To simplify the structure of real-time coherent Free-Space Optical (FSO) / wireless systems and reduce costs, this paper implements a low-cost FSO system based on bandpass Delta-Sigma Modulation (DSM) technology. A 256-ary Quadrature Amplitude Modulation (256-QAM) signal with a bandwidth of 400 MHz and a rate of 1.6 Gbit/s is generated to realize a 10 Gbit/s Polarization Division Multiplexed-Quadrature Phase Shift Keying (PDM-QPSK) signal after DSM. The real-time transmission in a 50 m FSO link with a sensitivity of -46.2 dBm is achieved. The experimental results indicate that the combination of coherent detection and DSM is a promising technology for future optical wireless scenarios.