the output signal gets shifted depending upon the input as shown
in figure below. PSK signals are mainly of two types, namely BPSK and QPSK,
according to the number of phase shifts. The other one is DPSK which changes the
phase according to the previous value.

Binary Phase-Shift Keying

The simplest form of PSK is binary phase-shift keying (BPSK), where N = 1 and M
= 2. Therefore, with BPSK, two phases (M=2N=21 = 2) are possible for the carrier.
The two phases are labelled {A, B} corresponding to either Φ=0 or Φ=180 degrees
and the same amplitude. One phase represents a logic 1, and the other phase represents
a logic 0. As the input digital signal changes state (i.e., from a 1 to a 0 or from a 0 to a
1), the phase of the output carrier shifts between two angles that are separated by 180°.
The phase of the signal remains constant during each bit and changes when the bit
change.
Hence, other names for BPSK are phase reversal keying (PRK) and biphase
modulation. BPSK is a form of square-wave modulation of a continuous wave (CW)
signal.

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

BPSK MODULATOR:
Figure below shows a simplified block diagram of a BPSK transmitter. A balanced
modulator (mixer) is used to generate PSK, at high data rate. The balanced
modulator acts as a phase reversing switch. Depending on the logic condition of the
digital input, the carrier is transferred to the output either in phase or 180° out of
phase with the reference carrier oscillator.

For lower data rate, simple analogue switch controlled by data can be used to
generate PSK.

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

PSK DEMODULATOR:
Since information is generally conveyed through phase variations, then only
coherent PSK is possible, i.e. coherent carrier phase must be generated at the
receiver.

Carrier Recovery (Squaring Loop):

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

a squarer circuit removes the 180° phase shifts in the modulated carrier

The zero-crossing detector (ZDC) changes sinewave into rectangular waves with
frequency 2c

where the phase 2(t) is now approximately phase locked to (t) of the incoming
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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

BANDWIDTH CONSIDERATIONS OF BPSK:

In a BPSK modulator. the carrier input signal is multiplied by the binary data.
If + 1 V is assigned to a logic 1 and -1 V is assigned to a logic 0, the input carrier
(sin ωct) is multiplied by either a + or - 1.

The output signal is either + 1 sin ωct or -1 sin ωct the first represents a signal that is
in phase with the reference oscillator, the latter a signal that is 180° out of phase
with the reference oscillator. Each time the input logic condition changes, the output
phase changes.

Mathematically, the output of a BPSK modulator is proportional to

BPSK output = [sin (2πfbt)] x [sin (2πfct)]
where
fb = maximum fundamental frequency of binary input (hertz)
fc = reference carrier frequency (hertz)

Example 1:
For a BPSK modulator with a carrier frequency of 70 MHz and an input bit rate of
5 Mbps, determine the maximum and minimum upper and lower side frequencies,
draw the output spectrum, determine the minimum Nyquist bandwidth, and calculate
the baud.

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

Solution

Minimum lower side frequency (LSF):

LSF=70MHz - 5MHz = 65MHz
Maximum upper side frequency (USF):
USF = 70 MHz + 5 MHz = 75 MHz
Therefore, the output spectrum for the worst-case binary input conditions is as
follows: The minimum Nyquist bandwidth (B) is
B = 75 MHz - 65 MHz = 10 MHz

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

Advantages of BPSK:

Disadvantages of BPSK:
Bandwidth efficiency is given by

Applications of BPSK:

Differential PSK (DPSK)

There is possibility in PSK detection that the receiver will receive “0” as “1” or “1”
as “0” (i.e. data complement) due to the initial phase of the carrier (phase ambiguity).
To solve this problem, a technique called differential PSK (DPSK) is used.

DPSK Modulator
• DPSK is a technique of BPSK, in which there is no reference phase signal.
Here, the transmitted signal itself can be used as a reference signal.
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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

• The serial data input is given to the XNOR gate and the output is again fed
back to the other input through 1-bit delay (i.e exclusive-NORing previous
output code with present data). The output of the XNOR gate along with
the carrier signal is given to the conventional PSK modulator with 0 o and 180o
phase shift for logic “0” and logic “1”.

The coded output x(n) is obtained from transmitted data using:

x(n) = modified bit transmitted

d(n) = input bit
x(n-1) =previous transmitted bit delayed by 1 bit duration Tb

Example 2:
Find the output coded sequence and carrier phase for data input d(n)=11011001 ….

Solution:
For any random choice of the initial content of the D flip-flop the DPSK will work
successfully. For example if the initial state of D-FF is logic “1” then: x(0)=1 and

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

DPSK Demodulator
The decoder of DPSK does not need correct phase of the carrier and hence DPSK
is sometimes called noncoherent PSK (DPSK=noncoherent PSK)

• The extra requirement for DPSK noncoherent detection is the analogue delay line of
Tb. The DPSK is delayed by Tb and then multiplied with itself. That signal is made
to confine to lower frequencies with the help of LPF then it is passed to threshold
comparator to recover the original binary data as the output.

Example 3:
With the help of data of previous example, explain the operation of DPSK detector.

Solution:
−𝐴cos𝜔𝑡 &+ 𝐴cos𝜔𝑡will be used for 180 & 0 phase shift then;

H.W-1:
Repeat encoder and decoder operations when x(0)=0, and comment to the results.

Bandwidth of DPSK

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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

Advantages of DPSK:
1- Noncoherent detection (no carrier generation)
2- No phase ambiguity (possible data complement) since the decoded data is
independent of random initial choice of the state of D-FF (x(0))
3-The BW of DPSK requirement is low evaluated to BPSK modulation.
4- non-consistent receivers are simple and inexpensive to construct, therefore
extensively used in wireless communication.

Disadvantages of DPSK:
1- The bit error rate is high in DPSK contrast to BPSK.
2- The interference of noise in DPSK is more.
3- This modulation employs two consecutive bits intended for its response. Thus
error in primary bit makes error within a subsequent bit as well as consecutively
error spreads.

Differential Phase Shift Keying Applications

The applications of DPSK mainly include wireless communications like RFID,
WLANs, and Bluetooth.

Problems
Q1: For the binary sequence 11111011111, sketch the resulting waveforms from
sinusoidal modulation if the modulation type: ASK, BPSK, FSK.
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Digital Communication, 3rd year, Chapter 3 Prof. Dr. Asmaa H. Majeed

Q2: Find the output coded sequence and carrier phase of DPSK modulator for input
data d(n)=01011101. Then show the detection process of DPSK demodulator.
Assume x (0) =0.

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