We propose a simplified coherent detection system, which achieved phase diversity reception based on a single balanced photodetector (BPD) by alternately detecting the in-phase (I) and quadrature (Q) information components of a signal in the time domain. The simulation results show that a 25 Gbit/s non-return-to-zero signal has a receiving sensitivity of -39.97 dBm at a bit-error-rate threshold of 10 ^-3 after 25 km of standard single-mode fiber transmission. The Gram-Schmidt orthogonalization process algorithm is used to compensate the mismatch angle between the I and Q components caused by the frequency offset between the signal light and the local oscillator (LO) light. When the frequency offset is less than 1/5 of the symbol rate (±5 GHz), the sensitivity penalty is about 3.2 dB. In addition, a single PD can be used to replace the BPD if the LO optical power is high, and the sensitivity penalty is about 3 dB. The scheme provides a solution for low-cost coherent detection.