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A receiver which employs coherent, or synchronous, detection must have knowledge of the phase of the received signal. In general, the receiver acquires this knowledge from sighals received previously over the channel. The result of this measurement process is a noisy phase reference which is then used by the receiver in the detection of the incoming signals. In this paper the effect of using baud decisions to direct the phase measurement process (decision-directed measurement) is investigated by means of Monte Carlo computer simulation of a coherent, binary communication system employing either orthogonal or phase-reversal signaling. The results are compared with those obtained from similar systems using nondecision-directed measurement. Analytical verification of the computer results is provided for particular cases. Error rates are given at several signal-to-noise ratios. The central conclusion of this study is that detection using a phase reference obtained through the decision-directed technique outlined herein results in system error rates which are generally lower, at all signal-to-noise ratios, than error rates of corresponding nondecision-directed phase measurement schemes. A "run-away" phenomenon, in which the receiver by committing errors loses the reference phase and never regains it, is not encountered.