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A 20-Gb/s quadrature phase-shift keying (QPSK) homodyne coherent system was analyzed and experimentally demonstrated. This system is based on subcarrier optical phase-locked loop (SC-OPLL) and it works in real time. In this letter, we present a brief optical receiver analysis and the transmission system performance in terms of simulations and experimental results. We also show that the QPSK system works but it is affected by loop propagation delay effects due to the prototypal implementation. The delay loop effects can be mitigated by an SC-OPLL integrated realization.