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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:

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

1 2 ... 203 204 205 206 207 208 209 210 211 212 213 ... 317 318

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Agilent Technologies E8257C PSG Guía de usuario Pagina 208