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//
// Copyright 2015, Oliver Jowett <oliver@mutability.co.uk>
//
// This file is free software: you may copy, redistribute and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation, either version 2 of the License, or (at your
// option) any later version.
//
// This file is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <math.h>
#include <unistd.h>
#include "uat.h"
#include "fec.h"
static void make_atan2_table();
static void read_from_stdin();
static int check_sync_word(uint16_t *phi, uint64_t pattern, int16_t *center);
static int process_buffer(uint16_t *phi, int len, uint64_t offset);
static int demod_adsb_frame(uint16_t *phi, uint8_t *to, int *rs_errors);
static int demod_uplink_frame(uint16_t *phi, uint8_t *to, int *rs_errors);
static void demod_frame(uint16_t *phi, uint8_t *frame, int bytes, int16_t center_dphi);
static void handle_adsb_frame(uint64_t timestamp, uint8_t *frame, int rs);
static void handle_uplink_frame(uint64_t timestamp, uint8_t *frame, int rs);
#define SYNC_BITS (36)
#define ADSB_SYNC_WORD 0xEACDDA4E2UL
#define UPLINK_SYNC_WORD 0x153225B1DUL
// relying on signed overflow is theoretically bad. Let's do it properly.
#ifdef USE_SIGNED_OVERFLOW
#define phi_difference(from,to) ((int16_t)((to) - (from)))
#else
inline int16_t phi_difference(uint16_t from, uint16_t to)
{
int32_t difference = to - from; // lies in the range -65535 .. +65535
if (difference >= 32768) // +32768..+65535
return difference - 65536; // -> -32768..-1: always in range
else if (difference < -32768) // -65535..-32769
return difference + 65536; // -> +1..32767: always in range
else
return difference;
}
#endif
int main(int argc, char **argv)
{
make_atan2_table();
init_fec();
read_from_stdin();
return 0;
}
static void dump_raw_message(char updown, uint8_t *data, int len, int rs_errors)
{
int i;
fprintf(stdout, "%c", updown);
for (i = 0; i < len; ++i) {
fprintf(stdout, "%02x", data[i]);
}
if (rs_errors)
fprintf(stdout, ";rs=%d", rs_errors);
fprintf(stdout, ";\n");
}
static void handle_adsb_frame(uint64_t timestamp, uint8_t *frame, int rs)
{
dump_raw_message('-', frame, (frame[0]>>3) == 0 ? SHORT_FRAME_DATA_BYTES : LONG_FRAME_DATA_BYTES, rs);
fflush(stdout);
}
static void handle_uplink_frame(uint64_t timestamp, uint8_t *frame, int rs)
{
dump_raw_message('+', frame, UPLINK_FRAME_DATA_BYTES, rs);
fflush(stdout);
}
static uint16_t iqphase[65536]; // contains value [0..65536) -> [0, 2*pi)
void make_atan2_table()
{
unsigned i,q;
union {
uint8_t iq[2];
uint16_t iq16;
} u;
for (i = 0; i < 256; ++i) {
double d_i = (i - 127.5);
for (q = 0; q < 256; ++q) {
double d_q = (q - 127.5);
double ang = atan2(d_q, d_i) + M_PI; // atan2 returns [-pi..pi], normalize to [0..2*pi]
double scaled_ang = round(32768 * ang / M_PI);
u.iq[0] = i;
u.iq[1] = q;
iqphase[u.iq16] = (scaled_ang < 0 ? 0 : scaled_ang > 65535 ? 65535 : (uint16_t)scaled_ang);
}
}
}
static void convert_to_phi(uint16_t *buffer, int n)
{
int i;
// unroll the loop. n is always > 2048, usually 36864
for (i = 0; i+8 <= n; i += 8) {
buffer[i] = iqphase[buffer[i]];
buffer[i+1] = iqphase[buffer[i+1]];
buffer[i+2] = iqphase[buffer[i+2]];
buffer[i+3] = iqphase[buffer[i+3]];
buffer[i+4] = iqphase[buffer[i+4]];
buffer[i+5] = iqphase[buffer[i+5]];
buffer[i+6] = iqphase[buffer[i+6]];
buffer[i+7] = iqphase[buffer[i+7]];
}
for (; i < n; ++i)
buffer[i] = iqphase[buffer[i]];
}
void read_from_stdin()
{
char buffer[65536*2];
int n;
int used = 0;
uint64_t offset = 0;
while ( (n = read(0, buffer+used, sizeof(buffer)-used)) > 0 ) {
int processed;
convert_to_phi((uint16_t*) (buffer+(used&~1)), ((used&1)+n)/2);
used += n;
processed = process_buffer((uint16_t*) buffer, used/2, offset);
used -= processed * 2;
offset += processed;
if (used > 0) {
memmove(buffer, buffer+processed*2, used);
}
}
}
// Return 1 if word is "equal enough" to expected
static inline int sync_word_fuzzy_compare(uint64_t word, uint64_t expected)
{
uint64_t diff;
if (word == expected)
return 1;
diff = word ^ expected; // guaranteed nonzero
// This is a bit-twiddling popcount
// hack, tweaked as we only care about
// "<N" or ">=N" set bits for fixed N -
// so we can bail out early after seeing N
// set bits.
//
// It relies on starting with a nonzero value
// with zero or more trailing clear bits
// after the last set bit:
//
// 010101010101010000
// ^
// Subtracting one, will flip the
// bits starting at the last set bit:
//
// 010101010101001111
// ^
// then we can use that as a bitwise-and
// mask to clear the lowest set bit:
//
// 010101010101000000
// ^
// And repeat until the value is zero
// or we have seen too many set bits.
// >= 1 bit
diff &= (diff-1); // clear lowest set bit
if (!diff)
return 1; // 1 bit error
// >= 2 bits
diff &= (diff-1); // clear lowest set bit
if (!diff)
return 1; // 2 bits error
// >= 3 bits
diff &= (diff-1); // clear lowest set bit
if (!diff)
return 1; // 3 bits error
// >= 4 bits
diff &= (diff-1); // clear lowest set bit
if (!diff)
return 1; // 4 bits error
// > 4 bits in error, give up
return 0;
}
#define MAX_SYNC_ERRORS 4
// check that there is a valid sync word starting at 'phi'
// that matches the sync word 'pattern'. Place the dphi
// threshold to use for bit slicing in '*center'. Return 1
// if the sync word is OK, 0 on failure
int check_sync_word(uint16_t *phi, uint64_t pattern, int16_t *center)
{
int i;
int32_t dphi_zero_total = 0;
int zero_bits = 0;
int32_t dphi_one_total = 0;
int one_bits = 0;
int error_bits;
// find mean dphi for zero and one bits;
// take the mean of the two as our central value
for (i = 0; i < SYNC_BITS; ++i) {
int16_t dphi = phi_difference(phi[i*2], phi[i*2+1]);
if (pattern & (1UL << (35-i))) {
++one_bits;
dphi_one_total += dphi;
} else {
++zero_bits;
dphi_zero_total += dphi;
}
}
dphi_zero_total /= zero_bits;
dphi_one_total /= one_bits;
*center = (dphi_one_total + dphi_zero_total) / 2;
// recheck sync word using our center value
error_bits = 0;
for (i = 0; i < SYNC_BITS; ++i) {
int16_t dphi = phi_difference(phi[i*2], phi[i*2+1]);
if (pattern & (1UL << (35-i))) {
if (dphi < *center)
++error_bits;
} else {
if (dphi >= *center)
++error_bits;
}
}
//fprintf(stdout, "check_sync_word: center=%.0fkHz, errors=%d\n", *center * 2083334.0 / 65536 / 1000, error_bits);
return (error_bits <= MAX_SYNC_ERRORS);
}
#define SYNC_MASK ((((uint64_t)1)<<SYNC_BITS)-1)
int process_buffer(uint16_t *phi, int len, uint64_t offset)
{
uint64_t sync0 = 0, sync1 = 0;
int lenbits;
int bit;
uint8_t demod_buf_a[UPLINK_FRAME_BYTES];
uint8_t demod_buf_b[UPLINK_FRAME_BYTES];
// We expect samples at twice the UAT bitrate.
// We look at phase difference between pairs of adjacent samples, i.e.
// sample 1 - sample 0 -> sync0
// sample 2 - sample 1 -> sync1
// sample 3 - sample 2 -> sync0
// sample 4 - sample 3 -> sync1
// ...
//
// We accumulate bits into two buffers, sync0 and sync1.
// Then we compare those buffers to the expected 36-bit sync word that
// should be at the start of each UAT frame. When (if) we find it,
// that tells us which sample to start decoding from.
// Stop when we run out of remaining samples for a max-sized frame.
// Arrange for our caller to pass the trailing data back to us next time;
// ensure we don't consume any partial sync word we might be part-way
// through. This means we don't need to maintain state between calls.
lenbits = len/2 - (SYNC_BITS + UPLINK_FRAME_BITS);
for (bit = 0; bit < lenbits; ++bit) {
int16_t dphi0 = phi_difference(phi[bit*2], phi[bit*2+1]);
int16_t dphi1 = phi_difference(phi[bit*2+1], phi[bit*2+2]);
sync0 = ((sync0 << 1) | (dphi0 > 0 ? 1 : 0)) & SYNC_MASK;
sync1 = ((sync1 << 1) | (dphi1 > 0 ? 1 : 0)) & SYNC_MASK;
if (bit < SYNC_BITS)
continue; // haven't fully populated sync0/1 yet
// see if we have (the start of) a valid sync word
// It would be nice to look at popcount(expected ^ sync)
// so we can tolerate some errors, but that turns out
// to be very expensive to do on every sample
// when we find a match, try to demodulate both with that match
// and with the next position, and pick the one with fewer
// errors.
// check for downlink frames:
if (sync_word_fuzzy_compare(sync0, ADSB_SYNC_WORD) || sync_word_fuzzy_compare(sync1, ADSB_SYNC_WORD)) {
int startbit = (bit-SYNC_BITS+1);
int shift = (sync_word_fuzzy_compare(sync0, ADSB_SYNC_WORD) ? 0 : 1);
int index = startbit*2+shift;
int skip_0, skip_1;
int rs_0 = -1, rs_1 = -1;
skip_0 = demod_adsb_frame(phi+index, demod_buf_a, &rs_0);
skip_1 = demod_adsb_frame(phi+index+1, demod_buf_b, &rs_1);
if (skip_0 && rs_0 <= rs_1) {
handle_adsb_frame(offset+index, demod_buf_a, rs_0);
bit = startbit + skip_0;
continue;
} else if (skip_1 && rs_1 <= rs_0) {
handle_adsb_frame(offset+index+1, demod_buf_b, rs_1);
bit = startbit + skip_1;
continue;
} else {
// demod failed
}
}
// check for uplink frames:
else if (sync_word_fuzzy_compare(sync0, UPLINK_SYNC_WORD) || sync_word_fuzzy_compare(sync1, UPLINK_SYNC_WORD)) {
int startbit = (bit-SYNC_BITS+1);
int shift = (sync_word_fuzzy_compare(sync0, UPLINK_SYNC_WORD) ? 0 : 1);
int index = startbit*2+shift;
int skip_0, skip_1;
int rs_0 = -1, rs_1 = -1;
skip_0 = demod_uplink_frame(phi+index, demod_buf_a, &rs_0);
skip_1 = demod_uplink_frame(phi+index+1, demod_buf_b, &rs_1);
if (skip_0 && rs_0 <= rs_1) {
handle_uplink_frame(offset+index, demod_buf_a, rs_0);
bit = startbit + skip_0;
continue;
} else if (skip_1 && rs_1 <= rs_0) {
handle_uplink_frame(offset+index+1, demod_buf_b, rs_1);
bit = startbit + skip_1;
continue;
} else {
// demod failed
}
}
}
return (bit - SYNC_BITS)*2;
}
// demodulate 'bytes' bytes from samples at 'phi' into 'frame',
// using 'center_dphi' as the bit slicing threshold
static void demod_frame(uint16_t *phi, uint8_t *frame, int bytes, int16_t center_dphi)
{
while (--bytes >= 0) {
uint8_t b = 0;
if (phi_difference(phi[0], phi[1]) > center_dphi) b |= 0x80;
if (phi_difference(phi[2], phi[3]) > center_dphi) b |= 0x40;
if (phi_difference(phi[4], phi[5]) > center_dphi) b |= 0x20;
if (phi_difference(phi[6], phi[7]) > center_dphi) b |= 0x10;
if (phi_difference(phi[8], phi[9]) > center_dphi) b |= 0x08;
if (phi_difference(phi[10], phi[11]) > center_dphi) b |= 0x04;
if (phi_difference(phi[12], phi[13]) > center_dphi) b |= 0x02;
if (phi_difference(phi[14], phi[15]) > center_dphi) b |= 0x01;
*frame++ = b;
phi += 16;
}
}
// Demodulate an ADSB (Long UAT or Basic UAT) downlink frame
// with the first sync bit in 'phi', storing the frame into 'to'
// of length up to LONG_FRAME_BYTES. Set '*rs_errors' to the
// number of corrected errors, or 9999 if demodulation failed.
// Return 0 if demodulation failed, or the number of bits (not
// samples) consumed if demodulation was OK.
static int demod_adsb_frame(uint16_t *phi, uint8_t *to, int *rs_errors)
{
int16_t center_dphi;
int frametype;
if (!check_sync_word(phi, ADSB_SYNC_WORD, &center_dphi)) {
*rs_errors = 9999;
return 0;
}
demod_frame(phi + SYNC_BITS*2, to, LONG_FRAME_BYTES, center_dphi);
frametype = correct_adsb_frame(to, rs_errors);
if (frametype == 1)
return (SYNC_BITS + SHORT_FRAME_BITS);
else if (frametype == 2)
return (SYNC_BITS + LONG_FRAME_BITS);
else
return 0;
}
// Demodulate an uplink frame
// with the first sync bit in 'phi', storing the frame into 'to'
// of length up to UPLINK_FRAME_BYTES. Set '*rs_errors' to the
// number of corrected errors, or 9999 if demodulation failed.
// Return 0 if demodulation failed, or the number of bits (not
// samples) consumed if demodulation was OK.
static int demod_uplink_frame(uint16_t *phi, uint8_t *to, int *rs_errors)
{
int16_t center_dphi;
uint8_t interleaved[UPLINK_FRAME_BYTES];
if (!check_sync_word(phi, UPLINK_SYNC_WORD, &center_dphi)) {
*rs_errors = 9999;
return 0;
}
demod_frame(phi + SYNC_BITS*2, interleaved, UPLINK_FRAME_BYTES, center_dphi);
// deinterleave and correct
if (correct_uplink_frame(interleaved, to, rs_errors) == 1)
return (UPLINK_FRAME_BITS+SYNC_BITS);
else
return 0;
}