TABLE OF CONTENTS There are two aspects to tracking: carrier tracking and code tracking. Phase-shift keying demodulation requires a frequency lock or a phase lock of the received carrier, or intermediate frequency, to the receiver local oscillator frequency. A normal phase lock loop cannot be used because the carrier is suppressed in PSK modulation (however, in some systems a residual carrier is transmitted for tracking). Code tracking is necessary to suppress the spreading code, allowing the data to be demodulated through a bandpass or lowpass filter that is narrow compared to the spreading code bandwidth, and to suppress narrowband interference and other noncoherently spread DSSS signals on the same channel. First we discuss carrier tracking.
Two common configurations for a BPSK demodulator are squaring loop and Costas loop. A squaring loop demodulator is shown in Figure 3.17. The IF signal is squared or doubled by other means to multiply the frequency by two. The phase is also multiplied by two, canceling the modulation since twice the 180 data shifts leaves no phase shift. The doubled frequency is locked to a VCO in a standard phase lock loop, and the VCO's output is divided by two to provide the missing carrier at the IF frequency. The reconstructed carrier can be used to provide I and Q baseband data outputs. The doubled frequency PLL does have a performance penalty compared to a PLL at the original IF, because of squaring the noise together with the carrier. The PLL in the...
