Agilent Technologies E8267D PSG - Manual (Page 208)

194 Chapter 7
Custom Real Time I/Q Baseband
Working with Differential Data Encoding
The following illustration shows a 4QAM modulation I/Q State Map.
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:
1 2
3 4
1st Symbol 
Data = 00000000

Distinct values: +1, +1
4th Symbol 
Data = 00000011

Distinct values: +1, –1
2nd Symbol 
Data = 00000001

Distinct values: –1, +1
3rd Symbol 
Data = 00000010

Distinct values: –1, –1
transmittedbit i
  databit i 1
–
 
= databit i
  