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162 Chapter 7
Custom Real Time I/Q Baseband
Working with Differential Data Encoding
Differential Data Encoding
In real-time I/Q baseband digital modulation waveforms, data (1’s and 0’s) are encoded, modulated onto a
carrier frequency and subsequently transmitted to a receiver. In contrast to differential encoding, differential
data encoding modifies the data stream prior to I/Q mapping. Where differential encoding encodes the raw
data by using symbol table offset values to manipulate I/Q 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:
transmittedbit i() databit i 1–()= databit i()⊕
1
01 001 100
1
1
0
raw (unencoded) data
differentially encoded data
11
11
1
1
1
111
0
0
0
change =
no change =
0