Predistortion Scheme Research Articles

Novel Baseband Equivalent Model for Digital Predistortion of Wideband Frequency-Multiplier-Based Millimeter Wave Sources

This article proposes a novel digital predistortion (DPD) scheme for mitigating the effects of nonlinear distortion caused by frequency multipliers when driven with wideband vector modulated signals. An effective pruning strategy is applied to limit the number of coefficients that consequently reduces the complexity of the DPD scheme while maintaining excellent linearization capacity. Extensive tests are conducted using different millimeter-wave frequency multipliers for proof-of-concept validation. These multipliers include two frequency doublers (25 and 28 GHz), a tripler (63 GHz), and a quadrupler (25 GHz), each driven by vector-modulated signals with an instantaneous bandwidth of up to 400 MHz. Using 44 coefficients or less, the proposed DPD approach allows for excellent frequency-doubler distortion cancellation and improves the error vector magnitude (EVM) and adjacent channel power ratio (ACPR) from about 21% and 21 dB to 1.8%-0.8% and 47-54 dB for the different test signals. Similarly, in the case of the frequency tripler, the EVM and ACPR improve from 8% and 30 dB to less than 1.7% and 45-51 dB after applying the proposed DPD scheme with 57 coefficients under different test signals. Finally, in the case of the frequency quadrupler (composed of two cascaded frequency doublers), the proposed DPD scheme achieves the EVM and ACPR of 0.9%-1.6% and 53-45 dB, respectively, with a higher number of coefficients.

MIMO SWIPT Systems With Power Amplifier Nonlinearities and Memory Effects

In this letter, we study the impact of nonlinear high power amplifier (HPA) on simultaneous wireless information and power transfer (SWIPT), for a point-to-point multiple-input multiple-output communication system. We derive the rate-energy (RE) region by taking into account the HPA nonlinearities and its associated memory effects. We show that HPA significantly degrades the achievable RE region, and a predistortion technique is investigated for compensation. The performance of the proposed predistortion scheme is evaluated in terms of RE region enhancement. Numerical results demonstrate that approximately 24% improvement is obtained for both power-splitting and time-splitting SWIPT architectures.

Frequency Multiplier-Based Millimeter-Wave Vector Signal Transmitter Using Digital Predistortion With Constrained Feedback Bandwidth

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.

Artificial Neural-Network-Based Pre-Distortion for High Loss-Budget 60-km Long-Reach Passive Optical Network

High launch optical power can compensate for severe fading and power loss in long-reach passive optical networks (LR-PONs); however, it also aggravates nonlinear degradation, which necessitates the use of complex DSP-based nonlinear compensation techniques at optical network users (ONUs). DSP-related techniques also necessitate the use of additional hardware/software components by the receiver, which can greatly increase implementation costs and energy consumption, particularly when dealing with large-scale ONU deployment. This is the first study to propose artificial neural network (ANN)-based pre-distortion to eliminate the need for complex DSP at ONUs in a high-launch-power LR-PON, thereby permitting the use of a simplified architecture at the user end. In the first phase of the study, the proposed ANN-based pre-distortion scheme was implemented in a single-channel IMDD OFDM LR-PON, which achieved a data rate of >55 Gbps over 60-km transmission with a loss budget of 30 dB without the need for optical inline- or pre-amplification. In the second phase of experiments, the same scheme was applied to a 4-channel wavelength division multiplexing (WDM) OFDM LR-PON. Here, the proposed scheme achieved data rates of >200 Gbps using launch power of 18 dBm per lane, resulting in a loss budget of roughly 29 dB over 60-km single mode fiber transmission.

Nonlinear predistortion scheme based on Gaussian kernel-aided deep neural networks channel estimator for visible light communication system

A scheme for Gaussian kernel-aided deep neural networks nonlinear predistortion (GK-DNNPD), which could effectively reduce the computational complexity of the receivers, is experimentally demonstrated. Compared with lookup table (LUT) PD, the GK-DNNPD could increase the Q -factor of 8 pulse amplitude modulation visible light communication (VLC) system by 1.56 dB at 1.335 Gbps. We experimentally proved that GK-DNNPD could increase the bitrate under hard-decision forward error correction from 1.335 to 1.385 Gbps. This is the first time that GK-DNNs are utilized for PD in the field of VLC systems. Meanwhile, GK-DNNPD requires less data for training than LUT, and the space complexity of the model is lower than LUT as well, which provides GK-DNNPD with the potential to be applied in practical VLC systems.

A robust multi‐sampling rate digital predistortion for ultra‐broadband power amplifiers

AbstractIn this letter, a robust multisampling rate digital predistortion (DPD) scheme is proposed to linearize ultra‐broadband power amplifiers (PAs) under the limited DAC sampling rate. In the ultra‐broadband scenarios, it is no longer affordable to employ DACs with 3‐5 times over‐sampling rate, which will inevitably lead to spectrum aliasing for the sake of high order basis function and compromise the DPD performance. In order to improve the modeling accuracy, the input signal with low sampling rate will be up‐sampled firstly to generate nonaliasing basis functions. Then, after filtering and down‐sampling operation, the coefficients are extracted with low sampling rate. Moreover, the conventional behavioral model is modified with the linear‐spline basis functions to improve the stability and complexity. Measurements on Doherty PA and class‐AB PA with 100 MHz, 200 MHz, and 500 MHz bandwidth input signal at 2.1 GHz carrier frequency show the EVM can be improved to 0.71%, 0.94%, and 1.86% respectively.

Analysis and Compensation of Nonidealities in Frequency Multiplier-Based High-Frequency Vector Signal Generators

This paper expounds on the application of frequency multipliers for the frequency upconversion of amplitude- and vector-modulated signals to high frequencies. First, a complex baseband equivalent Volterra series behavioral model is derived for representing the envelope transformation of frequency multipliers. Based on this model, a low-complexity baseband digital predistortion (DPD) scheme is developed to mitigate unwanted distortions exhibited by the frequency multipliers. The proposed DPD scheme consists of three modules: 1) a polynomial predistorter (PD) is first employed to mitigate the memoryless (ML) distortions associated with the frequency conversion; 2) a $D$ th order root module is included to correct for the phase modulation (PM)–PM distortions exhibited by the frequency multiplier with a frequency multiplication factor of $D$ ; and 3) a pruned Volterra-based PD is added to correct for residual distortions attributed to the output stage. As a proof of concept using a low-complexity experimental setup, the proposed DPD scheme is used to linearize several frequency multipliers, namely, a frequency doubler with an output at 12.5 GHz, a frequency quadrupler to produce an output at 25 GHz, and a frequency tripler with an output at 63 GHz. The proposed DPD scheme showed excellent capacity to linearize these frequency multipliers when driven by orthogonal frequency division multiplexing signals with modulation bandwidths up to 400 MHz with less than 56 coefficients.

A 16-bit 12-GS/s Single-/Dual-Rate DAC With a Successive Bandpass Delta-Sigma Modulator Achieving <−67-dBc IM3 Within DC to 6-GHz Tunable Passbands

This paper presents a hybrid digital-to-analog converter (DAC) architecture with a tunable bandpass delta-sigma modulator (DSM) to synthesize channelized signals over a wide frequency range with high linearity and a low in-band noise floor. Due to the mostly digital architecture, this DAC topology favors technology scaling. The DSM uses a successive pipeline structure with time-interleaving techniques to achieve a 12-GS/s data rate and overall 16-bit DAC resolution. To improve the linearity at high frequencies, an inverse-sinc-shaped digital pre-distortion (DPD) scheme is used to better approximate and compensate the timing errors. The DAC can be configured for single- or dual-rate operation modes to tradeoff different input data rates with DAC linearity and spectral images. The prototype was fabricated in 65-nm CMOS technology with an analog area of 0.1 mm2 and 60 dBc up to a 4.2-GHz signal frequency at 12 GS/s in the single-rate mode thanks to the hybrid structure and proposed DPD techniques.

Exploiting low complex digital predistortion to improve wideband nonlinear satellite downlinks

SummaryWith the increasing demand for higher data rate, increasing throughput requires wider bandwidth. Due to the nonlinear effect of power amplifier, severe nonlinear distortion effects will appear onboard satellite. Digital predistortion (DPD) scheme is usually employed to compensate for the distortions and memory effects introduced by traveling wave tube amplifier (TWTA) and output multiplexing filter (OMUX). In conventional predistorter, the signal loss of band‐limited feedback signal output through OMUX is usually ignored. Actually, it will affect the linear effect of TWTA. In order to solve the problem as to improve wideband nonlinear satellite downlink, this paper introduces a novel spectral extrapolation method based on deep neural network to recover the band‐limited feedback signal. On this basis, an advanced orthogonal matching pursuit algorithm is adopted in the nonlinear TWTA model construction to further reduce the DPD complexity. The proposed setup effectively compensates the distortions and is well suited for systems that generate data bits on satellites.

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.

Integration of a GFDM-Based 5G Transceiver in a GPON Using Radio Over Fiber Technology

This paper reports the integration and experimental performance analysis of a GFDM-based 5G transceiver in a gigabit passive optical network (GPON), using radio over fiber technology. The proposed architecture enables to simultaneously transport two 5G candidates RF signals through an active GPON under real channel conditions. One signal is generated by a GFDMbased 5G prototype transceiver at 735 MHz, whereas the second one is synthetized by a vector signal generator at 26 GHz. A dualdrive Mach-Zehnder modulator has been utilized in the optical line terminal to modulate both signals, with the purpose of mitigating the interference between them. Particularly for the GFDM-based 735 MHz signal, a modulation error ratio (MER) of 40 dB has been obtained at RF-driven signal up to -9 dBm. Furthermore, the use of a digital predistortion scheme has been efficiently employed to reduce the impact of the nonlinear distortions and enhance MER. The 26-GHz RF signal, aimed for the 5G millimeter wave band, has been investigated as a function of error vector magnitude (EVM) for bitrates up to 1 Gbit/s. EVMRMS of 2.18% and 5.70% have been obtained for 100 Mbit/s and 1 Gbit/s, respectively. Finally, the latency and throughput measurements of the baseband signal originally running over GPON have shown no significant penalties.

Pre-Distortion Scheme to Enhance the Transmission Performance of Organic Photo-Detector (OPD) Based Visible Light Communication (VLC)

Organic photo-detector (OPD) is lightweight and physically flexible. It can transform plastic or glass into smart surfaces. Visible light communication (VLC) is considered as one of the promising technologies for the future wireless communications. VLC combines the advantages of lighting and communication simultaneously. Besides, it can be deployed using existing lighting infrastructure; hence little extra cost is needed to provide this wireless communication. As the OPD has a large optical detection area, it would be interesting to employ the OPD as VLC receiver (Rx). If the OPD can be used in the VLC systems, it can benefit the Internet-of-Thing (IoT) networks by providing efficient human-to-machine and machine-to-machine communications. However, implementing VLC using OPD Rx is challenging. Due to the slow charging and discharging processes of the OPD, the optical-to-electrical response speed is limited. Besides, the OPD requires external bias to be operated. Here, we propose and demonstrate using pre-distortion scheme for pulse-amplitude-modulation (PAM) signal to a self-powered PCBM:P3HT OPD to enhance the VLC transmission performance. The operation of the self-powered PCBM:P3HT OPD is discussed. Both experiments using PAM-2 and PAM-4 (with and without pre-distortion) are evaluated. The performance, requirement and capability limitations of the pre-distortion schemes are also analyzed and discussed by using numerical simulations.

A Novel Generalized Parallel Two-Box Structure for Behavior Modeling and Digital Predistortion of RF Power Amplifiers at LTE Applications

This paper presents a generalized parallel two-box structure that is proposed for modeling and digital predistortion of power amplifiers and wireless transmitters exhibiting memory effects. The proposed predistortion scheme consists of two separable boxes; the first is utilized to model the static behavior of the power amplifier, while the second is proposed to consider the memory effect and nonlinear distortion of the power amplifier. The coefficients of the proposed model are identified by applying an indirect learning structure and a least square method. The validation of the proposed model is carried out using the simulation of the power amplifier and the digital predistortion excited by a 64QAM signal in the advanced design system software. According to the simulation results, the criterion of adjacent channel power ratio reduced by about 16 dB. The simulation results reveal an adjacent channel power ratio of almost − 48 dB. Indeed, the proposed model leads to a better performance in terms of spectral regrowth in comparison with the memory polynomial model, and it also reduces the number of coefficients by approximately 22%. This proposed model enables a more accurate modeling of nonlinear distortion and memory effects compared to previous linearization methods.

Wideband Over-the-Air RF Self-Interference Cancellation by an EML-Based Optical System With Baseband Predistortion

In this paper, we propose and experimentally demonstrate an optical self-interference cancellation (OSIC) system with baseband predistortion for wideband over-the-air in-band full-duplex (IBFD) communication. The key factors of OSIC bandwidth limitation are analyzed, and the solution toward it is proposed for the first time. Experimental results show that, by traditional OSIC under over-the-air condition, only narrow even zero band is with enough cancellation depth to satisfy IBFD. But with our proposed baseband predistortion scheme, deep OSIC depth is achieved over the whole 1 GHz/5 GHz baseband within the total 7–25 GHz available band of horn antennas. The employment of baseband predistortion obviously broadens the cancellation bandwidth of an EML-based OSIC system. The largest OSIC bandwidth for over-the-air IBFD communication is demonstrated so far.

Predistortion for Wideband Nonlinear Satellite Downlinks

In this letter, an efficient predistortion scheme is proposed to compensate imperfections in a satellite channel due to the nonlinear power amplifier and the output channel filter. The predistorter is a Wiener system consisting of the linear and nonlinear components, which are trained independently. The predistorter is well suited to systems where the data bits are generated onboard the satellite. The proposed setup effectively compensates the distortions and has lower complexity compared with other strategies.

Digital Pre-Distortion of Carrier Frequency Offset for Reliable Wi-Fi Enabled IoTs

The Internet of Things (IoTs) will change the requirements for wireless connectivity significantly, mainly with regard to service coverage, data rate, and energy efficiency. Therefore, to improve robustness and reliability, WiFi-enabled IoT devices have been developed to use narrowband communication. However, narrowband transmission in WiFi such as IEEE 802.11ah causes relatively higher frequency error due to the reduced subcarrier space, which is larger than legacy wireless local area networks (WLANs) in 2.4/5 GHz frequencies. In a direct conversion receiver, this error degrades the signal quality due to the presence of direct current (DC) offset cancellation circuits. In this paper, a digital carrier frequency offset (CFO) predistortion scheme is proposed for a reliable communication link in dense networks. Evaluation results demonstrate that the proposed scheme can improve received signal quality in terms of packet error rate and error vector magnitude.

Robustness analysis of a parallel two-box digital polynomial predistorter for an SOA-based CO-OFDM system

The linearization performance of various digital baseband pre-distortion schemes is evaluated in this paper for a coherent optical OFDM (CO-OFDM) transmitter employing a semiconductor optical amplifier (SOA). In particular, the benefits of using a parallel two-box (PTB) behavioral model, combining a static nonlinear function with a memory polynomial (MP) model, is investigated for mitigating the system nonlinearities and compared to the memoryless and MP models. Moreover, the robustness of the predistorters under different operating conditions and system uncertainties is assessed based on a precise SOA physical model. The PTB scheme proves to be the most effective linearization technique for the considered setup, with an excellent performance-complexity tradeoff over a wide range of conditions.

Sub-Band Pre-Distortion for PAPR Reduction in Spectral Efficient 5G Mobile Fronthaul

We propose a pre-distortion scheme in transmitter DSP to reduce the peak-to-average power ratio (PAPR) in multi-intermediate-frequency (IF)-over-fiber mobile fronthaul (MFH). In optical spectral efficient MFH, multiple modulated sub-bands are transmitted with different IFs serving multi-in multi-out and carrier aggregation in 5G. By applying phase pre-distortion on sub-bands in the transmitter DSP, we reduce the PAPR by more than 2.2 dB with 1% complementary cumulative distribution function value in MFH for 5G mobile networks and significantly increase the transmitted signal power and quality without modifying the system linearity and resolution. The proposed scheme generates small latency. It is DSP efficient and compatible with current mobile devices, without additional noise. With 32 sub-bands served, we demonstrated a 2-dB sensitivity improvement experimentally by applying the proposed scheme.

A 12-Bit 2 GS/s Dual-Rate Hybrid DAC With Pulse-Error Pre-Distortion and In-Band Noise Cancellation Achieving > 74 dBc SFDR and <−80 dBc IM3 up to 1 GHz in 65 nm CMOS

This paper presents a 12-bit 2 GS/s dual-rate hybrid DAC using bandwidth- and linearity-enhancement techniques. The proposed pulsed-error pre-distortion scheme enhances DAC linearity at both low and high signal frequencies by leveraging the fine time-and-voltage resolution from the oversampling path of the hybrid DAC structure. To further widen the DAC bandwidth, a noise-cancellation scheme is proposed to suppress the quantization noise of the delta-sigma modulator within DC to the 1 GHz band without increasing modulator complexity. The analytical derivations and numerical simulations of the proposed techniques are provided to demonstrate the technique’s effectiveness, as well as its practical design constraints. A proof-of-concept silicon prototype is implemented in 65 nm standard CMOS that achieves SFDR of 74–98 dBc over the DC-1GHz Nyquist band. In a two-tone measurement, the IM3 product is measured from 101 dBc to 80 dBc across the Nyquist band. The SFDR improvement after enabling the proposed pulsed pre-distortion scheme measures from 67.5 to 74.4dBc, while the in-band noise floor demonstrates 6 dB suppression at the Nyquist band edge after enabling the noise-cancellation scheme.

A 41dB Gain Control Range 6th-Order Band-Pass Receiver Front-End Using CMOS Switched FTI

A 41dB gain control range 6 th -order band-pass receiver front-end (RFE) using CMOS switched frequency translated impedance (FTI) is presented in a 40 nm CMOS technology. The RFE consists of a frequency tunable RF band-pass filter (BPF), IQ gm cells, and IQ TIAs. The RF BPF has wide gain control range preserving constant filter Q and pass band flatness due to proposed pre-distortion scheme. Also, the RF filter using CMOS switches in FTI blocks shows low clock leakage to signal nodes, and results in low common mode noise and stable operation. The baseband IQ signals are generated by combining baseband Gm cells which receives 8-phase signal outputs down-converted at last stage of FTIs in the RF BPF. The measured results of the RFE show 36.4 dB gain and 6.3 dB NF at maximum gain mode. The pass-band IIP3 and out-band IIP3@20 MHz offset are -10 dBm and +12.6 dBm at maximum gain mode, and +14 dBm and +20.5 dBm at minimum gain mode, respectively. With a 1.2 V power supply, the current consumption of the overall RFE is 40 mA at 500 MHz carrier frequency.