SOQPSK is a family of constant envelope continuous phase modulation (CPM) waveforms defined by Mr. T. Hill37, 38, 39, 40. The details of SOQPSK-TG are described in paragraphs 2.4.3.2 and 2.4.3.2.1 of this document. The SOQPSK‑TG signal amplitude is constant and the phase trajectory is determined by the coefficients in Table 2-4. Therefore, SOQPSK‑TG can be implemented using a precision phase or frequency modulator with proper control of the phase trajectory. Figure A-22 illustrates the measured phase trajectory of an SOQPSK‑TG signal. The vertical lines correspond approximately to the “bit” decision times.
Figure A-23. SOQPSK-TG Power Spectrum (5 Mb/s).
The power spectrum of a random 5 Mb/s SOQPSK‑TG signal is shown in Figure A‑23. The 60 dBc bandwidth of this 5 Mb/s signal was about 7.34 MHz. Note that the maximum power level is about -19 dBc.
Figure A-24. BEP versus Eb/No performance of 5 Mb/s SOQPSK-TG.
Figure A-24 shows the measured bit error probability (BEP) versus signal energy per bit/noise power per Hz (Eb/N0) of two SOQPSK-TG modulator/demodulator combinations including non-linear amplification and differential encoding/decoding in an additive white Gaussian noise environment (AWGN) with no fading. Other combinations of equipment may have different performance. Phase noise levels higher than those recommended in Chapter 2 can significantly degrade the BEP performance.
