Differential Phase Shift Keying (DPSK)

Differential Phase Shift Keying:-

It is Non Coherent version of Phase Shift Keying. It eliminates the need for a Coherent reference signal at the Receiver, by combining the following two basic operations at the Receiver.
1. Differential encoding
2. PSK
Hence given the name DPSK.

DPSK Generator:-

The Figure shows the Block Diagram of DPSK Transmitter. It consists of a logic Network (we can use EX-OR also) EX-NOR gate and a one bit-delay element so as to convert the Raw Binary sequence {bk} into a differentially encoded sequence {dk}.Now this sequence is amplitude level encoded and then used to modulate a carrier of frequency fc(similar to PSK Generator), there by producing the desired DPSK signal.

Now the generation of DPSK signal is explained with an example sequence

Input sequence : {bk} : 1 0 0 1 0 0 1 1
One bit delayed bit {dk-1} : 1
Differentially encoded sequence {dk}: 1 1 0 1 1 0 1 1 1
Transmitted Phase : 0 0 π 0 0 π 0 0 0
:0o 0o 180o 0o 0o 180o 0o 0o 0o

Now the corresponding phases has been transmitted from the Transmitter in the form of a PSK signal, Now this transmitted DPSK signal will be received at the Receiver

Now At the Receiving end
DPSK input to the Receiver : 0o 0o 180o 0o 0o 180o 0o 0o 0o

now the phase at the current instance is compared with its next phase , if both are same the received bit is a ‘1’ otherwise a ‘0’.
Therefore the Received sequence is : 1 0 0 1 0 0 1 1.

From the DPSK generator {dk} = {bk} Xnor {dk}.
The Transmitted phase has been chosen based on PSK scheme
for binary ‘1’ ——–> 0o
for binary ‘0’———> 180o

DPSK Receiver:-

The Received signal is delayed by one bit time, then compared with the next signaling element in the Balanced Modulator, here Balanced Modulator acts as Phase Comparator .
If the Phases are same a logic ‘1’ is identified and for different phases a logic ‘0’. If the reference phase is incorrectly assumed only the first demodulated bit is in error .

Differential Encoding can be implemented with higher than Binary digital Modulation schemes. Although the Differential algorithms are much more complicated than for DPSK.

The Primary Advantage of DPSK is the Simplicity with which it can be implemented. No carrier recovery circuit is needed. A Disadvantage of DPSK is that it requires 1 dB and 3 dB more Signal to Noise Ratio to the same bit error rate as that of absolute PSK.

DPSK Generator and Detector are Collectively called as DPSK system.

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Author: Lakshmi Prasanna

Completed M.Tech in Digital Electronics and Communication Systems and currently working as a faculty.