diff options
Diffstat (limited to 'gsm-receiver/src/lib/decoder/sch.c')
| -rw-r--r-- | gsm-receiver/src/lib/decoder/sch.c | 333 |
1 files changed, 333 insertions, 0 deletions
diff --git a/gsm-receiver/src/lib/decoder/sch.c b/gsm-receiver/src/lib/decoder/sch.c new file mode 100644 index 0000000..6f141dd --- /dev/null +++ b/gsm-receiver/src/lib/decoder/sch.c @@ -0,0 +1,333 @@ +#include "system.h" +#include <stdio.h> +#include <stdlib.h> +#include <unistd.h> +#include <string.h> +#include "gsm_constants.h" + +/* + * Synchronization channel. + * + * Timeslot Repeat length Frame Number (mod repeat length) + * 0 51 1, 11, 21, 31, 41 + */ + +/* + * Parity (FIRE) for the GSM SCH. + * + * g(x) = x^10 + x^8 + x^6 + x^5 + x^4 + x^2 + 1 + */ +#define DATA_BLOCK_SIZE 25 +#define PARITY_SIZE 10 +#define TAIL_BITS_SIZE 4 +#define PARITY_OUTPUT_SIZE (DATA_BLOCK_SIZE + PARITY_SIZE + TAIL_BITS_SIZE) + +static const unsigned char parity_polynomial[PARITY_SIZE + 1] = { + 1, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1 +}; + +static const unsigned char parity_remainder[PARITY_SIZE] = { + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 +}; + + +static void parity_encode(unsigned char *d, unsigned char *p) +{ + + unsigned int i; + unsigned char buf[DATA_BLOCK_SIZE + PARITY_SIZE], *q; + + memcpy(buf, d, DATA_BLOCK_SIZE); + memset(buf + DATA_BLOCK_SIZE, 0, PARITY_SIZE); + + for (q = buf; q < buf + DATA_BLOCK_SIZE; q++) + if (*q) + for (i = 0; i < PARITY_SIZE + 1; i++) + q[i] ^= parity_polynomial[i]; + for (i = 0; i < PARITY_SIZE; i++) + p[i] = !buf[DATA_BLOCK_SIZE + i]; +} + + +static int parity_check(unsigned char *d) +{ + + unsigned int i; + unsigned char buf[DATA_BLOCK_SIZE + PARITY_SIZE], *q; + + memcpy(buf, d, DATA_BLOCK_SIZE + PARITY_SIZE); + + for (q = buf; q < buf + DATA_BLOCK_SIZE; q++) + if (*q) + for (i = 0; i < PARITY_SIZE + 1; i++) + q[i] ^= parity_polynomial[i]; + return memcmp(buf + DATA_BLOCK_SIZE, parity_remainder, PARITY_SIZE); +} + + +/* + * Convolutional encoding and Viterbi decoding for the GSM SCH. + * (Equivalent to the GSM SACCH.) + * + * G_0 = 1 + x^3 + x^4 + * G_1 = 1 + x + x^3 + x^4 + * + * i.e., + * + * c_{2k} = u_k + u_{k - 3} + u_{k - 4} + * c_{2k + 1} = u_k + u_{k - 1} + u_{k - 3} + u_{k - 4} + */ +#define CONV_INPUT_SIZE PARITY_OUTPUT_SIZE +#define CONV_SIZE (2 * CONV_INPUT_SIZE) +#define K 5 +#define MAX_ERROR (2 * CONV_INPUT_SIZE + 1) + + +/* + * Given the current state and input bit, what are the output bits? + * + * encode[current_state][input_bit] + */ +static const unsigned int encode[1 << (K - 1)][2] = { + {0, 3}, {3, 0}, {3, 0}, {0, 3}, + {0, 3}, {3, 0}, {3, 0}, {0, 3}, + {1, 2}, {2, 1}, {2, 1}, {1, 2}, + {1, 2}, {2, 1}, {2, 1}, {1, 2} +}; + + +/* + * Given the current state and input bit, what is the next state? + * + * next_state[current_state][input_bit] + */ +static const unsigned int next_state[1 << (K - 1)][2] = { + {0, 8}, {0, 8}, {1, 9}, {1, 9}, + {2, 10}, {2, 10}, {3, 11}, {3, 11}, + {4, 12}, {4, 12}, {5, 13}, {5, 13}, + {6, 14}, {6, 14}, {7, 15}, {7, 15} +}; + + +/* + * Given the previous state and the current state, what input bit caused + * the transition? If it is impossible to transition between the two + * states, the value is 2. + * + * prev_next_state[previous_state][current_state] + */ +static const unsigned int prev_next_state[1 << (K - 1)][1 << (K - 1)] = { + { 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2}, + { 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2}, + { 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2}, + { 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2}, + { 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2}, + { 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2}, + { 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2}, + { 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2}, + { 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2}, + { 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2}, + { 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2}, + { 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2}, + { 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2}, + { 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2}, + { 2, 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1}, + { 2, 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1} +}; + + +static inline unsigned int hamming_distance2(unsigned int w) +{ + + return (w & 1) + !!(w & 2); +} + + +static void conv_encode(unsigned char *data, unsigned char *output) +{ + + unsigned int i, state = 0, o; + + // encode data + for (i = 0; i < CONV_INPUT_SIZE; i++) { + o = encode[state][data[i]]; + state = next_state[state][data[i]]; + *output++ = !!(o & 2); + *output++ = o & 1; + } +} + + +static int conv_decode(unsigned char *data, unsigned char *output) +{ + + int i, t; + unsigned int rdata, state, nstate, b, o, distance, accumulated_error, + min_state, min_error, cur_state; + + unsigned int ae[1 << (K - 1)]; + unsigned int nae[1 << (K - 1)]; // next accumulated error + unsigned int state_history[1 << (K - 1)][CONV_INPUT_SIZE + 1]; + + // initialize accumulated error, assume starting state is 0 + for (i = 0; i < (1 << (K - 1)); i++) + ae[i] = nae[i] = MAX_ERROR; + ae[0] = 0; + + // build trellis + for (t = 0; t < CONV_INPUT_SIZE; t++) { + + // get received data symbol + rdata = (data[2 * t] << 1) | data[2 * t + 1]; + + // for each state + for (state = 0; state < (1 << (K - 1)); state++) { + + // make sure this state is possible + if (ae[state] >= MAX_ERROR) + continue; + + // find all states we lead to + for (b = 0; b < 2; b++) { + + // get next state given input bit b + nstate = next_state[state][b]; + + // find output for this transition + o = encode[state][b]; + + // calculate distance from received data + distance = hamming_distance2(rdata ^ o); + + // choose surviving path + accumulated_error = ae[state] + distance; + if (accumulated_error < nae[nstate]) { + + // save error for surviving state + nae[nstate] = accumulated_error; + + // update state history + state_history[nstate][t + 1] = state; + } + } + } + + // get accumulated error ready for next time slice + for (i = 0; i < (1 << (K - 1)); i++) { + ae[i] = nae[i]; + nae[i] = MAX_ERROR; + } + } + + // the final state is the state with the fewest errors + min_state = (unsigned int) - 1; + min_error = MAX_ERROR; + for (i = 0; i < (1 << (K - 1)); i++) { + if (ae[i] < min_error) { + min_state = i; + min_error = ae[i]; + } + } + + // trace the path + cur_state = min_state; + for (t = CONV_INPUT_SIZE; t >= 1; t--) { + min_state = cur_state; + cur_state = state_history[cur_state][t]; // get previous + output[t - 1] = prev_next_state[cur_state][min_state]; + } + + // return the number of errors detected (hard-decision) + return min_error; +} + + +int decode_sch(const unsigned char *buf, int * t1_o, int * t2_o, int * t3_o, int * ncc_o, int * bcc_o) +{ + + int errors, t1, t2, t3p, t3, ncc, bcc; + unsigned char data[CONV_SIZE], decoded_data[PARITY_OUTPUT_SIZE]; + + // extract encoded data from synchronization burst + /* buf, 39 bit */ + /* buf + 39 + 64 = 103, 39 */ + memcpy(data, buf, SCH_DATA_LEN); + memcpy(data + SCH_DATA_LEN, buf + SCH_DATA_LEN + N_SYNC_BITS, SCH_DATA_LEN); + + // Viterbi decode + if (errors = conv_decode(data, decoded_data)) { + // fprintf(stderr, "error: sch: conv_decode (%d)\n", errors); + DEBUGF("ERR: conv_decode %d\n", errors); + return errors; + } + + // check parity + if (parity_check(decoded_data)) { + // fprintf(stderr, "error: sch: parity failed\n"); + DEBUGF("ERR: parity_check failed\n"); + return 1; + } + + // Synchronization channel information, 44.018 page 171. (V7.2.0) + ncc = + (decoded_data[ 7] << 2) | + (decoded_data[ 6] << 1) | + (decoded_data[ 5] << 0); + bcc = + (decoded_data[ 4] << 2) | + (decoded_data[ 3] << 1) | + (decoded_data[ 2] << 0); + t1 = + (decoded_data[ 1] << 10) | + (decoded_data[ 0] << 9) | + (decoded_data[15] << 8) | + (decoded_data[14] << 7) | + (decoded_data[13] << 6) | + (decoded_data[12] << 5) | + (decoded_data[11] << 4) | + (decoded_data[10] << 3) | + (decoded_data[ 9] << 2) | + (decoded_data[ 8] << 1) | + (decoded_data[23] << 0); + t2 = + (decoded_data[22] << 4) | + (decoded_data[21] << 3) | + (decoded_data[20] << 2) | + (decoded_data[19] << 1) | + (decoded_data[18] << 0); + t3p = + (decoded_data[17] << 2) | + (decoded_data[16] << 1) | + (decoded_data[24] << 0); + + t3 = 10 * t3p + 1; + + // modulo arithmetic t3 - t2 mod 26 +// tt = ((t3 + 26) - t2) % 26; + +// fn = (51 * 26 * t1) + (51 * tt) + t3; + + /* + * BSIC: Base Station Identification Code + * BCC: Base station Color Code + * NCC: Network Color Code + * + * FN: Frame Number + */ + +// printf("bsic: %x (bcc: %u; ncc: %u)\tFN: %u\n", bsic, bsic & 7, +// (bsic >> 3) & 7, fn); + +// if (fn_o) +// *fn_o = fn; +// if (bsic_o) + if (t1_o && t2_o && t3_o && ncc_o && bcc_o) { + *t1_o = t1; + *t2_o = t2; + *t3_o = t3; + *bcc_o = bcc; + *ncc_o = ncc; + } + + return 0; +} |