The performance of bandlimited offset-quaternary-phase-shift-keying (OQPSK) systems in the presence of noisy phase reference and additive Gaussian noise is analyzed. Expressions for error probability calculation are obtained. A new search method for computing the detection loss and the required SNR of the phase reference is proposed. It is shown that when the rolloff factor for the Nyquist filtering is large, OQPSK surpasses QPSK, but when the rolloff factor is small, QPSK surpasses OQPSK.>

In this paper, the performance of direct sequence QPSK spread-spectrum systems using complex adaptive filters in the presence of pulsed CW interference is analyzed. Both adaptive prediction error filters and adaptive transversal filters with two-sided taps are considered. It is shown that the time constant of the tap weight adaptation in the interference off-interval is usually much greater than the time constant in the on-interval, and that this is beneficial for the system since it results in retaining the rejection property of the filter. Under steady-state conditions, the tap weights are calculated. Analytical expressions for the signal-to-noise ratio improvement under the least favorable interference condition are given. It is shown that the performance of the two-sided transversal filter is better than that of the prediction error filter.

In this paper, the performance of QPSK systems using complex transversal filters with additional decision-feedback taps, in the presence of Gaussian noise and a single CW interferer, is analyzed. Both one-sided (with lagging taps) and two-sided transversal filters with additional decision-feedback taps are considered. Analytic expressions for the tap weights and the minimum mean square errors are obtained. The effect of error propagation on the error probability is discussed and an approximate solution for the error probability is obtained. The transient behavior of the filters using the LMS adaptation algorithm is analyzed. It is shown that if the filter is used for rejecting CW interference only, the one-sided decision-feedback filter is preferred.

In the presence of narrow-band interference, the performance of PN spread-spectrum communication systems can he improved by using various interference rejection filters. In this paper, both prediction error filters and transversal filters with two-sided taps are used and the performance of each is analyzed. Analytic expressions for the signal-to-noise ratio improvement factors are obtained. If the frequency of the jamming tone is near the carrier frequency of the spread-spectrum signal, the performance of the transversal filter with two-sided taps is better than that of the prediction error filter with the same number of taps.