88 lines
3.1 KiB
C
Executable File
88 lines
3.1 KiB
C
Executable File
/*
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* High Speed modem to transfer data in a 2,7kHz SSB channel
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* =========================================================
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* Author: DJ0ABR
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*
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* (c) DJ0ABR
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* www.dj0abr.de
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*
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*/
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/*
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* The total length of the FEC-secured part is 255,
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* this is a requirement of the Shifra FEC routine, which
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* is the best FEC that I have seen so far, highly recommended
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*/
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// total "on the air" frame size
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// the total length must be a multiple of 2 and 3, so QPSK and 8PSK symbols fit into full bytes
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// this is the case with a total length of 258
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#define HEADERLEN 3
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#define FECBLOCKLEN 255
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#define UDPBLOCKLEN (HEADERLEN + FECBLOCKLEN)
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/* !!! IMPORTANT for GNU RADIO !!!
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* the UDP payload size for TX MUST be exactly UDPBLOCKLEN (258 in this case) or
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* the transmitter will not align bits to symbols correctly !
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*
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* RX payload size is not that important. But the currect size for
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* QPSK is UDPBLOCKLEN*8/2 = 1032 and for 8PSK UDPBLOCKLEN*8/3 = 688
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* so we can use 344 which are 2 blocks for 8PSK and 3 blocks for QPSK
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* */
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// size of the elements inside an FECblock
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// sum must be 255
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#define FECLEN 32 // supported: 16,32,64,128
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#define STATUSLEN 2
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#define CRCLEN 2
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#define PAYLOADLEN (FECBLOCKLEN - FECLEN - CRCLEN - STATUSLEN)
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#define CRCSECUREDLEN (PAYLOADLEN + STATUSLEN)
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#define DATABLOCKLEN (PAYLOADLEN + CRCLEN + STATUSLEN)
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// the header is not FEC secured therefore we give some room for bit
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// errors. Only 24 out of the 32 bits must be correct for
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// a valid frame detection
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extern uint8_t header[HEADERLEN];
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typedef struct {
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// the total size of the following data must be 255 - 32 = 223 bytes
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// the FEC is calculated on FRAME with a length of 223 and returns
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// a data block with length 255.
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// we use a 10 bits frame counter -> 1024 values
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// so we can transmit a data block with a maximum
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// size of 255 * 1024 = 261kByte. With the maximum modem speed
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// this would be a transmission time of 5,8 minutes which
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// is more then enough for a single data block
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uint8_t counter_LSB; // lower 8 bits of the frame counter
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// the status byte contains these information:
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// bit 0..3 : 4 bit (16 values) frame type information
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// bit 4 : first frame of a block if "1"
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// bit 5 : last frame of a block if "1"
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// bit 6..7 : MSB of the frame counter
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uint8_t status;
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// payload
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uint8_t payload[PAYLOADLEN];
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// CRC16
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uint8_t crc16_MSB;
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uint8_t crc16_LSB;
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} FRAME;
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