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Lattice Semiconductor
Functional Description
On the other hand, if the decoder is configured in block mode, the number of tracebacks and traceback length
depends on the parameters of the decoder. The user has to specify the termination method that was used for the
convolutional coding to enable the decoder to start from the correct initial state.
In dynamic puncturing mode, only block decoding is permitted.
Termination Modes
Convolutional encoders employ two block terminations methods: Zero Flushing and Tail Biting. In Zero Flushing
mode, a series of zeros are added to the end of each block at the input of the convolutional encoder. In Tail Biting
mode, the last few bits of each block are used to initialize the state of the encoder, before encoding that block. Both
modes are widely used in various telecommunication standards.
Lattice’s Block Viterbi decoder IP supports both of these termination methods. The choice of termination method is
decided by the user and it must be exactly the same as what was used in the convolutional encoder.
Number of Tracebacks and Traceback Length
The accuracy of decoding depends to some extent on the starting node of a traceback operation. Usually, if the
data was encoded using the Zero Flushing scheme and if the traceback length is equal to block length, the trace-
back can start at state 0. For all other schemes or for a continuous decoder, starting the traceback from zero state
may not lead to right results. A reliable starting state can be determined by performing an additional traceback
operation. The Block Viterbi Decoder can be configured to perform either 1 or 2 tracebacks by setting the parame-
ter Number of Tracebacks in the IP GUI. For some configurations, the number of tracebacks can be selected
by the user and for others, it is set automatically inside the decoder.
If Number of Tracebacks is equal to 1, the decoder performs length-L traceback starting from state 0 and does
decoding. If the Number of Tracebacks is equal to 2, the decoder performs a length-L traceback from state 0 to
determine a reliable starting point for second traceback. From that starting point, it performs a second length-L
traceback and does decoding. For continuous decoders and block decoders with Tail Biting termination mode,
Number of Tracebacks is internally set to 2. For block decoders with Zero Flushing termination mode, Number
of Tracebacks can be set to either 1 or 2 by the user.
The traceback length is typically close to 7 to 9 times the constraint length ( K ) in most applications. Lattice’s Viterbi
Decoder IP allows the user to specify any traceback length between 3 K and 14 K for most configurations; however,
the Traceback Length is restricted to be a multiple of puncture block size for fixed puncturing decoders. When
the Termination Mode is set to “Tail Biting”, the traceback length is internally set by the core to Block
Length * k / n . When the decoder operates in dynamic puncture mode and Number of Tracebacks is set to 1,
the Traceback Length should be a common multiple of all possible input rates and between 8. and 128. For
example, if Max Input Rate is 4, the possible input rates are 1, 2, 3 and 4. Therefore, the Traceback Length
can only be in the set {12, 24, 36, ..., 116, 128}.
Block Length
For block decoders, the block length is implicitly specified using the input signals ibstart and ibend . All the data
between ibstart and ibend pulses, including both the ends, are taken to be part of the block. When ibstart is
pulled high for one clock cycle the input data is read in as the first data of the block. The decoder continues to read
the data in consecutive clock cycles into a block until it encounters a one clock cycle pulse in the ibend port. The
block size has to be one of the legal values as given in Table 2-1 , for the decoder to function correctly.
Table 2-1. Legal Values for Block Size
Termination Number of
Mode Tracebacks
None
Fixed
Puncturing
Dynamic
Zero Flushing
Zero Flushing
Tail Biting
1
2
2
8 to 128
>8
8 to 128
8 to 128* k / n , multiples of n
> 8, multiples of n
8 to 128* k / n , multiples of n
> 8, Traceback Length * outrate/inrate
> 8, multiples of outrate
Not Applicable
IPUG32_02.7, June 2010
11
Block Viterbi Decoder User’s Guide
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相关PDF资料
VTERB-DECO-XP-N1
IP CORE VITERBI DECODER XPGA
VTP110F
POLYSWITCH PTC RESET 1.1A STRAP
VTP175LF
POLYSWITCH PTC RESET 1.75A STRAP
VTP210GF
POLYSWITCH PTC RESET 2.1A STRAP
VTP210SF
POLYSWITCH PTC RESET 2.1A STRAP
W51-A121B1-10
CIRCUIT BREAKER THERM 10A ILLUM
W54-XC2A4B10-40
CIRCUIT BREAKER THERMAL 40AMP
WV-089047-10-9
LABEL ID PRODUCTS
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