13 J J Modulation and Coding

Before we can complete our link design, we need to select a modulation and coding technique. Modulation is the process by which an input signal varies the characteristics of a radio frequency carrier (usually a sine wave). These characteristics are amplitude, phase, frequency, and polarization. Demodulation of the signal at the receiver consists of measuring the variations in the characteristics of the received carrier and deducing what the original signal was. Amplitude modulation, though common in terrestrial services, seldom appears in satellite systems because it requires larger (and more costly) transmitters. Phase or frequency modulation techniques are preferred, because the transmitter can operate at saturation for maximum power efficiency.

Figure 13-8 shows the most common modulation techniques used in satellite systems. Binary phase shift keying (BPSK) consists of setting the carrier phase at 0 deg to transmit a binary 0, and setting the phase at 180 deg to transmit a binary 1. Quad-riphased phase shift keying (QPSK), takes two bits at a time to define one of four symbols. Each symbol corresponds to one of four carrier phases: 0 deg, 90 deg, 180 deg, or 270 deg. Note that the symbol rate is one half the bit rate, thus reducing the spectrum width by one half.

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