6-10
Real-Time I/Q Baseband Generator User’s and Programming Guide
Theory of Operation
ESG Family Signal Generators
Understanding Differential Data Encoding
Options UN8 and 202
Understanding Differential Data Encoding
In digital modulation formats such as GSM, digital data (1’s and 0’s) are encoded,
modulated onto a carrier frequency and subsequently transmitted to a receiver. In contrast
to differential encoding (described on page 2-30), 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.
How Differential Data Encoding Works
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 to Access and Apply Differential Data Encoding
You can apply differential data encoding to a custom modulation by pressing
Mode
>
Real Time I/Q BaseBand
(if this key is present) >
Custom
>
Diff Data Encode Off On
until
On
is
highlighted.
You can apply differential data encoding to a GSM modulation by pressing
Mode
>
Real Time I/Q BaseBand
(if this key is present) >
TDMA
,
GSM
>
Modify Standard
>
Diff Data Encode Off On
until
On
is highlighted.
transmittedbit i
( )
databit i
1
–
(
)
=
databit i
( )
⊕
1 0 1 0 0 1 1 0 0
1
1 0
raw (unencoded) data
differentially encoded data
1 1
1
1
1
1
1 1 1
1
0
0
0
change =
no change =
0
Summary of Contents for ESG series
Page 2: ...ii ...
Page 446: ...I 16 Index Index ...
