11-10-2016, 04:04 PM
1458667829-TUTORIAL2DifferentialEncoding.pdf (Size: 19.52 KB / Downloads: 619)
One of the common difficulties in communications engineering is that often the same terms are used to
represent different ideas. Differential Encoding is one such term. We will describe here two common
ways this term is used and unless you know these differences, they can lead to confusion.
Differential Encoding – Is used to provide polarity reversal protection
Bit streams going through the many communications circuits in the channel can be
un-intentionally inverted. Most signal processing circuits can not tell if the whole stream is inverted.
This is also called phase ambiguity. Differential Encoding is used to protect against this possibility. It is
one of the simplest form of error protection coding done on a baseband sequence prior to modulation.
A Differential Coding system consists of a modulo 2 adder operation as shown below.
din = Data sequence in
eout = Differentially Encoded data sequence out
Differential Encoding - Used to provide phase reference
Differential Encoding as in DEBPSK or DPSK) is an another way to apply this same concept.
Although the concept is exactly the same, here the purpose of Differential Encoding is to enocode the
information in terms of phase changes. DPSK or DEBPSK is used in situations where coherent
detection is not possible and the signal phase transitions are used to decode the data. We will look at the
details of this modulation in a later release.
Summary:
Main purpose of Differential Encoding is to protect against polarity reversals of input bit sequences.
Hence Differentially Encoded data sequences have a slightly superior error performance. Differential
Encoding is also used to provide a way to decode a BPSK signal, called DEBPSK or DPSK. The
operation is exactly the same as described above.
In software simulation, there is no chance that a bit sequence will be reversed. So when
creating a simulation model of a Differentially Encoded sequence, we can ignore this baseband coding.
However, when modeling a DPSK system, we need to consider its implication since it is essential to the
system.