Group Digital Predistortion With Bias Voltage Adjustment for MIMO Transmitters

This article presents the methodology for designing triple-mode Doherty power amplifier (DPA) with multi-band operation and extended high efficiency range. It is illustrated that three different Doherty operation modes can be realized by simply re-setting the gate bias voltages while the DPA circuits keep unchanged. By employing a novel reciprocal gate bias DPA architecture, five operation bands have been achieved by the proposed DPA with larger than 9 dB high efficiency power range. The active load modulation process and theoretical performance of proposed DPA are analyzed in detail for each mode. For verification, a commercial GaN transistors-based DPA is designed and implemented with 1.76–1.96 GHz band in Mode I, 1.13–1.20/2.48–2.58 GHz bands in Mode II and 0.7–0.76/2.34–2.40 GHz bands in Mode III. The fabricated DPA achieves 7.0–10.5 dB saturation gain, 44.18–46.86 dBm peak output power and 41.9%–61.5% back-off efficiency. When stimulated by a 10 MHz long term evolution (LTE) modulation signal with 8.6 dB peaking-to-average ratio (PAPR), the adjacent channel power ratio (ACPR) of the fabricated DPA is better than −45 dBc after digital pre-distortion at 0.73, 1.18 and 1.85 GHz.

In this article, a fully integrated dual-band gallium nitride (GaN) Doherty power amplifier (DPA) based on multi-resonant circuits (MRCs) is presented. A lumped T-type network in combination with the output capacitances of the main and auxiliary transistors is able to realize the quarter-wavelength transformer (QWT) in DPA, and its component values are totally decided by the operating frequency, given a certain characteristic impedance. MRCs are proposed to exhibit the required component values at two different frequencies. By replacing each component in the T-type network with an MRC, a dual-band QWT can be achieved. An integrated dual-band DPA is implemented in a commercial 0.25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> GaN-HEMT process to validate the proposed method. The fabricated DPA shows a saturated output power of 43.8–44.7/44.4–44.8 dBm, a 6-dB back-off drain efficiency (DE) of 41%–46%/41.5%–47%, and a saturated DE of 49%–52%/56%–59% in 3.35–3.6/4.8–5.2 GHz, with a compact size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.8\times3.5$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Applying a 100-MHz OFDM signal with a 7.8-dB peak-to-average power ratio (PAPR), an average efficiency of 37%/40% and an average power of 37/37 dBm are measured at 3.5/5.0 GHz, and the adjacent channel power ratio (ACPR) is better than −49 dBc after digital predistortion (DPD). To the best of our knowledge, the proposed DPA is the first demonstration of fully integrated dual-band GaN DPAs.

Digital predistortion (DPD) is known as an efficient solution to suppress nonlinear distortion of power amplifiers (PAs) in wireless transmitters to achieve both high linearity and power efficiency. In multiple input and multiple output (MIMO) applications, the power gains of multiple transmitting channels are usually different. Meanwhile, the channel crosstalk cannot be ignored, given the isolations between the physical channels are limited. Accordingly, the linearization performances of the conventional DPD methods are usually degraded. In this article, we propose a novel DPD method for MIMO transmitters, where both the crosstalk and different power gains of multiple channels are considered. In this method, the expected predistorted signals, which would be used to train the DPD model, are firstly obtained by an iterative algorithm derived from the contraction mapping theorem. Then, the coefficients of a MIMO-DPD model are extracted by fitting the DPD model to the obtained predistorted signal. Experimental results verify that the extracted coefficients can achieve an improved linearization performance for the MIMO transmitter with crosstalk between the channels, especially in the case where the channel gains are different. Specifically, for a $2 \times 2$ MIMO transmitter with -20 dB crosstalk and 1.2 dB difference between the two channel gains, the proposed DPD improves the adjacent channel power ratio (ACPR) from -31.4 dBc to -47.1 dBc, which outperforms the conventional DPD by 6.8 dB.

This paper investigates the issue of high complexity of digital predistortion for the nonlinear distortion introduced due to crosstalk in addition to the nonlinear distortion generated by a power amplifier (PA) in 2 × 2 MIMO transmitters. This paper proposes a less complex, novel model for digital predistortion (DPD) for linearizing multiple-input multiple-output (MIMO) transmitters. This model is constructed by taking account of both linear and nonlinear crosstalks, along with the fact that the nonlinearity generated due to crosstalk would be lesser than the nonlinearity already available due to PA. For proof-of-concept, it is shown for two different scenarios that proposed model provides similar linearization performance as compared to 2 × 2 PH model with less number of coefficients.

An adaptive digital predistortion (PD) technique is proposed for linearization of power amplifiers (PAs) in multiple-input multiple-output (MIMO) transmitters. Instead of constructing a separate feedback path at each PA, we propose a PD technique based on a combined feedback, which adds all the PAs' outputs to form a single feedback. We develop an algorithm for finding the multiple PA characteristics jointly from the combined feedback. The PD parameters are calculated from the identified PA characteristics. Although the proposed method requires more computations for PA identification than the conventional method, its hardware implementation is much simpler and can overcome the crosstalk problem of a radio frequency (RF) selector, which is necessary for multiple feedback selection. The simulation results show that, despite its simple structure, the linearization performance of the proposed method is almost the same as the separate multiple feedback method.

In massive multiple-input–multiple-output (MIMO) systems, circuit differences among power amplifiers (PAs) lead to poor nonlinear behavior consistency. This makes it difficult to linearize multiple PAs with shared digital predistortion (DPD). Pre-tuning circuit differences and improving circuit performance with low-power, low-cost analog devices is a desirable method. This article proposes a technology for the co-optimization of nonlinear behavior consistency and efficiency/linearity of PAs. This technology uses the passive adjustable mismatch network to fine-tune the load impedance of PAs. Two strategies are proposed for configuring network parameters. Their contribution is to achieve co-optimization, as well as fast selection of network parameters. In the theoretical part, a numerical model is presented to demonstrate the mechanism of tuning circuit differences. Based on simulation and test, the relationship between nonlinear behavior consistency, efficiency, and linearity, as well as the impact of circuit differences on them, is pointed out. At the same time, an efficient co-optimization method is proposed. The feasibility of this method is verified by numerical simulation and test of nine Doherty PAs from the same batch. In the test, when the shared DPD signal excites multiple PAs, the maximum improvement in adjacent channel power ratio (ACPR) is 7.73 dB; when the original signal excites multiple PAs, the maximum improvement in efficiency and normalized mean square error (NMSE) is 7.25% and 4.12 dB, respectively. In addition, this technology shows excellent adaptability in the dynamic carrier frequency range of 2340–2480 MHz.

In this work, a Doherty power amplifier is presented for IMT-Advanced system at 450–470MHz frequency band. An imperfect Quarter-wave transmission line, which consisted of two shunt capacitors and a series microstrip line, is adopted to replace the bulky impedance inverter quarter-wave transmission line for designing the Doherty power amplifier (PA). Both the traditional and proposed structures for the Doherty power amplifiers were implemented. Comparing with the traditional Doherty amplifier, the size of the circuit and the manufacturing costs are decreased obviously. The measurement results of the proposed Doherty PA exhibit good performance as well as the traditional Doherty power amplifier. For a input signal with a 7.8dB Peak to Average Power Ratio (PAPR), the measured drain efficiency of the proposed Doherty power amplifier is 48.5% at an average output power of 43.0 dBm, which is a 7.8dB backed — off the peak output power. Moreover, in order to improve the linearity of the proposed Doherty power amplifier, the augmented Hammerstein and polynomial predistorter are used to linearize the proposed Doherty power amplifier. After digital predistortion, the adjacent channel leakage ratio (ACLR) reaches −45.5dBc at an average output power of 43.0 dBm, which can satisfy the requirements of the IMT-Advanced systems.

Nonlinearity problems for power amplifiers are well known and many linearization techniques have been proposed in the literature, especially digital baseband predistortion. Orthogonal Frequency Division Multiplexing (OFDM) has a very high Peak-to-Average Power Ratio (PAPR), making it very sensitive to non linearities. We have chosen the Active Constellation Extension (ACE) to reduce the crest factor, which is not sufficient. In this paper, we show that the association of ACE with digital predistortion helps to improve the Adjacent Channel Power Ratio (ACPR) and so permits a good linearity improvement. The association is first validated in simulation. In measurements, for 42.5dBm RMS output power, which represents the nominal power, the ACPR is increased from 32dBc to 49dBc. For a given ACPR of 35dBc, the RMS output power is increased from 40.7dBm to 42.7dBm.

Multiple Input Multiple Output (MIMO) systems with limited Radio Frequency (RF) chains play a pivotal role in the Fifth Generation (5G) of wireless networks. However, the transmitter of Generalized Spatial Modulation MIMO (GSM-MIMO) systems that characterized by a single RF chain and multiple active antennas is capable of reducing both the energy consumption and the transmitter cost. In this paper, combining GSM-MIMO systems with the fully digital Geometric Mean Decomposition (GMD)-based precoding scheme and Analog Beamforming (ABF) into Hybrid Beamforming (HBF) regime is presented for Millimeter-wave (mmWave) massive MIMO systems which is a key enabler for 5G networks. In addition, applying the norm-based user selection algorithm in GSM-MIMO scheme with GMD-based hybrid precoding is proposed, and referred to as Multiuser Steered GSM-MIMO (MUS-GSM-MIMO) scheme. Simulation results demonstrate that the proposed schemes are capable of achieving considerable performance gains over the conventional GSM-MIMO schemes, while avoiding the overwhelming costs of multiple RF chains. Therefore, the proposed schemes are very efficient and highly suitable for large-scale and 5G wireless networks.

We have demonstrated a frequency multiplier-based multiplying transmitter (MT) architecture, which shows the transmission of wideband modulated signals from the lower frequency band to the millimeter-wave (mm-wave) through the frequency multiplier directly is a viable option. To compensate for the severe nonlinear distortions exhibited by the frequency multiplier in a more efficient way, a two-stage cascaded digital predistortion (DPD) scheme with constrained feedback bandwidth is proposed. The complexity-reduced generalized memory polynomial (GMP) and $N$ th-order power/root module are used for nonlinear behavioral modeling. Furthermore, the spectral extrapolation and band-divided (BD) modeling methods are employed to achieve more accurate linearization performance of the frequency multiplier over the limited feedback bandwidth. The MT experimental system with an active frequency quadrupler at the Q-band was set up to evaluate the linearization capacity of the proposed BD $N$ th-order cascaded complexity-reduced GMP (BD-N-CGMP) DPD scheme. In the test for 24-MHz bandwidth 16 quadrature amplitude modulation (QAM) signal, the frequency quadrupler’s linearized output at 48.6 GHz with error vector magnitude (EVM) of 1.94% and adjacent channel power ratio (ACPR) of −47.2 dBc could be achieved, even when the feedback bandwidth was limited from 480 to 78 MHz. This demonstrated system significantly decreased the requirement of the digital-to-analog converters (DACs) and analog-to-digital converters (ADCs) sampling rate, and showed promising performances for future MT architecture at mm-wave and sub-terahertz (sub-THz) frequencies with large signal bandwidth.

Digital predistortion (DPD) is an important technique that is commonly used in wireless transmitters to reduce the out-of-band emissions caused by power amplifier (PA) nonlinearities. This frequency-domain goal is generally achieved by learning a baseband equivalent time-domain inverse transfer function of the PA and applying it to the transmitted digital baseband signal. In this work, we take advantage of the frequency-domain nature of orthogonal frequency-division multiplexing (OFDM) signals by injecting cancellation tones into the guard-band subcarriers to perform DPD. We experimentally evaluate our OFDM-based DPD (ODPD) method using a Doherty PA and show that, when compared to a standard generalized memory polynomial solution, the ODPD can achieve better suppression of out-of-band emissions with lower complexity. Moreover, when combined with a neural network model of the PA, our proposed ODPD method only requires oversampling the transmitted signal by a factor of two, which has important implications for the analog transceiver front-end.

Multiple-input multiple-output (MIMO) signaling is one of the key technologies of current mobile communication systems. However, the complex and expensive radio frequency (RF) chains have always limited the increase of MIMO scale. In this paper, we propose a MIMO transmission architecture based on a dual-polarized reconfigurable intelligent surface (RIS), which can directly achieve modulation and transmission of multichannel signals without the need for conventional RF chains. Compared with previous works, the proposed architecture can improve the integration of RIS-based transmission systems. A prototype of the dual-polarized RIS-based MIMO transmission system is built and the experimental results confirm the feasibility of the proposed architecture. The dual-polarized RIS-based MIMO transmission architecture provides a promising solution for realizing low-cost ultra-massive MIMO towards future networks.

A new load modulation combiner (LMC) having the capability of back-off control is proposed for a Doherty power amplifier (DPA). The proposed LMC consists of two transmission lines tied at the combining node, and a shunt element across the other ends of the transmission lines. The shunt element of the LMC becomes virtually open-circuited at the peak power level and allows the carrier amplifier to have an arbitrary load impedance at the low power level. The operating principle and circuit structure of the DPA based on the proposed LMC are presented in detail. The proposed DPA was implemented using GaN-HEMT for the 3.5 - 4.0 GHz band. Using a 5G NR signal with a peak-to-average power ratio (PAPR) of 7.9 dB and signal bandwidth of 100 MHz, the implemented DPA showed drain efficiency (DE) of 49.6 - 54.4% and power gain of 11.1 - 13.0 dB at an average output power of 34.5 dBm. An adjacent channel leakage power ratio (ACLR) was obtained as −50.4 - −45.0 dBc at the average power level with a digital pre-distortion (DPD) technique.

In this paper, a novel modified Doherty power amplifier (DPA) that allows for extended output back-off (OBO) range, improved power utilization factor (PUF), and increased peak power over a broad bandwidth is proposed. It starts with an in-depth analysis of the modified DPA circuitry that first revealed the ability to control the impedances seen by the main transistor versus frequency by properly setting the magnitude and phase of the ratio between the peaks of the combining currents. This is then exploited to maximize the peak power and enhance the achievable bandwidth. Furthermore, a complex-to-real output matching network is incorporated in the auxiliary path and its parameters are carefully chosen to produce proper load modulation while satisfying the required peak combining current ratio. For validation purpose, a DPA prototype is designed to operate over the frequency range of 1.35-2.05 GHz with an OBO of 9 dB. Under the continuous-wave excitation, the DPA prototype maintained the drain efficiency (DE) of 52%-55% and 65%-75% at 9-dB OBO and peak power, respectively, over the entire targeted band. In addition, the measured peak power and PUF were about 42 dBm and higher than 0.9, respectively. The linearizability of the DPA prototype using digital predistortion (DPD) with memory was assessed while being driven with a 40-MHz carrier-aggregated signal with a peak-to-average power ratio of 8.9 dB and at an average output power of 33 dBm. When the carrier frequency is swept over the entire band, the measurement revealed an adjacent channel leakage ratio of better than -46.5 dBc after DPD with an average DE of 49.3%-53%.

This article first analyzes the nonlinear behavior of frequency-multiplier (FM)-based radio frequency (RF) beamforming transmitters TXs) over a wide range of steering angles. Then, based on this analysis, a digital predistortion (DPD) scheme is proposed to linearize FM-based TX arrays. Specifically, a dual-input behavioral model of FM-based TX arrays is derived. This dual-input model is then transformed into an angle-dependent single-input–single-output (SISO) model. Next, the effect of load modulation on FM nonlinearity is studied using a circuit-inspired behavioral model, and a very distinctive feature is revealed. Notably, it is shown that FM-based RF beamforming arrays have reduced sensitivity to load modulation when compared to last-stage power-amplifier (PA)-based arrays. This is first validated in simulation using a frequency doubler and PA terminated with load impedances extracted from an 8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 8 electromagnetically simulated array. Furthermore, experimental results are obtained 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">$\times $ </tex-math></inline-formula> 2 planar frequency-doubler-based RF beamforming TX operated at 28 GHz and driven with up to 400-MHz-wide orthogonal frequency-division multiplexing (OFDM) signals where the subcarriers are modulated using up to 256-QAM. It is experimentally demonstrated that a single set of SISO DPD coefficients can significantly improve the adjacent channel power ratio (ACPR) and error vector magnitude (EVM) over a wide steering range. Specifically, for the case of a 100-MHz signal, the ACPR and EVM improved from 20 dBc and 23% to, at worst, 48 dBc and 1%, respectively, when the array is steered from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-40^\circ $ </tex-math></inline-formula> to 40° and measured along the main beam direction. In light of the FM-based arrays reduced sensitivity to load modulation, this article also investigates different TX observation receiver (TOR) methods for DPD function training, namely, far-field-based TOR, coupler-based TOR, and single-element-based TOR, and their impacts on the linearizability of the FM-based RF beamforming TXs.