81 lines
2.7 KiB
Plaintext
81 lines
2.7 KiB
Plaintext
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Viterbi.asm Soft Decision GSM Viterbi Decoder
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Analog Devices, Inc.
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DSP Division
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Three Technology Way
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P.O. Box 9106
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Norwood, MA 02062
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21-JUNE-2001 BJM
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This directory contains an example ADSP-2191 single-core subroutine
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that implements a soft decision half-rate, soft-decision, GSM Viterbi decoder.
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Files contained in this directory:
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VITERBI.dpj VisualDSP project file
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VITERBI.asm ADSP-2191 source for Viterbi
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ADSP-2191.ldf Linker description file
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GSM_REC.dat Soft decision data for Viterbi
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_________________________________________________________________
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CONTENTS
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I. FUNCTION/ALGORITHM DESCRIPTION
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II. IMPLEMENTATION DESCRIPTION
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III. DESCRIPTION OF INPUT DATA
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I. FUNCTION/ALGORITHM DESCRIPTION
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The project Viterbi.dpj contains an implementation of a single-core
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subroutine that implements a half-rate, soft-decision, GSM Vitertbi decoder.
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II. IMPLEMENTATION DESCRIPTION
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1. METRIC UPDATE:
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-----------------
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The metric update section accumulates branch distance metrics into path metrics.
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The lowest path metric at the end of processing is considered to denote the
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path most likely to contain the correct decoding of the input.
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Each element of the state_trans[] array represents a state transition.
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Each of the 16-bits in an element of state_trans[] represents one of the
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16 possible new states, and indicates which of two possible states
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is the most likely previous state.
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2. TRACE BACK:
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--------------
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The traceback traces back through the state_trans[] array. Starting
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with a new state, the bits of "state" are rotated to compute the
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position of the bit in the current state_trans[] array element that
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represents this new state. This bit indicates which state was previous.
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We update the state to the previous state using this bit.
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During any state transition, the most significant of the four bits in
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"state" is the most recent input bit to the convolutional encoder.
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This is the bit which is added to the output of the decoder.
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III. DESCRIPTION OF INPUT DATA
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1. INPUT SAMPLES:
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-----------------
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The Viterbi decoder routine expects input data which conforms to the following criteria:
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The class 1 bits are encoded with the 1/2 rate convolutional code defined by
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the polynomials:
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G0 = 1 + D3+ D4
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G1 = 1 + D + D3+ D4
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Encoded outputs are transmitted as signed antipodal analog signals. They are received at
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the decoder and quantized. The quantized number is represented in a 2's
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compliment giving the range of -8 to 7. The process of quantizing a binary analog signal
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with a multi-bit qunatizer is called soft decision. This soft decision is stored in the
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array 'SOFT_DEC_INPUT'.
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