Redesign of the AX.25 decoder
This commits introduces a significant redesign of the AX.25 decoding block. Due to the poor AX.25 sync flag, the decoders exhibited too many false alarms. To deal with this problem, we introduced the quadrature demod filter block, that tried to measure the SNR based on the running variance of the signal. The problem with that was that the user should have to fine tune two parameters, the one related with the amplitude of the incoming signal and the second one related with the window that the calculations should take place. This solution worked until now, but we can always increase the performance. The new AX.25 decoder stores the bitstream in a queue and always tries to exlpoit possible valid frames in the queue. If now sync flags have been encountered, the queue is flushed. After a valid frame extraction, bits corresponding to this frame are also deleted from the queue. This technique requires more memory and CPU, but it increases a lot the decoding performance.
This commit is contained in:
Manolis Surligas
parent
6e996fc85a
commit
767e4e3d38
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@ -197,7 +197,10 @@ namespace gr
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out[i++] = fcs & 0xFF;
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out[i++] = (fcs >> 8) & 0xFF;
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memset (out + i, AX25_SYNC_FLAG, postamble_len);
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for(size_t j = preamble_len; j < i; j++) {
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printf("0x%02x ", out[j]);
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}
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printf("\n");
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return i + postamble_len;
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}
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@ -30,406 +30,299 @@
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namespace gr
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{
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namespace satnogs
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{
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namespace satnogs
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{
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ax25_decoder_bm::sptr
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ax25_decoder_bm::make (const std::string& addr, uint8_t ssid, bool promisc,
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bool descramble, size_t max_frame_len)
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{
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return gnuradio::get_initial_sptr (
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new ax25_decoder_bm_impl (addr, ssid, promisc, descramble,
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max_frame_len));
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}
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ax25_decoder_bm::sptr
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ax25_decoder_bm::make (const std::string &addr, uint8_t ssid, bool promisc,
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bool descramble, size_t max_frame_len)
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{
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return gnuradio::get_initial_sptr (
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new ax25_decoder_bm_impl (addr, ssid, promisc, descramble, max_frame_len));
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}
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/*
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* The private constructor
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*/
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ax25_decoder_bm_impl::ax25_decoder_bm_impl (const std::string& addr,
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uint8_t ssid, bool promisc,
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bool descramble,
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size_t max_frame_len) :
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gr::sync_block ("ax25_decoder_bm",
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gr::io_signature::make (1, 1, sizeof(uint8_t)),
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gr::io_signature::make (0, 0, 0)),
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d_promisc (promisc),
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d_descramble (descramble),
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d_max_frame_len (max_frame_len),
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d_state (NO_SYNC),
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d_shift_reg (0x0),
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d_dec_b (0x0),
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d_prev_bit_nrzi (0),
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d_received_bytes (0),
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d_decoded_bits (0),
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d_lfsr (0x21, 0x0, 16),
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d_frame_buffer (
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new uint8_t[max_frame_len + AX25_MAX_ADDR_LEN
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+ AX25_MAX_CTRL_LEN + sizeof(uint16_t)])
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{
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/* Valid PDUs output message port */
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message_port_register_out (pmt::mp ("pdu"));
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/*
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* Valid invalid (wrong CRC, different destination Callsign/SSID,
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* wrong frame size)PDUs output message port
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*/
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message_port_register_out (pmt::mp ("failed_pdu"));
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}
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/*
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* The private constructor
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*/
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ax25_decoder_bm_impl::ax25_decoder_bm_impl (const std::string &addr,
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uint8_t ssid, bool promisc,
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bool descramble,
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size_t max_frame_len) :
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gr::sync_block ("ax25_decoder_bm",
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gr::io_signature::make (1, 1, sizeof(uint8_t)),
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gr::io_signature::make (0, 0, 0)),
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d_promisc (promisc),
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d_descramble (descramble),
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d_max_frame_len (max_frame_len),
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d_state (NO_SYNC),
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d_shift_reg (0x0),
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d_dec_b (0x0),
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d_prev_bit_nrzi (0),
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d_received_bytes (0),
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d_decoded_bits (0),
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d_lfsr (0x21, 0x0, 16),
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d_frame_buffer (
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new uint8_t[max_frame_len + AX25_MAX_ADDR_LEN + AX25_MAX_CTRL_LEN
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+ sizeof(uint16_t)]),
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d_start_idx(0)
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{
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/* Valid PDUs output message port */
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message_port_register_out (pmt::mp ("pdu"));
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/*
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* Valid invalid (wrong CRC, different destination Callsign/SSID,
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* wrong frame size)PDUs output message port
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*/
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message_port_register_out (pmt::mp ("failed_pdu"));
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}
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size_t
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ax25_decoder_bm_impl::descramble_and_decode (const uint8_t* in,
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size_t nitems)
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{
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size_t i;
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switch (d_state)
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{
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case NO_SYNC:
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for (i = 0; i < nitems; i++) {
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descramble_and_decode_1b (in[i]);
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if (d_shift_reg == AX25_SYNC_FLAG) {
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enter_sync_state ();
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return i + 1;
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}
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}
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return nitems;
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case IN_SYNC:
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/*
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* Most of the transmitters repeat several times the AX.25 SYNC
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* In case of G3RUH this is mandatory to allow the self synchronizing
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* scrambler to settle
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*/
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for (i = 0; i < nitems; i++) {
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descramble_and_decode_1b (in[i]);
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d_decoded_bits++;
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if(d_decoded_bits == 8) {
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/* Perhaps we are in frame! */
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if(d_shift_reg != AX25_SYNC_FLAG) {
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enter_decoding_state();
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return i+1;
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}
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d_decoded_bits = 0;
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}
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}
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return nitems;
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case DECODING:
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for (i = 0; i < nitems; i++) {
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descramble_and_decode_1b (in[i]);
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if (d_shift_reg == AX25_SYNC_FLAG) {
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LOG_DEBUG("Found frame end");
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enter_frame_end();
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return i+1;
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}
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else if ((d_shift_reg & 0xfc) == 0x7c) {
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/*This was a stuffed bit */
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d_dec_b <<= 1;
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}
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else if((d_shift_reg & 0xfe) == 0xfe) {
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LOG_DEBUG("Invalid shift register value %u", d_received_bytes);
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reset_state ();
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return i+1;
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}
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else{
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d_decoded_bits++;
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if(d_decoded_bits == 8) {
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d_frame_buffer[d_received_bytes] = d_dec_b;
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d_received_bytes++;
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d_decoded_bits = 0;
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/*Check if the frame limit was reached */
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if (d_received_bytes >= d_max_frame_len) {
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LOG_DEBUG("Wrong size");
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message_port_pub (
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pmt::mp ("failed_pdu"),
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pmt::make_blob (d_frame_buffer, d_max_frame_len));
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reset_state ();
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return i+1;
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}
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}
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}
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void
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ax25_decoder_bm_impl::decode ()
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{
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while (1) {
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bool cont = false;
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switch (d_state)
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{
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case NO_SYNC:
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for (size_t i = 0; i < d_bitstream.size (); i++) {
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decode_1b (d_bitstream[i]);
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if (d_shift_reg == AX25_SYNC_FLAG) {
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d_bitstream.erase (d_bitstream.begin (),
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d_bitstream.begin () + i + 1);
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enter_sync_state ();
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d_start_idx = 0;
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cont = true;
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break;
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}
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return nitems;
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case FRAME_END:
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for (i = 0; i < nitems; i++) {
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descramble_and_decode_1b (in[i]);
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d_decoded_bits++;
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if (d_decoded_bits == 8) {
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/* Repetitions of the trailing SYNC flag finished */
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if (d_shift_reg != AX25_SYNC_FLAG) {
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reset_state();
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return i + 1;
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}
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d_decoded_bits = 0;
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}
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}
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return nitems;
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default:
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LOG_ERROR("Invalid decoding state");
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reset_state();
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return nitems;
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}
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}
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size_t
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ax25_decoder_bm_impl::decode (const uint8_t* in, size_t nitems)
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{
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size_t i;
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switch (d_state)
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{
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case NO_SYNC:
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for (i = 0; i < nitems; i++) {
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decode_1b (in[i]);
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if (d_shift_reg == AX25_SYNC_FLAG) {
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enter_sync_state ();
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return i + 1;
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}
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}
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return nitems;
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case IN_SYNC:
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/*
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* Most of the transmitters repeat several times the AX.25 SYNC
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* In case of G3RUH this is mandatory to allow the self synchronizing
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* scrambler to settle
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*/
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for (i = 0; i < nitems; i++) {
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decode_1b (in[i]);
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d_decoded_bits++;
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if (d_decoded_bits == 8) {
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/* Perhaps we are in frame! */
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if (d_shift_reg != AX25_SYNC_FLAG) {
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enter_decoding_state ();
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return i + 1;
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}
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d_decoded_bits = 0;
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}
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}
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return nitems;
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case DECODING:
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for (i = 0; i < nitems; i++) {
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decode_1b (in[i]);
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if (d_shift_reg == AX25_SYNC_FLAG) {
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enter_frame_end ();
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return i + 1;
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}
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else if ((d_shift_reg & 0xfc) == 0x7c) {
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/*This was a stuffed bit */
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d_dec_b <<= 1;
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}
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else if ((d_shift_reg & 0xfe) == 0xfe) {
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LOG_DEBUG("Invalid shift register value");
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reset_state ();
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return i + 1;
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}
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else {
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d_decoded_bits++;
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if (d_decoded_bits == 8) {
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d_frame_buffer[d_received_bytes] = d_dec_b;
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d_received_bytes++;
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d_decoded_bits = 0;
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/*Check if the frame limit was reached */
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if (d_received_bytes >= d_max_frame_len) {
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LOG_DEBUG("Wrong size");
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message_port_pub (
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pmt::mp ("failed_pdu"),
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pmt::make_blob (d_frame_buffer, d_max_frame_len));
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reset_state ();
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return i + 1;
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}
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}
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}
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}
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return nitems;
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case FRAME_END:
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for (i = 0; i < nitems; i++) {
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decode_1b (in[i]);
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d_decoded_bits++;
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if (d_decoded_bits == 8) {
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/* Repetitions of the trailing SYNC flag finished */
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if (d_shift_reg != AX25_SYNC_FLAG) {
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reset_state ();
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return i + 1;
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}
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d_decoded_bits = 0;
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}
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}
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return nitems;
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default:
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LOG_ERROR("Invalid decoding state");
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reset_state ();
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return nitems;
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if(cont) {
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continue;
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}
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}
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/*
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* Our virtual destructor.
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*/
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ax25_decoder_bm_impl::~ax25_decoder_bm_impl ()
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{
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delete [] d_frame_buffer;
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}
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void
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ax25_decoder_bm_impl::reset_state ()
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{
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d_state = NO_SYNC;
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d_dec_b = 0x0;
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d_shift_reg = 0x0;
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d_decoded_bits = 0;
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d_received_bytes = 0;
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d_prev_bit_nrzi = 0;
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}
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void
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ax25_decoder_bm_impl::enter_sync_state ()
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{
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d_state = IN_SYNC;
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d_dec_b = 0x0;
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d_shift_reg = 0x0;
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d_decoded_bits = 0;
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d_received_bytes = 0;
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}
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void
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ax25_decoder_bm_impl::enter_decoding_state ()
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{
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uint8_t tmp;
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d_state = DECODING;
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d_decoded_bits = 0;
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d_shift_reg = 0x0;
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/*
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* Due to the possibility of bit stuffing on the first byte some special
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* handling is necessary
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*/
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tmp = d_dec_b;
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d_dec_b = 0x0;
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for (size_t i = 0; i < 8; i++) {
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d_shift_reg = (d_shift_reg >> 1) | (((tmp >> i) & 0x1) << 7);
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d_dec_b = (d_dec_b >> 1) | (((tmp >> i) & 0x1) << 7);
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if ((d_shift_reg & 0xfc) == 0x7c) {
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/*This was a stuffed bit */
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d_dec_b <<= 1;
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}
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else{
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d_bitstream.clear ();
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return;
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case IN_SYNC:
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/*
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* Most of the transmitters repeat several times the AX.25 SYNC
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* In case of G3RUH this is mandatory to allow the self synchronizing
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* scrambler to settle
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*/
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for (size_t i = d_start_idx; i < d_bitstream.size (); i++) {
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decode_1b (d_bitstream[i]);
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d_decoded_bits++;
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if (d_decoded_bits == 8) {
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/* Perhaps we are in frame! */
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if (d_shift_reg != AX25_SYNC_FLAG) {
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d_start_idx = i + 1;
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enter_decoding_state ();
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cont = true;
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break;
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}
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d_decoded_bits = 0;
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}
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}
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}
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if(cont) {
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continue;
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}
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d_start_idx = d_bitstream.size ();
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return;
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case DECODING:
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for (size_t i = d_start_idx; i < d_bitstream.size (); i++) {
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decode_1b (d_bitstream[i]);
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if (d_shift_reg == AX25_SYNC_FLAG) {
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LOG_DEBUG("Found frame end");
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if (enter_frame_end ()) {
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d_bitstream.erase (d_bitstream.begin (),
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d_bitstream.begin () + i + 1);
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}
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cont = true;
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break;
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}
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else if ((d_shift_reg & 0xfc) == 0x7c) {
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/*This was a stuffed bit */
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d_dec_b <<= 1;
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}
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else if ((d_shift_reg & 0xfe) == 0xfe) {
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LOG_DEBUG("Invalid shift register value %u", d_received_bytes);
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reset_state ();
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cont = true;
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break;
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}
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else {
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d_decoded_bits++;
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if (d_decoded_bits == 8) {
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d_frame_buffer[d_received_bytes++] = d_dec_b;
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d_decoded_bits = 0;
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if(d_decoded_bits == 8) {
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d_frame_buffer[0] = d_dec_b;
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d_decoded_bits = 0;
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d_received_bytes = 1;
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}
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else {
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d_received_bytes = 0;
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}
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}
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void
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ax25_decoder_bm_impl::enter_frame_end ()
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{
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uint16_t fcs;
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uint16_t recv_fcs = 0x0;
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/* First check if the size of the frame is valid */
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if (d_received_bytes < AX25_MIN_ADDR_LEN + sizeof(uint16_t)) {
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d_dec_b = 0x0;
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d_shift_reg = 0x0;
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d_decoded_bits = 0;
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d_received_bytes = 0;
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d_state = FRAME_END;
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/*Check if the frame limit was reached */
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if (d_received_bytes >= d_max_frame_len) {
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LOG_DEBUG("Wrong size");
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reset_state ();
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cont = true;
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break;
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}
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}
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}
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}
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if(cont) {
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continue;
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}
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d_start_idx = d_bitstream.size ();
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return;
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default:
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LOG_ERROR("Invalid decoding state");
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reset_state ();
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return;
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}
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}
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}
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/* Check if the frame is correct using the FCS field */
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fcs = ax25_fcs (d_frame_buffer, d_received_bytes - sizeof(uint16_t));
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recv_fcs = (((uint16_t) d_frame_buffer[d_received_bytes - 1]) << 8)
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| d_frame_buffer[d_received_bytes - 2];
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/*
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* Our virtual destructor.
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*/
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ax25_decoder_bm_impl::~ax25_decoder_bm_impl ()
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{
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delete[] d_frame_buffer;
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LOG_DEBUG("Left over: %lu", d_bitstream.size());
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}
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if (fcs == recv_fcs) {
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message_port_pub (
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pmt::mp ("pdu"),
|
||||
pmt::make_blob (d_frame_buffer,
|
||||
d_received_bytes - sizeof(uint16_t)));
|
||||
}
|
||||
else {
|
||||
message_port_pub (
|
||||
pmt::mp ("failed_pdu"),
|
||||
pmt::make_blob (d_frame_buffer,
|
||||
d_received_bytes - sizeof(uint16_t)));
|
||||
LOG_DEBUG("Wrong crc");
|
||||
}
|
||||
d_dec_b = 0x0;
|
||||
d_shift_reg = 0x0;
|
||||
d_decoded_bits = 0;
|
||||
d_received_bytes = 0;
|
||||
d_state = FRAME_END;
|
||||
}
|
||||
void
|
||||
ax25_decoder_bm_impl::reset_state ()
|
||||
{
|
||||
d_state = NO_SYNC;
|
||||
d_dec_b = 0x0;
|
||||
d_shift_reg = 0x0;
|
||||
d_decoded_bits = 0;
|
||||
d_received_bytes = 0;
|
||||
}
|
||||
|
||||
/**
|
||||
* Performs descrambling and NRZI decoding of an input bit.
|
||||
* The decoded bit is then shifted in front of the d_shift_reg and the d_dec_b
|
||||
* variables. This shift in front is due to the LS bit first transmission
|
||||
* of the Ax.25 protocol.
|
||||
*
|
||||
* @param in input bit
|
||||
*/
|
||||
inline void
|
||||
ax25_decoder_bm_impl::descramble_and_decode_1b (uint8_t in)
|
||||
{
|
||||
uint8_t descr_bit;
|
||||
uint8_t dec_bit;
|
||||
void
|
||||
ax25_decoder_bm_impl::enter_sync_state ()
|
||||
{
|
||||
d_state = IN_SYNC;
|
||||
d_dec_b = 0x0;
|
||||
d_shift_reg = 0x0;
|
||||
d_decoded_bits = 0;
|
||||
d_received_bytes = 0;
|
||||
}
|
||||
|
||||
in &= 0x1;
|
||||
void
|
||||
ax25_decoder_bm_impl::enter_decoding_state ()
|
||||
{
|
||||
uint8_t tmp;
|
||||
d_state = DECODING;
|
||||
d_decoded_bits = 0;
|
||||
d_received_bytes = 0;
|
||||
|
||||
/*
|
||||
* Due to the possibility of bit stuffing on the first byte some special
|
||||
* handling is necessary
|
||||
*/
|
||||
if ((d_shift_reg & 0xfc) == 0x7c) {
|
||||
/*This was a stuffed bit */
|
||||
d_dec_b <<= 1;
|
||||
d_decoded_bits = 7;
|
||||
}
|
||||
else {
|
||||
d_frame_buffer[0] = d_dec_b;
|
||||
d_decoded_bits = 0;
|
||||
d_received_bytes = 1;
|
||||
}
|
||||
}
|
||||
|
||||
bool
|
||||
ax25_decoder_bm_impl::enter_frame_end ()
|
||||
{
|
||||
uint16_t fcs;
|
||||
uint16_t recv_fcs = 0x0;
|
||||
|
||||
/* First check if the size of the frame is valid */
|
||||
if (d_received_bytes < AX25_MIN_ADDR_LEN + sizeof(uint16_t)) {
|
||||
reset_state ();
|
||||
return false;
|
||||
}
|
||||
|
||||
/*
|
||||
* Check if the frame is correct using the FCS field
|
||||
* Using this field also try to correct up to 2 error bits
|
||||
*/
|
||||
if (frame_check ()) {
|
||||
message_port_pub (
|
||||
pmt::mp ("pdu"),
|
||||
pmt::make_blob (d_frame_buffer, d_received_bytes - sizeof(uint16_t)));
|
||||
reset_state ();
|
||||
return true;
|
||||
}
|
||||
else {
|
||||
message_port_pub (
|
||||
pmt::mp ("failed_pdu"),
|
||||
pmt::make_blob (d_frame_buffer, d_received_bytes - sizeof(uint16_t)));
|
||||
LOG_DEBUG("Wrong crc");
|
||||
reset_state ();
|
||||
return false;
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
inline void
|
||||
ax25_decoder_bm_impl::decode_1b (uint8_t in)
|
||||
{
|
||||
|
||||
/* In AX.25 the LS bit is sent first */
|
||||
d_shift_reg = (d_shift_reg >> 1) | (in << 7);
|
||||
d_dec_b = (d_dec_b >> 1) | (in << 7);
|
||||
}
|
||||
|
||||
bool
|
||||
ax25_decoder_bm_impl::frame_check ()
|
||||
{
|
||||
uint16_t fcs;
|
||||
uint16_t recv_fcs = 0x0;
|
||||
uint8_t orig_byte;
|
||||
|
||||
/* Check if the frame is correct using the FCS field */
|
||||
fcs = ax25_fcs (d_frame_buffer, d_received_bytes - sizeof(uint16_t));
|
||||
recv_fcs = (((uint16_t) d_frame_buffer[d_received_bytes - 1]) << 8)
|
||||
| d_frame_buffer[d_received_bytes - 2];
|
||||
if (fcs == recv_fcs) {
|
||||
return true;
|
||||
}
|
||||
return false;
|
||||
}
|
||||
|
||||
int
|
||||
ax25_decoder_bm_impl::work (int noutput_items,
|
||||
gr_vector_const_void_star &input_items,
|
||||
gr_vector_void_star &output_items)
|
||||
{
|
||||
int ret;
|
||||
const uint8_t *in = (const uint8_t*) input_items[0];
|
||||
|
||||
if (noutput_items < 1) {
|
||||
return noutput_items;
|
||||
}
|
||||
|
||||
|
||||
if (d_descramble) {
|
||||
for (int i = 0; i < noutput_items; i++) {
|
||||
/* Perform NRZI decoding */
|
||||
dec_bit = (~((in - d_prev_bit_nrzi) % 2)) & 0x1;
|
||||
d_prev_bit_nrzi = in;
|
||||
|
||||
|
||||
/* Descramble the input bit */
|
||||
descr_bit = d_lfsr.next_bit_descramble (dec_bit);
|
||||
|
||||
/* In AX.25 the LS bit is sent first */
|
||||
d_shift_reg = (d_shift_reg >> 1) | (descr_bit << 7);
|
||||
d_dec_b = (d_dec_b >> 1) | (descr_bit << 7);
|
||||
uint8_t b = (~((in[i] - d_prev_bit_nrzi) % 2)) & 0x1;
|
||||
d_prev_bit_nrzi = in[i];
|
||||
b = d_lfsr.next_bit_descramble (b);
|
||||
d_bitstream.push_back (b);
|
||||
}
|
||||
|
||||
inline void
|
||||
ax25_decoder_bm_impl::decode_1b (uint8_t in)
|
||||
{
|
||||
uint8_t dec_bit;
|
||||
in &= 0x1;
|
||||
|
||||
}
|
||||
else {
|
||||
for (int i = 0; i < noutput_items; i++) {
|
||||
/* Perform NRZI decoding */
|
||||
dec_bit = (~((in - d_prev_bit_nrzi) % 2)) & 0x1;
|
||||
d_prev_bit_nrzi = in;
|
||||
|
||||
/* In AX.25 the LS bit is sent first */
|
||||
d_shift_reg = (d_shift_reg >> 1) | (dec_bit << 7);
|
||||
d_dec_b = (d_dec_b >> 1) | (dec_bit << 7);
|
||||
uint8_t b = (~((in[i] - d_prev_bit_nrzi) % 2)) & 0x1;
|
||||
d_prev_bit_nrzi = in[i];
|
||||
d_bitstream.push_back (b);
|
||||
}
|
||||
}
|
||||
decode();
|
||||
return noutput_items;
|
||||
}
|
||||
|
||||
int
|
||||
ax25_decoder_bm_impl::work (int noutput_items,
|
||||
gr_vector_const_void_star &input_items,
|
||||
gr_vector_void_star &output_items)
|
||||
{
|
||||
int ret;
|
||||
const uint8_t *in = (const uint8_t *) input_items[0];
|
||||
|
||||
if(noutput_items < 1) {
|
||||
return noutput_items;
|
||||
}
|
||||
if (d_descramble) {
|
||||
ret = descramble_and_decode (in, noutput_items);
|
||||
}
|
||||
else {
|
||||
ret = decode (in, noutput_items);
|
||||
}
|
||||
return ret;
|
||||
}
|
||||
|
||||
} /* namespace satnogs */
|
||||
} /* namespace satnogs */
|
||||
} /* namespace gr */
|
||||
|
||||
|
|
|
|||
|
|
@ -24,6 +24,7 @@
|
|||
|
||||
#include <satnogs/ax25_decoder_bm.h>
|
||||
#include <gnuradio/digital/lfsr.h>
|
||||
#include <deque>
|
||||
|
||||
namespace gr
|
||||
{
|
||||
|
|
@ -35,7 +36,7 @@ namespace gr
|
|||
private:
|
||||
typedef enum
|
||||
{
|
||||
NO_SYNC, IN_SYNC, DECODING, FRAME_END
|
||||
NO_SYNC, IN_SYNC, DECODING
|
||||
} decoding_state_t;
|
||||
|
||||
/**
|
||||
|
|
@ -53,6 +54,8 @@ namespace gr
|
|||
size_t d_decoded_bits;
|
||||
digital::lfsr d_lfsr;
|
||||
uint8_t *d_frame_buffer;
|
||||
std::deque<uint8_t> d_bitstream;
|
||||
size_t d_start_idx;
|
||||
|
||||
void
|
||||
reset_state ();
|
||||
|
|
@ -60,18 +63,16 @@ namespace gr
|
|||
enter_sync_state ();
|
||||
void
|
||||
enter_decoding_state ();
|
||||
void
|
||||
bool
|
||||
enter_frame_end ();
|
||||
|
||||
size_t
|
||||
descramble_and_decode (const uint8_t *in, size_t nitems);
|
||||
size_t
|
||||
decode (const uint8_t *in, size_t nitems);
|
||||
void
|
||||
decode ();
|
||||
|
||||
inline void
|
||||
descramble_and_decode_1b (uint8_t in);
|
||||
inline void
|
||||
decode_1b (uint8_t in);
|
||||
bool
|
||||
frame_check();
|
||||
|
||||
|
||||
public:
|
||||
|
|
|
|||
Loading…
Reference in New Issue