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.

Definition

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 Bd. 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 (at least ten ones "1").

Data Display

There are two different types of data transmissions - synchronous and asynchronous.

In the synchronous data mode, the data is bit synchronously displayed as 7 bit characters, while in the asynchronous mode, different character lengths are possible using start, stop and parity bits. The parity, start and stop bits, as well as the special "null" characters, inserted by the modem and not being part of the data stream, are ignored by the display.

The decoder stops displaying data after the EOM sequence is received and goes to Sync state and resynchronizes.

In Options/Frame Format the user data rate and interleaver length can be set.

In Options/Diversity a value between Time / Frequency and Frequency Only diversity can be selected. The value must be set correctly for all user data rates except 2400 and 1200 bps.

In Options/Message Type all the different character lengths and number of extracted data bits can be set.

Tuning the decoder

The Polarity of the decoder should be set according to the signal. When the received signal is in NOR polarity (USB), the Doppler frequency should be tuned to 393.75 Hz using the Offset, all the 39 tones are at the right side of the Doppler tone; when the signal is in INV polarity (LSB), the Doppler frequency should be tuned to 3206.25 Hz using the Offset, with all the 39 tones at the left side of it.

The correct decoding process is indicated by a Confidence value, with 100 for a 100% correctly decoded message. However, finally getting a useful data output can only be achieved by selecting the proper message type, frame format (user data rate / interleaver), diversity and signal center.