kopia lustrzana https://github.com/M17-Project/M17_Implementations
203 wiersze
4.9 KiB
C
203 wiersze
4.9 KiB
C
//--------------------------------------------------------------------
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// M17 C library - decode/viterbi.c
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//
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// This file contains:
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// - the Viterbi decoder
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//
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// Wojciech Kaczmarski, SP5WWP
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// M17 Project, 29 December 2023
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//--------------------------------------------------------------------
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#include <stdio.h>
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#include <string.h>
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#include <m17/m17.h>
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static uint32_t prevMetrics[NUM_STATES];
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static uint32_t currMetrics[NUM_STATES];
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static uint32_t prevMetricsData[NUM_STATES];
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static uint32_t currMetricsData[NUM_STATES];
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static uint16_t viterbi_history[244];
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/**
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* @brief Decode unpunctured convolutionally encoded data.
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*
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* @param out Destination array where decoded data is written.
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* @param in Input data.
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* @param len Input length in bits.
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* @return Number of bit errors corrected.
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*/
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uint32_t viterbi_decode(uint8_t* out, const uint16_t* in, const uint16_t len)
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{
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if(len > 244*2)
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fprintf(stderr, "Input size exceeds max history\n");
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viterbi_reset();
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size_t pos = 0;
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for(size_t i = 0; i < len; i += 2)
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{
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uint16_t s0 = in[i];
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uint16_t s1 = in[i + 1];
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viterbi_decode_bit(s0, s1, pos);
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pos++;
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}
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return viterbi_chainback(out, pos, len/2);
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}
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/**
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* @brief Decode punctured convolutionally encoded data.
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*
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* @param out Destination array where decoded data is written.
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* @param in Input data.
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* @param punct Puncturing matrix.
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* @param in_len Input data length.
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* @param p_len Puncturing matrix length (entries).
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* @return Number of bit errors corrected.
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*/
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uint32_t viterbi_decode_punctured(uint8_t* out, const uint16_t* in, const uint8_t* punct, const uint16_t in_len, const uint16_t p_len)
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{
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if(in_len > 244*2)
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fprintf(stderr, "Input size exceeds max history\n");
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uint16_t umsg[244*2]; //unpunctured message
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uint8_t p=0; //puncturer matrix entry
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uint16_t u=0; //bits count - unpunctured message
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uint16_t i=0; //bits read from the input message
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while(i<in_len)
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{
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if(punct[p])
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{
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umsg[u]=in[i];
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i++;
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}
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else
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{
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umsg[u]=0x7FFF;
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}
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u++;
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p++;
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p%=p_len;
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}
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return viterbi_decode(out, umsg, u) - (u-in_len)*0x7FFF;
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}
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/**
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* @brief Decode one bit and update trellis.
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*
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* @param s0 Cost of the first symbol.
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* @param s1 Cost of the second symbol.
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* @param pos Bit position in history.
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*/
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void viterbi_decode_bit(uint16_t s0, uint16_t s1, const size_t pos)
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{
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static const uint16_t COST_TABLE_0[] = {0, 0, 0, 0, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF};
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static const uint16_t COST_TABLE_1[] = {0, 0xFFFF, 0xFFFF, 0, 0, 0xFFFF, 0xFFFF, 0};
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for(uint8_t i = 0; i < NUM_STATES/2; i++)
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{
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uint32_t metric = q_abs_diff(COST_TABLE_0[i], s0)
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+ q_abs_diff(COST_TABLE_1[i], s1);
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uint32_t m0 = prevMetrics[i] + metric;
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uint32_t m1 = prevMetrics[i + NUM_STATES/2] + (0x1FFFE - metric);
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uint32_t m2 = prevMetrics[i] + (0x1FFFE - metric);
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uint32_t m3 = prevMetrics[i + NUM_STATES/2] + metric;
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uint8_t i0 = 2 * i;
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uint8_t i1 = i0 + 1;
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if(m0 >= m1)
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{
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viterbi_history[pos]|=(1<<i0);
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currMetrics[i0] = m1;
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}
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else
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{
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viterbi_history[pos]&=~(1<<i0);
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currMetrics[i0] = m0;
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}
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if(m2 >= m3)
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{
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viterbi_history[pos]|=(1<<i1);
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currMetrics[i1] = m3;
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}
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else
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{
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viterbi_history[pos]&=~(1<<i1);
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currMetrics[i1] = m2;
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}
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}
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//swap
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uint32_t tmp[NUM_STATES];
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for(uint8_t i=0; i<NUM_STATES; i++)
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{
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tmp[i]=currMetrics[i];
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}
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for(uint8_t i=0; i<NUM_STATES; i++)
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{
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currMetrics[i]=prevMetrics[i];
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prevMetrics[i]=tmp[i];
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}
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}
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/**
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* @brief History chainback to obtain final byte array.
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*
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* @param out Destination byte array for decoded data.
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* @param pos Starting position for the chainback.
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* @param len Length of the output in bits.
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* @return Minimum Viterbi cost at the end of the decode sequence.
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*/
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uint32_t viterbi_chainback(uint8_t* out, size_t pos, uint16_t len)
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{
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uint8_t state = 0;
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size_t bitPos = len+4;
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memset(out, 0, (len-1)/8+1);
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while(pos > 0)
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{
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bitPos--;
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pos--;
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uint16_t bit = viterbi_history[pos]&((1<<(state>>4)));
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state >>= 1;
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if(bit)
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{
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state |= 0x80;
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out[bitPos/8]|=1<<(7-(bitPos%8));
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}
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}
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uint32_t cost = prevMetrics[0];
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for(size_t i = 0; i < NUM_STATES; i++)
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{
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uint32_t m = prevMetrics[i];
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if(m < cost) cost = m;
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}
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return cost;
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}
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/**
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* @brief Reset the decoder state. No args.
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*
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*/
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void viterbi_reset(void)
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{
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memset((uint8_t*)viterbi_history, 0, 2*244);
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memset((uint8_t*)currMetrics, 0, 4*NUM_STATES);
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memset((uint8_t*)prevMetrics, 0, 4*NUM_STATES);
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memset((uint8_t*)currMetricsData, 0, 4*NUM_STATES);
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memset((uint8_t*)prevMetricsData, 0, 4*NUM_STATES);
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}
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