Phase Shift Keying(PSK) in Digital Communications

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What is Phase Shift Keying?

Phase Shift Keying (PSK) is a digital modulation technique used to transmit data by modifying the phase of a carrier wave. This method is widely used in communication systems, including wireless networks, satellite communication, and fiber optic transmission.

How Does PSK Work?

1. Carrier Wave Generation: A high-frequency carrier wave is generated.

2. Data Encoding: Digital data (0s and 1s) is encoded onto the carrier wave by shifting its phase.

3. Phase Shift: The phase of the carrier wave is shifted according to the data being sent.

4. Transmission: The modulated carrier wave is transmitted through the channel.

5. Demodulation: The received signal is demodulated to extract the original data.

Types of PSK

1. Binary Phase Shift Keying (BPSK): 2-phase shifts (0° and 180°) for binary data.

2. Quadrature Phase Shift Keying (QPSK): 4-phase shifts (0°, 90°, 180°, and 270°) for 2-bit data.

3. 8-Phase Shift Keying (8PSK): 8-phase shifts for 3-bit data.

4. Differential Phase Shift Keying (DPSK): Phase shifts are relative to the previous symbol.

In phase shift keying, the phase of the carrier signal changes concerning binary data.

Binary phase shift keying(BPSK):

There are two phases:

1.   Zero-degree phase

2.   180-degree phase

Let s1(t)=Accos(2πfct+00)—b(t)=1

S2(t)=Accos(2πfct+π)—b(t)=0

Logic 0=+1— Accos2πfct

Logic 1=-1—- Accos2πfct

Isolation distance of phase Shift Keying

Compared to ASK, and FSK, the isolation distance for phase shift keying PSK is high.

For binary data b(t)

B(t)=r

c(t)= Accos2πfct

s1(t)= Accos2πfct b(t)=1

s2(t)=- Accos2πfct b(t)=0

b(t)=bipolar NRZ

logic 1=+1r

logic 0= -1r

s(t)=b(t) Accos2πfct.

BPSK Modulator:

BPSK Modulator
BPSK Wave forms

Coherent BPSK Demodulator:

Coherent BPSK Demodulator

Case-1:  if s(t)= Accos2πfct      s(t)=s1(t)

c(t)= Accos2πfct         b(t)=1

Now for PM, s1(t)= Ac2 cos 22πfct

Output is Ac2/2

Case-2: if s(t)=s2(t) b(t)=0

s2(t)= -Ac2 cos 22πfct

output is -Ac2/2

The bandwidth of phase shift keying is 2fb

Bandwidth of Phase Shift Keying

Differential Phase Shift Keying:

Differential Phase shift keying

d(t)=b(t) exclusive OR with d(t-tb)

DPSK Wave forms

DPSK Demodulator:

The first-bit duration (0 to Tb)

s(t)=s1(t)

the output of PM is s(t)= – Accos2πfcts1(t)

= -Ac2 cos 24πfct

DPSK Demodulator

The second-bit duration (Tb to 2Tb)

The output of PM is s(t)= – Accos2πfcts1(t)s1(t)

=-Ac2/2- Ac2cos4πfct /2

For the second bit duration

Output= Ac2/2 + Ac2cos2πfct/2

QPSK Demodulator:

QPSK Demodulator

I=1,Q=0

s(t)= Acsinwct + Accoswct

I-channel:

Output of PM= Ac 2sin2 wct- Ac 2coswct+ sinwct

            = Ac 2/2 – Ac 2/2 cos2wct –  Ac 2/2 sin2wct

            = Ac 2/2

Q-channel:

Output of phase modulation= Ac 2sinwct coswct + Ac 2coswct

 = – Ac 2/2 – Ac 2/2 cos2wct + Ac 2/2 sin2wct

 = – Ac 2/2

Bandwidth of QPSK:

BW=1/Tb

     = Rb or fb

Bandwidth of QPSK

Example:

A voice signal sample at the rate of 8000 samples per second and each sample is encoded into 8 bits using PCM. The binary data is transmitted into free space after modulation. Determine the bandwidth of the modulated signal when the modulation is ASK, BPSK, FSK with f1=10 MHz, f2=8MHz

Solution: Rb=nfs

=8 x 8000

=64000 bits/sec

fb=64 KHz

Bandwidth of ASK = 2fb=128 KHz

Bandwidth of BPSK = 2fb= 128 KHz

Bandwidth of FSK = (f1-f2)+2fb=2 MHz+128 KHz

                                      = 2.128 MHz

Bandwidth of QPSK=fb=64 KHz.

Advantages of PSK

1. High Spectral Efficiency: More data is transmitted within a given bandwidth.

2. Resistance to Noise and Interference: PSK signals are less susceptible to noise and interference.

3. Simple Implementation: PSK is relatively easy to implement using digital circuits.

Limitations of PSK

1. Sensitive to Phase Noise: PSK signals can be affected by phase noise in the transmitter or receiver.

2. Requires Coherent Detection: PSK signals require coherent detection, which can be complex to implement.

Applications of Phase Shift Keying:

  1. Wireless LANs (Wi-Fi): PSK is widely used in Wi-Fi networks to transmit data wirelessly between devices and routers. It enables high-speed internet connections for laptops, smartphones, and other devices.
  2. Bluetooth Communication: PSK is used in Bluetooth technology for short-range wireless communication between devices due to its efficiency and low power consumption.
  3. Satellite Communication: PSK is crucial for reliable data transmission between ground stations and Earth-orbiting satellites.
  4. Digital Television Broadcasting: PSK is utilized in digital television broadcasting standards like DVB-S and DVB-T2. It helps efficiently transmit high-quality video and audio signals to viewers’ homes.
  5. RFID (Radio-Frequency Identification): PSK in RFID systems identifies and tracks objects using radio waves in applications like inventory management, access control, and contactless payments.

Conclusion

Phase Shift Keying is a widely used and effective modulation technique for digital communication systems. Its advantages make it a popular choice for various applications, but its limitations should also be considered. By understanding PSK, you can better appreciate the technology behind modern communication systems.

FAQs of Phase Shift Keying:

1. What is Phase Shift Keying (PSK)?

  • PSK is a digital modulation technique where the phase of a carrier wave is changed to represent digital data. 
  • It’s a widely used method for transmitting information over wireless and wired communication channels.

2. How does Phase Shift Keying (PSK) work?

  • In PSK, the phase of the carrier signal is shifted according to the digital data being transmitted. 
  • Each unique phase shift corresponds to a specific symbol or bit pattern, allowing the receiver to decode the information.

3. What are the different types of PSK?

The most common types of PSK include:

  • Binary Phase Shift Keying (BPSK): Binary 0 and 1 are represented by two phases
  • Quadrature Phase Shift Keying (QPSK): QPSK leverages four phase states to achieve a higher data rate, transmitting two bits per symbol.
  • 8-PSK: Eight phases are used, representing three bits per symbol.
  • 16-PSK: Sixteen phases are used, representing four bits per symbol.

4. What are the advantages of PSK?

PSK offers several advantages in digital communication:

  • Good noise immunity: It’s relatively resistant to noise and interference.
  • Bandwidth efficiency: It offers increased data-carrying capacity within a given bandwidth compared to other modulation schemes.
  • Simple implementation: It’s relatively easy to implement in hardware and software.

5. What are the applications of PSK?

PSK is used in a wide range of applications, including:

  • Wireless LANs (Wi-Fi)
  • Bluetooth communication
  • Satellite communication
  • Digital television broadcasting
  • RFID (Radio-Frequency Identification)
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