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Chapter 7 179
Custom Real Time I/Q Baseband
Working with Differential Data Encoding
mapping at the point of modulation, differential data encoding uses the transition from one bit value
to another to encode the raw data.
Differential data encoding modifies the raw digitized data by creating a secondary, encoded data
stream that is defined by changes in the digital state, from 1 to 0 or from 0 to 1, of the raw data
stream. This differentially encoded data stream is then modulated and transmitted.
In differential data encoding, a change in a raw data bit’s digital state, from 1 to 0 or from 0 to 1,
produces a 1 in the encoded data stream. No change in digital state from one bit to the next, in
other words a bit with a value of 1 followed by another bit with a value of 1 or a bit with a value
of 0 followed by the same, produces a 0 in the encoded data. For instance, differentially encoding the
data stream containing 01010011001010 renders 1111010101111.
Differential data encoding can be described by the following equation:
For a bit-by-bit illustration of the encoding process, see the following illustration:
How Differential Encoding Works
Differential encoding employs offsets in the symbol table to encode user-defined modulation schemes.
The Differential State Map editor is used to introduce symbol table offset values, which in turn
cause transitions through the I/Q State Map based on their associated data value. Whenever a data
value is modulated, the offset value stored in the Differential State Map is used to encode the data
by transitioning through the I/Q State Map in a direction and distance defined by the symbol table
offset value.
Entering a value of +1 causes a 1-state forward transition through the I/Q State Map. As an example,
consider the following data/symbol table offset values. These symbol table offsets result in one of the
transitions shown.
transmittedbit i() databit i 1–()= databit i()⊕
1
01 00 1 100
1
1
0
raw (unencoded) data
differentially encoded data
11
11
1
1
1
111
0
0
0
change =
no change =
0