IS-95A Rev Ch Viterbi Decoder (CDMA Reference Blockset)

Decode a convolutionally encoded information sequence optimally for the Access and reverse Traffic channels

Library

IS-95 Base Station Receiver

Description

This block decodes a convolutionally encoded information sequence optimally for the Access and reverse Traffic channels. For the Access channel and Rate Set I of the Traffic channel, a 1/3 rate Viterbi decoder is used. For Rate Set II of the Traffic channel, a 1/2 rate Viterbi decoder is used.

The Viterbi algorithm searches through the trellis, which defines all possible allowed sequences determined by the encoder, for the most probable sequence. This is done by updating a likelihood metric for each possible choice of the decoded sequence and selecting the paths corresponding to the best metric choices. Finally, it outputs the decoded data along with the metric for the final detected path. The decoder performs a traceback along the survivor paths in the trellis to generate the output bits.

Note This traceback is started in the zero state, so the last 8 bits of the sequence being decoded must be 0.

For this block, the Frame In input size is equal to the maximum possible size of 576 (Rate Set II, Traffic Full Rate). The number of relevant input bits applied to the convolution decoder depends on the channel type, rate set, and rate as specified in the table below. Similarly, the output frame size is equal to the maximum possible size (Rate Set II, Traffic Full Rate). The number of relevant output bits is also specified in the table below. The relevant bits start from the beginning of a frame. The block ignores input bits beyond the relevant bits, and sets output bits beyond the relevant bits to zero.

Channel Type
Number of Relevant Input Bits
Number of Relevant Output Bits

Access
288
96

Traffic, Rate Set I, Full Rate
576
192

Traffic, Rate Set I, Half Rate
288
96

Traffic, Rate Set I, Quarter Rate
144
48

Traffic, Rate Set I, Eighth Rate
72
24

Traffic, Rate Set II, Full Rate
576
288

Traffic, Rate Set II, Half Rate
288
144

Traffic, Rate Set II, Quarter Rate
144
72

Traffic, Rate Set II, Eighth Rate
72
36

Channel Type	Number of Relevant Input Bits	Number of Relevant Output Bits
Access	288	96
Traffic, Rate Set I, Full Rate	576	192
Traffic, Rate Set I, Half Rate	288	96
Traffic, Rate Set I, Quarter Rate	144	48
Traffic, Rate Set I, Eighth Rate	72	24
Traffic, Rate Set II, Full Rate	576	288
Traffic, Rate Set II, Half Rate	288	144
Traffic, Rate Set II, Quarter Rate	144	72
Traffic, Rate Set II, Eighth Rate	72	36

Inputs

Rate

Integer scalar that specifies the data rate of the input signal. To indicate a rate fraction of Full, Half, Quarter, or Eighth, use an input value of 0, 1, 2, or 3, respectively. The table below shows all valid channel types and rate fractions, along with their associated data rates and Rate input values.


Channel Type	Data Rate (bps)	Rate Input Value
Access (always Half rate)	4800	1
Traffic, Rate Set I, Full	9600	0
Traffic, Rate Set I, Half	4800	1
Traffic, Rate Set I, Quarter	2400	2
Traffic, Rate Set I, Eighth	1200	3
Traffic, Rate Set II, Full	14400	0
Traffic, Rate Set II, Half	7200	1
Traffic, Rate Set II, Quarter	3600	2
Traffic, Rate Set II, Eighth	1800	3

Frame In

Real vector of size 576 containing the input frame to be decoded.

Outputs

Frame Out: Binary vector of size 288 containing the decoded frame.
Metric: Real scalar representing the final path metric of the Viterbi decoder.

Dialog Box

Parameters

Rate set: The rate set, either Rate Set I or Rate Set II.
Channel type: The channel type, either Access or Traffic.
Ratio of trace-back length to frame length: Real scalar that specifies the maximum number of bits that the block should trace back in the trellis structure while decoding, as a fraction of the frame length. The maximum value is 1. If the product of this ratio and the frame length is not an integer, then the product is truncated to an integer.
Ratio of decoding length to frame length: Real scalar that specifies the number of bits decoded in a traceback operation as a fraction of the frame length. If the product of this ratio and the frame length is not an integer, then the product is truncated to an integer. The decoding length to frame length ratio should be equal to or less than the traceback length to frame length ratio. When the decoding length is less than the traceback length, the decoded bits are the last bits traced back.

Specification References

IS-95A 6.1.3.1.3
J-STD-008 2.1.3.1.3

IS-95A Rev Ch Short Code Despreader IS-95A Rev Ch Walsh Correlator