This article discusses the reduction of nonlinearities in analog-to-digital (A/D) converters using digital signal processing (DSP). Also modeling of certain essential nonlinearities is considered in detail. The main focus is on wideband radio receivers, such as the emerging cognitive radio applications, where a collection of signals at different frequency channels is converted to digital domain as a whole. Therefore, the overall dynamic range can easily be in the order of tens of dBs and thus even mild nonlinear distortion can cause strong carriers to block weaker signal bands. In this article, a mathematical model for clipping distortion due to improper input signal conditioning is derived through Fourier analysis. Additionally, stemming from the analysis an adaptive DSP-based post-processing method for reducing the effects of clipping and integral nonlinearity (INL) in A/D converters is presented with illustrative examples using both computer simulations and laboratory radio signal measurements.

This article addresses the analysis and digital signal processing (DSP)-based mitigation of phase noise and sampling clock jitter in orthogonal frequency division multiplexing (OFDM) radios. In the phase noise studies, the basic direct-conversion receiver architecture case is assumed with noisy downconverting oscillator. In the sampling jitter case, on the other hand, the so-called direct-radio-frequency-sampling receiver architecture is deployed utilizing bandpass sub-sampling principle. The basis for the DSP-based impairment mitigation techniques is first formed using analytical receiver modeling with incoming OFDM waveform, where the effects of both oscillator phase noise and sampling clock jitter are mapped to certain type subcarrier cross-talk and distortion compared to ideal receiver case. Then iterative detection principles and interpolation techniques are developed to essentially estimate and cancel the subcarrier distortion. Also some related practical aspects, like channel estimation, are addressed. The performance of the proposed mitigation techniques is analyzed and verified with extensive computer simulations. In the simulations, realistic phase-locked-loop-based oscillator models are used for phase noise and sampling clock jitter. In addition, different received signal conditions like plain additive white Gaussian noise channel and extended ITU-R Vehicular A multipath channel are considered for practical purposes. Altogether the obtained results indicate that the effects of oscillator and sampling clock instabilities can be efficiently reduced using the developed signal processing techniques.

The present paper presents a very efficient technique for enhancing the pointing accuracy in beam-waveguide (BWG) antennas and its application to the deep space antenna DSA2 of the European Space Agency. The proposed technique permits to achieve a twofold result: on the one hand, it provides a solution to the beam aberration issue, arising when the antenna simultaneously receives from and transmits to a spacecraft moving in the transversal direction. On the other hand, it allows to perform a fast conical scan to enhance the pointing accuracy of the antenna. Both results are achieved by simple linear displacements of feeds and mirrors located in the lower part of the BWG, with a very limited deterioration of the antenna gain. The required displacements of feeds and mirrors are determined through a fast optimization algorithm, based on a top-down approach, which requires repeated physical–optics analyses of the lower part of the beam waveguide only, with a significant reduction in the computing time.

In this paper the non-ideal and frequency-dependent behavior of the hardware components of the ultra-wideband (UWB) transmitter RF-front end is analyzed. Both digital and analytical methods to compensate for this frequency-dependent behavior are investigated, in order to optimize the radiated wave, so that its spectrum perfectly matches the given mask. An example of spectral system optimization with analog devices is shown and verified through measurement results.

The main implementation impairments degrading the performance of direct-conversion radio transmitters are in-phase/quadrature (I/Q) mismatch, local oscillator (LO) leakage, and power amplifier (PA) nonlinear distortion. In this article, we propose a recursive least-squares-based learning algorithm for joint digital predistortion (PD) of frequency-dependent PA and I/Q modulator impairments. The predistorter is composed of a parallel connection of two parallel Hammerstein (PH) predistorters and an LO leakage compensator, yielding a predistorter which as a whole is fully linear in the parameters. In the parameter estimation stage, proper feedback signal from the transmitter radio frequency (RF) stage back to the digital parts is deployed, combined with the indirect learning architecture and recursive least-squares training. The proposed structure is one of the first techniques to explicitly consider the joint estimation and mitigation of frequency-dependent PA and I/Q modulator impairments. Extensive simulation and measurement analysis is carried out to verify the operation and efficiency of the proposed PD technique. In general, the obtained results demonstrate linearization and I/Q modulator calibration performance clearly exceeding the performance of current state-of-the-art reference techniques.

This paper deals with a robust RFSID (Radio Frequency Self IDentification) system for road-to-car communications. The RFSID-based system operates in Ku-band and consists of a fixed transmitter, located at the road side, and of a receiver unit placed on the moving target, i.e. a car in its first proposed application. A slotted waveguide antenna array is used to illuminate the moving object at the desired position, whereas a four-patch array antenna is adopted at the receiver side. Both the antennas have been designed using numerical simulations based on a Finite Difference Time Domain (FDTD) algorithm. When the moving object crosses the antenna beam a triggering pulse is generated by the receiver; such a pulse can be used to reset or update the electronics aboard the vehicle and to log specific information coming from location-based systems (LBSs), into the car equipments. A digital transmission of gold sequences and a post-processing unit have been exploited so far to increase the robustness and the accuracy of the system. At this stage of the development the system benefit of extensive field tests, being adopted for some years by many top Formula 1 racing teams as a lap trigger system, used to reset the on-board electronics when the car crosses either the finishing line and peculiar path reference points. The temporal accuracy exhibited is better than 1 ms with a coverage of about 90 m.

The measurements of physical level parameters can become the area where decisions about cognitive radio will have the most striking effect. Field-programmable gate array (FPGA) enables real-time analyses of physical layer data to satisfy constraints like dynamic spectrum allocations, data throughput, and the coding rate. Cognitive radio will be based on simple network management techniques, using remote procedure calls. Intelligent knowledge-base system (IKBS) techniques will be used to search the parameter space in selecting changes to the system. Worldwide Interoperability for Microwave Access (WiMAX) PHY-layer functions will be managed cognitively by a FPGA-based controller to optimize the performance of the system. Instead of simple bit loading methods, the global multi-criteria optimization promise possibility to adapt more parameters with respect to several objectives. In this paper, the application of particle swarm optimization to fixed WiMAX-OFDM (Orthogonal Frequency Division Multiplexing) parameter adaptation is presented and compared with the greedy bit loading algorithm.

The present paper presents the design and development of a dual-channel microwave rotary joint using coaxial waveguide as primary waveguide and rectangular waveguide as the secondary waveguide. Design is presented at 5.85–7.02 GHz with 20% bandwidth and at 14–14.5 GHz bands with 3.6% bandwidth using dual channel mode transducers exciting transverse electromagnetic (TEM) mode in the coaxial waveguide. Rectangular to coaxial waveguide transitions employing multi-stepped doorknob transitions are used to excite the TEM mode from the rectangular waveguide. The measured results for electrical parameters such as return loss, insertion loss of the dual-channel rotary joint are presented with 360° rotation of the rotary part with respect to the stator part.

This paper presents an overview of the principles suitable for wireless power supply of devices with a small power input in picocells. This means predominantly different types of small electric devices in the space of rooms. Basic principles, namely electromagnetic induction and electromagnetic wave, are explained using examples of developed systems. Different types of wireless power systems are compared with respect to efficiency, frequency, power, and transmission distance.

A detailed nonlinear analysis and design of a distributed voltage controlled oscillator (DVCO) is presented in this work. The analysis and design of the DVCO is performed using harmonic balance-based simulation techniques and envelope transient simulation. These techniques are used to make a complete nonlinear study of the behavior of the DVCO. Using the proposed nonlinear tools, a DVCO operating in the frequency tuning range between 1 and 2 GHz has been implemented in order to support the simulation results. The average output power and consumption along the frequency band for the designed DVCO are 5.2 dBm and 60.4 mW, respectively.

This paper presents a detailed experimental analysis of the cyclostationary properties of low-frequency (LF) noise sources of microwave bipolar devices, in order to improve the LF noise description in compact models. Such models are used to help designers on predicting circuit performances such as phase and amplitude noise in oscillators. We start by reviewing the most relevant experimental and simulation results on the subject, and then investigate the model of conductance fluctuations proposed to explain the 1/f noise of carbon resistors. This simple linear case serves as a basis for understanding the complex case of a non-linear device under large-signal periodic operation. We then present the large-signal small-signal analysis of a pumped junction, focusing on the process of converting the fundamental LF noise process, a current fluctuation, into voltage fluctuations. We show why a stationary noise model would lead to an increase of the voltage noise observed around DC when the device is pumped, while the voltage noise would decrease if a cyclostationary model was used. A great amount of experimental data is presented not only to support our analysis, but also as a mean to distinguish between the two noise processes under consideration: stationary or cyclostationary. The goal of our noise measurement technique was to maximize the difference between those two concepts. Throughout the paper, we revisit some known concepts and show how some experimental results may lead to misinterpretations.