pager_flex_sync.cc

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/*
 * Copyright 2004,2006 Free Software Foundation, Inc.
 * 
 * This file is part of GNU Radio
 * 
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 * 
 * GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <pager_flex_sync.h>
#include <pageri_flex_modes.h>
#include <pageri_bch3221.h>
#include <pageri_util.h>
#include <gr_io_signature.h>
#include <gr_count_bits.h>

pager_flex_sync_sptr pager_make_flex_sync()
{
    return pager_flex_sync_sptr(new pager_flex_sync());
}

// FLEX sync block takes input from sliced baseband stream [0-3] at specified 
// channel rate.  Symbol timing is established based on receiving one of the
// defined FLEX protocol synchronization words.  The block outputs one FLEX frame
// worth of bits on each output phase for the data portion of the frame. Unused phases
// get all zeros, which are considered idle code words.

pager_flex_sync::pager_flex_sync() :
    gr_block ("flex_sync",
    gr_make_io_signature (1, 1, sizeof(unsigned char)),
    gr_make_io_signature (4, 4, sizeof(unsigned char))),
    d_sync(10) // Fixed at 10 samples per baud (@ 1600 baud)
{
    enter_idle();
}

void pager_flex_sync::forecast(int noutput_items, gr_vector_int &inputs_required)
{
    // samples per bit X number of outputs needed
    int items = noutput_items*d_spb;
    for (unsigned int i = 0; i < inputs_required.size(); i++)
        inputs_required[i] = items;
}

int pager_flex_sync::index_avg(int start, int end)
{
    // modulo average
    if (start < end)
        return (end + start)/2;
    else
        return ((end + start)/2 + d_spb/2) % d_spb;
}

bool pager_flex_sync::test_sync(unsigned char sym)
{
    // 64-bit FLEX sync code:
    // AAAA:BBBBBBBB:CCCC
    //
    // Where BBBBBBBB is always 0xA6C6AAAA
    // and AAAA^CCCC is 0xFFFF
    // 
    // Specific values of AAAA determine what bps and encoding the
    // packet is beyond the frame information word
    //
    // First we match on the marker field with a hamming distance < 4
    // Then we match on the outer code with a hamming distance < 4

    d_sync[d_index] = (d_sync[d_index] << 1) | (sym < 2);
    gr_int64 val = d_sync[d_index];
    gr_int32 marker = ((val & 0x0000FFFFFFFF0000ULL)) >> 16;

    if (gr_count_bits32(marker^FLEX_SYNC_MARKER) < 4) {
        gr_int32 code = ((val & 0xFFFF000000000000ULL) >> 32) |
                         (val & 0x000000000000FFFFULL);

        for (int i = 0; i < num_flex_modes; i++) {
            if (gr_count_bits32(code^flex_modes[i].sync) < 4) {
                d_mode = i;
                return true;
            }
        }

        // Marker received but doesn't match known codes
        // All codes have high word inverted to low word
        unsigned short high = (code & 0xFFFF0000) >> 16;
        unsigned short low = code & 0x0000FFFF;
        unsigned short syn = high^low;
        if (syn == 0xFFFF)
            fprintf(stderr, "Unknown sync code detected: %08X\n", code);
    }

    return false;
}

void pager_flex_sync::enter_idle()
{
    d_state = ST_IDLE;
    d_index = 0;
    d_start = 0;
    d_center = 0;
    d_end = 0;
    d_count = 0;
    d_mode = 0;
    d_baudrate = 1600;
    d_levels = 2;
    d_spb = 16000/d_baudrate;
    d_bit_a = 0;
    d_bit_b = 0;
    d_bit_c = 0;
    d_bit_d = 0;
    d_hibit = false;
    fflush(stdout);
}

void pager_flex_sync::enter_syncing()
{
    d_start = d_index;
    d_state = ST_SYNCING;
}

void pager_flex_sync::enter_sync1()
{
    d_state = ST_SYNC1;
    d_end = d_index;
    d_center = index_avg(d_start, d_end); // Center of goodness
    d_count = 0;
}

void pager_flex_sync::enter_sync2()
{
    d_state = ST_SYNC2;
    d_count = 0;
    d_baudrate = flex_modes[d_mode].baud;
    d_levels = flex_modes[d_mode].levels;
    d_spb = 16000/d_baudrate;

    if (d_baudrate == 3200) {
        // Oversampling buffer just got halved
        d_center = d_center/2;

	// We're here at the center of a 1600 baud bit
	// So this hack puts the index and bit counter
	// in the right place for 3200 bps.
        d_index = d_index/2-d_spb/2;         
	d_count = -1;                
    }				     
}

void pager_flex_sync::enter_data()
{
    d_state = ST_DATA;
    d_count = 0;
}

void pager_flex_sync::parse_fiw()
{
    // Nothing is done with these now, but these will end up getting
    // passed as metadata when mblocks are available

    // Bits 31-28 are frame number related, but unknown function
    // This might be a checksum
    d_unknown2 = pageri_reverse_bits8((d_fiw >> 24) & 0xF0);
	
    // Cycle is bits 27-24, reversed
    d_cycle = pageri_reverse_bits8((d_fiw >> 20) & 0xF0);

    // Frame is bits 23-17, reversed
    d_frame = pageri_reverse_bits8((d_fiw >> 16) & 0xFE);

    // Bits 16-11 are some sort of marker, usually identical across
    // many frames but sometimes changes between frames or modes
    d_unknown1 = (d_fiw >> 11) & 0x3F;

    //printf("CYC:%02i FRM:%03i\n", d_cycle, d_frame);
}

int pager_flex_sync::output_symbol(unsigned char sym)
{
    // Here is where we output a 1 or 0 on each phase according
    // to current FLEX mode and symbol value.  Unassigned phases
    // are zero from the enter_idle() initialization.
    //
    // FLEX can transmit the data portion of the frame at either
    // 1600 bps or 3200 bps, and can use either two- or four-level
    // FSK encoding.
    //
    // At 1600 bps, 2-level, a single "phase" is transmitted with bit
    // value '0' using level '3' and bit value '1' using level '0'.
    //
    // At 1600 bps, 4-level, a second "phase" is transmitted, and the 
    // di-bits are encoded with a gray code:
    //
    // Symbol	Phase 1  Phase 2
    // ------   -------  -------
    //   0         1        1
    //   1         1        0
    //   2         0        0
    //   3         0        1
    //
    // At 1600 bps, 4-level, these are called PHASE A and PHASE B.
    //
    // At 3200 bps, the same 1 or 2 bit encoding occurs, except that
    // additionally two streams are interleaved on alternating symbols.
    // Thus, PHASE A (and PHASE B if 4-level) are decoded on one symbol,
    // then PHASE C (and PHASE D if 4-level) are decoded on the next.
    
    int bits = 0;
    
    if (d_baudrate == 1600) {
	d_bit_a = (sym < 2);
	if (d_levels == 4)
	    d_bit_b = (sym == 0) || (sym == 3);

	*d_phase_a++ = d_bit_a;
	*d_phase_b++ = d_bit_b;
	*d_phase_c++ = d_bit_c;
	*d_phase_d++ = d_bit_d;
	bits++;
    }
    else {
	if (!d_hibit) {
	    d_bit_a = (sym < 2);
	    if (d_levels == 4)
		d_bit_b = (sym == 0) || (sym == 3);
	    d_hibit = true;
	}
	else {
	    d_bit_c = (sym < 2);
	    if (d_levels == 4)
		d_bit_d = (sym == 0) || (sym == 3);
	    d_hibit = false;

	    *d_phase_a++ = d_bit_a;
	    *d_phase_b++ = d_bit_b;
	    *d_phase_c++ = d_bit_c;
	    *d_phase_d++ = d_bit_d;
	    bits++;
	}
    }

    return bits;
}

int pager_flex_sync::general_work(int noutput_items,
    gr_vector_int &ninput_items,
    gr_vector_const_void_star &input_items,
    gr_vector_void_star &output_items)
{
    const unsigned char *in = (const unsigned char *)input_items[0];
    d_phase_a = (unsigned char *)output_items[0];
    d_phase_b = (unsigned char *)output_items[1];
    d_phase_c = (unsigned char *)output_items[2];
    d_phase_d = (unsigned char *)output_items[3];

    int i = 0, j = 0;
    int ninputs = ninput_items[0];

    while (i < ninputs && j < noutput_items) {
        unsigned char sym = *in++; i++;
        d_index = ++d_index % d_spb;
    
        switch (d_state) {
            case ST_IDLE:
		// Continually compare the received symbol stream
		// against the known FLEX sync words.
                if (test_sync(sym))
                    enter_syncing();
                break;
    
            case ST_SYNCING:
		// Wait until we stop seeing sync, then calculate
		// the center of the bit period (d_center)
                if (!test_sync(sym))
                    enter_sync1();
                break;
    
            case ST_SYNC1:
		// Skip 16 bits of dotting, then accumulate 32 bits
		// of Frame Information Word.
                if (d_index == d_center) {
		    d_fiw = (d_fiw << 1) | (sym > 1);
                    if (++d_count == 48) {
			// FIW is accumulated, call BCH to error correct it
			pageri_bch3221(d_fiw);
			parse_fiw();
                        enter_sync2();  
		    }
                }
                break;
    
            case ST_SYNC2:
		// This part and the remainder of the frame are transmitted
		// at either 1600 bps or 3200 bps based on the received
		// FLEX sync word. The second SYNC header is 25ms of idle bits
		// at either speed.
                if (d_index == d_center) {
                    // Skip 25 ms = 40 bits @ 1600 bps, 80 @ 3200 bps
                    if (++d_count == d_baudrate/40)
                        enter_data();
                }
                break;
    
            case ST_DATA:
		// The data portion of the frame is 1760 ms long at either 
		// baudrate.  This is 2816 bits @ 1600 bps and 5632 bits @ 3200 bps.
		// The output_symbol() routine decodes and doles out the bits
		// to each of the four transmitted phases of FLEX interleaved codes.
                if (d_index == d_center) {
		    j += output_symbol(sym);		    
                    if (++d_count == d_baudrate*1760/1000)
                        enter_idle();
		}
                break;

            default:
                assert(0); // memory corruption of d_state if ever gets here
                break;
        }
    }

    consume_each(i);
    return j;
}