Adjacent Channel Power Ratio Improvement Research Articles

ACPR Improvement in Large Phased Arrays With Complex Modulated Waveforms

Phased array transmit systems use a number of antennas to combine RF power amplifiers (PAs) in free space. If all the RF channels are identical, then the transmit spectrum in the far field, including both linear and nonlinear components, would be a scaled version of the output spectrum of each channel. However, it is found that random variations in the nonlinear components (AM–AM and AM–PM) between the channels improve the nonlinearity of the overall array as the number of elements increases. In this article, detailed measurements are done on an 8- to 256-element phased array and show that the adjacent channel power ratio (ACPR), which is one metric of linearity, improves with the number of elements at a fixed backoff from the 1-dB compression point. Also, for a100-Mbaud 64-QAM signal and a fixed ACPR of −32 dBc, a 256-element phased array can be operated at $P_{1\,\text {dB}} - 2$ dB, while an eight-element phased array needs to be operated at $P_{1\,\text {dB}} - 4$ dB to achieve the same ACPR level. These improvements were found to occur at broadside (no scan) for any scan angle and for different modulated waveforms (64-QAM, 16-QAM, QPSK, and so on). This result has great implications on the overall efficiency of 5G phased arrays since it implies that large phased arrays can be operated at less backoff than the small phased arrays (or single antennas with high-PAs). Thus, in practical implementations and taking the ACPR as the figure of merit, large phased arrays are ~2 dB more efficient than what is predicted by standard system simulations that assume the same nonlinear response for all the phased array channels.

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.

Beam-Oriented Digital Predistortion for 5G Massive MIMO Hybrid Beamforming Transmitters

In this paper, we propose a beam-oriented digital predistortion (BO-DPD) technique for power amplifiers (PAs) in hybrid beamforming massive multiple-input multiple-output (MIMO) transmitters, which can achieve linearization of the transmitted signal in the main beam direction and address the DPD implementation issue in the hybrid beamforming array. In massive MIMO hybrid beamforming transmitters, the conventional DPD to linearize each PA is impractical to implement for that the number of digital chains is less than the number of PAs; however, the BO-DPD can resolve this issue by constructing and linearizing the “virtual” main beam signal rather than single PAs. According to the beamforming weights for array elements, the feedback path combines all PA’s outputs to estimate the main beam signal for DPD processing. In addition, an average estimation method is also presented to broaden the linearized angle range around the main beam direction. A $4 \times 16$ (four subarrays and 16 antennas in each subarray) uniform linear array (ULA) simulation along with a $1 \times 2$ ULA at 2.5 GHz and a $2 \times 2$ ULA at 3.5-GHz experimental tests was performed. The experimental results show that the proposed technique and linearization angle range broadening method achieve up to 15-dB adjacent channel power ratio improvement and 10° linearization angle range in the main beam direction.

Linearization and Imbalance Correction Techniques for Broadband Outphasing Power Amplifiers

This paper presents adaptive phase-only memory digital pre-distortion (DPD) and gain/phase/delay imbalance correction techniques for outphasing power amplifiers (PAs). Least squares adaptation is used for the phase-only DPD and imbalance correction techniques. The performance of these techniques is verified by co-simulations and measurements of an outphasing PA implemented with two identical class-F PAs and a Chireix combiner. The Chireix combiner with compensating stubs added to achieve high efficiency introduces nonlinearity. The proposed adaptive phase-only memory DPD corrects for the nonlinearity due to the Chireix combiner. The proposed gain/phase/delay imbalance correction technique achieves total lower/upper adjacent channel power ratio (ACPR) improvements of 43.8/37.6 dB for 5-MHz and 46.2/38.3 dB for 10-MHz long-term-evolution signals in measurements. The fractional-sample delay imbalance correction reduces distortion at low input amplitudes and demonstrates on the order of a 10-dB ACPR improvement compared to full-sample delay imbalance correction.

Performance of feed‐forward linearizers of power amplifiers in OFDM systems under complex gain errors

SummaryAdaptive feed‐forward (FF) linearization schemes of power amplifiers provide high distortion cancelation and adjacent channel power ratio (ACPR) reduction. However, FF schemes are very sensitive to imbalances in the circuit parameters used to achieve signal cancelation. In this paper, an FF linearization circuit is analyzed using the orthogonalization of the nonlinear model and considering the effects of complex gain errors in the signal and distortion cancelation loops on the overall performance of the FF linearizer. The analysis enables the effective signal‐to‐distortion ratio (SDR) and ACPR to be estimated from FF circuit model. It is shown that complex gain errors in the distortion cancelation loop have a more significant effect on in‐band distortion than out‐of‐band distortion, which means that the design of FF linearizers based on ACPR improvement is not optimal. Copyright © 2015 John Wiley & Sons, Ltd.

Improvement of Memory Effects and ACPR of Power Amplifiers in CDMA Cellular Mobile and OFDM WLAN Transmitters

In this article, improvement of the memory effects and adjacent channel interference introduced by the nonlinearity of the power amplifier in the both Code division multiple access (CDMA) and Orthogonal frequency division multiplexing based (OFDM) transmitters is evaluated through Feedforward linearization method. For CDMA system, more than 17.27 dB increasing in OIP3 is and at least 25 dB improvement in adjacent channel power ratio (ACPR) is obtained. For OFDM system, achieved improvement through Feedforward linearization method is at least 39.03 dB in third output intercept point (OIP3 ), more than 26.6 dB in ACPR and 5.44 dB in peak-to- average ratio. It is observed that the designed Feedforward circuit can efficiently improve the nonlinearity caused by memory effects and saturation of the power amplifiers of CDMA cellular mobile telephony and OFDM wireless local area network (WLAN) transmitters.

A Time Misalignment Tolerant 2D-Memory Polynomials Predistorter for Concurrent Dual-Band Power Amplifiers

This letter proposes a modified 2-D memory polynomial predistorter for concurrent dual band power amplifiers, which is tolerant to the time misalignment between the two bands. The new model is derived from the prior 2-D DPD model by substituting the interpolation expression into it. For performance validation, the proposed method is applied to a dual-band transmitter. In the presence of 4.87 samples time-misalignment existing between the two input signals, the proposed method achieves normalized mean square error of -40 dB and adjacent channel power ratio improvement of about 15 dB in the dual bands. Further experiment shows the proposed 2-D predistorter outperforms 2D-GMP in terms of complexity and signal quality.

Joint mitigation of nonlinearity and modulator imperfections in dual‐band concurrent transmitter using neural networks

Reported, for the first time is a predistortion scheme for a concurrent dual-band transmitter with a dual-band concurrent Doherty power amplifier in the presence of modulator I/Q imbalance and DC offsets. Inverse PA modelling for predistortion is achieved using real valued time delayed neural networks. The model has been validated for a 10 W GaN based dual-band concurrent Doherty power amplifier for wideband code division multiple access signals at 1960 and 3500 MHz, respectively, and an adjacent channel power ratio improvement of 18 dB is reported.

Digital techniques to mitigate feedback impairment and noise of look-up table digital predistortion

In this article, some new effects of feedback impairment and noise on look-up table (LUT) digital predistortion (DPD) are presented. Several digital techniques are proposed to mitigate these effects. A smoothing filter (SMF) for LUTs is used to eliminate fluctuations of the transfer function of the overall DPD amplification system. By combining SMF method with iterative average (IA) method, the LUTs iterative process becomes stable and well converged. A postcompensator is established according to the proposed two-box model for feedback impairment. For the demonstration, both simulation and experiments are carried out based on a Hammerstein-type PA. Simulation results give some preliminary cognition of these new effects and the effectiveness of proposed techniques. Experimental tests are performed on an S-band amplifier excited with single-carrier WCDMA signal. The adjacent channel power ratio (ACPR) at 5-MHz offset is −48 dBc after DPD with SMF method and postcompensator. A total of 8 dB extra improvement of ACPR is obtained compared with that with neither SMF nor postcompensator. The result clearly shows that the proposed digital techniques are qualified for LUT DPD system, especially when it suffers significant feedback impairment and noise. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.

Power amplifier characterization: an active load-pull system based on six-port reflectometer using complex modulated carrier

An original measurement system for nonlinear RF power-transistor characterization is presented. This new setup enables measurement and optimization of output power, power-added efficiency (PAE), or linearity using active fundamental tuning and six-port reflectometers as vector network analyzers. High- and low-frequency bias-tees are inserted at both ports of transistors in order to control source and load impedances at the baseband (envelope) frequency. Experimental results at 1.575 GHz show an adjacent channel power ratio improvement of 20 dB for a commercial GaAs MESFET power transistor operating in class AB. Moreover, the output power and PAE are increased by 1 dB and ten points, respectively

ACPR Improvement Limitations of Predistortion Linearizer for Nonlinear RF Power Amplifiers

This paper investigates limitations of adjacent channel power ratio (ACPR) improvement in predistortion (pre-D) linearizer used with nonlinear RF power amplifiers (PAs) when the PA model is not perfectly acquired in pre-D design. The error between the physical PA and the nonlinear model is expanded by pre-D function and its power spectral density (PSD) works as limitations in ACPR improvement of the pre-D linearizer. An analytical estimation of ACPR limitations in RF PAs driven by digitally modulated input signal is derived using a formulation of autocorrelation function. The analysis technique is validated with the example of the memory polynomial PA model with the quasi-memoryless pre-D linearizer. The technique is also verified by comparing predicted ACPR limitation with measured limitation for a RF PA with 802.1 Ig input signal.

Adaptive methods to preserve power amplifier linearity under antenna mismatch conditions

Under antenna mismatch conditions at high output power, voltage clipping (due to collector voltage saturation) is the main cause of power amplifier linearity degradation. To preserve linearity under mismatch three adaptive methods are presented that make use of the detected minimum collector peak voltage. This detected signal controls either the amplifier output power, load-line, or supply voltage. These concepts are generalized analytically, and calculated results compare well to simulations. Measurements demonstrate am error vector magnitude reduction of 5% and an adjacent channel power ratio improvement of 10 dB at a voltage standing wave ratio of 4 for an EDGE amplifier with adaptively controlled output power. These adaptive methods offer a cost and size effective alternative to the use of an isolator.