Real-time FPGA Implementation of a Digital Self-interference Canceller in an Inband Full-duplex Transceiver

Full-duplex is a communications engineering scheme that allows a single device to transmit and receive at the same time, using the same frequency for both tasks. Compared to traditionally used half-duplex, where the transmission and reception is divided temporally or spectrally, the spectral efficiency may theoretically be doubled in full-duplex operation. However, the technology suffers from a profound problem, namely the self-interference (SI) signal, which is the name given to the signal a node transmits and simultaneously also receives. Making the full-duplex technology feasible demands that the SI signal is mitigated with SI cancellers. Such cancellers reconstruct an estimate of the SI signal and subtract the estimate from the received signal, thus suppressing the SI. For the SI signal to be diminished as much as possible, canceller solutions should be deployed in both analog and digital domains. This thesis presents a digital real-time implementation of a novel nonlinear self-interference canceller, based on splines interpolation. This canceller utilizes a Hammerstein model to identify the SI signal, taking advantage of a FIR filter for the identification of the SI channel, and splines interpolation to model the nonlinear effects of the transceiver circuitry. The new canceller solution promises great reduction in computational complexity compared to traditional algorithms with little to no sacrifice in cancellation performance. The algorithm was implemented for a National Instruments USRP SDR device using LabVIEW Communications System Design Suite 2.0. The LabVIEW program provides the required connectivity to the USRP platform, as the SDR lacks a user interface. In addition, the functionality of the SDR is determined in LabVIEW, by creating code that is then run on the USRP, or more specifically, on the built-in FPGA of the device. The FPGA is where the SI canceller is executed, in order to ensure real-time operation. Even though the USRP device employs a high-end FPGA with plenty of resources, the canceller implementation needs to be simplified nonetheless, for example by approximating magnitudes of complex values and by decreasing the sample rate of the canceller. With the simplifications, the implementation utilizes only 34.9 % of available slices on the FPGA and only 34.6 % of the DSP units. Measurements with the canceller show that it is capable of SI cancellation of up to 48 dB, which is on par with state-of-the-art real-time SI cancellations in literature. Furthermore, it was demonstrated that the canceller is capable of bidirectional communication in various circumstances

A Swept-Sine-Type Single Measurement to Estimate Intermodulation Distortion in a Dynamic Range of Audio Signal Amplitudes

n/

Modelação comportamental e pré-distorção digital de transmissores de rádio-frequência

Doutoramento em Engenharia ElectrotécnicaNos atuais sistemas de telecomunicações, os transmissores de rádio-frequência são desenvolvidos tendo maioritariamente em conta a eficiência da conversão da potência fornecida da fonte em potência de rádio-frequência. Este tipo de desenho resulta em amplificadores de potência com características de transmissão não-lineares, que distorcem severamente o envelope de informação no processo de amplificação, gerando distorção fora da banda. Para corrigir este problema utiliza-se um processo de compensação não linear, sendo que a pré-distorção digital se tem favorecido pela sua flexibilidade e precisão. Este método é tipicamente aplicado de uma forma cega, por força bruta até se obter a compensação desejada. No entanto, quando o método se mostra ineficaz, como se verificou em amplificadores de potência baseados em transístores de nitreto de gálio, é difícil saber o que modificar nos sistemas para os tornar de novo úteis. De forma a compreender e desenhar sistemas de pré-distorção digital robustos é necessário, por um lado, perceber o comportamento dos amplificadores de rádio-frequência, por outro, perceber as limitações e relações entre os modelos digitais e o comportamento real do amplificador. Nesse sentido, esta tese explora e descreve estas relações de forma a suportar a escolha de modelos de pré-distorção, desenvolve novos modelos baseados no comportamento dos transístores, e propõe métodos de caracterização para os amplificadores de RF.In current telecommunication systems, the main concern when developing the radio frequency transmitter is power efficiency. This type of design generally leads to a highly nonlinear transmission characteristic, mainly due to the radio frequency power amplifier. This nonlinear transmission severely distorts the information envelope, leading to spectral regrowth, out-of-band distortion. To correct this problem a nonlinear compensation process is employed. For this application, digital predistortion is generally favored for its flexibility and accuracy. Digital predistortion is mostly applied in a blind manner, using brute force until the desired compensation is achieved. Because of this, when the method fails, as it has in gallium nitride based power amplifiers, it is difficult to modify the system to achieve the desired results. To understand and design robust predistortion systems, it is both necessary to have knowledge of the power amplifiers’ behavior, on one hand, and understand the limitations and relations between the digital models and these behaviors, on the other. To do this, this thesis explores and describes these relationships, granting support to the digital predistortion model choice, it further develops new predistortion models based on the physics of the transistors’ behaviors, and it proposes methods for the characterization of radio frequency power amplifiers

A Fast Gradient Approximation for Nonlinear Blind Signal Processing

When dealing with nonlinear blind processing algorithms (deconvolution or post-nonlinear source separation), complex mathematical estimations must be done giving as a result very slow algorithms. This is the case, for example, in speech processing, spike signals deconvolution or microarray data analysis. In this paper, we propose a simple method to reduce computational time for the inversion of Wiener systems or the separation of post-nonlinear mixtures, by using a linear approximation in a minimum mutual information algorithm. Simulation results demonstrate that linear spline interpolation is fast and accurate, obtaining very good results (similar to those obtained without approximation) while computational time is dramatically decreased. On the other hand, cubic spline interpolation also obtains similar good results, but due to its intrinsic complexity, the global algorithm is much more slow and hence not useful for our purpose

Regularized System Identification

This open access book provides a comprehensive treatment of recent developments in kernel-based identification that are of interest to anyone engaged in learning dynamic systems from data. The reader is led step by step into understanding of a novel paradigm that leverages the power of machine learning without losing sight of the system-theoretical principles of black-box identification. The authors’ reformulation of the identification problem in the light of regularization theory not only offers new insight on classical questions, but paves the way to new and powerful algorithms for a variety of linear and nonlinear problems. Regression methods such as regularization networks and support vector machines are the basis of techniques that extend the function-estimation problem to the estimation of dynamic models. Many examples, also from real-world applications, illustrate the comparative advantages of the new nonparametric approach with respect to classic parametric prediction error methods. The challenges it addresses lie at the intersection of several disciplines so Regularized System Identification will be of interest to a variety of researchers and practitioners in the areas of control systems, machine learning, statistics, and data science. This is an open access book