Wireless data transmission has a wide range of applications in modern society and the currently used radio frequency (RF) spectrum is saturating due to the rapidly growing demand for data traffic. The optical spectrum, which consists of the visible light (VL) and the infrared (IR) spectra, provides an unlicensed band which is approximately 2600 times wider than the RF band. Light fidelity (LiFi) technology, which is originally based on light emitting diodes (LEDs), has been proposed as one of the solutions to the bandwidth shortage problem. In LiFi systems, data is conventionally modulated onto the intensity of the LED at the transmitter, and then demodulated using direct detection at the receiver. This principle is called intensity modulation / direct detection (IM/DD). While the IM/DD system is primarily limited by the modulation bandwidth of the light source, LiFi systems with coherent detection (CD) have drawn much attention in recent years as they provide a better receiver sensitivity. In this thesis, the principles of CD LiFi systems have been studied and two novel structures for CD LiFi systems to enhance its performance have been presented. Firstly, the channel direct current (DC) gain is derived from the Gaussian beam profile of the laser diodes (LDs). Based on the different beam profiles and channel DC gain characteristics, a performance comparison between the CD and the IM/DD LiFi systems has been presented. Both the bit error ratio (BER) and the power efficiency performances have been compared. The CD LiFi system provides a significant gain over its IM/DD counterpart. The photomixing process, which refers to the mixing between the signal laser and the local oscillator (LO), has been revisited. A revised photomixing efficiency formula has been derived and further generalised to the photomixing of multiple signals. Secondly, a novel structure combining spatial multiplexing (SMX) and CD, named CD SMX, of LiFi systems has been proposed. It has been demonstrated that due to the signal matching requirement of coherent detection, the proposed CD SMX system is free from inter-channel interference (ICI). The BER performance of the scheme has been studied via both theoretical analysis and numerical simulations. A considerably large power gain of over 30 dB and a signal-to-noise ratio (SNR) gain of about 10 dB over the IM/DD spatial modulation (SM) system (which is also ICI-free), have been reported. It has also been demonstrated that the proposed CD SMX system could outperform its IM/DD counterpart even when the CD SMX system operates at a higher bit rate, showing that it is desirable for high-speed next generation communication systems. Thirdly, an alternative structure of the CD LiFi system, namely the self-coherent (SC) LiFi system, has been presented. The SC LiFi system is able to reduce the receiver’s complexity by using simple direct detection with a single photodiode (PD), without losing information. It has been demonstrated via theoretical analysis and numerical simulations, that the SNR loss due to the lack of the local oscillator (LO) at the receiver could be controlled within 3 dB. It has also been reported that the SC LiFi system is robust against random orientations of the receiver, while a slight misplacement would largely degrade the conventional CD LiFi system. This gives the SC LiFi system the potential to support user mobility.