Wensheng Pan

Simplified Approach to Optimized Iterative Clipping and Filtering for PAPR Reduction of OFDM Signals

Iterative clipping and filtering (ICF) is a well-known technique to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. Recently, Wang and Luo investigated the clipped signal and proposed a modified algorithm called optimized ICF (OICF). This is an optimal algorithm since it can achieve the required PAPR reduction with minimum in-band distortion and far fewer iterations. However, OICF needs to solve a convex optimization problem with O(N3) complexity, where N represents the number of subcarriers. In this paper, instead of analyzing the clipped signal, we study the clipping noise and propose a simplified OICF algorithm. In the new algorithm, solving the convex optimization problem is approximated by some simple algebraic operations and the computational complexity reduces to O(N). Simulation results show that after three iterations, the original OICF algorithm can achieve the desired PAPR while the simplified one exhibits almost the same performance: for a 128-subcarrier and quadrature phase shift keying (QPSK) modulated OFDM system, the PAPR-reduction performance difference between the two algorithms are 5×10-3dB at a 10-4 clipping probability and the bit-error-rate performance difference is 6×10-3 dB at a 10-7 error probability.

A General Digital Predistortion Architecture Using Constrained Feedback Bandwidth for Wideband Power Amplifiers

Digital predistortion (DPD) is one of the most effective techniques to mitigate the distortions caused by power amplifier (PA) nonlinearity and memory effects. As the input signal bandwidth increases, the required bandwidth on the DPD feedback channel becomes even larger, i.e., normally five times the signal bandwidth. However, the DPD feedback bandwidth is often restricted by the nonideal electronic components, e.g., the anti-aliasing filter and associated circuits, which therefore introduce bandwidth mismatch between the PA model basis functions and the feedback signal, and thus degrade the linearization performances of the DPD. This paper presents a general DPD architecture for wideband PA systems with constrained feedback bandwidth. By using linear operations to cancel the bandwidth mismatch between the proposed model and the PA feedback signal, the full-band PA model parameters can be estimated with bandwidth-limited observations. This estimated PA model is subsequently used with the PA input signal to extract the DPD function by applying the direct learning algorithms. The proposed DPD architecture reduces the feedback bandwidth to less than two times that of the input signal, while it maintains its linearization performance, as in the full-band case. Experiments are performed on the 20- and 100-MHz long-term evolution advanced signals to demonstrate the effectiveness of the proposed PA behavior modeling and DPD linearization performances with limited feedback bandwidth.

A New Digital Predistortion for Wideband Power Amplifiers With Constrained Feedback Bandwidth

This letter presents a new digital predistortion (DPD) solution for wideband power amplifiers (PAs) with restricted feedback bandwidth and relatively low sampling rate. By integrating a microwave cavity filter into the feedback path, the feedback bandwidth is reduced efficiently. A PA parameter extraction method is then proposed to identify the PA model, using the bandwidth-constrained signals. By applying the extracted parameters to the direct learning architecture, the DPD function can be obtained, which can linearize nonlinear distortion over the sampling bandwidth. Experiments demonstrate that a 22 dB adjacent channel leakage ratio improvement is acquired for a 100 MHz Long Term Evolution-advanced signal, even when the feedback bandwidth is restricted from 500 MHz to 100 MHz, which remarkably reduces the ADC sampling rate from 1105.92 Msps to 368.64 Msps.

A Predistortion Algorithm Based on Accurately Solving the Reverse Function of Memory Polynomial Model

This letter presents a predistortion algorithm which identifies the power amplifier (PA) model and accurately calculates the predistortion function from the model by a new method. The method constructs a univariate polynomial and finds its roots by QR factorization. Although additional calculations are required, the new algorithm can compensate the nonlinearity of the PA more precisely and reduce the normalized mean square error (NMSE) performance of the new algorithm to the NMSE of the PA modeling. Simulation results demonstrate that the new algorithm outperforms the conventional algorithm by 4dB in the adjacent channel leakage ratio (ACLR) performance.

Nonlinear distortion suppression for active analog self-interference cancellers in full duplex wireless communication

Self-interference (SI) cancellation is one of the most important techniques for full duplex wireless communication. As an indispensable component, the active analog canceller (AC) needs to mitigate the analog SI to ensure it meets the analog-to-digital converter (ADC) sampling requirement. However, the active AC cancellation performance is often restricted by the nonideal electronic components, e.g., the tunable attenuator, the phase shifter and associated circuits, which introduce nonlinear distortions to the residual SI signal that enters the receive chain, as the transmit power increases. This paper presents a full duplex architecture with AC nonlinearity modeling and suppression by including an extra feedback loop right after the power amplifier and an AC nonlinear distortion modeling and cancellation process followed by SI channel estimation and digital SI cancellation. By analyzing the feedback signal feeding the AC and the residual signal after AC cancellation, the AC nonlinear distortion can be estimated and subsequently suppressed by subtracting its estimates from the received samples. The distortion-suppressed signal is then processed by the refined digital SI cancellation with channel estimation and mitigation to further improve the cancellation performance. Experiments are performed on 20-MHz Long Term Evolution-advanced signals to demonstrate the effectiveness of the proposed full duplex architecture with signal power ranging from -5 dBm to 23 dBm at the PA output while from -23 dBm to 5 dBm at the receiver front end.

A Wideband Doherty Power Amplifier With 100 MHz Instantaneous Bandwidth for LTE-Advanced Applications

In this letter, a 698-862 MHz wideband Doherty power amplifier (DPA) with 100 MHz instantaneous bandwidth is presented. The electrical memory effect of the wideband DPA is reduced by the LC resonant circuits employed in the drain bias networks. The influence of the integrated Doherty combiner is compensated by the broadband output matching networks, which were designed by applying the simplified real frequency technique. Driven with 100 MHz LTE-advanced signals, the adjacent channel leakage ratio (ACLR) and ACLR asymmetry of the proposed DPA at 20 MHz offset are improved by about 4 dB and 3 dB respectively, compared to the wideband DPA with conventional bias networks. After digital predistortion (DPD) linearization, the proposed DPA shows an ACLR of lower than -48 dBc at an average output power of 42.5 dBm, and high drain efficiency of over 42% across the operating frequency range.

Full duplex 2×2 MIMO radios

This paper presents the novel design and implementation of our 2 ×2 full duplex MIMO radios for LTE system. Our design first uses multi-tap analog cancellation to reduce self-interference in RF domain. 4 multi-tap cancellers are used respectively for the 4 self-interference chains existed in a radio. RF power detectors and ADCs are introduced to indicate the residual self-interference power arriving at receivers. Multidimensional gradient descent search algorithm aiming at minimizing residual self-interference power is used to find out the optimal setting for the adjustable attenuation, time delay and phase shift of each tap in multi-tap cancellers. In the further digital cancellation, due to the fact that the non-linear component and noise arisen from transmitters are significant in residual self-interference and difficult to be cancelled, we use additional receivers to get the feedback of local transmitted RF signals, and therefore the non-linear component and noise hided in them are known and can easily be cancelled. The feedback makes it not necessary to estimate non-linear component and transmitter noise in digital cancellation algorithm and thus makes the algorithm simple. Actually, a simple channel estimation based method utilizing the feedback can accurately reconstruct self-interference. Then the reconstructed self-interference is subtracted from received signals to complete the digital cancellation. Our implementation can cancel the self-interference caused by 23dBm LTE transmitted signals to the receiver noise floor of -90dBm and achieve 220Mbps full duplex communication.

A new digital predistortion using indirect learning with constrained feedback bandwidth for wideband power amplifiers

Digital predistortion (DPD) is one of the most effective techniques to mitigate the power amplifier (PA) nonlinear distortion. The DPD feedback bandwidth is often restricted by the nonideal electronic components, e.g., the anti-aliasing filter, which introduces bandwidth mismatch between model basis function and feedback signal thus degrades the linearization performance. This paper presents a new DPD solution for wideband PA systems with constrained feedback bandwidth. By including a linear operation into the PA identification, the PA model can be accurately estimated. Subsequently, the DPD parameters are extracted using the PA model estimated output and PA input signal by applying indirect learning algorithm. Experiments demonstrate a 23-dB adjacent channel leakage ratio improvement is acquired on a 100-MHz Long Term Evolution-advanced signal with the feedback bandwidth reduced from 500 MHz to 140 MHz.

Digitally Assisted Analog Interference Cancellation for In-Band Full-Duplex Radios

In this letter, a digitally assisted analog interference cancellation architecture is proposed for the prevailing full-duplex radios, which simultaneously transmit and receive signals on the same carrier frequency. An auxiliary transmit chain is deployed to generate a canceling signal and subtract from the self-interference (SI) signal at the receiver front end to prevent saturating the analog-to-digital converter. To ensure the canceling signal a close approximation of the SI signal, an observation chain is used to extract the transmitter response. Correspondingly, a two-step modeling process is inserted to the auxiliary transmit chain to recover both the nonlinear behavior and the multi-path channel between the transmit and receive chains. Experimental results have validated the superior performance of this architecture, particularly for transmitters of high output power and strong nonlinear distortions.

Novel Linearization Architecture with Limited ADC Dynamic Range for Green Power Amplifiers

Design of high-efficiency power amplifier (PA) is one of the key challenges to realize green radios, wherein digital predistortion (DPD) is deployed to reduce the PAs power back-off and thus increase its power efficiency. As the bandwidth of the transmit signal increases, stringent requirements are posed on the DPD linearization performance with limited sampling rate and dynamic range for the analog-to-digital converter (ADC) in the DPD feedback channel. In this paper, under a fixed ADC sampling rate, novel DPD architecture is proposed to compensate for the PA nonlinearity with limited ADC dynamic range. In the feedback channel of the proposed architecture, an extra radio frequency (RF) cancellation chain is introduced to eliminate the linear component of the PA amplified signal, and thus the requirement on the ADC dynamic range can be significantly reduced. Subsequently, by accurately estimating the loop delay and attenuation of the cancellation chain, the baseband replica of the RF cancelling signal is recovered, and the original PA output signal is rebuilt to estimate the DPD coefficients. Finally, experiments show that for the long term evolution (LTE)-advanced signals, the proposed architecture can achieve an adjacent channel leakage ratio lower than -47.6 dBc, which outperforms the conventional DPD by about 3.4 dB, with the effective bits of the ADC being reduced by 4.4 and a power added efficiency of 43.8% with 7.3 dB power back-off being observed for a fabricated Doherty PA with 50-dBm saturation power.