You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
251 lines
8.7 KiB
251 lines
8.7 KiB
/* K=9 r=1/3 Viterbi decoder for PowerPC G4/G5 Altivec vector instructions |
|
* 8-bit offset-binary soft decision samples |
|
* Copyright Aug 2006, Phil Karn, KA9Q |
|
* May be used under the terms of the GNU Lesser General Public License (LGPL) |
|
*/ |
|
#include <stdio.h> |
|
#include <stdlib.h> |
|
#include <memory.h> |
|
#include <limits.h> |
|
#include "fec.h" |
|
|
|
typedef union { unsigned char c[2][16]; vector unsigned char v[2]; } decision_t; |
|
typedef union { unsigned short s[256]; vector unsigned short v[32]; } metric_t; |
|
|
|
static union branchtab39 { unsigned short s[128]; vector unsigned short v[16];} Branchtab39[3]; |
|
static int Init = 0; |
|
|
|
/* State info for instance of Viterbi decoder */ |
|
struct v39 { |
|
metric_t metrics1; /* path metric buffer 1 */ |
|
metric_t metrics2; /* path metric buffer 2 */ |
|
void *dp; /* Pointer to current decision */ |
|
metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */ |
|
void *decisions; /* Beginning of decisions for block */ |
|
}; |
|
|
|
/* Initialize Viterbi decoder for start of new frame */ |
|
int init_viterbi39_av(void *p,int starting_state){ |
|
struct v39 *vp = p; |
|
int i; |
|
|
|
for(i=0;i<32;i++) |
|
vp->metrics1.v[i] = (vector unsigned short)(1000); |
|
|
|
vp->old_metrics = &vp->metrics1; |
|
vp->new_metrics = &vp->metrics2; |
|
vp->dp = vp->decisions; |
|
vp->old_metrics->s[starting_state & 255] = 0; /* Bias known start state */ |
|
return 0; |
|
} |
|
|
|
void set_viterbi39_polynomial_av(int polys[3]){ |
|
int state; |
|
|
|
for(state=0;state < 128;state++){ |
|
Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0; |
|
Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0; |
|
Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & abs(polys[2])) ? 255 : 0; |
|
} |
|
Init++; |
|
} |
|
|
|
/* Create a new instance of a Viterbi decoder */ |
|
void *create_viterbi39_av(int len){ |
|
struct v39 *vp; |
|
|
|
if(!Init){ |
|
int polys[3] = { V39POLYA, V39POLYB, V39POLYC }; |
|
|
|
set_viterbi39_polynomial_av(polys); |
|
} |
|
vp = (struct v39 *)malloc(sizeof(struct v39)); |
|
vp->decisions = malloc(sizeof(decision_t)*(len+8)); |
|
init_viterbi39_av(vp,0); |
|
return vp; |
|
} |
|
|
|
/* Viterbi chainback */ |
|
int chainback_viterbi39_av( |
|
void *p, |
|
unsigned char *data, /* Decoded output data */ |
|
unsigned int nbits, /* Number of data bits */ |
|
unsigned int endstate){ /* Terminal encoder state */ |
|
struct v39 *vp = p; |
|
decision_t *d = (decision_t *)vp->decisions; |
|
int path_metric; |
|
|
|
/* Make room beyond the end of the encoder register so we can |
|
* accumulate a full byte of decoded data |
|
*/ |
|
endstate %= 256; |
|
|
|
path_metric = vp->old_metrics->s[endstate]; |
|
|
|
/* The store into data[] only needs to be done every 8 bits. |
|
* But this avoids a conditional branch, and the writes will |
|
* combine in the cache anyway |
|
*/ |
|
d += 8; /* Look past tail */ |
|
while(nbits-- != 0){ |
|
int k; |
|
|
|
k = (d[nbits].c[endstate >> 7][endstate & 15] & (0x80 >> ((endstate>>4)&7)) ) ? 1 : 0; |
|
endstate = (k << 7) | (endstate >> 1); |
|
data[nbits>>3] = endstate; |
|
} |
|
return path_metric; |
|
} |
|
|
|
/* Delete instance of a Viterbi decoder */ |
|
void delete_viterbi39_av(void *p){ |
|
struct v39 *vp = p; |
|
|
|
if(vp != NULL){ |
|
free(vp->decisions); |
|
free(vp); |
|
} |
|
} |
|
|
|
int update_viterbi39_blk_av(void *p,unsigned char *syms,int nbits){ |
|
struct v39 *vp = p; |
|
decision_t *d = (decision_t *)vp->dp; |
|
int path_metric = 0; |
|
vector unsigned char decisions = (vector unsigned char)(0); |
|
|
|
while(nbits--){ |
|
vector unsigned short symv,sym0v,sym1v,sym2v; |
|
vector unsigned char s; |
|
void *tmp; |
|
int i; |
|
|
|
/* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */ |
|
s = (vector unsigned char)vec_perm(vec_ld(0,syms),vec_ld(5,syms),vec_lvsl(0,syms)); |
|
|
|
symv = (vector unsigned short)vec_mergeh((vector unsigned char)(0),s); /* Unsigned byte->word unpack */ |
|
sym0v = vec_splat(symv,0); |
|
sym1v = vec_splat(symv,1); |
|
sym2v = vec_splat(symv,2); |
|
syms += 3; |
|
|
|
for(i=0;i<16;i++){ |
|
vector bool short decision0,decision1; |
|
vector unsigned short metric,m_metric,m0,m1,m2,m3,survivor0,survivor1; |
|
|
|
/* Form branch metrics |
|
* Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255, |
|
* the XOR operations constitute conditional negation. |
|
* the metrics are in the range 0-765 |
|
*/ |
|
m0 = vec_add(vec_xor(Branchtab39[0].v[i],sym0v),vec_xor(Branchtab39[1].v[i],sym1v)); |
|
m1 = vec_xor(Branchtab39[2].v[i],sym2v); |
|
metric = vec_add(m0,m1); |
|
m_metric = vec_sub((vector unsigned short)(765),metric); |
|
|
|
/* Add branch metrics to path metrics */ |
|
m0 = vec_adds(vp->old_metrics->v[i],metric); |
|
m3 = vec_adds(vp->old_metrics->v[16+i],metric); |
|
m1 = vec_adds(vp->old_metrics->v[16+i],m_metric); |
|
m2 = vec_adds(vp->old_metrics->v[i],m_metric); |
|
|
|
/* Compare and select */ |
|
decision0 = vec_cmpgt(m0,m1); |
|
decision1 = vec_cmpgt(m2,m3); |
|
survivor0 = vec_min(m0,m1); |
|
survivor1 = vec_min(m2,m3); |
|
|
|
/* Store decisions and survivors. |
|
* To save space without SSE2's handy PMOVMSKB instruction, we pack and store them in |
|
* a funny interleaved fashion that we undo in the chainback function. |
|
*/ |
|
decisions = vec_add(decisions,decisions); /* Shift each byte 1 bit to the left */ |
|
|
|
/* Booleans are either 0xff or 0x00. Subtracting 0x00 leaves the lsb zero; subtracting |
|
* 0xff is equivalent to adding 1, which sets the lsb. |
|
*/ |
|
decisions = vec_sub(decisions,(vector unsigned char)vec_pack(vec_mergeh(decision0,decision1),vec_mergel(decision0,decision1))); |
|
|
|
vp->new_metrics->v[2*i] = vec_mergeh(survivor0,survivor1); |
|
vp->new_metrics->v[2*i+1] = vec_mergel(survivor0,survivor1); |
|
|
|
if((i % 8) == 7){ |
|
/* We've accumulated a total of 128 decisions, stash and start again */ |
|
d->v[i>>3] = decisions; /* No need to clear, the new bits will replace the old */ |
|
} |
|
} |
|
#if 0 |
|
/* Experimentally determine metric spread |
|
* The results are fixed for a given code and input symbol size |
|
*/ |
|
{ |
|
int i; |
|
vector unsigned short min_metric; |
|
vector unsigned short max_metric; |
|
union { vector unsigned short v; unsigned short s[8];} t; |
|
int minimum,maximum; |
|
static int max_spread = 0; |
|
|
|
min_metric = max_metric = vp->new_metrics->v[0]; |
|
for(i=1;i<32;i++){ |
|
min_metric = vec_min(min_metric,vp->new_metrics->v[i]); |
|
max_metric = vec_max(max_metric,vp->new_metrics->v[i]); |
|
} |
|
min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,8)); |
|
max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,8)); |
|
min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,4)); |
|
max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,4)); |
|
min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,2)); |
|
max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,2)); |
|
|
|
t.v = min_metric; |
|
minimum = t.s[0]; |
|
t.v = max_metric; |
|
maximum = t.s[0]; |
|
if(maximum-minimum > max_spread){ |
|
max_spread = maximum-minimum; |
|
printf("metric spread = %d\n",max_spread); |
|
} |
|
} |
|
#endif |
|
|
|
/* Renormalize if necessary. This deserves some explanation. |
|
* The maximum possible spread, found by experiment, for 8 bit symbols is about 3825 |
|
* So by looking at one arbitrary metric we can tell if any of them have possibly saturated. |
|
* However, this is very conservative. Large spreads occur only at very high Eb/No, where |
|
* saturating a bad path metric doesn't do much to increase its chances of being erroneously chosen as a survivor. |
|
|
|
* At more interesting (low) Eb/No ratios, the spreads are much smaller so our chances of saturating a metric |
|
* by not not normalizing when we should are extremely low. So either way, the risk to performance is small. |
|
|
|
* All this is borne out by experiment. |
|
*/ |
|
if(vp->new_metrics->s[0] >= USHRT_MAX-5000){ |
|
vector unsigned short scale; |
|
union { vector unsigned short v; unsigned short s[8];} t; |
|
|
|
/* Find smallest metric and splat */ |
|
scale = vp->new_metrics->v[0]; |
|
for(i=1;i<32;i++) |
|
scale = vec_min(scale,vp->new_metrics->v[i]); |
|
|
|
scale = vec_min(scale,vec_sld(scale,scale,8)); |
|
scale = vec_min(scale,vec_sld(scale,scale,4)); |
|
scale = vec_min(scale,vec_sld(scale,scale,2)); |
|
|
|
/* Subtract it from all metrics |
|
* Work backwards to try to improve the cache hit ratio, assuming LRU |
|
*/ |
|
for(i=31;i>=0;i--) |
|
vp->new_metrics->v[i] = vec_subs(vp->new_metrics->v[i],scale); |
|
t.v = scale; |
|
path_metric += t.s[0]; |
|
} |
|
d++; |
|
/* Swap pointers to old and new metrics */ |
|
tmp = vp->old_metrics; |
|
vp->old_metrics = vp->new_metrics; |
|
vp->new_metrics = tmp; |
|
} |
|
vp->dp = d; |
|
return path_metric; |
|
}
|
|
|