17 March 2015

MIL STD-188-110 Appendix B, 39-Tones (M-39)

"MIL-STD-188-110 39 Tone is a non-mandatory part of the MIL-STD-188-110 military standard for use by all departments and agencies of the Department of Defense. 
The modulation technique used in this mode consists of differential quadrature phase shift keying (QDPSK) of 39 orthogonal sub carriers in the range from 675Hz to 2812.5 Hz, and an additional unmodulated Doppler reference tone at 393.75Hz. The modulation speed (symbol rate) is always 44.44 baud. Through the transmission of redundant information on certain tones, different user data rates can be achieved within a range of 75 to 2400 bps.
This mode uses FEC and interleaving to combat the effects of fading, frequency shift, multipath, and burst noise.

The user data is transmitted using a continuous frame structure with a variable block length (number of symbols), depending on user data rate and message type. Each transmission starts with a preamble, consisting of three phases, followed by block synchronization and data segments. The data block immediately follows the next block synchronization segment defining again the start of the next data block. This repeated frame structure enables synchronization of the demodulator at any time of transmission.
The end of transmission is determined by an EOM sequence".

Radioscanner.ru reports analysis of the 188-110 App.B waveform at this page: 
although the sample has some problems of digitization (maybe due to PC sound card) and wrong sampling rate. According to http://www.radioscanner.ru/info/article538/ MIL 39-tones has a native sampling of multiple of 3600 hertz so I resampled my recording obtaining the right K = 17/64 along with the expected values for baudrate and channel separation. 

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