Communication reliability in high-mobility millimeter-wave (mmWave) systems is fundamentally challenged by rapid channel variations, manifested in short channel coherence times, and phase noise, which necessitate frequent channel estimation and phase tracking. To overcome these limitations, we introduce Direction-Shift Keying (DSK), a geometry-based modulation scheme that exploits the relative stability of propagation directions rather than the rapidly varying amplitude and phase of the wireless channel. DSK conveys information through distinguishable directions of arrival (DoAs), enabled by a distributed antenna system (DAS), thereby shifting the communication principle from channel-dependent to geometry-dependent signaling. We first derive the structure of the optimal DSK detector for a mobile device equipped with an arbitrary number of antennas. We then establish the fundamental law governing the temporal stability of propagation directions, termed Direction Coherence Time (DCT), and show that DCT scales as $d/v$, where $d$ is the transmitter--receiver distance and $v$ is the relative velocity. In contrast, conventional Channel Coherence Time (CCT) scales as $λ/v$, where $λ$ denotes the wavelength, revealing a coherence-time advantage on the order of $d/λ$, potentially exceeding four orders of magnitude in mmWave systems. Furthermore, we establish the inherent invariance of DSK to common phase rotations, providing resilience to common oscillator phase noise without explicit phase tracking. Numerical results corroborate the theoretical findings and demonstrate the potential of geometry-based signaling for reliable high-mobility mmWave communications.

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