Research Of RoF Access Technology Facing 5G

The next generation mobile communication system has higher requirements for transmission rate,number of access users and multi services support.The frequency bands will also be higher,and the usage scenarios of millimeter wave will become more frequent.To ensure signal's coverage,the number of active antenna units(AAU)will be greatly increased.So designing forward link which has large capasity and simple structure is of great significance for reducing network cost and improving transmission rate.Currently,the intensity Modulation-direct detection structure(IM-DD)is widely used in the forward link.In the IM-DD link,the optical carrier is modulated by electronic signal in terms of intensity.In the receiver of IM-DD link,the electronic signal is recovered by a photodiode.Due to the nonlinearity of the intensity modulator,the dynamic range of the IM-DD link is limited,which leads to a series of problems such as the inability to support high-order modulation formats.The radio-over-fiber(Ro F)links based on coherent detection support multi-dimensional modulation modes such as amplitude and phase,avoiding the nonlinearity caused by using intensity modulators.In terms of link capacity,the link also has the advantage of supporting higher-order modulation formats.In terms of transmission quality,coherent detection links with large dynamic range are also more suitable for upstream links with large difference in receiving radio frequency power.In terms of link gain,coherent detection has higher receiving sensitivity.Coherent detection also has the advantage of supporting frequency conversion.By frequency shifting of local oscillator light,the RF signal can be easily downconverted in the optical domain,avoiding the use of frequency conversion devices in the electrical domain.In this thesis,after analyzing the transmission characteristics of common optoelectronic devices and the influence of laser phase noise on coherent detection,three optical-borne radio frequency transmission schemes based on coherent detection are proposed:(1)Ro F link based on self-coherent detection: This link uses self-coherent structure,which has advantages of large transmission capacity and high spectral utilization.Through VPI simulation,the 16 QAM signal with 2.5 Gbps bit rate is transmitted simultaneously.The EVM of received signal is about 3%.(2)Down-conversion Ro F link based on self-coherent detection: For millimeter wave application scenarios,the link combines optical down-conversion unit.Through VPI simulation,the link can transmit 16 QAM signals with a bit rate of 2.5 Gbps and a carrier frequency of 40 GHz.Through optical down-conversion,the carrier frequency of the signal is finally converted to 2GHz.(3)Down-conversion Ro F link based on heterodyne coherent detection: This link uses additional heterodyne light source to compensate for the deficiency of self-coherent detection's gain.The link combines the optical down-conversion unit and the phase noise cancellation unit in the receiver.Through VPI simulation,a 16 QAM signal with a bit rate of 2.5 Gbps is finally transmitted,and the signal is downconverted from 30 GHz to 2 GHz.