The Effects of Filtering and Limiting on the Performance of QPSK, Offset QPSK, and MSK Systems

The effects on spectral densities, symbol waveshapes and Peversus performances, resulting from the addition of filtering followed by hard limiting on raised cosine filtered QPSK, offset QPSK, and MSK point-to-point radio systems are studied. A mathematical model and physical insight are presented into the crosstalk phenomenon between quadrature channels, created in the systems by the effect of limiting on filtered signals. This crosstalk is shown to result whether the filtering is ideal or otherwise. Computergenerated and measured eye diagrams showing crosstalk as predicted on a filtered, then limited, offset QPSK signal are given. Measured and computed spectral density results are given which are in close agreement with each other, indicating that the computer model provides a good representation of the real system. In addition, an explanation of the shape of the power spectra associated with filtered, then limited, modulated signals is provided by studying the symbol waveshapes of these signals. Using the spectral density results and the performance findings, it is shown that for a microwave system which: 1) incorporates an amplitude limiting amplifying device in the transmitter, 2) must operate within the FCC limits for radiated spectrum, and 3) must operate at a spectral efficiency greater than 1 bit/s/Hz, offset QPSK modulation is the best choice of the three modulation methods studied.