Modulation formats: There are many different modulation formats used in satellite communication links. Examples include QPSK, OQPSK, MSK, GMSK and CPFSK. Each format has its own issues that must be investigated.
QPSK vs. offset QPSK: To maximize the output power in the downlink, the idea is to operate the amplifier as close to saturation as possible. To avoid destroying the signal information it is common to employ so called constant envelope modulation techniques such as FM and QPSK. In both cases, the information is in the carrier, and hard limiting the signal does no harm. But it is a little more complicated than this. To keep out unwanted signals, and to limit the spectral occupancy of the signal, each channel is band limited at the transmitting earth station using shaping such as the raised-cosine spectrum. This causes standard QPSK, which can have +/- 180-degree transitions, to have significant amplitude variations during these transitions. QPSK is therefore more sensitive to the filtering and limiting process. A common variation is Offset QPSK. Here, the quadrature channel is delayed by ½ of a data bit. Thus the phase transitions can never be more than 90 degrees, which alleviates this problem. Figure 5 shows the SystemView by Elanix frequency spectra of both cases at the final output of a saturated amplifier. While the spectra of both signals are the same for the basic two signals, we see that the QPSK output has higher side lobes that can affect other channels.
Figure 5. Frequency power spectra of QPSK and offset QPSK after filtering and limiting.
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