Coherent light fidelity (LiFi) systems which uses coherent detection at the receiver are attracting more and more interest as they provide many advantages over the conventional intensity modulation / direct detection (IM/DD) LiFi systems. Recent efforts to boost the performance of IM/DD LiFi systems include the usage of multiple-input-multiple-output (MIMO) technology, among which spatial multiplexing (SMX) has gained certain attention. This paper combines coherent detection and spatial multiplexing, and proposes a novel scheme called CD SMX (coherent-detection spatial-multiplexing) LiFi systems to improve LiFi performances. Compared with CD single-input-single-output (SISO) LiFi systems and CD spatial modulation (SM) LiFi systems, the proposed scheme provides a spectral efficiency gain. Unlike IM/DD or radio frequency (RF) SMX systems, the proposed CD SMX LiFi system is free from inter-channel interferences (ICI) due to the matching properties of coherent detection. The performance of the proposed CD SMX system is studied via both theoretical analysis and computer simulations. It is reported that the proposed system achieves a large gain over its IM/DD counterpart, and a moderate gain over a coherent system with SM.

This paper compares an intensity modulation/direct detection (IM/DD) light fidelity (LiFi) system using direct current biased optical orthogonal frequency-division multiplexing (DCO-OFDM) and a coherent LiFi system using homodyne detection. The performance metrics of bit error ratio (BER) and the power efficiency are compared between the two systems. In addition, the channel direct current (DC) gain characteristics of the two systems are investigated. Simulation results show that the coherent system achieves a better BER performance than the IM/DD system, but the transmitter and the receiver of the coherent system need to be perfectly matched. Also, within the same coverage area, the coherent system is more power efficient than the IM/DD system.