From 8d2bc49fb9e0c9a5fbd75aa3cad207608e72bf99 Mon Sep 17 00:00:00 2001 From: Piotr Krysik Date: Tue, 30 Jun 2009 23:03:33 +0200 Subject: moved gsm-receiver into directory - preparation to move to airprobe --- gsm-receiver/src/lib/decoder/cch.c | 482 +++++++++++++++++++++++++++++++++++++ 1 file changed, 482 insertions(+) create mode 100644 gsm-receiver/src/lib/decoder/cch.c (limited to 'gsm-receiver/src/lib/decoder/cch.c') diff --git a/gsm-receiver/src/lib/decoder/cch.c b/gsm-receiver/src/lib/decoder/cch.c new file mode 100644 index 0000000..f1da56d --- /dev/null +++ b/gsm-receiver/src/lib/decoder/cch.c @@ -0,0 +1,482 @@ +//TODO: this file shouldn't be part of the GSM Receiver +#include "system.h" +#include +#include +#include +#include +#include + +//#include +//#include +#include +//#include "burst_types.h" +#include "cch.h" +#include "fire_crc.h" + + +/* + * GSM SACCH -- Slow Associated Control Channel + * + * These messages are encoded exactly the same as on the BCCH. + * (Broadcast Control Channel.) + * + * Input: 184 bits + * + * 1. Add parity and flushing bits. (Output 184 + 40 + 4 = 228 bit) + * 2. Convolutional encode. (Output 228 * 2 = 456 bit) + * 3. Interleave. (Output 456 bit) + * 4. Map on bursts. (4 x 156 bit bursts with each 2x57 bit content data) + */ + + +/* + * Parity (FIRE) for the GSM SACCH channel. + * + * g(x) = (x^23 + 1)(x^17 + x^3 + 1) + * = x^40 + x^26 + x^23 + x^17 + x^3 + 1 + */ + +static const unsigned char parity_polynomial[PARITY_SIZE + 1] = { + 1, 0, 0, 0, 0, 0, 0, 0, + 0, 0, 0, 0, 0, 0, 1, 0, + 0, 1, 0, 0, 0, 0, 0, 1, + 0, 0, 0, 0, 0, 0, 0, 0, + 0, 0, 0, 0, 0, 1, 0, 0, + 1 +}; + +// remainder after dividing data polynomial by g(x) +static const unsigned char parity_remainder[PARITY_SIZE] = { + 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1 +}; + + +/* +static void parity_encode(unsigned char *d, unsigned char *p) { + + 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 SACCH channel. + */ + +/* + * Convolutional encoding: + * + * 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 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 *output, unsigned char *data) { + + 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; +} + + +/* + * GSM SACCH interleaving and burst mapping + * + * Interleaving: + * + * Given 456 coded input bits, form 4 blocks of 114 bits: + * + * i(B, j) = c(n, k) k = 0, ..., 455 + * n = 0, ..., N, N + 1, ... + * B = B_0 + 4n + (k mod 4) + * j = 2(49k mod 57) + ((k mod 8) div 4) + * + * Mapping on Burst: + * + * e(B, j) = i(B, j) + * e(B, 59 + j) = i(B, 57 + j) j = 0, ..., 56 + * e(B, 57) = h_l(B) + * e(B, 58) = h_n(B) + * + * Where h_l(B) and h_n(B) are bits in burst B indicating flags. + */ + +/* +static void interleave(unsigned char *data, unsigned char *iBLOCK) { + + int j, k, B; + + // for each bit in input data + for(k = 0; k < CONV_SIZE; k++) { + B = k % 4; + j = 2 * ((49 * k) % 57) + ((k % 8) / 4); + iBLOCK[B * iBLOCK_SIZE + j] = data[k]; + } +} + */ + + +#if 0 +static void decode_interleave(unsigned char *data, unsigned char *iBLOCK) { + + int j, k, B; + + for(k = 0; k < CONV_SIZE; k++) { + B = k % 4; + j = 2 * ((49 * k) % 57) + ((k % 8) / 4); + data[k] = iBLOCK[B * iBLOCK_SIZE + j]; + } +} + +#endif + +/* +static void burstmap(unsigned char *iBLOCK, unsigned char *eBLOCK, + unsigned char hl, unsigned char hn) { + + int j; + + for(j = 0; j < 57; j++) { + eBLOCK[j] = iBLOCK[j]; + eBLOCK[j + 59] = iBLOCK[j + 57]; + } + eBLOCK[57] = hl; + eBLOCK[58] = hn; +} + */ + + +static void decode_burstmap(unsigned char *iBLOCK, unsigned char *eBLOCK, + unsigned char *hl, unsigned char *hn) { + + int j; + + for(j = 0; j < 57; j++) { + iBLOCK[j] = eBLOCK[j]; + iBLOCK[j + 57] = eBLOCK[j + 59]; + } + *hl = eBLOCK[57]; + *hn = eBLOCK[58]; +} + + +/* + * Transmitted bits are sent least-significant first. + */ +static int compress_bits(unsigned char *dbuf, unsigned int dbuf_len, + unsigned char *sbuf, unsigned int sbuf_len) { + + unsigned int i, j, c, pos = 0; + + if(dbuf_len < ((sbuf_len + 7) >> 3)) + return -1; + + for(i = 0; i < sbuf_len; i += 8) { + for(j = 0, c = 0; (j < 8) && (i + j < sbuf_len); j++) + c |= (!!sbuf[i + j]) << j; + dbuf[pos++] = c & 0xff; + } + return pos; +} + + +#if 0 +int get_ns_l3_len(unsigned char *data, unsigned int datalen) { + + if((data[0] & 3) != 1) { + fprintf(stderr, "error: get_ns_l3_len: pseudo-length reserved " + "bits bad (%2.2x)\n", data[0] & 3); + return -1; + } + return (data[0] >> 2); +} + +#endif + + +static unsigned char *decode_sacch(GS_CTX *ctx, unsigned char *burst, unsigned int *datalen) { + + int errors, len, data_size; + unsigned char conv_data[CONV_SIZE], iBLOCK[BLOCKS][iBLOCK_SIZE], + hl, hn, decoded_data[PARITY_OUTPUT_SIZE]; + FC_CTX fc_ctx; + + data_size = sizeof ctx->msg; + if(datalen) + *datalen = 0; + + // unmap the bursts + decode_burstmap(iBLOCK[0], burst, &hl, &hn); // XXX ignore stealing bits + decode_burstmap(iBLOCK[1], burst + 116, &hl, &hn); + decode_burstmap(iBLOCK[2], burst + 116 * 2, &hl, &hn); + decode_burstmap(iBLOCK[3], burst + 116 * 3, &hl, &hn); + + // remove interleave + interleave_decode(&ctx->interleave_ctx, conv_data, (unsigned char *)iBLOCK); + //decode_interleave(conv_data, (unsigned char *)iBLOCK); + + // Viterbi decode + errors = conv_decode(decoded_data, conv_data); + //DEBUGF("conv_decode: %d\n", errors); + if (errors) + return NULL; + + // check parity + // If parity check error detected try to fix it. + if (parity_check(decoded_data)) + { + FC_init(&fc_ctx, 40, 184); + unsigned char crc_result[224]; + if (FC_check_crc(&fc_ctx, decoded_data, crc_result) == 0) + { + errors = -1; + DEBUGF("error: sacch: parity error (%d)\n", errors); + return NULL; + } else { + DEBUGF("Successfully corrected parity bits!\n"); + memcpy(decoded_data, crc_result, sizeof crc_result); + errors = 0; + } + } + + if((len = compress_bits(ctx->msg, data_size, decoded_data, + DATA_BLOCK_SIZE)) < 0) { + fprintf(stderr, "error: compress_bits\n"); + return NULL; + } + if(len < data_size) { + fprintf(stderr, "error: buf too small (%d < %d)\n", + sizeof(ctx->msg), len); + return NULL; + } + + if(datalen) + *datalen = (unsigned int)len; + return ctx->msg; +} + + +/* + * decode_cch + * + * Decode a "common" control channel. Most control channels use + * the same burst, interleave, Viterbi and parity configuration. + * The documentation for the control channels defines SACCH first + * and then just keeps referring to that. + * + * The current (investigated) list is as follows: + * + * BCCH Norm + * BCCH Ext + * PCH + * AGCH + * CBCH (SDCCH/4) + * CBCH (SDCCH/8) + * SDCCH/4 + * SACCH/C4 + * SDCCH/8 + * SACCH/C8 + * + * We provide two functions, one for where all four bursts are + * contiguous, and one where they aren't. + */ +unsigned char *decode_cch(GS_CTX *ctx, unsigned char *burst, unsigned int *datalen) { + + return decode_sacch(ctx, burst, datalen); +} + + +#if 0 +unsigned char *decode_cch(GS_CTX *ctx, unsigned char *e, unsigned int *datalen) { + + return decode_sacch(ctx, e, e + eBLOCK_SIZE, e + 2 * eBLOCK_SIZE, + e + 3 * eBLOCK_SIZE, datalen); +} +#endif -- cgit v1.2.3