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

In wireless full-duplex communication systems, selfinterference (SI) cancellation is a key technique to ensure simultaneous transmission and reception on the same carrier frequency. During the SI cancellation, it has been shown that the oscillator phase noise dramatically affects the cancellation capability and thus needs consideration and suppression. In this paper, a novel time-domain digital SI cancellation approach is proposed to cancel the transmitter and receiver phase noises along with the incoming SI signal, without estimating either the SI channel response or the phase noise associated coefficients. For proofof-concept evaluation, simulations are performed on orthogonal frequency division multiplexing (OFDM)-modulated signals of 20 MHz bandwidth with 16 QAM constellations to verify the effectiveness of the proposed method. It is demonstrated that the proposed approach provides at least 3 dB improvement on the SI cancellation capability and at least 5 dB increase on the interference tolerance over the conventional cancellation approach that uses the frequency domain SI channel estimation.

Nonlinear self-interference (SI) cancellation is essential for mitigating the impact of transmitter-side nonlinearity on overall SI cancellation performance in flexible duplex systems, including in-band full-duplex (IBFD) and sub-band full-duplex (SBFD). Digital SI cancellation (SIC) must address the nonlinearity in the power amplifier (PA) and the in-phase/quadrature-phase (IQ) imbalance from up/down converters at the base station (BS), in addition to analog SIC. In environments with rich signal reflection paths, however, the required number of delayed taps for time-domain nonlinear SI cancellation increases exponentially with the number of multipaths, leading to excessive complexity. This paper introduces a novel, low-complexity, frequency domain nonlinear SIC, suitable for flexible duplex systems with multiple-input and multiple-output (MIMO) configurations. The key approach involves decomposing nonlinear SI into a nonlinear basis and categorizing them based on their effectiveness across any flexible duplex setting. The proposed algorithm is founded on our analytical results of intermodulation distortion (IMD) in the frequency domain and utilizes a specialized pilot sequence. This algorithm is directly applicable to orthogonal frequency division multiplexing (OFDM) multi-carrier systems and offers lower complexity than conventional digital SIC methods. Additionally, we assess the impact of the proposed SIC on flexible duplex systems through system-level simulation (SLS) using 3D ray-tracing and proof-of-concept (PoC) measurement.

In this paper, a full-duplex transceiver with a two-stage analog interference cancellation architecture is proposed for the prevailing wireless full-duplex communication, which enables simultaneous transmission and reception on the same frequency. This two-stage cancellation architecture jointly combines the structures of two typical analog self-interference (SI) cancellation approaches, i.e., the radio frequency (RF)-tapping and the baseband-tapping approaches, and mitigates the SI signal in two steps to provide an improved analog cancellation performance with reduced restrictions on the RF components required to build the cancellation architecture. The stage-I cancellation uses a dual-tap analog canceling circuit to mitigate only the direct leakage and one refection component within the SI signal to yield a residual multipath SI of reduced dynamic range. In the stage-II cancellation, a nonlinear model is particularly designed to build a multipath canceling signal in digital domain, which is capable of characterizing the joint effect of the multipath SI propagation channel, the transmitter nonlinearity, and the receiver nonlinearity of high accuracy. The digital canceling signal is then transmitted through an auxiliary transmit chain to cancel the residual SI from the stage-I cancellation to further improve the overall analog cancellation performance. Simulations are explicitly performed on a variety of wideband signals to verify cancellation capability of the proposed full-duplex transceiver. For proof-of-concept verification, experiments are also performed on our self-designed testbed to validate the cancellation performances of each cancellation stage.

Recent works have demonstrated the feasibility and potential of full-duplex (FD) wireless systems to double the spectral efficiency of half-duplex (HD) systems. Self-interference (SI) cancellation is the key to FD communication and the residual SI is the major factor determining the performance of an FD radio. This paper presents a novel frequency domain based approach, for the reconstruction of SI signal in digital domain. Unlike the existing time domain reconstruction approach, which requires convolution, the proposed approach is simple and uses FFT processing for SI signal reconstruction. By means of computational complexity evaluation, it is shown that the proposed reconstruction approach offers 61% reduction in the floating point operations required to reconstruct SI signal. Furthermore, via detailed simulations on the performance of digital SI cancellation under fading channels, it is demonstrated that the SI suppression achieved with the proposed approach is not only comparable to existing approach, but with frequency domain SI channel estimates, it can offer 5-7 dB better cancellation under highly frequency selective fading conditions, suggesting its suitability for long range transmission.

In full-duplex multipath self-interference (SI), each of the resolvable multipath SI components consists of a group of non-resolvable (NR) components with similar delays. Since the transceiver cannot distinguish the NR components in reality, the canceller can only generate the approximate constructed SI to imperfectly cancel the received SI. This leads to residual SI remaining. This paper analyzes the effect of the NR components on the SI cancellation, and proposes a practical scheme to eliminate the residual SI caused by the NR components. First, a closed-form expression of the residual SI is obtained. Next, the average power of the residual SI and the SI cancellation ratio are derived to characterize the effect of NR components on SI cancellation in the multipath SI channel. Last, a switch scheme is proposed to eliminate the residual SI.

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.

In this paper, a novel communication scheme that combines beamforming, energy harvesting, and cooperative non-orthogonal multiple access (NOMA) system is introduced. In the proposed scheme, NOMA is first combined with beamforming to serve more than one user in each beamforming vector. Later, simultaneous wireless information and power transfer (SWIPT) technique is exploited to encourage the strong user to relay the weak user's information messages in full-duplex (FD) manner. However, one major drawback of FD communications is the self-interference (SI) signal due to signal leakage from terminal's output to the input. Three different cases are adopted. In the first case, the SI signal is assumed to be fully cancelled at the strong user using SI cancellation techniques. In the second case, SI is only cancelled at the information decoding circuits, while being harvested in the energy harvester to provide extra energy. The third case investigates the impact of imperfect SI cancellation on system performance is investigated. It is proved that introducing SWIPT doesn't only motivate users to collaborate, but also mitigates the impact of the SI signal. In addition, the problem of total sum rate maximisation is formulated and solved locally by means of two-step difference of convex functions (DC)-based procedure. Furthermore, the outage probability of both weak and strong user is derived and provided in closed-form expressions. Numerical simulations are presented to illustrate the sum rate gain of the proposed scheme if compared with existing schemes, and to verify the derived formulas.

To improve the throughput of underwater acoustic (UWA) networking, the In-band full-duplex (IBFD) communication is one of the most vital pieces of research. The major drawback of IBFD-UWA communication is Self-Interference (SI). This paper presents a digital SI cancellation algorithm for asynchronous IBFD-UWA communication system. We focus on two issues: one is asynchronous communication dissimilar to IBFD radio communication, the other is nonlinear distortion caused by power amplifier (PA). First, we discuss asynchronous IBFD-UWA signal model with the nonlinear distortion of PA. Then, we design a scheme for asynchronous IBFD-UWA communication utilizing the non-overlapping region between SI and intended signal to estimate the nonlinear SI channel. To cancel the nonlinear distortion caused by PA, we propose an Over-Parameterization based Recursive Least Squares (RLS) algorithm (OPRLS) to estimate the nonlinear SI channel. Furthermore, we present the OPRLS with a sparse constraint to estimate the SI channel, which reduces the requirement of the length of the non-overlapping region. Finally, we verify our concept through simulation and the pool experiment. Results demonstrate that the proposed digital SI cancellation scheme can cancel SI efficiently.

In this letter, for the first time, we propose to employ the adaptive frequency-domain pre-equalization (FDPE) into an electro-absorption modulator laser (EML)-based optical self-interference cancellation (OSIC) system to suppress the time-varying multipath radio frequency (RF) self-interference (SI) signal, which is implemented for in-band full-duplex (IBFD) communication. Faced with time-varying multipath wireless channel, the OSIC system estimates multipath channel response using a least square estimation algorithm, and adjusts the coefficients of frequency-domain pre-equalizer adaptively to combat time-varying multipath SI. Based on the proposed FDPE-aided OSIC, the IBFD transmission of OFDM RF signals with time-varying multipath channel is demonstrated experimentally, achieving 30-dB OSIC depth over 1.22-GHz broad baseband.

Analog self-interference(SI) cancellation in full-duplex systems can be implemented with either fixed or adjustable delay arrangement. In this paper, the theoretical residual SI power in multi-tap analog SI cancellation with arbitrary time delay arrangement for each tap is derived, and the cancellation performance with fixed and optimal adjustable delay arrangement are compared by simulations. The influence of the bandwidth, the carrier frequency, the number of taps and the maximum delay of the SI channel on the cancellation performance are investigated. In consideration of the complexity of the optimal method, a novel algorithm for adjustable delay arrangement is proposed based on the cross-correlation of the reference signal and the SI signal.

An adaptive photonic-assisted wideband self-interference cancellation (SIC) scheme based on self-mixing for frequency-modulated continuous-wave (FMCW) radar is proposed. The signals are modulated onto the optical carrier using a dual-polarization quadrature phase shift keying (DP-QPSK) modulator. After adjusting the delay and amplitude of the reference signal in the optical domain, the self-interference (SI) signal is eliminated through a balanced photodetector (BPD). The output signal from the BPD and the reference signal are mixed and then downconverted to zero intermediate frequency. By checking the delay difference between the signal of interest (SOI) and the SI signal, the two signals are distinguished. The Hooke-Jeeves (HJs) algorithm takes the power of the SI signal after self-mixing as the objective function, adaptively optimizing the delay and amplitude of the reference signal until the objective function is minimized. Experimental results show that the scheme can successfully recover the SOI, with the SI signal being eliminated more than 30 dB. The scheme achieved cancellation depths of 33.2 and 31.9 dB for broadband signals in Ku-band with bandwidths of 500 MHz and 1 GHz, respectively. Additionally, we demonstrated the cancellation of signals within the 4–40 GHz range, with a bandwidth of 100 MHz, achieving a cancellation depth >30 dB.

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.

In full duplex (FD), which improves frequency efficiency and achieves flexible resource assignment between the uplink and downlink, self-interference (SI) from a transmitter should be suppressed by using antenna isolation, an analog SI canceler, and digital SI canceler (DSIC) to a level such that the data or control channel satisfies the required block error rate (BLER). This paper proposes an alternately iterative DSIC (AI-DSIC) structure for orthogonal frequency division multiplexing (OFDM) using FD. We first present the required SI suppression level considering SI, quantization noise of an analog-to-digital converter, and nonlinear distortions of a power amplifier and RF receiver circuitries for a direct conversion transceiver using FD. Then, we propose an AI-DSIC structure that iterates alternately the generation of the SI estimate, the downlink symbol estimate, and removal of each from the received signal in the downlink including SI. We investigate the average BLER performance of the AI-DSIC for OFDM using FD based on link-level simulations under the condition that the derived required signal-to-SI ratio is satisfied.

In this paper, we focus on the self-interference (SI) cancellation in the presence of phase-noise in full-duplex systems. Reducing the SI needs the estimation of the SI channel and the phase-noise. In particular, the time-varying phase-noise process is approximated by an expansion over a basis. As the unknown intended signal received from the other transceiver can limit the estimation performance if considered as additive noise, we incorporate it in the joint estimation of the SI channel, the intended channel and the phase-noise process. The proposed algorithm maximizes the likelihood function by fully exploiting the known transmitted data and the second-order statistic of the intended signal.

This article investigates novel adaptive self-interference cancellation solutions and the total integrated cancellation performance of a mobile single-antenna inband full-duplex transceiver. First, novel self-adaptive digital self-interference cancellation algorithms are described, with an emphasis on tracking of time-varying self-interference coupling channel in a mobile device as well as on structural ability to suppress also nonlinear self-interference with highly nonlinear mobile power amplifiers. This leads to an advanced self-adaptive nonlinear digital canceller which utilizes a novel orthogonalization procedure for nonlinear basis functions, together with low-cost LMS-based parameter learning. The achievable self-interference cancellation performance is then evaluated with actual RF measurements using mobile device scale RF components, in particular a highly nonlinear PA. The measurements also incorporate a novel self-adaptive RF cancellation circuit in order to realistically assess the total integrated cancellation performance. The reported results show that highly efficient self-interference cancellation can be achieved also in a mobile device, despite a heavily nonlinear PA and limited computing and hardware resources. The proposed cancellation solutions, when integrated together, show that 100 dB of self-interference can be cancelled using a 20 MHz LTE waveform, while the SI can be attenuated by over 110 dB with a narrower bandwidth of 1.4 MHz, all measured at 2.4 GHz ISM band. Furthermore, these results are achieved using a highly nonlinear transmitter power amplifier and fully adaptive canceller structures which can track a rapidly changing coupling channel in a mobile full-duplex device.