Xin Quan

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.

Effect of phase noise on digital self-interference cancellation in wireless full duplex

Oscillator phase noise in a full duplex radio causes the mismatch between the self-interference (SI) signal and the cancelling signal, and thus degrades performance of the digital SI cancellation (SIC). In this paper, we analyze the effect of phase noise on digital SIC in wireless full duplex. We consider an OFDM-based full duplex radio corrupted by the phase noise at both the transmitter and the receiver, which are modeled as independent Wiener processes. A closed-form expression for the cancellation ability of a common digital SIC scheme is derived, in terms of the interference-to-noise ratio (INR), the SI subcarrier spacing and the oscillators 3dB coherence bandwidth. The theoretical analysis and simulations reveal that the digital SIC ability degrades with the increase of the ratio of the oscillators 3dB coherence bandwidth to the signal bandwidth, which determines the upper bound of the digital SIC ability.

Impacts of Phase Noise on Digital Self-Interference Cancellation in Full-Duplex Communications

In full-duplex (FD) radios, phase noise leads to random phase mismatch between the self-interference (SI) and the reconstructed cancellation signal, resulting in possible performance degradation during SI cancellation. To explicitly analyze its impacts on the digital SI cancellation, an orthogonal frequency division multiplexing (OFDM)-modulated FD radio is considered with phase noises at both the transmitter and receiver. The closed-form expressions for both the digital cancellation capability and its limit for the large interference-to-noise ratio (INR) case are derived in terms of the power of the common phase error, INR, desired signal-to-noise ratio (SNR), channel estimation error and transmission delay. Based on the obtained digital cancellation capability, the achievable rate region of a two-way FD OFDM system with phase noise is characterized. Then, with a limited SI cancellation capability, the maximum outer bound of the rate region is proved to exist for sufficiently large transmission power. Furthermore, a minimum transmission power is obtained to achieve

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

\beta

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

-portion of the cancellation capability limit and to ensure that the outer bound of the rate region is close to its maximum.

Performance Analysis of Direct-Learning Digital Predistortion With Loop Delay Mismatch in Wideband Transmitters

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.

Suppression of Analog Self-Interference Canceller Nonlinearities in MIMO Full Duplex

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.

Eigen Domain Interference Rejection Combining Algorithm for MIMO Systems

Orthogonal frequency-division multiplexing (OFDM) modulation, which is commonly used in modern digital communication systems due to its flexibility and high spectral efficiency, is sensitive to transmitter nonlinearities introduced by the power amplifiers (PAs). As a key technology in wireless transmitting systems, digital predistortion (DPD) is widely applied to compensate for the transmitter nonlinearities and ensure high power-added efficiency in base stations and mobile devices for cellular systems. In a DPD architecture, the transmitter input and output signals are sampled, aligned, and processed to extract the DPD parameters that would be used to predistort the source signal before transmission to counteract the transmitter nonlinearities. However, the alignment accuracy in DPD is limited by nonideal electronic components and the associated circuitry, which introduces unknown loop delay mismatch and, thus, degrades the overall linearization performance. In this paper, we analyze the effect of a loop delay mismatch on PA model identification and, hence, on DPD linearization performance of the wideband OFDM signals. The expression and upper limit for the normalized mean square error (NMSE) are derived in terms of the loop delay mismatch, transmission signal bandwidth, and signal-to-noise ratio (SNR) of the feedback channel. The expression for the adjacent channel leakage ratio (ACLR) performance is also derived to predict the DPD linearization performance with the presence of the loop delay mismatch. The theoretical analysis reveals that the performance degradation increases with the bandwidth of the OFDM signal and the modulus of the loop delay mismatch. Experiments are performed on the Long-Term Evolution (LTE) Advanced signals with bandwidth ranging from 20 to 100 MHz to evaluate the degradation effect of the loop delay mismatch on the NMSE and ACLR performances.

Digital predistortion architecture with reduced ADC dynamic range

This paper presents the nonlinear effects of the analog self-interference (SI) canceller and a practical solution to suppress the nonlinear distortions in multiple-input multiple-output (MIMO) full-duplex wireless communication systems. Due to the inherent nonlinearities of the active radio frequency (RF) components used to tune the attenuations and the phase shifts in the analog SI cancellers, nonlinear distortions are introduced to the residual SI signal by the analog cancellation and enter the receive chain. In this paper, we build an extra feedback chain for each analog canceller to obtain the reference signal coupled from the transmitted RF signal and observe the characteristics of the canceller. Using the observations, we develop a nonlinear model including all the cancellers at each receive antenna with the presence of MIMO SI channel to estimate the nonlinear components. By subtracting the model estimates from the received signal instantaneously, the corresponding nonlinear distortions are effectively mitigated. After the nonlinearity mitigation, a linear digital cancellation scheme based on the Maximum Likelihood Estimation is applied to further reduce the residual SI signal caused by the MIMO SI channel to the noise floor. Experiments are performed on a 2x2 MIMO full-duplex testbed to validate the effectiveness of the proposed nonlinearity modeling and suppression method using Long Term Evolution signals of 20-MHz bandwidth.

Analysis and Experimental Verification of Digital Self-Interference Cancelation for Co-time Co-frequency Full-Duplex LTE

In this letter, we investigate an eigen domain interference rejection combining (E-IRC) algorithm for multiple-input-multiple-output (MIMO) systems suffering from narrowband interference (NBI). The received signals are transformed into the eigen domain, which is obtained by eigenvalue decomposition of the time domain autocorrelation matrix of the NBI, and the interference is rejected in the eigen domain via multiple antennas. V-BLAST and Alamouti schemes are considered in this letter. For the V-BLAST scheme, the E-IRC algorithm is an extension of the one for SIMO system. For the Alamouti scheme, we redesign the transmission codeword, which can preserve the orthogonality in the eigen domain, and propose an NBI rejection algorithm based on the spatial whitening filter.