Key Technologies Of Digital And Analog Fiber Transmission For 5G Mobile Fronthaul

With 3GPP Release 15 finalized in 2018,the first commercially available 5G standard was established.Then,the scenarios of 5G are further enriched and the deployment of 5G is facilitated globally with the freeze of Release 16 in 2020.The networking of traditional distributed radio access network（D-RAN）is based on the macro base stations,which are equipped with full baseband functions.To save the CAPEX and the OPEX of the radio access network,the centralized radio access network（C-RAN）is reconstructed for the standalone 5G network,where the baseband function is de-coupled and assigned to central unit（CU）,distributed unit（DU）,and radio unit（RU）,and the data transmission between DU and RU is carried by the fiber-based fronthaul.The transport network is a priority in 5G deployment,and fronthaul transmission should provide large capacity,high spectral efficiency,low latency,and high signal fidelity while also maintaining the low-cost property,which is a challengeable section in 5G networking.The technologies for fronthaul can be divided into three categories,the digital transmission of common public radio interface（CPRI）and evolved CPRI（e CPRI）,the analog transmission of radio over fiber（Ro F）,and the digital-analog-integrated co-transmission.To meet the demands of fronthaul transmission,the research of this thesis is carried out with regard to these three techniques.The motivation,academic contribution,and innovation of our work are as follows:1.Twisted 4-ary pulse amplitude modulation for CPRI digital fronthaulCarrying the high-bandwidth CPRI fronthaul based on next-generation passive optical network（NG-PON）is actively explored by the industry.However,due to the limitation of the access network cost,the use of low-bandwidth components will deteriorate the high-bandwidth signals,and the introduction of high-order modulation formats such as PAM4also results in a decline of the ability to resist noise.Therefore,it is a huge challenge for the next generation PON to meet the stringent bit error rate of 10^-12 specified by CPRI.To improve the transmission reliability and power budget of high-speed PON systems,hence meeting the stringent BER requirement of CPRI,this thesis,for the first time,proposes and demonstrates the twisted 4-ary pulse amplitude modulation（T-PAM4）.The transmitter can generate T-PAM4 symbols with a DAC working at 2 sample-per-symbol and a specially designed level mapping.The receiver can realize two-dimensional decision on T-PAM4 with 2 sample-per-symbol,which significantly improves the noise-resistance performance.The experimental results prove T-PAM4 can bring a 5-d B sensitivity gain.This solution has low computational complexity and low hardware cost.2.Flexible quantization for e CPRI digital fronthaulCompared to the CPRI standard,e CPRI as the latest standard for the 5G fronthaul delivers the frequency-domain IQ signal samples and is characterized by less bandwidth consumption.Whereas employing e CPRI results in the dynamic behavior of fronthaul traffic caused by the load fluctuation of the wireless network.To cover the maximum requested line rate,the over-fluctuated traffic can lead to redundant bandwidth allocation and limited efficiency.Besides,the wireless channel features the time-varying and frequency-selective power fading,which significantly enhances the quantization noise of the fronthaul interface.Regarding these problems,this thesis studies the following solutions.1)The data redundancy of the quantized frequency-domain IQ signals is theoretically analyzed,and the flexible quantization resolution scheme is proposed to mitigate the dynamic property of fronthaul traffic and avoid redundant bandwidth deployment.This scheme takes the advantages of the functional splitting in e CPRI,in which the quantization resolution of the IQ signal can be adaptively adjusted according to the wireless signal quality of the IQ signal and the real-time fronthaul load.The main contribution of this work is to set up a wireless access simulation system that fully complies with the 3GPP standards,the results can well reflect the efficiency of the proposal in practice.The system implements the baseband functions of the Low-MAC and physical layer and the propagation of the standardized wireless channel.The fronthaul data generated by the system is experimentally transmitted through the optical link.The results show that the peak line rate of the fronthaul can be reduced by～40%at the expense of 1.2～1.9%terminal speed during full-load period,which greatly loosens the demand for fronthaul bandwidth and improves spectral efficiency.Besides,this solution is aimed at the widely deployed e CPRI for 5G fronthaul and is more valuable for practical application compared to the CPRI-oriented transmission.2)For the first time,this thesis theoretically analyzes the impact of wireless channel fading acting on e CPRI fronthaul quantization noise and proposes a scheme to compensate for the IQ signal using the existing channel estimation results or demodulation reference signal of the wireless system.With low computational complexity,this technology can suppress the fronthaul quantization noise by up to 6.5 d B,which significantly improves the fidelity of the e CPRI fronthaul for the wireless signal and outperforms the existing CPRI-aimed time-domain compensation scheme.3.Segment-wise time division multiplexing for analog fronthaul transmissionThe analog-Ro F-based fronthaul offers higher transmission spectral efficiency than the digital one.For the analog fronthaul interface,the multiplexing technology combining wireless IQ signals into one single high-speed analog signal is a critical issue,where the low-cost analog TDM is a preferred technical candidate.Aiming at 5G multi-input-multi-output（MIMO）scenarios and low latency,an arraying method that interleaves the samples of the MIMO signal has been applied to the TDM scheme to shorten the multiplexing latency.This technology relies on a large number of guard interval time slots and frequency conversion structures to eliminate interference between sampling points after fiber transmission,which leads to a decrease in transmission efficiency and an increase in complexity,respectively.The corresponding work in this thesis is as follows:1)For the first time,this thesis theoretically analyzes the impairment of the MIMO signal after the analog TDM fronthaul transmission.Regarding the issue of bandwidth waste due to too many guard time slots in MIMO-interleaved TDM,an improved scheme that takes signal segment as TDM granularity（Se-TDM）is proposed,which has a low-complexity system structure and achieves a satisfying trade-off between spectral efficiency and latency performance.In our analog TDM transmission experiment with an equivalent162-Gbps CPRI rate,the proposed scheme nails a 21%gain of spectral efficiency and upgrades QAM order from 64 to 256.2)This thesis presents for the first time a frequency-conversion-free MIMO-interleaved scheme,which further simplifies the fronthaul multiplexing structure.Theoretical analysis and experiments prove that this proposal can achieve the same interference cancellation effect as the prior art.The baseband IQ signals are directly multiplexed in this scheme,making it easier to shrink the bandwidth of the multiplexed signal and improve the spectral efficiency.4.Spectral null filling for digital-analog-integrated co-transmissionThe co-transmission of digital and analog signals on a single wavelength can combine the advantages of the two and save both optic facilities and wavelength resources,hence better supporting diverse access scenarios.Factors that affect the feasibility of the co-transmission schemes include complex hardware structures,low spectral efficiency,and that the signals are incompatible with industrial standards.Regarding these issues,the corresponding solution in this thesis is as follows:This thesis proposes a co-transmission scheme using spectral-null-filling technology.The innovation of this work lies in that the co-transmission is based on leveraging the spectral null at 28 GHz provided by the 56-Gbps PAM4 signal to insert the 5G millimeter-wave RF signal,therefore a frequency-gap-less transmission achieving high spectral efficiency is realized.The digital signal in the scheme is generated by only a low-cost and low-resolution DAC,and the digital and analog radio frequency（RF）signals comply with the NG-EPON and 5G standards respectively.The effect of fiber dispersion on the quality of the analog RF signal in the co-transmission system is theoretically derived and the strategy of RF band selection is demonstrated to maximize the bandwidth for the analog signal transmission.Based on the firstly proposed integrated transmitter structure,the 56-Gbps PAM4 and the superimposed 10×400-MHz analog RF signal are successfully delivered over 25 km,which is the largest transmission capacity ever reported for the intensity-modulation-direct-detection-based digital-analog-integrated transmission scheme.In conclusion,this thesis conducts a series of researches on the key technologies for the fronthaul transmission through theoretical analysis,simulation,and experiment,providing referential solutions for the fiber-based 5G mobile fronthaul.