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High frequency (HF) transmitter implemented with the multioctave broadband power amplifier (PA) enables long-distance communication and has strong survivability. However, it is faced with the problem of harmonics because of the nonlinearity of the PA. We first consider canceling the harmonics using a harmonic cancellation digital predistortion (HC-DPD) technique, instead of lossy and bulky analog filter banks. A uniform complex-valued Hilbert transform-based polynomial (HTBP) function is proposed for the modeling of the harmonics, as well as the fundamental component in the real-valued input and output signals. In order to validate this idea, detailed experiments were performed for carrier frequencies located at 5 and 10 MHz, respectively. After the proposed HC-DPD, the harmonics were all canceled to satisfy the linearization requirements. In particular, the improvement of the second harmonic is more than 40 dB, which is nearly 20 dB better than that of the memoryless polynomial (MLP) model. Furthermore, the experimental results demonstrate that the proposed technique is scalable, in which the model coefficients extracted at a specific power level, signal bandwidth, and carrier frequency can be extended to other scenarios. Both the second and third harmonics were canceled for being below −45 dBc after the HC-DPD. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Directional modulation (DM) implemented at the transmitter side is a physical layer security technology based on the multi-antenna structure. Current DM methods paid little attention to the issue of the nonlinear distortion of power amplifiers (PAs), which may lead to poor amplifier efficiency and worsen the signal’s error vector magnitude (EVM). For the first time, we consider linearization of a DM transmitter using the digital predistortion (DPD) technique. We modify the paralleled DM DPD model to a novel cascaded model based on the characteristics of the DM transmitter. This modified method applies a common DPD to mitigate the nonlinear distortion of all paths and an auxiliary DPD to fine-tune each path, so as to significantly reduce the complexity. In order to validate this idea, two-element uniform linear array (ULA) and four-element ULA experiments were performed, in which 16QAM 20-MHz bandwidth signals with a peak-to-average power ratio (PAPR) of about 8.0 dB were generated for commercial part PAs. Experimental results show that the modified approach has almost the same linearization performance as the paralleled DM DPD technique but reduces the computational complexity significantly. [ABSTRACT FROM AUTHOR]

This paper proposes an error variation ranking (EVR)-based pruning method to reduce the complexity of memory polynomials (MPs) for power amplifier behavioral modeling. During the EVR pruning, the variation of prediction error caused by removing each term is calculated and ranked as a quantification factor to show the term's importance. The dominant terms are then selected based on their ranking positions among all terms. This method is verified by comparing its results with all other possible selections under the same conditions. When it is used to prune digital predistorters, approximately 74% of the terms in the MP model and 78% of the terms in the 2-D digital-predistortion model can be removed with negligible deterioration of the prediction and linearization performance. Moreover, further discussion is presented to strategize the configuration of MP models based on the EVR pruning results. [ABSTRACT FROM AUTHOR]

Cross-band modulation effects in the concurrent dual-band transmitters are quite noticeable for the linearization. This paper proposes a novel two-dimensional modified memory polynomial (2D-MMP) technique to compensate for nonlinear distortion in the concurrent dual-band transmitters. By taking into account the cross-band modulation effects, the 2D-MMP model is developed by properly modifying the envelope terms of the conventional memory polynomial (MP) model. This makes the model complexity of the 2D-MMP model significantly reduced compared to the prior 2D-DPD model in previous work. However, the linearization ability of the 2D-MMP model is still retained. Simulations prove that the 2D-MMP model is quite robust to different PA models. Experimental measurements were carried out for a dual-band class-AB PA exhibiting mild nonlinearity and a highly nonlinear dual-band Doherty PA. The results show that the 2D-MMP model can achieve nearly the same linearization accuracy compared to the 2D-DPD model, with much less coefficients. Less than -52 dBc adjacent channel power ratios (ACPRs) for the dual-band class-AB power amplifiers (PAs) and -50 dBc ACPRs for the dual-band Doherty PA in the dual bands were achieved. [ABSTRACT FROM AUTHOR]

Measurement errors (in-phase/quadrature imbalance, dc offset, and nonlinearity) in the feedback path can adversely affect the linearization performance of digital predistorter (DPD) for RF power amplifiers (PAs). In this paper, a generalized analysis for the Volterra-series DPD system is presented in the presence of feedback measurement errors. It shows that the DPD coefficients are biased due to these errors. A modified least squares (MLS) method is then proposed for DPD coefficients extraction, which can eliminate the detrimental effect of feedback measurement errors without using a post-compensator. The proposed MLS method has the advantage of being free of behavioral modeling for the feedback path or the post-compensator. However, it can still achieve comparable performance as the state-of-the-art. The performance of the MLS method is validated with both simulations and experiments. The measurement results show that, when a nonideal feedback path is employed to capture the PA output, the proposed MLS method can still ensure a high linearization performance of the DPD, and the results are nearly the same as that when an ideal feedback path is used. [ABSTRACT FROM PUBLISHER]