IS-95A Frame Quality Detector (CDMA Reference Blockset)

Determine the quality of a received frame of data and output the decoded data and a frame quality indicator

Library

IS-95A Common

Description

This is a hierarchical block that determines if there are any frame errors using a syndrome detection procedure or via a metric comparison procedure for the Access and Traffic channels. In the case of the Sync and Paging channels, this block does not determine frame errors since the data is encoded over multiple frames.

The syndrome detection procedure uses the IS-95A Syndrome Detector block to compute a syndrome for the CRC encoded received data frame. A zero value for the syndrome indicates a correct frame has been received. The syndrome detection procedure is applicable to the Access channel and the Traffic channel full and half rates in Rate Set I and all rates in Rate Set II. When the block uses the syndrome detection procedure, it also removes the CRC and tail bits.

The metric comparison procedure is applicable to the quarter and eighth rate Traffic channel in Rate Set I where a CRC check is not provided in the frame. In this case, the metric detection procedure compares the soft-value metric at the output of the Viterbi decoder to a threshold. If the metric value equals or exceeds the threshold, it is assumed that the decoded frame is received correctly.

The Frame In port size is equal to the maximum possible size of 288 (Rate Set II, Traffic Full Rate). Similarly, the Frame Out port size is equal to the maximum possible size of 268 (Rate Set II, Traffic Full Rate). The following table gives the number of relevant output bits, in a frame, for all channel types. The block sets to zero any unused output bits. The CRC and tail bits immediately follow the relevant information bits.

Channel Type
Number of Relevant Inputs Bits
Number of CRC Bits
Number of Tail Bits
Number of Relevant Output Bits

Sync, Eighth Rate
32
0
0
32

Paging, Full Rate
192
0
0
192

Paging, Half Rate
96
0
0
96

Access
96
0
8
88

Traffic, Rate Set I, Full Rate
192
12
8
172

Traffic, Rate Set I, Half Rate
96
8
8
80

Traffic, Rate Set I, Quarter Rate
48
0
8
40

Traffic, Rate Set I, Eighth Rate
24
0
8
16

Traffic, Rate Set II, Full Rate
288
12
8
268

Traffic, Rate Set II, Half Rate
144
10
8
126

Traffic, Rate Set II, Quarter Rate
72
8
8
56

Traffic, Rate Set II, Eighth Rate
36
6
8
22

Channel Type	Number of Relevant Inputs Bits	Number of CRC Bits	Number of Tail Bits	Number of Relevant Output Bits
Sync, Eighth Rate	32	0	0	32
Paging, Full Rate	192	0	0	192
Paging, Half Rate	96	0	0	96
Access	96	0	8	88
Traffic, Rate Set I, Full Rate	192	12	8	172
Traffic, Rate Set I, Half Rate	96	8	8	80
Traffic, Rate Set I, Quarter Rate	48	0	8	40
Traffic, Rate Set I, Eighth Rate	24	0	8	16
Traffic, Rate Set II, Full Rate	288	12	8	268
Traffic, Rate Set II, Half Rate	144	10	8	126
Traffic, Rate Set II, Quarter Rate	72	8	8	56
Traffic, Rate Set II, Eighth Rate	36	6	8	22

Inputs

Rate

Integer scalar that specifies the data rate for 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)	Input Value
Sync (always Eighth rate)	1200	3
Paging, Full	9600	0
Paging, Half	4800	1
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 288 containing the input frame with the CRC and tail bits.

Metric

Real scalar representing the final soft metric value computed in the IS-95A Fwd Ch Viterbi Decoder using the Viterbi algorithm.

Outputs

Quality Indicator: Integer scalar that indicates the decision as to whether the frame was in error. A value of 0 indicates no error and 1 indicates an error. In the cases of Sync, Paging, and Access channels, the value is set to -1.
Frame Out: Binary vector of size 268 representing the data frame after removing CRC and tail bits, if any.

Dialog Box

Parameters

Rate set: The rate set, either Rate Set I or Rate Set II.
Channel type: The channel type, either Sync, Paging, Access, or Traffic.
Metric threshold for quarter rate: Real scalar that specifies the threshold value against which the metric from the IS-95A Fwd Ch Viterbi Decoder is compared for the Traffic channel quarter rate frame. This is only applicable in Rate Set I.
Metric threshold for eighth rate: Real scalar that specifies the threshold value against which the metric from the IS-95A Fwd Ch Viterbi Decoder is compared for the Traffic channel eighth rate frame. This is only applicable in Rate Set I.

Specification References

IS-95A 6.1.3.3.2, 7.1.3.5.2,
J-STD-008 2.1.3.3.2.1, 3.1.3.5.2.1

IS-95A CRC Generator IS-95A Fwd Ch Base Station Transmitter Interface