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This paper introduces a concept of a generator coherency-based System Protection Scheme (SPS), addressing interarea oscillation events. The scheme uses wide-area generator speed measurements combined with discrete Fourier transform (DFT) analysis to determine coherency. A test case simulated in the NPCC 68-bus system is used to introduce the scheme's operating principle. The test case is also used to investigate the scheme suitability for different well-established digital signal processing (DSP) methods. Further aspects, such as evolving/sliding window functions, window size, and frequency resolution are analyzed. For the detection of interarea modes, parametric DSP methods turned out to be the most feasible. In general, evolving windows are faster than sliding windows. However, evolving windows require a trigger and were therefore rejected for the design proposed here. A lower frequency resolution may shorten the response time at the expense of the quality of the estimates of the instantaneous speed deviation and the generator coherency. The design proposed here is based on a high-frequency resolution at the cost of a lower response time.