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Mode A

  • RF bandwidth: 750 Hz
  • Symbol rate: 600 Sym/s
  • Root raised-cosine pulse shaping, $\alpha=0.25$
  • Content: dual bit-rate digital voice
  • No FEC, only CRC for bit error detection
  • Frame length: 40 ms (frames come continuously without ramping power up/down)
  • Synchronization frames every 20th frame (0.8 s)

Unsorted thoughts:

  • 256-QAM requires less than 3.7 degree of phase error.
  • What requirements for phase noise of synthesizer? ⇒ [1]
  • Digital predistortion necessary?
  • Bigger Eb/No difference between high/low bit-rate subframes possible?
  • Send synchronization frame in silent frames, too.


During one frame the modulation is switched between 4-QAM and 256-QAM. The four states of the 4-QAM are a direct subset of the 256-QAM states. Thus, synchronization is kept during one frame. The high bit-rate voice data stream is desigend to fall over at approximately the power level analog FM does. The low bit-rate stream is more robust and can be demodulated at lower signal levels.

modulation bits/
ideal Eb/No estim. min. power¹
4-QAM (=QPSK) 2 4 dB @ 0.01 BER -139 dBm
256-QAM 8 16 dB @ 0.01 BER -127 dBm

¹ for NF=2dB


Synchronization Frame (SF)

Every 20th frame is a synchronization frame. It contains a long sync sequence for robust signal detection and frequency+time synchronization. The high quality audio frame is exactly the same as in a normal frame (see below). However, the low quality audio is disrupted during a SF.

content sync meta data high quality audio + CRC
symbols 10 symbols/
16.67 ms
4 symbols/
6.67 ms
10 symbols/
16.67 ms
modulation (256-QAM) 256-QAM
bits (80 bit) 32 bit 80 bit

Normal Frame (NF)

A normal frame contains a short sync sequence mainly for packet synchronization. A low quality audio stream modulated with 4-QAM ensures robust demodulation at low signal levels, while the high quality audio stream is modulated with 256-QAM.

content sync low quality audio + CRC high quality audio + CRC
symbols 3 symbols/
5.0 ms
11 symbols/
18.33 ms
10 symbols/
16.67 ms
modulation (256-QAM) 4-QAM 256-QAM
bits (24 bit) 18 bit + 4 bit 80 bit


3 symbols

Low quality audio + CRC

Codec2 450 bit/s:

High quality audio + CRC

Codec2 1700 bit/s:

Meta data

Up to 45 characters of meta data, e.g. the call sign, can be transmitted. Each SF contains three characters, thus a minimum speed of 3.75 characters per second can be achieved. Each frame is numbered with 4 bit (0…15). A 4 bit CRC (tbd) over both position and message is appended - if an error is detected, the meta data message should be ignored (voice data may still be ok).

content position message CRC
bits 4 bit 3×8 bit 4 bit
protocol/rel-0/mode_a.txt · Last modified: 2019/03/14 23:02 by stefan