Linear Cancellation Research Articles

Integrated Linear and Nonlinear Digital Cancellation for Full Duplex Communication

Full duplex (FD) communication has the potential to double the throughput of wireless links while occupying the same bandwidth of half duplex communication. To enable FD communication, the self-interference (S1) signal is to be suppressed down to the noise floor. S1 cancellation can be achieved via antenna suppression, active analog cancellation, and digital cancellation. At high transmit power levels, the nonlinear behavior of the transceiver hardware degrades linear S1 channel estimation; hence, S1 cancellation results in reduced total suppression. In this article, we review nonlinear DSIC solutions in the literature, and we present a comprehensive integrated linear and nonlinear digital S1 cancellation framework with nested and residual approaches, where nonlinear algorithms, memory polynomial, support vector regression, and orthonormalized least mean squares algorithms are implemented together with linear cancellation. The performance of the proposed integrated framework is evaluated in comparison with well-known nonlinear digital cancellation techniques by implementing all techniques on the same software defined FD radio setup, consisting only of a single antenna and a WARP v3 board. It is shown that the integrated residual approach can enhance the total S1 suppression performance significantly by up to 7 dB as compared to linear-only digital S1 cancellation and by up to 4 dB with respect to existing nonlinear-only techniques observed at moderate power levels, and the integrated nested approach can provide an improvement of up to 6 dB over linear-only and 3 dB over nonlinear-on-ly digital S1 cancellation for high power settings. All schemes are also evaluated in terms of training overhead and computational complexity, providing insights about the trade-off between cost and performance at different power levels.

A novel phase noise compensation algorithm combining the DF algorithm with LCSC algorithm in CO-OFDM systems

A novel phase noise (PN) compensation algorithm based on the decision feedback (DF) algorithm and the linear combination self cancellation (LCSC) algorithm is proposed to improve the system performance degradation caused by laser linewidth in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. In this proposed LCSC-DF algorithm, the LCSC algorithm is used to precode the subcarrier information at the transmitter and decode the demodulation information and inter-carrier interference (ICI) related information at the receiver. And then the pilot information is used to obtain the final compensation signal by the improved DF algorithm. The simulation results show that the PN compensation performance of the proposed LCSC-DF algorithm is better than that of the DF algorithm. Furthermore, with the increase of the signal to noise ratio (SNR), its bit error rate (BER) performance approaches to that of the SC-DF algorithm at the larger PN linewidth. The subcarriers utilization ratio of the proposed algorithm is higher than that of the SC-DF algorithm. As a result, the proposed algorithm can effectively improve the performance of the system.

Nonlinear digital self-interference cancellation for full duplex communication

Full duplex (FD) communication is a promising solution for enhancing the data rates of wireless networks, thanks to the simultaneous bilateral spectral use. Nevertheless, FD communication is still challenged by the self-interference (SI) problem. Existing linear SI cancellation techniques to fight this challenge are observed to be less effective at relatively higher power levels, due to the nonlinear behavior introduced by the transmitter and receiver hardware. In this paper, we propose an effective nonlinear digital cancellation solution and demonstrate its performance on a real software defined FD radio set-up. The proposed nonlinear solution is based on a memory polynomial model, which is also integrated with linear cancellation and both are implemented at the baseband level. The performance of the FD radio with integrated nonlinear and linear cancellation is observed via laboratory tests, as well as tests with a channel emulator under time varying channel conditions. It is shown that the proposed nonlinear cancellation provides about 3 dB improvement over linear only cancellation at low and moderate power levels, and up to 5 dB improvement at high power levels without any modification to hardware, unlike the existing methods. The proposed nonlinear cancellation does not introduce any extra communication overhead, since nonlinear coefficients are estimated only once for each transmit power level, stored and reused. Under time varying channel conditions, the performance of nonlinear estimation and cancellation is affected only by the performance of linear channel estimation, which can be addressed by choosing the packet size in accordance with channel coherence time.

Bidirectional satellite communication under same frequency transmission with non‐linear self‐interference reduction algorithm

Two-way (on-frequency) relaying using simple amplify-and-forward processing at the satellite is an emerging technology that allows doubling of system spectral efficiency for certain networking applications. However, the satellite channel is non-linear and compensation for the self-interfering signal is more difficult than on linear channels, especially when memory effects are recognised. In this paper, we model two-user, asynchronous, bidirectional relaying for uncoded transmission, and proposed a bidirectional satellite communication algorithm under same frequency transmission. First, we employ a table addressed by side-information and the symbol to be detected to model the action of the channel. Then, we employ a linear adaptive canceler followed by a smaller table lookup process to cancel the self-interference and to make decisions. In the simulation part, we focus in the results section is upon 16-amplitude phase shift keying (APSK), widely employed in the DVB-S2 standard for bandwidth-efficient operation. Compared with the existing bidirectional satellite communication algorithm, our proposed algorithm have the far less complexity and training time at the similar performance.

An Architecture for Capturing the Nonlinear Distortion of Analog Self-Interference Cancellers in Full-Duplex Radios

Analog self-interference (SI) cancellation is a key process to enable full-duplex wireless communications, which generates a cancelling signal (a copy of the SI) at the local receiver and subtracts from the received signal to cancel the SI imposed by the local transmitter. However, due to the inherent nonlinearities of analog canceller (AC), the SI cancellation performance is often restricted by introducing extra nonlinear distortion to the receiver. In this letter, an observation architecture is proposed to capture the AC nonlinear distortion by deploying an auxiliary chain identical to the AC and a linear cancellation approach. The captured nonlinear distortion could be used as a reference in the digital domain to mitigate the AC-induced nonlinear interference at the receiver. Experiments verify that the proposed architecture is capable of capturing the weak AC-induced distortion and suppressing it to close the receiver noise floor.

Hierarchical Precoding in a Realistic Ultradense Heterogeneous Environment Exceeding the Degrees of Freedom

Cell densification is a widely used approach to increase the spectral efficiency per area of cellular networks. Such Ultradense Networks (UDNs) consisting of small cells are often coordinated by macro base stations (BSs). With universal frequency reuse interference from the macro BS limits the system spectral efficiency. In this work we exploit the degrees of freedom at the macro BS to apply interference coordination. We propose a hierarchical precoding strategy in the spatial domain in order to project interference from the macro BS into the subspace of small cell users enabling linear cancellation. The macro BS interference towards small cell users is aligned within the joint null space of users served by the macro BS. Compared to classical interference alignment, our scheme does not require coordination between macrocells and small cells. We present three algorithms: in the first the interference is minimized by iterative alignment, in the second the uncoordinated interference from the small cells is considered, and in the third iterative Minimum Mean Square Error (MMSE) technique is used. We provide numerical evaluation, complexity analysis, and robustness analysis of these algorithms based on a realistic channel model showing the benefit of hierarchical precoding compared to the uncoordinated case.

Multi-channel differencing adaptive noise cancellation with multi-kernel method

Although a various of existing techniques are able to improve the performance of detection of the weak interesting signal, how to adaptively and efficiently attenuate the intricate noises especially in the case of no available reference noise signal is still the bottleneck to be overcome. According to the characteristics of sonar arrays, a multi-channel differencing method is presented to provide the prerequisite reference noise. However, the ingredient of obtained reference noise is too complicated to be used to effectively reduce the interference noise only using the classical linear cancellation methods. Hence, a novel adaptive noise cancellation method based on the multi-kernel normalized least-mean-square algorithm consisting of weighted linear and Gaussian kernel functions is proposed, which allows to simultaneously consider the cancellation of linear and nonlinear components in the reference noise. The simulation results demonstrate that the output signal-to-noise ratio (SNR) of the novel multi-kernel adaptive filtering method outperforms the conventional linear normalized least-mean-square method and the mono-kernel normalized least-mean-square method using the realistic noise data measured in the lake experiment.

Widely linear digital self-interference cancellation in direct-conversion full-duplex transceiver

This paper addresses the modeling and cancellation of self-interference in full-duplex direct-conversion radio transceivers, operating under practical imperfect radio frequency (RF) components. First, detailed self-interference signal modeling is carried out, taking into account the most important RF imperfections, namely, transmitter power amplifier nonlinear distortion as well as transmitter and receiver IQ mixer amplitude and phase imbalances. The analysis shows that after realistic antenna isolation and RF cancellation, the dominant self-interference waveform at the receiver digital baseband can be modeled through a widely linear transformation of the original transmit data, opposed to classical purely linear models. Such widely linear self-interference waveform is physically stemming from the transmitter and receiver IQ imaging and cannot be efficiently suppressed by classical linear digital cancellation. Motivated by this, novel widely linear digital self-interference cancellation processing is then proposed and formulated, combined with efficient parameter estimation methods. Extensive simulation results demonstrate that the proposed widely linear cancellation processing clearly outperforms the existing linear solutions, hence enabling the use of practical low-cost RF front ends utilizing IQ mixing in full-duplex transceivers.

MMSE-based partial crosstalk cancellation for upstream VDSL

In current DSL systems, crosstalk is a major source of performance degradation. For upstream VDSL scenarios with in-domain crosstalk and AWGN, it has been shown that the crosstalk can be effectively mitigated using a linear zero-forcing canceler. Furthermore, the complexity can be reduced by only canceling the crosstalk from major crosstalkers, which is referred to as partial crosstalk cancellation. However, such approach does not work well in scenarios that also have out-of-domain (alien) crosstalk. As alien crosstalk is spatially correlated, the zero-forcing canceler performs very poorly, hence alternative structures for the canceler have to be examined. In this paper, we first demonstrate that an MMSE-based linear and nonlinear crosstalk canceler provides improved performance compared to the zero-forcing canceler in this scenario and then present efficient algorithms to perform partial crosstalk cancellation with these MMSE-based cancelers.

Overestimation of force during matching of externally generated forces

If a weight is applied to a finger and the subject asked to produce the same force, the subject generates a force larger than the weight. That is, subjects overestimate the force applied by an external target when matching it. Details of this force overestimation are not well understood. We show that subjects overestimate small target weights, but not larger ones. Furthermore we show for the first time that the force overestimation consists of two components. The first component is a constant. The second component depends on the precise magnitude of the weight and is only present when subjects hold the target weight against gravity. We suggest that the two components are generated in different phases of the force-matching task, are due to different processes, and must have an influence on all proprioceptive judgements of force.

An Efficient Inter-Carrier Interference Cancellation Scheme for OFDM Systems with Frequency Estimation Errors

One of the major drawbacks of orthogonal frequency division multiplexing (OFDM) systems is their vulnerability to synchronization errors. To remedy the inter-carrier interference (ICI) effect caused by carrier frequency offset (CFO) estimation errors, this paper proposes a weighted linear parallel ICI cancellation (WLPICIC) equalizer. The optimal weights in the WLPICIC scheme are derived in closed-form expressions by maximizing the average signal-to-interference ratio (SIR) at the WLPICIC output of each sub-carrier. The simulation results show that the WLPICIC equalizer significantly improves the performance of OFDM systems with frequency estimation errors in both AWGN channels and frequency selective fading channels.

Signal Transmission with Known-Interference Cancellation [Lecture Notes

This paper outlines some of the main ideas for designing signaling schemes in the presence of a known interference. The paper shows that the straightforward application of conventional methods is ineffective and that while the linear cancellation of the interference using subspace methods is an appealing idea, has clear limitations as well. Quantization-based cancellation of the interference was found to perform very well, and makes it possible to approach fundamental information theoretic limits

Simultaneous cancellation of air and bone conduction tones at two frequencies: Extension of the famous experiment by von Békésy

Cancellation experiments between air conduction (AC) and bone conduction (BC) tones were conducted at two frequencies (0.7 and 1.1 kHz) and three levels (40, 50, and 60 dB HL) in three subjects. The tests were divided into three categories: (1) single tone cancellation, (2) simultaneous cancellation of two tones, and (3) cancellation of one tone while a disturbing tone was present. In total, each subject performed twelve cancellation tasks. The hypothesis is that the AC and BC sound transmission behaves as linear systems and they both excite the basilar membrane in the cochlea similarly. The cancellation results are presented as the deviations from this hypothesis; except for a few larger deviations, the intrasubject deviations were generally less than 0.5 dB and 5°. The results from all three test categories indicate that the hypothesis of linear transmission systems and similarity of cochlear stimulation by AC and BC holds. However, due to the subjects’ limited ability to conduct optimal cancellation and to imperfect methodology and equipment, the small deviations from perfectly linear cancellation that were observed do neither conclusively prove nor refute the possibility of small differences in the cochlear processing of AC and BC sound. Nonetheless, it is clear that if such differences in the processing of the two stimuli exist, they are small in magnitude.

Optimal Power/Rate Allocation and Code Selection for Iterative Joint Detection of Coded Random CDMA

Iterative interference cancellation of coded code-division multiple access (CDMA) using random spreading with linear cancellation is analyzed. If users are grouped into power classes and Shannon bound approaching codes are used, a geometric power distribution achieves the additive white Gaussian noise (AWGN) channel Shannon bound as the numbers of classes becomes large. The optimal distribution of the size of these classes is shown to be uniform. If users are grouped into different rate classes with equal powers among equal rate users, the Shannon bound for AWGN channels can be achieved with an arbitrary distribution of the classes sizes, provided that the size of the largest rate class obeys the mild condition that its ratio of size to processing gain is much smaller than the inverse of the signal-to-noise ratio (SNR). The case of equal powers and equal rates among all users is addressed as a "worst case" scenario. It is argued that simple repetition codes provide for a larger achievable capacity than stronger codes. It is shown that this capacity monotonically increases as the rate of the code decreases. A density evolution analysis is used to show that the achievable rates exceed those of a minimum-mean square error filter applied to the uncoded signals. This lower bound is tight for small ratios of bit energy to noise power, and otherwise the iterative cancellation receiver provides an appreciably larger capacity. Relating to recent result from the application of statistical mechanics it is shown that the repetition-coded system with iterative cancellation achieves the performance of an equivalent optimal joint detector for uncoded transmission

Iterative decoding and demodulation for single-antenna vector OFDM systems

Vector orthogonal frequency-division multiplexing (OFDM) for single-antenna systems generalizes OFDM in that it converts an intersymbol interference (ISI) channel into multiple ISI-free vector channels and involves vector channel matrices instead of channel coefficients in its one-tap equalization (demodulation). Over frequency-selective fading channels, vector OFDM with increasing vector length implicitly offers more signal space diversity advantages over OFDM. However, the complexity of the brute force demodulator increases exponentially in terms of the vector size. In this paper, iterative demodulation and decoding for convolutionally coded vector OFDM systems are studied, and in particular, low-complexity demodulation schemes that employ linear cancellation and feature linear complexity in terms of the vector length are proposed and compared. Following the introduction of demodulators with ideal linear cancellation as benchmark, demodulators with hard and soft interference cancellation and their initialization options are studied. The demodulator with soft interference cancellation dynamically updates the symbol estimates and their instantaneous reliability measure based on the code bit probabilistic information passed from preceding iterations. It alleviates error propagation that is detrimental to the demodulator with hard interference cancellation and shows performance close to that of the brute force demodulator, with great overall complexity reduction. Extensive simulation results demonstrate how the choice among the presented schemes and the vector length affect the convergence rate and the achievable performance of the iterative receivers

Performance evaluation of a generalized linear SIC for DS/CDMA signals

The linear successive interference canceler (LSIC) is a multiuser detector that separates code-division multiple-access (CDMA) signals in a multistage manner. In each stage, a user is detected, and its contribution is regenerated and canceled from the input of that stage. A user's spreading sequence is employed for despreading and respreading, and the magnitude of the despreader output is used as the amplitude estimate to reconstruct that user's signal. This paper describes a generalized version of the LSIC (GLSIC) that employs various types of linear filters for the despreading and respreading operations. We analyze the bit error rate (BER), asymptotic multiuser efficiency (AME), and the mean and variance of the amplitude estimates of the GLSIC. It is shown that for certain choices of linear filters, complete cancellation of a user can be achieved, irrespective of the reliability of the symbol estimates. We also demonstrate that, from a BER and AME viewpoint, it is not beneficial to use a linear canceler with a decorrelator or a minimum mean-square-error (MMSE) receiver.

The manipulation of the streamwise vortex instability in a natural convection boundary layer along a heated inclined flat plate

Flow instabilities leading to the formation of streamwise vortices in a natural convection boundary layer over a heated inclined plate submerged in a water tank are manipulated using spanwise arrays of surface-mounted heating elements. The flow over the plate is driven by a two-ply surface heater comprised of a uniform, constant- heat flux heater and a mosaic of 32 × 12 individually controlled heating elements that are used as control actuators. Surface temperature distributions are measured using liquid crystal thermography and the fluid velocity in cross-stream planes is measured using particle image velocimetry (PIV). Time-invariant spanwise-periodic excitation over a range of spanwise wavelengths leads to the formation of arrays of counter-rotating streamwise vortex pairs and to substantial modification of the surface temperature and heat transfer. The increase in surface heat transfer is accompanied by increased entrainment of ambient fluid and, as a consequence, higher streamwise flowrate. Subsequent spanwise-periodic merging of groups of vortices farther downstream retards the streamwise increase of the surface heat transfer rate. Finally, the suppression of small-amplitude spanwise disturbances by linear cancellation is demonstrated.

Adaptive cancellation of stationary interference in the presence of structured nonstationary interference

Adaptive linear cancellation of interference is a key technique for enhancing the performance of many current and planned radar and wireless communication systems. Present methods of cancellation have limited capability when the interference is strongly nonstationary. A maximum likelihood-based cancellation procedure is given for Gaussian interference with a partially known nonstationary covariance component. This procedure is designed for adaptive cancellation implemented by a systolic cascaded array and performs well in computer Monte Carlo experiments.

Multiuser soft interference canceler via iterative decoding for DSL applications

This paper presents a method to mitigate interference on a transmission system by another system with an overlapping frequency band. The systems are uncoordinated so that they cannot be made to transmit in orthogonal coding space. The transmission environment is an interference channel rather than the multiaccess channel often considered in other multiuser detection problems, for instance at the base station of a code division multiple access wireless system. Using the fact that the inputs are all from discrete constellations, it is possible to use a multiuser maximum-likelihood (ML) detector to decode the signals from the system of interest with low probability of error. However, the ML detector often requires impractical computation. A soft linear canceler is instead used to reduce the interference with a small loss of bandwidth in the system of interest. The method has a much lower complexity than a full-blown ML detector, and the soft canceler performance is close to an ML detector. The method is demonstrated on a very-high-speed digital subscriber lines system with a home local area network interference. The system of interest is using a multicarrier modulation transmission scheme such as discrete multitone, while the interfering system transmits using quadrature amplitude modulation.

Open-Loop Resonance-Cancellation Control for a Base-Excited Pendulum

A new open-loop control strategy applied to a planar pendulum subjected to the most severe com bination of base excitations, horizontal motion at the primary-resonance frequency, and vertical motion at the principal-parametric resonance frequency is developed. The control action is typical of many single-input control systems; the control authority in one direction (horizontal) is high, and the control authority in the or thogonal direction (vertical) is zero in a linear sense. Although the action of the controller is linearly orthog onal to part (vertical or parametric) of the system excitation, effects are transferred to this direction through nonlinear actuator action. Proper enhancement of the control input allows the nonlinear action to provide con trol authority over the parametric excitation. The dynamics at reduced orders, determined by a multiple-scales perturbation analysis, suggest the appropriate form for the control enhancement. The normal form of the sys tem provides information about how each parameter of the enhancement affects the steady-state pendulation of the system. In this case, heuristic arguments are used to reduce the dimension of the unknown enhance ment design parameters to a manageable size. The maximum pendulation angle of the steady-state motion of the system is one of the appropriate metrics for the system response; here it is used as the cost function for evaluation of the optimal enhancement gains. Because the size of the design parameter set has been re duced, a simple grid search is employed to find the optimal control. Relative to simple linear cancellation of the disturbance, the proposed approach reduces the response of the system by an order of magnitude for typical disturbance cases.