Linear Power Amplifier Research Articles

A Novel Multi-Band Reduced Sampling Rate and I/Q Compensation Technique for RF Power Amplifiers

In this paper, a novel multi-band reduced sampling rate technique is proposed for power amplifier (PA) linearization system. It is used to enhance performance and reduce hardware costs in spectrum aggregation discontinuous broadband systems and compensate in-phase/quadrature (I/Q) imbalance in the multi-band case. During the feedback loop process of sampling rate, a band-limited filter is introduced, and the frequency band extrapolation is carried out so that the spectrum outside the limited band is compensated effectively. Theoretical analysis is performed to compare the proposed method and the conventional methods. To evaluate the performance of the proposed method, a broadband PA is excited by a tri-band signal composed of a 20 MHz long-term evolution (LTE) signal at 1.31 GHz, a 20 MHz 4-carrier wideband code division multiple access (WCDMA) signal at 2.49 GHz, and a 20 MHz 2-carrier WiMAX signal at 2.58 GHz respectively. The experimental results demonstrate that the normalized mean square error (NMSE) can get high accuracy less than −41 dB, with peak-to-average power ratio (PAPR) equaling 9 dB accompanied by a large reduction in sampling rate. Error vector magnitude (EVM) is 1% better than band-limited memory polynomial (MP) model and 2.3% better than conventional MP model.

Independently Tunable Linearizer Based on Characteristic Self-Compensation of Amplitude and Phase

In this paper, an independently tunable linearizer (ITL) is proposed to offer a continuous tuning and individual compensation of AM/AM and AM/PM distortion of power amplifiers (PAs). Two new operating modes of linearizer have been extended from conventional linearizer which has a correlated expansion characteristic of gain and phase, a derivative factor of the linearizer's forward S-parameter is presented to characterize the modes and aided the ITL design. By synthesizing these modes, an ITL prototype is designed and fabricated at a center frequency of 3.5 GHz. The restrictions of the derivative factors during the design are analyzed and presented first, then the ITL is tested with a continuous wave (CW) and several Long Term Evolution (LTE) signals with different bandwidth and Peak to Average Power Ratio (PAPR), respectively. The results show that the independent tuning range of AM/AM and AM/PM is measured by more than 3.4 dB and 33 degrees, and the ITL can significantly improve the linearity of PAs with wideband stimulated up to a bandwidth of 55 MHz. Two different PAs achieve over -40 dBc of Adjacent Channel Power Ratio (ACPR) with the ITL. Furthermore, the power consumption of ITL is less than 0.5 W, the performance improvement does not increase the cost compared to conventional analog linearization.

Energy and Spectrally Efficient Modulation Scheme for IoT Applications.

Due to the Internet of Things (IoT) requirements for a high-density network with low-cost and low-power physical (PHY) layer design, the low-power budget transceiver systems have drawn momentous attention lately owing to their superior performance enhancement in both energy efficiency and hardware complexity reduction. As the power budget of the classical transceivers is envisioned by using inefficient linear power amplifiers (PAs) at the transmitter (TX) side and by applying high-resolution analog to digital converters (ADCs) at the receiver (RX) side, the transceiver architectures with low-cost PHY layer design (i.e., nonlinear PA at the TX and one-bit ADC at the RX) are mandated to cope with the vast IoT applications. Therefore, in this paper, we propose the orthogonal shaping pulses minimum shift keying (OSP-MSK) as a multiple-input multiple-output (MIMO) modulation/demodulation scheme in order to design the low-cost transceiver architectures associated with the IoT devices. The OSP-MSK fulfills a low-power budget by using constant envelope modulation (CEM) techniques at the TX side, and by applying a low-resolution one-bit ADC at the RX side. Furthermore, the OSP-MSK provides a higher spectral efficiency compared to the recently introduced MIMO-CEM with the one-bit ADC. In this context, the orthogonality between the in-phase and quadrature-phase components of the OSP are exploited to increase the number of transmitted bits per symbol (bps) without the need for extra bandwidth. The performance of the proposed scheme is investigated analytically and via Monte Carlo simulations. For the mathematical analysis, we derive closed-form expressions for assessing the average bit error rate (ABER) performance of the OSP-MSK modulation in conjunction with Rayleigh and Nakagami-m fading channels. Moreover, a closed-form expression for evaluating the power spectral density (PSD) of the proposed scheme is obtained as well. The simulation results corroborate the potency of the conducted analysis by revealing a high consistency with the obtained analytical formulas.

Understanding Normalized Mean Squared Error in Power Amplifier Linearization

This letter provides a detailed analysis of the normalized mean squared error (NMSE) of an ideal orthogonal frequency-division multiplexing (OFDM) system, subject to soft clipping of the power amplifier (PA) output. Based on the central limit theorem, the OFDM baseband input is modeled as white Gaussian noise, and based on the Bussgang theorem, the NMSE is studied in some detail. Asymptotes describing the NMSE for the considered system are derived, when the system is working in the linear operation mode and when working under compression, respectively. Detailed yet simple expressions between the NMSE, average output power, saturation voltage of the PA, and the noise level of the additive channel or thermal noise are derived, which are believed to provide insight into the system's behavior.

High-Precision Joint In-Band/Out-of-Band Distortion Compensation Scheme for Wideband RF Power Amplifier Linearization

In this letter, a high-precision joint in-band/out-of-band (OOB) distortion compensation scheme is proposed to overcome the large modulated bandwidth issue for wideband radio frequency power amplifier linearization. It is achieved by dividing the linearization process into in-band predistortion and OOB suppression, respectively. Experimental results have proven that high-accuracy compensation can be realized by this scheme and the required bandwidth of the in-band and OOB distortion can be controlled flexibly, leading to significant reduction of the system bandwidth requirements.

An Integrated RF Power Delivery and Plasma Micro-Thruster System for Nano-Satellites

Progress in satellite technologies is ongoing and eventually finds applications back on Earth. Electric propulsion systems have been proven effective on large scale satellites (NASA DAWN) with better propellant efficiency than chemical or cold gas propulsion, and if miniaturized can be promising for long term operation of nano-satellites (e.g.CubeSats) in low Earth orbit or in deep space (NASA MarCO). However, the power supplies often used to power these electric thrusters, particularly those involving radiofrequency (RF) plasma, typically comprise power inefficient linear mode RF power amplifiers (e.g. class AB). These are simple systems designed to operate reliably under a wide range of loads for versatility, but they suffer low power efficiencies when operating at conditions significantly different from the nominal operating point. The required bulky and heavy thermal management components deem these electric propulsion systems impossible to fit on board nano-satellites. Here we present a compact and efficient switched mode dc-RF power inverter integrated with an electro-thermal plasma micro-thruster for nano-satellite (e.g.Cubesats) propulsion. The integrated system can serve as side panels and structural support of CubeSats, saving precious on-board volume for propellant and/or payloads. A complete assembly has been operationally tested in a space simulation system. This development opens a route for a new generation of power supplies applicable to the space sector, the microelectronics industry as well as the field of bioengineering.

Optimal Parameter Identification for Look-up Table based Band-limited Memory Polynomial Model using Direct and Indirect Learnings

The linearization of radio frequency power amplifiers (PAs) for wireless communication systems can be performed by a digital baseband predistorter (DPD). The DPD design includes the parameter identification of a post-inverse or a pre-inverse model. Such process depends on measurements of discrete-time complex-valued envelopes. With the adoption of larger envelope bandwidths and the PA operation at stronger nonlinear regimes, the requirements on the sampling frequency are very stringent. To relax the specifications on the analog-to-digital and digital-to-analog converters, a band-limited memory polynomial can be employed. To reduce the amount of calculations, polynomials can be replaced by linearly interpolated look-up tables (LUTs). Traditionally, a non optimal two step procedure is performed to identify the values to be stored in the LUTs. This work contribution is to propose an optimal identification technique that computes directly the values to be stored in the LUTs. The reported case study uses an LTE OFDMA envelope and Matlab simulation results show that the modeling accuracy can be significantly improved by the adoption of the proposed technique instead of the traditional one, quantified by reductions in normalized mean square error and adjacent channel power ratio of up to 10.8 dB and 12.4 dB, respectively.

A Multilevel Pulse-Modulated Polar Transmitter Based on a Doherty Power Amplifier and Memoryless Digital Predistortion

This letter presents an aliasingfree multilevel pulse modulation (MLPM) architecture to implement a linear Doherty power amplifier (PA). This letter is believed to be the first reported results of this type of Doherty architecture to achieve linear amplification rather than the usual situation of a continuous amplitude modulated carrier and continuous load modulation. Our results show that both high efficiency and good linearization can be achieved with simple memoryless digital predistortion. Doherty operation with MLPM is believed to have benefits in linearization due to the MLPM discrete drive as less power is dissipated by the PA, leading to less thermal-related memory effects. A prototype transmitter system is constructed using a commercially available Doherty PA for validation. Measurements are performed using a 64-QAM long-term evolution (LTE) downlink signal at 2140 MHz with 20-MHz bandwidth. The overall system achieved an output power level of 40.1 dBm with a power-added efficiency of 39.5% while passing the −45-dBc downlink spectral requirements of the LTE standard.

A Hardware-Efficient Feedback Polynomial Topology for DPD Linearization of Power Amplifiers: Theory and FPGA Validation

This paper describes a hardware-efficient feedback polynomial topology for digital predistortion (DPD) linearization of power amplifiers. Unlike the existing pruned Volterra-series DPD linearization that compensates the nonlinearities in parallel, our topology tailors a feedback memory block, such that the nonlinearities and memory effects can be constructed separately, minimizing the running complexity while significantly reducing the size of the coefficients extractor. Yet, the coefficients of the feedback memory block cannot be extracted in the direct form. To surmount it, a design methodology is developed with the aid of complexity-reduced Volterra-series model. Also, it is known that the least square estimation can extract the coefficients of the digital predistorter, but its pseudo-inverse operation between the inputs and outputs involves heavy matrix multiplications and division. With a computational complexity of O(N3), the coefficients extractor could hardly be implemented efficiently in the field-programmable gate array (FPGA). Here, we propose a division-free line-searched-based recursive least square algorithm for adaptive linear and nonlinear coefficient estimation, relaxing the computational complexity to O(N) and supporting adaptive estimation in the FPGA. Our DPD experiments demonstrate both identification and predistortion procedures fully implemented in the FPGA. The measured error vector magnitude is reduced from 10.1% to <3.2%, and the adjacent channel leakage ratio (ACLR) is improved from −28.4 to −46.1 dBc, for a 20-MHz 64-QAM orthogonal frequency division multiplexing signal. For carrier-aggregation signals, the ACLR is improved from −35.8 to −45.3 dBc.

Low-Cost RFin–RFoutPredistorter Linearizer for High-Power Amplifiers and Ultra-Wideband Signals

The 5G and beyond systems require the highly efficient linear power amplifier (PA) at the base station to deliver promised data rate in a power efficient manner. To facilitate higher data rates in future 5G communication, the demands for the wideband signals are continuously increasing, which creates adversity in employing digital predistortion (DPD) for the linearization of RF PA in ultra-wideband systems. System bandwidth constraint of the conventional DPD does not provide sufficient linearization of wideband signals. In this paper, we present two types of predistorter, first is RFin-RFout analog predistorter, in which linearization is performed by RF components in the analog domain, and the second is hybrid RF predistorter which takes the advantage of advanced DSP platform for improving the performance of RFin-RFout analog predistorter. The proposed predistorters eliminate the constraint on the system bandwidth of the conventional DPD. The requirements of data converters and reconstruction filters are relaxed in the analog predistorter (APD) architecture, whereas hybrid digitally controlled (HDC) APD eliminates the need of analog components by compensating the delay digitally. In order to validate this concept, a ZX60V-63+ PA from minicircuits is tested with 160-MHz eight-component carrier-long term evolution signal. The proposed APD architecture is able to deliver an adjacent channel power ratio (ACPR) of -46.4 dBc with an ACPR improvement of 13.4 dB at 1-dB back-off. Furthermore, with digital intervention, modified model, HDC-APD further provides a significant improvement in the linearization performance and delivers an ACPR of -53.5 dBc with an ACPR improvement of 20.5 dB.

A Reflection-Aware Unified Modeling and Linearization Approach for Power Amplifier Under Mismatch and Mutual Coupling

The behavior of a power amplifier (PA) is substantially affected by the output mismatch and mutual coupling in modern compact transmitters. To date, different works in the literature address the problem of coupling and mismatch separately, which are limited in terms of performance with a higher level of mismatch and mutual coupling. This paper identifies the reflection at the PA–antenna interface to be an effective parameter to unify the problem of mismatch and mutual coupling. Thus, a reflection-aware PA modeling and linearization method is proposed to compensate the adverse effect of mismatch and/or mutual coupling. The performance of the proposed method is investigated with a class AB and a Doherty PA under wide range of output mismatch and/or mutual coupling conditions with a 20-MHz WCDMA signal. The proposed method shows robust performance with an average adjacent channel power ratio better than 45 dBc. Such robust linearization performance under diverse output mismatch and mutual coupling conditions is highly desirable for modern and future communication systems that are subject to undergo rapid fluctuations under antenna matching and cross-coupling conditions.

Ultrasound Transmission Spectral Compensation Using Arbitrary Position and Width Pulse Sets

Ultrasonic testing systems are widely used for the analysis and characterization of materials. In many applications, such as ultrasonic imaging and spectroscopy, wide bandwidth, spectral flatness, and signal-to-noise ratio (SNR) are essential. The transfer function of the ultrasonic transducer is the bandwidth limiting factor. Pulse excitation has limitations in spectral coverage and achievable SNR. Spread spectrum (SS) signals combined with amplitude modulation are used to compensate the spectral losses and widen the bandwidth. Yet, SS signals require arbitrary waveform generation using digital-to-analog converters and linear power amplifiers, resulting in costly, large, and inefficient equipment. To overcome the aforementioned problems, we propose the use of bipolar arbitrary position and width pulses (APWP) sequences derived from nonlinear frequency modulated SS signals. APWP are obtained by an iterative optimization for the spectral flatness and bandwidth of the received signal. Comparisons of the proposed signals with known linear and nonlinear modulation signals are shown in terms of bandwidth and spectral flatness.

Study on Dynamic Body Bias Controls of RF CMOS Cascode Power Amplifier

This letter presents a highly linear cascode CMOS power amplifier (PA) that uses dynamic body linearizers based on envelope signal injection to the bodies of the common source and common gate power transistors. The linearizers allow the PA to have optimum AM–AM and AM–PM, which reduces the nonlinear distortions significantly. The two-stage PA is fabricated using a 0.18- $\mu \text{m}$ CMOS process with a printed circuit board output transformer. At 1.85 GHz, it delivers 27.7-dBm output power with 41.3% power added efficiency under a −33-dBc ACLR $_{\textsf {E-UTRA}}$ and a 4.7% error vector magnitude using long term evolution (LTE) signal.

Impacts of Crest Factor Reduction and Digital Predistortion on Linearity and Power Efficiency of Power Amplifiers

The linearity and power efficiency are two main factors of power amplifiers (PA), which can hardly be improved together. Digital predistortion (DPD) compensates for nonlinearity and memory effects of PAs in high efficiency zone. However, for advanced communication standards the high peak-to-average power ratio of the modulated signal constrains the PA operating point and thus limits the power efficiency. Crest factor reduction (CFR) techniques are applied to address this problem though they degrade the PA linearity relatively. In this brief, we compare different CFR approaches with respect to their performances in terms of linearity, PA power efficiency, and computational complexity. Classical hard clipping, clip-and-filter as well as joint CFR/DPD methods are considered. To complete the comparison, we propose a modeled CFR identified using indirect learning architecture combined with DPD. The simulation results validate its effectiveness on the trade-off among linearity, PA power efficiency and CFR computational complexity.

Comparative performance of some polynomial-based lowpass filters for microwave/digital transmission applications

ABSTRACTThis paper reports on the comparative performances of some polynomial based lowpass filters in the lumped elements, microstrip and defected ground structure (DGS) environment. The microwave designers are normally familiar with Butterworth, Chebyshev and Bessel filters. However, many more polynomials based, ripple and non-ripple types LPF have been suggested for the low-frequency applications. Some of these LPF, such as L-opt, H-type, Transitional Butterworth-Legendre (TBL), Pascal, Legendre group of LPF, are compared for their applications in analog microwaves, digital transmission, efficiency enhancement of the linear power amplifiers and five level partial response modulations. A method is reported to compute the ripple frequency for the Legendre group of LPF and Pascal LPF. Further, a design is suggested to significantly improve group delay performance of high selectivity filters.

Highly linear combining CMOS PA with AAAC

A highly linear and efficient CMOS power amplifier (PA) with adaptive auxiliary amplifier control (AAAC) is presented, which makes the common-gate transistor of an auxiliary amplifier operate in the saturation region for overall output powers. This leads to decrease the AM–PM distortion by reducing the variation of the output susceptance. In addition, the AAAC reduces the current consumption for all power ranges by controlling the auxiliary amplifier. The PA is fully integrated with all matching networks in a 0.18 μm CMOS process. It was measured with an 802.11n 64-QAM MCS7 signal. It achieved a maximum average power of 19.9 dBm with a power-added efficiency of 28% under an error-vector magnitude of −25 dB.

A &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${D}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Digital Transmitter with 64-QAM and OFDM Free-Space Constellation Formation

A ${D}$ -band I/Q RF-DAC featuring two 6-bit DAC elements driven in quadrature, each with its own on-die antenna, and a total effective isotropic radiated power (EIRP) of 13.2 dBm, is demonstrated in a 45-nm SOI-CMOS technology. The carrier signal is first amplified by the 30-dB gain LO path and is directly modulated by the 12 baseband bit streams, without linear upconversion or power amplification. QPSK, 8-PSK, 16-QAM, 32-QAM, and 64-QAM single-carrier (SC) and OFDM constellations are formed in free space and measured above the die at data rates up to 12 Gb/s and at distances over 15 cm. The large-signal bandwidth—from the carrier input pad to the output of the transmitter above the antennas—is 130–142 GHz and was obtained by sweeping the frequency of the ${D}$ -band external carrier, modulating it on die at 0.4 Gb/s using 16-QAM format, and measuring the error vector magnitude (EVM) with an instrumentation receiver. The highest data rate of 12 Gb/s was measured for QPSK SC modulation with a corresponding EVM of −12.2 dB. Lower maximum data rates of 7 Gb/s with −14.3-dB EVM and 3.6 Gb/s with −19-dB EVM were observed for 16-QAM and 64-QAM formats, respectively. Spectral shaping and OFDM transmission were also demonstrated at up to 2.5 Gb/s. The prototype consumes a total of 1.25 W, with an energy efficiency of 104 pJ/b.

IQ Imbalance Compensation and Digital Predistortion for Millimeter-Wave Transmitters Using Reduced Sampling Rate Observations

This paper expounds on the mitigation of the hardware nonidealities exhibited by millimeter-wave (mm-wave) transmitters (TXs) generating ultra-wideband signals. In particular, it tackles the IQ imbalance and nonlinear distortions attributed to the IQ modulator and power amplifier (PA) stages. For that, an offline frequency-domain-based IQ imbalance detection and compensation method using an interleaved multitone test signal with an interleaved I and Q frequency grid is first proposed. Then, the multitone signal settings were modified to enable the reduction of the required sampling rate of the analog-to-digital converter in the TX observation receiver (TOR) needed for IQ imbalance detection. Subsequently, a rearranged pruned Volterra series digital predistortion (DPD) scheme was exploited to separately set the nonlinearity order and linear and nonlinear memory depths for reducing the number of DPD coefficients needed for mm-wave PA linearization. The application of the proposed IQ imbalance detection and compensation method to a commercial IQ mm-wave modulator is stimulated by an interleaved multitone test signal spanning a modulation bandwidth of 4 GHz allowed for the reduction in the normalized mean squared errors (NMSEs) between the input and output signals from −14.0 to −37.7 dB at a complex baseband sampling rate of 8 GS/s and −37.1 and −36.5 dB with the sub-Nyquist complex baseband sampling rates of 1 and 500 MS/s, respectively. With the full and reduced complex baseband sampling rates of the TOR, the subsequent application of the rearranged DPD scheme was used to linearize a PA under test with an output 1 dB compression point of 23 dBm operating at 30 GHz. The PA stimulated by carrier aggregated orthogonal frequency division multiplexing signals with the bandwidths of 320 and 800 MHz is demonstrated the NMSEs of −36.5 and −32.4 dB and the adjacent channel power ratios of 50 and 45 dBc, using 77 and 66 DPD coefficients, respectively.

A low-power high-performance digital predistorter for wideband power amplifiers

In this paper, we present a low-power high-performance digital predistorter (DPD) for the linearization of wideband RF power amplifiers (PAs). It is based on the novel FIR memory polynomial (FIR-MP) predistorter model, which significantly augments the performance of the conventional memory polynomial predistorter with the use of complex baseband digital FIR filter prior to the memory polynomial. The adjacent channel leakage ratio (ACLR) performance comparison between the conventional MP and the proposed FIR-MP is done based on simulations with multi-carrier modulated signals of 20 and 80 MHz bandwidths. The PA models used for the simulations are extracted from the measurements of a commercial $$1\,\hbox {W}$$ GaAs HBT PA. At the ideal system-level simulations, the improvements in ACLR over the conventional MP are 7.2 and 15.6 dB, respectively, for 20 and 80 MHz signals. The choice of selection of various parameters of the predistorter along with the subsequent digital-to-analog converter (DAC) is presented. The impact of fixed-point representation is assessed using ACLR metrics, which shows that a wordlength of 14 bits is sufficient to obtain ACLR beyond $$45\,\hbox {dBc}$$ with a margin of $$10\,\hbox {dB}$$ . The proposed predistorter is synthesized in $$28\,\hbox {nm}$$ fully-depleted silicon-on-insulator (FDSOI) CMOS process. It is shown that with a fraction of the power and die area of that of the MP a huge improvement in ACLR is attained. With an overall power consumption of 8.2 and 88.8 mW, respectively, for 20 and 80 MHz signals, the FIR-MP DPD proves to be a suitable candidate for small-cell base station PA linearization.

A Novel Algorithm for Determining the Structure of Digital Predistortion Models

The generalized memory polynomial (GMP) model is one of the most commonly used models in digital predistortion to compensate for the nonlinearities and memory effects of power amplifiers (PAs). A very difficult, yet crucial, aspect of behavioral modeling is model dimensioning, i.e., determining nonlinear orders and memory depths. In this paper, we propose an algorithm based on hill-climbing (HC) heuristic to search for a GMP model structure, which provides the best tradeoff between modeling accuracy and complexity with different search criteria. Algorithm performance and convergence are evaluated for two different search criteria. Some optimization methods of the algorithms are then proposed to reduce the execution time by controlling its search path. The algorithm has been evaluated using real measurements from two different Doherty PAs. The solutions of the algorithm are proved robust in linearization of PA on the test bench. The results show that the proposed algorithm allows us to decrease the searching time by a factor typically greater than $10^4$ compared with exhaustive search.