Parallel Tone Operation

The first technique referenced here [MIL-STD-188-110 (Ref. 3)] operates from 75 to 2400 bps. The modulator accepts serial binary data and converts the bit stream into 16 parallel data streams. The signal element interval on each data bit stream is 13.33 ms and its modulation rate is 75 baud. The modulator provides a separate tone combination for initial synchronization and, if required, a separate tone for Doppler correction. Tone frequencies and bit locations are shown in Table 12.10. For data signaling rates of 75, 150, 300, and 600 bps at the modulator input, each data tone signal element is biphase modulated, as shown in the upper part of Figure 12.30. Each bit of the serial binary input signal is encoded, depending on the mark or space logic sense of the bit, into a phase change of the data tone signal element as listed in Table 12.11. For data signaling rates of 1200 and 2400 bps at the modulator input, each data tone signal is four-phase modulated (QPSK), as shown in the lower part of Figure 12.30. In this case, each dibit of the serial binary input signal is encoded, depending on the mark or space logic sense and the even or odd bit location of each bit, into a phase change of the data tone signal element as listed in Table 12.11. The phase change of a data tone signal element is relative to the phase of the immediately preceding signal element. This is called differential phase-shift keying. In-band diversity (see Section 12.8) is provided for all but the 2400-bps data rate.

There are two arguments in favor of the parallel tone approach. The first is that the transmitted signal element is long (13.33 ms) for all user data

TABLE 12.10 Data Tone Frequencies and Bit Locations for HF Data Modems

Tone Frequency (Hz)

Function

Even and Odd Bit Locations of Serial Binary Bit Stream, Encoded and Phase Modulated on Each Data Tone Employing:

Quadrature-Phase Modulation

Biphase Modulation

2400 bps

1200 bps

600 bps

300 bps

150 bps

75 bps

605 825 a

Continuous Doppler Tone Synchronization Slot

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