Digital Predistortion for Broadband Radio-over-Fiber Transmission Systems

Abstract Digital Predistortion for Broadband Radio-over-Fiber Transmission Systems Zichen Xuan Concordia University 2015 With the increase of the demand of high capacity wireless access, design of cost effective broadband wireless signal distribution system is required, particularly for future massive multi-input and multi-output (MIMO) wireless. Recently, Radio-over-Fiber (RoF) transmission systems have been revisited for broadband wireless signal distribution between central processing unit (CPU) and remote radio unit (RRU) (i.e., antenna towers). RoF, which is based on optical subcarrier modulation and thus an analog transmission system, fully utilize the advantages of broadband and low-loss fiber transmission, and also radio signal transmission. Unfortunately, RoF transmission systems are very susceptible to nonlinear distortions, which can be generated by all inline functional components of the RoF systems. However, two typical functions, i.e., optical subcarrier modulation and RF power amplification, are the two key sources of the nonlinear distortions. Various linearization techniques have been investigated for power RF amplifiers. It has been found that digital predistortion (DPD) linearization is one of the best approaches for RF bandwidth of up to 20 MHz. In this thesis, DPD linearization is explored for broadband RoF transmission systems. Instead of DPD implemented in baseband previously, a DPD linearization technique implemented in RF domain is investigated and demonstrated experimentally for broadband RoF transmission systems. Memory polynomial (MP) model is used for theoretical modeling of nonlinear RoF transmission systems, in which both nonlinear distortion and memory effect can be included. In order to implement the predistorter of the DPD using the MP model, least square (LS) method is used to extract the coefficients of the predistorter. Using the obtained coefficients, the trained predistorter is implemented and then verified in two experiments of directly modulated RoF transmission systems. In the first experiment, the DPD is verified in WiFi over fiber transmission systems, and more than 8 dB and 5.6 dB improvements of error vector magnitude (EVM) are achieved in back to back (BTB) and after 10 km single mode fiber (SMF) transmission. In the second experiment, both WiFi and ultra wide band (UWB) wireless signals are transmitted in the RoF system, which occupies over 2.4 GHz transmission bandwidth. It is shown that the implemented DPD leads to EVM improvements of 4.5 dB (BTB) and 3.1 dB (10 km SMF) for the WiFi signal, and 4.6 dB (BTB) and 4 dB (10 km SMF) for the UWB signal

Broadband Linearization Technologies for Broadband Radio-over-Fiber Transmission Systems

Wireless access networks consist of three sections, i.e., back-haul, front-haul, and wireless transmission, where the front-haul transmission systems are to distribute radio frequency (RF) signals to antenna towers. For current low-capacity wireless access, RF signals over coaxial cables, digital fiber-optic transmission, microwave point-to-point transmission, and narrowband radio-over-fiber (RoF) transmission have been used for the front-haul transmission systems. However, with the increase of demand of high capacity wireless access and also use of massive multiple-input and multiple-output (MIMO) antennas, low-cost, simple and broadband front-haul transmission systems are required in current 4G and in particular the future 5G wireless. RoF transmission system, which is based on optical subcarrier modulation, combines the advantages of both optical fiber and radio transmission, where the optical fiber has low loss, low cost, extremely high capacity, lightweight, and immunity to electromagnetic interference, and the radio transmission simplifies remote radio units (RRUs) at antenna towers. Furthermore, radio transmission based front-haul is transparent to RF signal frequency and wireless protocol, i.e., upgradable, in addition to simplified RRUs. Unfortunately, RoF is an analog optical transmission, and it is well known that any analog transmission is susceptible to nonlinear distortion. To be more specific, nonlinear distortion is the major limit for RoF transmission. In fact, all inline functional optical and electrical components used in RoF transmission systems may induce the nonlinear distortion. Specifically in RoF based front-haul systems, two key functions, i.e., RF power amplification and optical subcarrier modulation, are the main factors in introducing nonlinear distortions. The nonlinear distortions from RF power amplifiers (PAs) have been studied for decades. Therefore, the nonlinear distortions from the optical subcarrier modulation are the main concern in this thesis. The nonlinear distortions include harmonic distortions (HDs) and intermodulation distortions (IMDs). For narrow band RF signals, the HDs can be suppressed by RF filtering, but it may be impossible for the IMDs to be filtered out. For broadband RF signals, both HDs and IMDs could fall in the passband of RF signals and introduce crosstalk, and therefore both of them are required to be suppressed, i.e., linearization required. In the past decades, linearization for RF PAs has been investigated extensively, mainly focusing on signal processing based linearization, i.e., digital linearization. Unfortunately, the digital linearization is typically limited to the RF signals with up to 20 MHz bandwidth. Based on the current technologies of signal processing hardware, linearization for 1 GHz RF signals can be done, but the complexity and cost are beyond the practical applications. In order to explore broadband RoF transmission systems that support broadband front-haul, simple, low cost, and broadband linearization is pivotal. In this thesis, two linearization technologies for RoF transmission systems are investigated comprehensively, i.e., analog predistortion circuit (PDC) and dual wavelength optical linearization. Two novel PDCs are designed and investigated to suppress 3rd order IMD (IMD3) of RoF transmission systems. The PDCs have the advantages of broad bandwidth, compact size, and low cost. The first PDC is designed to have a bandwidth from 7 to 18 GHz, using two zero-bias Gallium Arsenide (GaAs) Schottky diodes as predistorter. The linearization using this PDC is verified in externally modulated RoF transmission systems. When a Mach-Zehnder modulator (MZM) is used for the optical subcarrier modulation, the input power at 1 dB compression point (P1dB) of the RoF transmission system is improved by 0.4 and up to 2.2 dB from 7 to 18 GHz. The spurious-free dynamic range (SFDR) is improved by more than ~10 dB from 7 to 14 GHz and ~6 dB from 15 to 18 GHz. When an electro-absorption modulator (EAM) is used, the input P1dB is improved by 0.8 and up to 3.8 dB from 8 to 17 GHz. The SFDR is improved by more than ~9 dB from 7 to 14 GHz and ~4 dB from 15 to 18 GHz. The second PDC is designed to have an ultra broad bandwidth from 10 MHz to 30 GHz, using a dual Schottky diode as the predistorter. The linearization using this PDC is investigated in both directly and externally modulated RoF transmission systems. The SFDR at 8 GHz is improved by 11.9 dB for a directly modulated RoF transmission. The SFDR is improved by more than 10 dB from 1 to 5 GHz and more than 5 dB from 1 to 30 GHz for an externally EAM modulated RoF transmission. Similarly, the SFDR is improved by more than 12 dB from 2 to 5 GHz and more than 5 dB from 2 to 30 GHz for an externally MZM modulated RoF transmission. When WiFi signals are transmitted over the externally modulated RoF systems for back-to-back (BTB) and 20 km single mode fiber (SMF), the error vector magnitudes (EVMs) are improved by 0.4 and up to 5.1 dB by using the PDC. Dual wavelength linearization (DWL) technique is investigated compressively to suppress 2nd and 3rd order nonlinearities of externally modulated RoF transmission systems simultaneously, including HDs and IMDs. The linearization is verified in both EAM and MZM modulated RoF transmission systems. Theoretical analysis is given for the first time to understand DWL technique. The experimental results agree with the theoretical analyses. In the externally EAM modulated RoF transmission systems, when the 2nd order nonlinearity is maximally suppressed, 11.5 and 1.8 dB improvements of the SFDRs with respect to HD2 and HD3 respectively are achieved by using DWL simultaneously. 8.5 and 1.3 dB improvements of the SFDRs with respect to IMD2 and IMD3 respectively are also achieved. Correspondingly, 3 and 4 dB improvements of the input and output P1dBs respectively are obtained. When the 3rd order nonlinearity is maximally suppressed, the SFDRs with respect to HD3 and IMD3 are improved by 8.1 and 20.4 dB, respectively, and corresponding 7.7 and 11.7 dB improvements of the input and output P1dBs respectively are achieved. Furthermore, IMD5 is also suppressed, and the SFDR5 with respect to IMD5 is improved by 7.1 dB. Moreover, the RoF transmission of WiFi signals at 2.4 and 5 GHz are also linearized by using DWL technique. 3.5 dB at 2.4 GHz and 2.8 dB at 5 GHz improvements of the EVMs are obtained. For an externally MZM modulated RoF transmission system, DWL is also investigated theoretically and experimentally. In the system, it is found that the SFDRs with respect to HD2 and HD3 are both improved at the same time when the even order nonlinearities are suppressed, in which the power of the RF signal and 3rd order nonlinearity is increased by the same level. Thus, the SFDR3 is still improved even the 3rd order nonlinearity is increased. Compared to using a single 1553 nm laser, the SFDRs with respect to HD2 and HD3 are improved by 38.4 and 12.1 dB