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246 lines
9.9 KiB
246 lines
9.9 KiB
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README_ofdm.txt
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David Rowe
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Created Mar 2018
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Introduction
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------------
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An Orthogonal Frequency Division Multiplexed (OFDM) modem designed for
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digital voice over HF SSB. Typical configuration for FreeDV 700D is
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700 bit/s voice, a rate 0.5 LDPC code, and 1400 bit/s raw data rate
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over the channel.
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The OFDM modem was first implemented in GNU Octave, then ported to C.
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Algorithm development is generally easier in Octave, but for real time
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work we need the C version. Automated units tests ensure the
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operation of the Octave and C versions are identical.
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Refs
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-----
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[1] Spreadsheet (OFDM tab) describing the waveform design,
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https://svn.code.sf.net/p/freetel/code/codec2-dev/octave/modem_codec_frame_design.ods
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[2] Towards FreeDV 700D, https://www.rowetel.com/?p=5573
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[3] FreeDV 700D - First Over The Air Tests, https://www.rowetel.com/?p=5630
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[4] Steve Ports an OFDM modem from Octave to C, https://www.rowetel.com/?p=5824
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Quickstart
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----------
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Built as part of codec2-dev, see README for build instructions.
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1. Generate 10 seconds of test frame bits, modulate, and play audio
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out of sound device (SoX v14.4.2):
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build_linux/src$ ./ofdm_mod --in /dev/zero --testframes 10 | play --type s16 --rate 8000 --channels 2 -
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2. Generate 10 seconds of uncoded test frame bits, modulate, demodulate, count errors:
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build_linux/src$ ./ofdm_mod --in /dev/zero --testframes 10 | ./ofdm_demod --in /dev/null --testframes --log demod_dump.txt
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Use Octave to look at plots of C modem operation:
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$ cd ../../octave
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$ octave --no-gui
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octave:1> ofdm_demod_c("../build_linux/src/demod_dump.txt")
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4. Run Octave versions of mod and demod (called tx and rx to avoid
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namespace clashes in Octave):
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$ cd ~/octave
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$ octave --no-gui
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octave:1> ofdm_tx("ofdm_test.raw","700D",10)
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octave:1> ofdm_rx("ofdm_test.raw")
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The Octave modulator ofdm_tx can simulate channel impairments, for
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example AWGN noise at an Eb/No of 4dB measured on 1400 bit/s raw
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uncoded bits:
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octave:1> ofdm_tx("ofdm_test.raw", "700D", 10, 4)
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The Octave versions use the same test frames as C so can interoperate.
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build_linux/src$ ./ofdm_demod --in ../../octave/ofdm_test.raw --out /dev/null --testframes --verbose 1
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5. Run mod/demod with LDPC FEC; 4 frame interleaver, 60 seconds, 3dB
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Eb/No, Eb/No measured on 700 bit/s payload data bits. For rate 1/2
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code this is equivalent to 0dB on 1400 bit/s uncoded bits (0dB
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Eb/No argument for ofdm_tx())
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octave:6> ofdm_ldpc_tx('ofdm_test.raw',"700D",4,60,3)
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octave:7> ofdm_ldpc_rx('ofdm_test.raw',"700D",4)
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C demodulator/LDPC decoder:
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build_linux/src$ ./ofdm_demod --in ../../octave/ofdm_test.raw --out /dev/null --verbose 1 --testframes --ldpc 1 --interleave 4
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6. Run C mod/demod with LDPC and 2 frames interleaving:
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build_linux/src$ ./ofdm_mod --in /dev/zero --ldpc --testframes 60 --interleave 2 | ./ofdm_demod --out /dev/null --testframes --ldpc 1 --verbose 1 --interleave 2
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7. Pass Codec 2 700C compressed speech through OFDM modem:
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build_linux/src$ ./c2enc 700C ../../raw/ve9qrp_10s.raw - --bitperchar | ./ofdm_mod --ldpc 1 --interleave 2 | ./ofdm_demod --ldpc 1 --interleave 2 | ./c2dec 700C - - --bitperchar | play --type s16 --rate 8000 --channels 1 -
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8. Listen to signal through simulated fading channel in C:
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build_linux/src$ ./c2enc 700C ../../raw/ve9qrp_10s.raw - --bitperchar | ./ofdm_mod --ldpc 1 --interleave 4 | ./cohpsk_ch - - -20 -Fs 8000 --slow -f -5 | aplay -f S16
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9. Run test frames through simulated channel in C:
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build_linux/src$ ./ofdm_mod --in /dev/zero --ldpc 1 --testframes 20 | ./cohpsk_ch - - -24 --Fs 8000 -f -10 --fast | ./ofdm_demod --out /dev/null --testframes --verbose 1 --ldpc 1
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10. Run codec voice through simulated fast fading channel, just where it starts to fall over:
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build_linux/src$ ./c2enc 700C ../../raw/ve9qrp.raw - --bitperchar | ./ofdm_mod --ldpc 1 --interleave 8 | ./cohpsk_ch - - -24 --Fs 8000 -f -10 --fast | ./ofdm_demod --ldpc 1 --verbose 1 --interleave 8 | ./c2dec 700C - - --bitperchar | aplay -f S16
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11. FreeDV 1600 on the same channel conditions, roughly same quality at 8dB higher SNR:
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build_linux/src$ ./freedv_tx 1600 ../../raw/ve9qrp_10s.raw - - | ./cohpsk_ch - - -30 --Fs 8000 -f -10 --fast | ./freedv_rx 1600 - - | aplay -f S16
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12. Using FreeDV API test programs:
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build_linux/src$ ./freedv_tx 700D ../../raw/hts1a.raw - --testframes | ./freedv_rx 700D - /dev/null --testframes
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build_linux/src$ ./freedv_tx 700D ../../raw/hts1a.raw - | ./freedv_rx 700D - - | aplay -f S16
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With long interleaver times use a longer source file to allow interleaver time to sync, especially on poor
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channels:
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build_linux/src$ ./freedv_tx 700D ../../raw/ve9qrp.raw - - --interleave 8 | ./cohpsk_ch - - -26 --Fs 8000 -f -10 --fast | ./freedv_rx 700D - - --interleave 8 | aplay -f S16
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FreeDV 2020 extensions
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----------------------
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13. 37 Carrier waveform with a (504,396) code:
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build_linux/src$ nc=37; ./ofdm_mod --in /dev/zero --testframes 300 --nc $nc --ldpc 2 --verbose 1 | ./cohpsk_ch - - -22.5 --Fs 8000 -f 10 --ssbfilt 1 | ./ofdm_demod --out /dev/null --testframes --nc $nc --verbose 1 --ldpc 2
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SNR3k(dB): 4.05 C/No: 38.8 PAPR: 10.8
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BER......: 0.0348 Tbits: 1044792 Terrs: 36345
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Coded BER: 0.0094 Tbits: 820908 Terrs: 7717
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14. 31 carrier waveform, (504,396) code, but only 312 data bits used, so
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we don't send unused data bits. This means we need less carriers
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(so more power per carrier), and code rate is increased slightly
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(sorta). Anyhoo, it works about 1.7dB better:
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build_linux/src$ nc=31; ./ofdm_mod --in /dev/zero --testframes 300 --nc $nc --ldpc 2 --verbose 1 -p 312 | ./cohpsk_ch - - -21.6 --Fs 8000 -f 10 --ssbfilt 1 | ./ofdm_demod --out /dev/null --testframes --nc $nc --verbose 1 --ldpc 2 -p 312
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SNR3k(dB): 2.21 C/No: 37.0 PAPR: 9.6
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BER......: 0.0505 Tbits: 874020 Terrs: 44148
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Coded BER: 0.0096 Tbits: 649272 Terrs: 6230
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Octave Acceptance Tests
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-----------------------
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Here are some useful tests for the Octave, uncoded modem.
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The rate 1/2 LDPC code can correct up to about 10% raw BER, so a good
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test is to run the modem at Eb/No operating points that produce just
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less that BER=0.1
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The BER2 measure truncates the effect of any start up transients,
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e.g. as the frequency offset is tracked out.
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1/ HF Multipath:
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octave:580> ofdm_tx("ofdm_test.raw","700D",60,4,'hf',20,-0.1)
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octave:581> ofdm_rx("ofdm_test.raw")
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BER2.: 0.0997 Tbits: 93752 Terrs: 9344
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2/ AWGN:
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octave:582> ofdm_tx("ofdm_test.raw","700D",60,0,'awgn')
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octave:583> ofdm_rx("ofdm_test.raw")
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BER2.: 0.0827 Tbits: 96846 Terrs: 8008
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C Acceptance Tests
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------------------
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Here are some useful tests for the LDPC coded C version of the modem, useful to verify any changes.
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1/ AWGN channel, -2dB:
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./ofdm_mod --in /dev/zero --ldpc --testframes 60 --txbpf | ./cohpsk_ch - - -20 --Fs 8000 -f -10 | ./ofdm_demod --out /dev/null --testframes --verbose 1 --ldpc 1
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SVN Rev 3671:
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SNR3k(dB): -1.85 C/No: 32.9 PAPR: 9.8
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BER......: 0.0815 Tbits: 98532 Terrs: 8031
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Coded BER: 0.0034 Tbits: 46368 Terrs: 157
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2/ Fading HF channel:
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./ofdm_mod --in /dev/zero --ldpc 1 --testframes 60 --txbpf | ./cohpsk_ch - - -24 --Fs 8000 -f -10 --fast | ./ofdm_demod --out /dev/null --testframes --verbose 1 --ldpc 1
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SVN Rev 3671:
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SNR3k(dB): 2.15 C/No: 36.9 PAPR: 9.8
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BER......: 0.1015 Tbits: 88774 Terrs: 9012
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Coded BER: 0.0445 Tbits: 41776 Terrs: 1860
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Note: 10% Raw BER operating point on both channels, as per design. To
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get a coded BER of around 1% on fading channels we need an interleaver
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of about 2x the fade duration (e.g. --interleaver 16), above examples
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are with interleaver == 1, so coded performance is not that great.
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C Code
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------
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ofdm.c - OFDM library
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codec2_ofdm.h - API header file for OFDM library
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ofdm_get_test_bits - generate OFDM test frames
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ofdm_mod - OFDM modulator command line program
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ofdm_demod - OFDM demodulator command line program, supports uncoded
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(raw) and LDPC coded test frames, LDPC decoding
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of codec data, interleaving, and can output LLRs to external
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LDPC decoder
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ofdm_put_test_bits - measure BER in OFDM test frames
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unittest/tofdm - Run C port of modem to compare with octave version (see octave/tofdm)
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cohpsk_ch - From COHPSK modem development, useful C channel simulator
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Octave Scripts
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--------------
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ofdm_lib - OFDM library
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ofdm_dev - used for modem development, run various simulations
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ofdm_tx - modulate test frames to a file of sample, cam add channel impairments
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ofdm_rx - demod from a sample file and count errors
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tofdm - Compares Octave and C ports of modem
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ofdm_ldpc_tx - OFDM modulator with interleaver and (224,112) LDPC code
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ofdm_ldpc_rx - OFDM demodulator with interleaver and (224,112) LDPC code, interleaver sync
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Specifications (Nominal FreeDV 700D configuration)
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--------------------------------------------------
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Modem.........: OFDM, pilot assisted coherent QPSK
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Payload bits/s: 700
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Text bits/s...: 25
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Unique Word...: 10 bits
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Carriers......: 17
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RF bandwidth..: 944 Hz
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Symbol period.: 18ms
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Cyclic Prefix.: 2ms (note 1)
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Pilot rate....: 1 in every 8 symbols
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Frame Period..: 160ms
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FEC...........: rate 1/2 (224,112) LDPC
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Interleaving..: adjustable, suggest 1,4,8 or 16 frames
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Operating point
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AWGN........: Eb/No -0.5dB SNR(3000Hz): -2.5dB (note 2)
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HF Multipath: Eb/No 4.0dB SNR(3000Hz): 2.0dB (note 3)
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Freq offset.......: +/- 20 Hz (sync range)
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Freq drift........: +/- 0.2 Hz/s (for 0.5 dB loss)
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Sample clock error: 1000 ppm
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Notes:
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1/ Modem can cope with 2ms of multipath
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2/ Ideal SNR(3000) = Eb/No + 10*log10(Rb/B)
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= -1 + 10*log10(1400/3000)
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= -4.3 dB,
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So we have about 1.8dB overhead for synchronisation, implementation loss,
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and the text channel.
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3/ "CCIR Poor" HF Multipath channel used for testing is two path, 1Hz Doppler,
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1ms delay.
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