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Differentially encoding and frequency offset compensating for a 16-quadrature amplitude modulation (16-QAM) vector signal in 60-GHz millimeter-wave radio-over-fiber (RoF) systems are experimentally investigated. The fourfold phase ambiguity issue in a non-data-aided signal communication transmission system with conventional square 16-QAM constellations is successfully tackled based on the differentially encoding scheme. Experimental results show that frequency offset compensation and carrier phase recovery are simultaneously achieved by utilizing the decision-directed carrier phase recovery algorithm. Up to <formula formulatype="inline"><tex Notation="TeX">$\sim\! \pm 1.3 \times 10^{-3}$</tex></formula> frequency offset times symbol rate product <formula formulatype="inline"><tex Notation="TeX">$\Delta f \cdot T_{s}$</tex></formula> is well compensated at the given bit-error-ratio (BER) value of <formula formulatype="inline"><tex Notation="TeX">$\sim\!\!10^{-3}$</tex></formula>.