Purpose This paper aims to overcome the main obstacle to compare the merits of the different control strategies for fixed-wing unmanned aerial vehicles (UAVs) to assess autopilot performances. Up to now, the published studies of control strategies have been carried out over disperse models, thus being complicated, if not impossible, to compare the merits of each proposal. The authors present a worked benchmark for autopilots studies, consisting of generalized models obtained by merging UAVs’ parameters gathered from selected literature (journals) with other parameters directly obtained by the authors to include some relevant UAVs whose models are not provided in the literature. To obtain them it has been used a dedicated software (from U.S. Air Force). Design/methodology/approach The proposed models have been constructed by averaging both the main aircraft defining parameters (model derivatives) and pole-zero locations of longitudinal transfer functions. The suitability of the used methodologies has been checked from their capability to fit the short period and the phugoid modes. Previous analytical model arrangement has been required to match a uniform set of parameters, as the inner state variables are neither the same along the different published models nor between the additional models the authors have here contributed. Besides, moving models between the space state representation and transfer function is not just a simple averaging process, as neither the parameters nor the model orders are the same in the different published works. So, the junction of the models to a common set of parameters requires some residual’s computation and transient responses assessment (even Fourier analysis has been included to preserve the dominance of the phugoid) to keep the main properties of the models. The least mean squares technique has been used to have better fittings between SISO model parameters with state–space ones. Findings Both the SISO (Laplace) and state-space models for the longitudinal transfer function of an “averaged” fixed-wing UAV are proposed. Research limitations/implications More complicated situations, such as strong wind conditions, need another kind of models, usually based on finite element method simulation. These particular models apply fluid dynamics to study aerostructural aircraft aspects, such as flutter and other aerolastic aspects, the behavior under icing conditions or other distributed parameter problems. Even some models aim to control other aspects than the autopilot, such as the trajectory prediction. However, these models are not the most suitable for the basic UAV autopilot design (early design), so they are outside the objective of this paper. Obviously, the here-considered UAVs are not all the existing ones, but the number is large enough to consider the result as a reliable and realistic representation. The presented study may be seen as a stepping stone, allowing to include other UAVs in future works. Practical implications The proposed models can be used as benchmarks, or as a previous step to produce improved benchmarks, in order to have a common and realistic scenario the compare the benefits of the different control actions in UAV autopilots continuously presented in the published research. Originality/value A work with the scope of the presented one, merging model parameters from literature with other (often referred in papers and websites) whose parameters have been obtained by the authors has been never published.

This paper sets out a study of the autopilot design for fixed wing Unmanned Aerial Vehicles ( UAVs) taking into account the aircraft stability, as well as the power consumption as a function of the selected control strategy. To provide some generality to the outcomes of this study, construction of a reference small-UAV model, based on averaging the main aircraft defining parameters, is proposed. Using such a reference model of small, fixed-wing UAVs, different control strategies are assessed, especially with a view towards enlarging the controllers' sampling time. A beneficial consequence of this sample time enlargement is that the clock rate of the UAV autopilots may be proportionally reduced. This reduction in turn leads directly to decreased electrical power consumption. Such energy saving becomes proportionally relevant as the size and power of the UAV decrease, with benefits of lengthening battery life and, therefore, the flight endurance. Additionally, through the averaged model, which is derived from both published data and computations made from actual data captured from real UAVs, it is shown that behavior predictions beyond that of any particular UAV model may be extrapolated.

A novel Doherty-like power amplifier (DPA) has been fabricated using 15 W, 2.7 GHz, GaN HEMT transistors. The quarter-wave transformer used in the classical DPA topology is replaced by a matching network including a Klopfenstein taper. From a practical prototype realization, this modification has demonstrated that the resulting DPA bandwidth (BW) is increased in comparison with the conventional topology while keeping the efficiency figures. Moreover, this design allows an easy tuning of the group delay through the output reactance of the taper, resulting in a more straightforward adjustments than other recently-published designs where the quarter-wave transformer is replaced by multi-section transmission lines (hybrid or similar). Experimental results have shown an average efficiency of 47.2% for the HSPA+ modulation centered at 2.25 GHz.

The benefits of organizing multichannel communications by fragmenting the data in bonded or in aggregated channels have been usually assessed at the media access control layer, being the network traffic the purpose of the studies. Despite the importance of the different kinds of channel organizations over the transmit signal peak-to average power ratio (PAPR) to size the back-off value of the power amplifier, little attention has been paid to it. In this paper the key aspect is the PAPR, which is fundamental to face transmitter designs optimizing the trade-off between linearity and the time-of-life of the batteries. The results are of interest not only in ad hoc cognitive radio networks, but also in some of the latest European Telecommunications Standards Institute and Institute of Electrical and Electronics Engineers standards supporting mobility in either wireless local area network environments or in cellular communications. A standardized radio link is used to model a realistic scenario to study different settings of the aggregate channels and to compare them with channel bonding alternatives. Besides, the suitability of some crest factor reduction techniques is here considered for different channel organizations. The results show how the channel organization (contiguous, regular and random spaced), either with bonded or aggregated strategies, affects the PAPR.

This paper presents a wideband GaN Doherty power amplifier (DPA) in the 2.6 GHz band with a 3 stages Wilkinson splitter where the impedance inverter is based on a tapered line instead of a quarter wave transformer. From the comparison with the same amplifier using a typical quarter-wave transformer as impedance inverter, and keeping both designs the same bandwidth (1.6 to 3.3 GHz, 65% of the fractional bandwidth), the Power Added Efficiency (PAE) is enhanced up to 50% (from 40.46% to 61.17% at 35 dBm input power) in the DPA with a tapered line. Besides, the transducer gain remains practically unaltered.

This paper presents how to apply order reduction in wide-band digital predistortion (DPD) linearizers using the principal component analysis (PCA) technique. This method is tested in a wireless backhauling transmitter where four 28 MHz adjacent subcarrier transmission of M-QAM signals are considered. The DPD has to counteract not only the PA nonlinear behavior, but also its dynamics. This may results critical when considering wideband signals since the number of coefficients required to model memory effects can grow dramatically. By applying the PCA technique, the number of essential parameters can be significantly reduced. In addition, a strategy to minimize the computational cost of finding the optimal coefficients is also presented. A test-bed for evaluating the DPD linearization performance of the RF subsystem when PCA is applied was deployed and experimental results are presented in this paper.

A Doherty power amplifier (DPA) is an effective structure born in 1936 which, after a scarce revival around year 2000), had been strengthened from 2005 because its capability to combine linear amplification with power efficiency. Despite the conceptual simplicity of its basic operation, a lot of practical drawbacks shrink the theoretical behavior, thus leading a significant number of research works to overcome them. The main objective in DPA research is to increase efficiency while maintaining linearity and filling the specified bandwidth. This paper presents a survey of the state of the art of DPA advanced design aspects. After a short review of the DPA operation principles, aspects regarding improvements for linearity, power efficiency and amplification bandwidth are introduced. Besides, some alternative structures and technologies, as well as practical design aspects and some trade-offs which the designer usually has to face are also presented.

With the software defined radio (SDR) concept a new communication technology was created to improve interoperability among different wireless networks, systems, and devices and some of the typical limitations in flexibility of classical radio implementations were removed, aiming at the same time at reducing the cost of technology components. For example, in a classical radio implementation, some hardware components like filters are required to cancel unwanted emissions, but good analog filters are costly and inflexible. Instead, an SDR can be considered a wireless system whose main functionality and operational parameters, like output power, frequency band, air interface including modulation techniques, security and performance characteristics, are implemented by means of software programs running on programmable devices that can dynamically reconfigure the system. In such a way there is the possibility to create multimode, multiband, and multi-functional network equipment that can be reconfigured, enhanced, and upgraded during their operation through software updates and hardware reconfiguration, instead of physical changes in the hardware. As detailed in the sections below, standards for wireless communications include adaptive modulation and coding, as well as scalable bandwidths and transmit power control. These facts require, both in user terminals and in base-stations, a degree of flexibility and re-configurability, software upgrading, and multimode-multiband compliance. Besides, some access techniques, especially CDMA, are very sensitive to precise frame synchronization. Software defined radio is a suitable tool to overcome issues like these.

This paper presents a new Digital Predistorter (DPD) to compensate for nonlinear distortion that arises in Envelope Tracking (ET) Power Amplifiers (PAs) driven by slew-rate limited versions of the real signal's envelope. The slower version of the transmitted signal's envelope is used to cope with the slew-rate and bandwidth limitations of Envelope Amplifiers (EAs). Unfortunately, the use of slower versions of the real signal's envelope as the drain modulator generates a special kind of nonlinear memory effects. This paper shows experimental results that prove that is possible to compensate for these nonlinear memory effects that appear when exciting the supply of a RF linear PA with a slew-rate limited version of the envelope.

This paper presents the design of a nonlinear autoregressive moving average (NARMA) digital adaptive predistorter (DAPD) for power amplifier (PA) linearization consisting of a low-complex closed-loop architecture. Both the predistortion function and the adaptation algorithm are fully implemented in a field-programmable gate array (FPGA) device, without the need for using any additional coprocessor. The proposed predistortion architecture is capable to compensate for both the PA nonlinear distortion and memory effects. Moreover, this DAPD allows almost real-time adaptation without interrupting the normal transmission. The computational complexity introduced by this DAPD is studied in this paper. The proposed theoretical design is implemented in an FPGA, whereas the linearization performance of the DAPD is validated through simulated and experimental results.

This paper presents a practical application of a method for generating slew-rate limited envelopes in order to drive the dynamic supply of envelope tracking (ET) power amplifiers (PAs). The proposed method results useful to generate slower versions of the transmitted signal's envelope to cope with the slew-rate and bandwidth limitations of envelope amplifiers. Moreover, this paper shows experimental results comparing the performance of ET when exciting the drain of a PA (based in a GaN transistor) with both the envelope of the signal and the slew-rate limited version of the envelope.

This paper presents a digital platform to generate, linearize and adjust complex baseband and intermediate frequency signals for operating in both envelope tracking and polar transmitters. The closed-loop nature of this field programmable gate array (FPGA)-based set-up allows introducing control strategies to overcome or mitigate some of the unwanted distortion arising from maladjustments in the aforementioned efficient transmitters. Real-time adaptive digital predistortion is also provided to improve the overall linearity of the system. In a first approach, memoryless predistortion has been considered; however, additional look-up-tables can be easily included to compensate for memory effects.

This paper assesses the PAPR for multimode transmitters (WiMAX, WiFi and Bluetooth), considering different combinations of modulations and access techniques (spreading codes and OFDM). Simulations include preamble, header and data fields. Results give hints for tailoring the back-off of power amplifiers, as well as for foreseeing the necessary slew-rate in polar transmitters.

This paper presents an exhaustive description of a field programmable gate-array (FPGA) based set-up for envelope tracking and dynamic biasing of RF power amplifiers (PA). The system includes digital adaptive predistortion (DAPD) and gain compensation deployed in a FPGA device. For testing purposes a GaN HEMT RF PA operating at 3.5 GHz was considered. Preliminary experimental results are provided showing the DAPD compensate for the steady-state PA nonlinear distortion arising as a consequence of varying the power supply.

This paper shows a method for generating slew-rate limited envelopes in order to accomplish the slew-rate restrictions of the envelope driver in envelope tracking transmitters. The proposed algorithm can run in real-time and it is implemented in a digital front-end of a Software Radio system. Simulation results and conclusions are provided to validate the methodology. Moreover, experimental results of an FPGA implementation are also provided.

The IEEE MTT-S Administrative Committee (AdCom) has recently passed a resolution of its Transnational Committee to create and develop the IEEE MTT-S International Workshop Series (IMWS), which will complement the existing workshops of the MTTS International Microwave Symposia. The purpose of this new platform is to boost and promote MTT-S technical and educational activities as well as MTT-S international exchanges and collaborations. Several workshops covering different topics of MTT-S interest will be developed each year at a worldwide scale, spreading in various continents and countries. Through its technical program, this edition of the workshop aims to stimulate the discussion and promotion of new ideas in the field of advanced Microwave and RF front ends for Software Defined Radio and Cognitive Radio technologies. The workshop format will consist of a single oral session paralleled by a permanent poster session. Submitted abstracts are expected to cover state-of-the-art, theoretical aspects, implementation and innovative applications in the field of RF front ends for software defined radio and cognitive radio. Participation of young researchers is strongly encouraged and supported via Young Researcher Grants. This conference/workshop is a two day conference where the first day is composed of invited speakers, and the second day is composed of regular submitted papers.