MIL-188-110A Serial (single tone) is one of the most popularly used modes in long-haul HF data modems. The mode is specified by the US Department of Defense (DoD) in the "Military Standard Interoperability and Performance Standards for Data Modems" (30. Sept. 1991).

This mode employs 8-ary phase shift keying (PSK) on a single carrier frequency (1800 Hz) as the modulation technique for data transmission. Serial binary information (raw data) accepted at the line side input is converted into a single 8-ary PSK-modulated output carrier. The modulation of this output carrier is a constant 2400 Baud waveform regardless of the actual user data rate.

MIL-188-110A single tone waveform has the following characteristics:

Each transmission of a MIL-188-110A message begins with a synchronization phase (preamble) – 0.6 seconds for message with ZERO or SHORT interleaver setting and 4.8 seconds for a message with LONG interleaver setting – followed by the data phase, which is of unlimited length. The data phase is structured in User Data – Channel Probe pairs. The phase value of a User Data symbol is unknown, whereas the Channel Probe has a predefined phase pattern.

The FEC and interleaver are used to combat the effects of fading, frequency drift, multi-path effects, and burst noise affecting the HF transmission. The known 8-phase symbols – the Channel Probe – are transmitted to keep the receiver, mainly the equalizer, on track. Thus the receiver can be resynchronized without the preamble being regularly repeated.

The MIL-188-110A decoder processes all the above settings except 2400 bps voice. Generally MIL-188-110A user data is transmitted in binary mode, i.e., in a transparent mode. Thus upper protocols layers are required to further process this data stream. For this reason the decoder displays the user data BINARY,  HEX, ASCII ASYNC or ASCII SYNC format selected from Options/Message Type....

In the HEX display mode, the decoded binary data is just displayed as it is.

In ASCII ASYNC mode, the bit stream is searched with an ASCII ASYNC structure, i.e., one start-bit (0), 8 data-bits and at least one stop-bit (1). The 8 data bits are transmitted with the LSB first. The display is stopped when more than 3 NULL characters are received or when the asynchronous data structure is violated more than 8 times.

In ASCII SYNC mode, each 8 bits (LSB first) represent one ASCII character.

Tuning the Decoder

The decoder processes signal in both SSB settings: USB and LSB. This can be selected with Polarity in the menu: NOR means USB and INV means LSB signal.

The center frequency of the decoder should be set to 1800 Hz when the receiver is correctly tuned to the transmitting station. Small frequency variations are automatically compensated in the decoder. The center frequency of the decoder can be adjusted to ±400 Hz from its normal setting. By using the bar-graph, any remaining frequency difference can be compensated by fine tuning of the receiver frequency or by adjusting the center frequency of the decoder.

On-going Synchronization

The decoder synchronises both on the preamble and on the probe sequence. This on-going feature makes sure that the decoder will synchronise at any time of the transmission.

Dedicated Phase Plane

The decoder uses an equalizer to demodulate the high-speed PSK signal. To visualize the power of equalizer, user can turn on the dedicated phase plane in Options –> Show Phase Plane. The points in yellow is after synchronization befor the equalizer and points in blue is after the equalizer.