Wavecom W61PC V7.5.0 - Manual (Page 184)

174  Transmission Modes WAVECOM Decoder W61PC/LAN Manual V7.5
The MIL-188-141A signal is an 8-tone MFSK signal in the range 750 - 2500 Hz spaced 250 Hz apart. Each
tone (symbol) is 8 ms long corresponding to 125 Baud and represents three bits giving a bit rate of 375
bps.
The MIL-188-141A bit-stream is structured in 24-bit frame, which includes three bits preamble for the
frame type and three 7-bit ASCII characters or just 21 bits unformatted binary data.
To increase robustness the 24-bit frame is Golay (24, 12) encoded, and then interleaved giving a total
frame length of 48 bits + 1 stuff bit. Each 49-bit code word is transmitted three times one after another to
combat burst interference.
In both specifications, especially according to MIL-188-141B Appendix B, the 21-bit ALE frame data can be
encrypted before transmission. This feature is named Link Protection. The data may be encrypted accord-
ing to different classified application levels: AL-1 to AL-4. Unencrypted data is transmitted with AL-0.
The MIL-188-141A mode decoder processes the signal automatically after the center frequency is set. It
synchronizes the signal in both USB and LSB polarities. There are two display modes selected the in Dis-
play Mode menu: All Frames and Valid Frames Only. The first option just displays the frames without
checking if the ALE protocol is strictly observed; the second option displays the frames according to the
ALE protocol defined according to MIL-188-141B (Appendix A), which is backwards compatible to MIL-188-
141A. In both options the frame data can be displayed in ASCII, HEX or both ASCII and HEX formats.
This can be set by using the Display menu.
Only unencrypted data can be displayed correctly with the mode decoder. For protected application data –
according to MIL-188-141B Appendix B (Link Protection) – the display may be meaningless, because a us-
er specific key is necessary for data decryption.
MIL-188-141B (Appendix C)
MIL-188-141B Appendix C defines the third generation (3G) high frequency (HF) radio technology includ-
ing advanced automatic link establishment (ALE), automatic link maintenance, and high performance data
link protocols. This advanced technology improves on the performance of previous similar techniques (e.g.
2G-ALE). The specification is issued by the US Department of Defense (DoD) in the "Interface Standard
Interoperability and Performance Standards for Medium and High Frequency Radio Systems" (1st March
1999).
Parameter Value
Frequency range HF
Operation modes Broadcast/Simplex FEC
Modulation 8-PSK
Symbol rate 2400 Bd
Center frequency 1800 Hz
Receiver settings DATA, CW, USB, LSB
Input format(s) AF, IF
This mode employs 5 constituent burst waveforms BW0 to BW4 for the various kinds of signaling required
in the system, so as to meet their distinctive requirements as to payload, duration, time synchronization,
and acquisition and demodulation performance in the presence of noise, fading, and multi-path. All of the
burst waveforms use the basic 8-ary PSK serial tone modulation of an 1800 hertz (Hz) carrier at 2400
symbols per second (Baud). The same modulation is used in MIL-188-110A serial mode.
The burst waveforms have the following preamble and data structure.
Waveform Burst duration Preamble Payload
BW0 613.33 ms, 1472 PSK symbols 160.0 ms, 384 PSK symbols 26 bits
BW1 1.30667 s, 3136 PSK symbols 240.0 ms, 576 PSK symbols 48 bits