Linearity improvement of VCSELs based radio over fiber systems utilizing digital predistortion

The article proposes a Digital Predistortion (DPD) methodology that substantially meliorates the linearity of limited range Mobile Front Haul links for the extant Long-Term Evolution (LTE) and future (5G) networks. Specifically, the DPD is employed to Radio over Fiber links that contrive of Vertical Cavity Surface Emitting Lasers (VCSELs) working at 850 nm. Both, Memory and Generalized Memory Polynomial models are implied to Single Mode (SM) and Multi-Mode (MM) VCSELs respectively. The effectiveness of the proposed DPD methodology is analyzed in terms of Normalized Mean Square Error, Normalized Magnitude, Normalized phase and Adjacent Channel Power Ratio. The demonstration has been carried out with a complete (Long Term Evolution) LTE frame of 10 ms having 5 MHz bandwidth with 64-QAM modulation configuration. Additionally, the effectuality of the proposed DPD technique is evaluated for varying levels of input power and link lengths. The experimental outcomes signify the novel capability of the implied DPD methodology for different link lengths to achieve higher system linearization

Digital Signal Processing Techniques Applied to Radio over Fiber Systems

The dissertation aims to analyze different Radio over Fiber systems for the front-haul applications. Particularly, analog radio over fiber (A-RoF) are simplest and suffer from nonlinearities, therefore, mitigating such nonlinearities through digital predistortion are studied. In particular for the long haul A-RoF links, direct digital predistortion technique (DPDT) is proposed which can be applied to reduce the impairments of A-RoF systems due to the combined effects of frequency chirp of the laser source and chromatic dispersion of the optical channel. Then, indirect learning architecture (ILA) based structures namely memory polynomial (MP), generalized memory polynomial (GMP) and decomposed vector rotation (DVR) models are employed to perform adaptive digital predistortion with low complexities. Distributed feedback (DFB) laser and vertical capacity surface emitting lasers (VCSELs) in combination with single mode/multi-mode fibers have been linearized with different quadrature amplitude modulation (QAM) formats for single and multichannel cases. Finally, a feedback adaptive DPD compensation is proposed. Then, there is still a possibility to exploit the other realizations of RoF namely digital radio over fiber (D-RoF) system where signal is digitized and transmits the digitized bit streams via digital optical communication links. The proposed solution is robust and immune to nonlinearities up-to 70 km of link length. Lastly, in light of disadvantages coming from A-RoF and D-RoF, it is still possible to take only the advantages from both methods and implement a more recent form knows as Sigma Delta Radio over Fiber (S-DRoF) system. Second Order Sigma Delta Modulator and Multi-stAge-noise-SHaping (MASH) based Sigma Delta Modulator are proposed. The workbench has been evaluated for 20 MHz LTE signal with 256 QAM modulation. Finally, The 6x2 GSa/s sigma delta modulators are realized on FPGA to show a real time demonstration of S-DRoF system. The demonstration shows that S-DRoF is a competitive competitor for 5G sub-6GHz band applications

Digital Predistortion of Millimeter-Wave Phased Antenna Arrays

The cost of deployment of reliable, high-throughput, fifth-generation (5G) millimeter-wave (mm-wave) base stations will depend significantly on the maximum equivalent isotropically radiated power (EIRP) that the base stations can transmit. High EIRP can be generated using active beamforming antenna arrays with large apertures and driven by an array of power amplifiers (PAs). However, given the tight half-wavelength lattice that the arrays must retain to ensure a wide beam steering range, the achievable EIRP quickly becomes thermally-limited. Efficient power amplification is thus imperative to low-cost and reliable beamforming antenna array design. This work considers the application of Digital Predistortion (DPD) as an efficiency-enhancement technique for mm-wave beamforming antenna arrays. Two RF beamforming configurations were considered and corresponding DPD schemes were investigated. First, a single-input single-output (SISO) DPD is proposed that can linearize a single-user RF beamforming array in the presence of non-idealities such as PA load modulation and variation of phase shifter gain with phase. The SISO DPD relies on a feedback signal which reflects a coherent summation of the PA outputs. The SISO DPD is then validated by measurement of a 4-element and 64-element array at 28 GHz with 800 MHz modulated signals using a single over-the-air feedback signal. The SISO DPD uses different sets of coefficients to cope with changes in both linear and non-linear distortions as the beam is steered. Second, the SISO DPD formulation is extended to multi-user RF beamforming to linearize multiple sub-arrays. In this configuration, non-negligible inter-user interference can affect the DPD training. To address the interference, a linear estimate of the interference is calculated and canceled for each user before extracting the SISO DPD coefficients in each sub-array. The SISO DPD with interference cancellation is validated by measurement of a 2-user 2x64-element subarray hybrid at 28 GHz with 800 MHz modulated signals across different combinations of steering angles for the two users

Digital predistortion of RF amplifiers using baseband injection for mobile broadband communications

Radio frequency (RF) power amplifiers (PAs) represent the most challenging design parts of wireless transmitters. In order to be more energy efficient, PAs should operate in nonlinear region where they produce distortion that significantly degrades the quality of signal at transmitter’s output. With the aim of reducing this distortion and improve signal quality, digital predistortion (DPD) techniques are widely used. This work focuses on improving the performances of DPDs in modern, next-generation wireless transmitters. A new adaptive DPD based on an iterative injection approach is developed and experimentally verified using a 4G signal. The signal performances at transmitter output are notably improved, while the proposed DPD does not require large digital signal processing memory resources and computational complexity. Moreover, the injection-based DPD theory is extended to be applicable in concurrent dual-band wireless transmitters. A cross-modulation problem specific to concurrent dual-band transmitters is investigated in detail and novel DPD based on simultaneous injection of intermodulation and cross-modulation distortion products is proposed. In order to mitigate distortion compensation limit phenomena and memory effects in highly nonlinear RF PAs, this DPD is further extended and complete generalised DPD system for concurrent dual-band transmitters is developed. It is clearly proved in experiments that the proposed predistorter remarkably improves the in-band and out-of-band performances of both signals. Furthermore, it does not depend on frequency separation between frequency bands and has significantly lower complexity in comparison with previously reported concurrent dual-band DPDs