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Фрагмент инструкции
Differential data encoding can be described by the following equation: transmittedbit i()= databit i( –1).databit i() For a bit-by-bit illustration of the encoding process, see the following illustration: 0 101 00 1 100 101 raw (unencoded) data change = no change = 1111010101111 differentially encoded data 162 Chapter 7 Custom Real Time I/Q Baseband Working with Differential Data Encoding 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. NOTE The following I/Q State Map illustrations show all possible state transitions using a particular symbol table offset value. The actual state-to-state transition depends on the state in which the modulation starts. Example Data Offset Value 1 00000000 +1 2 00000001 .1 3 00000010 +2 4 00000011 0 Example 1 Example 2 transition 1 state forward transition 1 state backward Example 4 no transition Example 3 transition 2 states forward Chapter 7 163 Custom Real Time I/Q Baseband Working with Differential Data Encoding Data Value Symbol Table Offset 00 01 10 11 +1 -1 +2 +0 Data = 0011100001 {{ {{ { 5th Symbol 1st Symbol 3rd Symbol 4th Symbol 2nd Symbol 1st 2nd 3rd 5th 4th When applied to the user-defined default 4QAM I/Q map, starting from the 1st symbol (data 00), the differential encoding transitions for the data stream (in 2-bit symbols) 0011100001 appear in the previous illustration. As you can see, the 1st and 4th symbols, having the same data value (00), produce the same state transition (forward 1 state). In differential encoding, symbol values do not define location; they define the direction and distance of a transition through the I/Q State Map. For instructions on configuring differential encoding, see “Understanding Differential Encoding” on page 160. 164 Chapter 7 Custom Real Time I/Q Baseband Working with Differential Data Encoding Using Differential Encoding Differential encoding is a digital-encoding technique that denotes a binary value by a signal change rather than a particular signal state. It is available for Custom Real Time I/Q Baseband mode. It is not available for waveforms generated by Arb Waveform Generator mode. The signal generator’s Differential State Map editor enables you to modify the differential state map associated with user-defined I/Q and user-defined FSK modulations. In this procedure, you create a user-defined I/Q modulation and then configure, activate, and apply differential encoding to the user-defined modulation. For more information, see “Understanding Differential Encoding” on page 160. This section includes information on following: • “Configuring User-Defined I/Q Modulation” • “Accessing the Differential State Map Editor” on page 166 • “Editing the Differential State Map” on page 166 Configuring User-Defined I/Q Modulation 1. Press Preset. 2. Press Mode > Custom > Real Time I/Q Baseband > Modulation Type > Define User I/Q > More (1 of 2) > Load Default I/Q Map > QAM > 4QAM. This loads a default 4QAM I/Q modulation and displays it in the I/Q Values editor. The default 4QAM I/Q modulation contains data that represent 4 symbols (00, 01, 10, and 11) mapped into the I/Q plane using 2 distinct values (1.000000 and .1.000000). These 4 symbols will be traversed during the modulation process by the symbol table offset values associated with each symbol of data. Chapter 7 165 Custom Real Time I/Q Baseband Working with Differential Data Encoding Accessing the Differential State Map Editor •Press Configure Differential Encoding. This opens the Differential State Mapeditor. At this point, you see the data for the 1st symbol (00000000) and the cursor prepared to accept an offset value.You are now prepared to create a custom differential encoding for the user-defined default 4QAM I/Q modulation. Data Symbol Table Offset Values Entry Area Editing the Differential State Map 1. Press 1 > Enter. This encodes the first symbol by adding a symbol table offset of 1. The symbol rotates forward through the state map by 1 value when a data value of 0 is modulated. 2. Press +/-> 1 > Enter. This encodes the second symbol by adding a symbol table offset of -1. The symbol rotates backward through the state map by 1 value when a data value of 1 is modulated. NOTE At this point, the modulation has one bit per symbol. For the first two data values (00000000 and...
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