Linearisation of Satcom Power Amplifiers

An L-Band high power and efficient solid state power amplifier (SSPA) designed for the European satellite navigation system (i.e., Galileo) is presented. The developed SSPA, based on European Gallium Nitride (GaN) technology, comprises all the circuits required to interface the module with the satellite bus (i.e., a Power Supply Unit, PSU), and to control its functionalities by remote telecomand and telemetry (i.e., an Electronic Power Conditioner unit, EPC). The Radio frequency Unit (RFU) together with the PSU and EPC are accommodated in a single box with limited volume and mass. In continuous wave operating mode, the SSPA delivers an output power higher than 300W at less than 3 dB of gain compression in the whole E1-Band (i.e., center frequency ƒ0 = 1.575GHz). Moreover, the demonstrated gain and power added efficiency, including the power dissipated by the PSU and EPC, are higher than 65 dB and 44 %, respectively.

Digital predistortion (DPD), based on complex-valued memory polynomials (MP), is established as an efficient method for power amplifier (PA) linearization. The DPD facilitates compliance of the telecommunication infrastructure to strict standard specifications (transmit spectrum mask (TSM), error vector magnitude (EVM), bit error rate (BER), [Formula: see text]) by making PA more linear, while at the same time reduces the running cost of the wireless infrastructure (at both Base Transceiver Station (BTS) and User Equipment (UE) sides) by making PA more power efficient. Even when DPD is utilized, signals with high peak-to-average power ratio (PAPR) produce out-of-band PA spectrum emission due to intermodulation products affecting all above-mentioned critical standard specified parameters. The novelty proposed in this paper is as follows. PA is restricted to operate within “reasonably above” PA linear region using PAPR reduction technique. The residual nonlinearity is taken care of by DPD. The combination of DPD and PAPR PA linearization methods is implemented on software-defined radio board. The necessary steps for efficient PA linearization are presented, compensating both out-of-band and in-band signal distortions. We achieved EVM = 2.0%, ACPR [Formula: see text]50[Formula: see text]dBc, at 10[Formula: see text]W LTE modulated PA output, antenna point and PA output power of 39.5[Formula: see text]dBm.

UHF Solid State Power Amplifiers (SSPAs) for use in government and commercial communications satellites have been developed. These SSPAs combine high-voltage gallium nitride (GaN) FET technology with predistortion linearization (PDL) to allow the achievement of both linearity and record high efficiency. A peak power of more than 170 W with a power added efficiency (PAE) of 90% has been achieved. The SSPAs can be operated at lower power for multi-carrier WCDMA operation. A PAE of > 80% was achieved with a WCDMA adjacent channel level ratio (ACLR) of -25 dB.

Solid State Power Amplifiers (SSPA) based on Gallium Nitride (GaN) and Gallium Arsenide (GaAs) devices and novel power combining technologies have a very significant potential for use in transmitters in space-based communication equipment and radars replacing expensive, bulky and often unreliable tubes and their associated power supplies. Recent advances in GaN-based Microwave and Millimeter-wave Integrated circuits (MMIC) have clearly demonstrated performance and reliability suitable for their use in high power SSPAs for emerging and existing needs. These include transmitter payloads ranging from commercial broadband communications satellites to earth observation and exploration radars and sensors. Novel power combining techniques offer very compact, highly efficient, robust and reliable implementation of high power amplifiers in frequency range from 17 to 96 GHz, especially in the newly designated frequency bands (Ka, Q, V, E and W) for both space and ground terminals. Since the successful deployment of Ka/Q-band SSPA for Alphasat Aldo Paraboni Propagation/Communication Experiment, the authors have developed and delivered many other SSPAs covering the frequency range of 17–6 GHz for high reliability applications in space payloads, airborne platforms and ground stations using both GaAs and GaN-based devices. Very promising results have been achieved and significant future prospects have been envisaged. Details of these amplifiers and future trends in space-based SSPAs will be presented at the conference.

An under-sampling digital predistortion based on a multi-tone mixing feedback technique (MTM-DPD) is presented in this article. In contrast to the under-sampling digital predistortion (DPD) based on conventional receivers, the proposed method could simultaneously reduce the sampling rate and acquisition bandwidth using a multi-tone mixing (MTM)-based feedback channel. The MTM-DPD is implemented with an evolved direct learning architecture, where the coefficients of a predistorter are estimated based on the aliased feedback signal, preprocessed input signal, and preprocessed kernels of the DPD model. To preprocess the input signal and kernels, the behavioral model of the MTM-based feedback channel and its corresponding identification algorithm are analyzed in detailed. Experimental results show that for a gallium nitride (GaN)-based power amplifier (PA) with 40-MHz input, under the excitation of a 17 tone LO signal with frequency interval 7.68 MHz, the MTM-DPD with sampling rate 15.36 MSPS and acquisition bandwidth 15.36 MHz could improve the adjacent channel power ratio (ACPR) from −30.1 to −46.5 dBc and the error vector magnitude (EVM) from 9.13% to 1.76%. Compared with the conventional full sampling DPD (FS-DPD), the MTM-DPD could reduce the cost of analog-to-digital converters (ADCs) in feedback channel significantly while maintaining satisfactory linearization performance.

This article presents the design and experimental characterization of an engineering model (EM) solid-state power amplifier (SSPA) based on European 0.5-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula>m gallium nitride (GaN) on silicon carbide technology, conceived for next-generation flexible L-band navigation payloads. Operating in the E1 band (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1575.42\pm 25$</tex-math> </inline-formula> MHz), the unit integrates a radio frequency tray (RFT) to amplify the useful signal, and an electronic power conditioner (EPC), which provides telecommand/telemetry functions, remote mode selection, and interfaces the module with the satellite primary bus. Continuous-wave measurements demonstrate an adjustable output power range from 50.8 to 54.8 dBm with efficiency above 41%, including EPC consumption and connector/isolator losses. The amplifier also exhibits excellent thermal stability, with output power variation below 0.1 dB across −20 °C to +65 °C temperature range. Under Galileo-like modulated signals, it ensures full compliance with the spectral emission masks, thanks to an embedded analog linearizer (LIN), while delivering up to 300-W average output power with efficiencies up to 48%. The SSPA is multipaction free and fulfills all the space derating rules in terms of junction temperature and voltage/current swings for the involved GaN active devices. Compared with previously reported GaN SSPAs, the proposed design uniquely combines embedded output power flexibility and analog linearization in a compact, lightweight, space-ready unit. In particular, its power flexibility enables a reduction of more than 130 W in dc power consumption in low-power mode, easing thermal management and enhancing reliability, thus offering a valid solution for implementing high-reliability next-generation reconfigurable satellite payloads entirely based on solid-state technology.

In this paper, we propose a systematic approach to designing a highly efficient two-stage power amplifier (PA) with improved linearity. Investigation into the sources of nonlinearity in gallium nitride (GaN) high electron mobility transistors (HEMT) has been essential prior to the development of the two-stage PA design with improved linearity. A prototype was designed and fabricated using 45W and 6W packaged CREE transistors as the main and driver stages respectively. A peak efficiency of about 70% was obtained, using a continuous wave stimulus, over a bandwidth of 200MHz. When driven with a 20MHz WCDMA signal, at 800MHz, an adjacent channel leakage power ratio (ACLR) of 46dBc and an error vector magnitude (EVM) of about 1.5% were recorded at a peak output power of 39dBm without the use of digital pre-distortion (DPD). These results represent an improvement of about 10dB in ACLR and 2% in EVM for the two-stage PA using nonlinear driver as compare to those obtained using a linear driver.

We investigate the influence of both AM/PM conversion and HPA (high power amplifier) nonlinearity by evaluating BER degradations, using an SSPA (solid state power amplifier) for the multi-carrier CDMA (MC-CDMA) system downlink. A simple model of AM/PM conversion to cover individual differences of SSPAs is given for the evaluation. It is shown that AM/PM characteristics have influence on BER degradations and individual differences of SSPAs are considerable for the IBO (input back-off) configuration. The results might give insights for the design of MC-CDMA systems.

The Communications Satellite network in Japan is the principal means of transmitting emergency news, live programs, and other content. NHK has been developing satellite news gathering (SNG) equipment to achieve more stable operations, and has now developed a compact, outdoor solid state power amplifier (SSPA) for Ku-band satellite communications. It is believed to be the smallest SSPA of its kind in the world in the 80 W operating-power class and features a semi-integrated architecture with separable power-supply and amplifier components that each comply with International Air Transport Association (IATA) regulations. This improves mobility enabling the SSPA to be transported quickly to any location for SNG. NHK expects the introduction of solid-state amplifier devices to contribute significantly to stable satellite transmission compared to conventional TWT power amplifiers. This paper provides an overview of the newly-developed compact outdoor SSPA and reports on satellite transmission testing using this equipment.

One of the severe drawbacks of orthogonal fre-quency division multiplexing (OFDM) is high Peak-to-Average Power Ratio (PAPR) of transmitted OFDM signals. During modulation the sub-carriers are added together with same phase which increases the value of PAPR, leading to more interference and limits power efficiency of High Power Amplifier (HPA), it’s requires power amplifier’s (PAs) with large linear oper-ating ranges but such PAs are difficult to design and costly to manufacture. Therefore, to reduce PAPR various methods have been proposed. As a promising scheme, partial transmit sequences (PTS) provides an effective solution for PAPR reduction of OFDM signals. In this paper, we propose a PAPR reduction method for an OFDM system with variation of phases based on PTS schemes and Solid State Power Amplifiers (SSPA) of Saleh model in conjunction with digital predistortion (DPD), in order to improve the performance in terms of PAPR, the HPA linearity and for the sake of mitigating the in-band distortion and the spectrum regrowth. The simulation results show that the proposed algorithm can not only reduces the PAPR significantly, but also improves the out-of-band radiation and decreases the computational complexity.

The design of conventional digital predistortion (DPD) requires an analogue-to-digital converter (ADC) with a sampling frequency that is multiple times the signal bandwidth, which is extremely challenging for sub-Nyquist sampling systems with undersampled signals. To address this, this paper proposes a neural network (NN)-assisted wideband power amplifier (PA) DPD method for sub-Nyquist sampling systems, wherein a dual-stage architecture is designed to handle the ambiguity caused by subsampled communications signals. In the first stage, the time-delayed polynomial reconstruction method is employed to estimate the wideband DPD nonlinearity coarsely with the undersampled signals with limited pilots. In the second stage, an NN-based DPD method is proposed for the virtual training of the DPD, which learns the up-sampled DPD behavior by taking advantage of the pre-estimated DPD model and the input data signals, which reduces the length of the training sequence significantly and refines the DPD behavior efficiently. Simulation results demonstrate the efficacy of the proposed method in tackling the wideband PA nonlinearity and its ability to outperform the conventional method in terms of power spectrum, error vector magnitude, and bit error rate.

In most of the applications, maximum power handling capability of gallium nitride (GaN)-based solid-state power amplifiers (SSPAs) is limited due to limitation of overall thermal management system. To utilize full power handling capability of GaN high power amplifiers, thermal management is a great concern to achieve best performance and more importantly to ensure required device reliability. This chapter presents thermal design aspects of GaN high power amplifiers at device and circuit design levels. Different thermal management schemes of high power amplifiers are discussed for efficient heat dissipation from GaN devices with high power densities.

The analysis and implementation of a dual-band direct-conversion millimeter-wave transmitter is presented. The proposed dual-band modulator uses high-impedance loads at two frequencies for the local oscillator driving amplifiers and power amplifier (PA) pre-amplifiers. To correct the amplitude and phase imbalance of the in-phase and quadrature channels at Q- and W-bands, the paper suggests a digitally controlled quadrature error correction stage to finely calibrate these errors and lower the error vector magnitude (EVM) without increasing the baseband digital-to-analog converter resolution. The transmitter is fabricated in 0.12- μm silicon-germanium BiCMOS process and occupies an area of 5.5 mm 2 . At 45-GHz carrier frequency, error correction improves the sideband suppression ratio (SSR) by 10 dB at 2.4-MHz IF frequency. The EVM is lowered from 3.21% to 2.38% for a 64-QAM constellation at a 2.4-Ms/s symbol rate. At 94-GHz carrier frequency, the SSR is improved from 24 to 41 dBc at 2.4-MHz IF frequency, and the EVM is lowered by more than 1% for a 16-QAM constellation at 3-Ms/s symbol rate. A dual-band PA breakout measurement indicates the peak power-added efficiency of 15% with 14-dBm maximum saturated power (Psat) at 43 GHz and 9-dBm Psat at 82 GHz is achieved.

An ultrasmall-size and lightweight C-band SSPA (solid state power amplifier) has been developed for onboard use. The mass and dimensions of the SSPA are 0.85 kg and 106 x 170 x 67 mm, respectively. The output power of the SSPA is 20 W with an efficiency of 36-40 percent covering the temperature range of -15 to +60 C and the frequency bandwidth of 260 MHz. It is the smallest and lightest SSPA in the world, and its efficiency is among the top level in the world. This paper describes the design features and the performance of the SSPA. (Author)

This study proposes a switchless dual-power-mode (DPM) power amplifier (PA), which operates independently in high-powermode (HPM) and low-power mode (LPM) to improve the efficiency. The DPM PA optimizes the output impedances using bias control of the PA and the proposed bidirectional output matching structure with a transformer (TF) for each power mode with no switches for isolation between the LPM and the HPM with high passive efficiency. The proposed output structure realizes a high impedance in the nonoperating mode network to ensure that the operating mode network realizes the optimal impedance by utilizing bidirectional matching of the OFF characteristics of transistors and TF output for each mode. The differential DPM PA is implemented using a 2- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \mathrm {m}$ </tex-math></inline-formula> InGaP/GaAs heterojunction bipolar transistor (HBT) process on a nine-layer printed circuit board with low insertion loss to demonstrate the proposed structure. The HPM and LPM, operating independently, achieved the gains of 24–25.5 and 11.4–12.9 dB and the saturated output power of 34.2–34.8 and 23.2–23.5 dBm, with a peak power-added efficiency (PAE) of 44.4%–49% and 32%–34.4%, respectively, under a continuous-wave (CW) signal test of over 5.15–5.925 GHz. The proposed PA error vector magnitude (EVM) is tested with 160 MHz of a 64-quadrature amplitude modulator (QAM) signal and achieves the average output powers of 24.5 and 14 dBm (EVM = −32 dB) and a PAE of 10.5% and 10.1% for HPM and LPM with digital predistortion, respectively.