Research And Implementations On Adaptive Filtering Algorithms Of Second-Order Noncircular Signals

Complex-valued description of signals and systems is benefiting numerous research subjects from science to engineering since it can carry out the underlying amplitude and phase information in a more concise way than real-valued representation.However,noncircularity,which is the distinctive statistical property of complex-valued variables,is often neglected in previous researches.In this paper,two major reasons that lead to noncircularity,in-phase/quadrature(I/Q)imbalance and orthorgonal frequency division multiplexing/offset quadrature amplitude modulation(OFDM/OQAM),are considered.A number of complex-valued adaptive filtering approaches are proposed to address the associated real-world challenges of measuring and compensating I/Q imbalance impairment,and cancelling self-interference in full-duplex transceivers.Comprehensive performance analysis,and standard-compliant simulations and experiments of the proposed methods are provided,demonstrating their theoretical novelties and practicalities.The main contents of this paper are listed as follows.A second order statistics based I/Q imbalance measurement for OFDM-based wireless local area network(WLAN)transmitters is proposed.This instrument-level two-stage framework can decouple the effects of the frequency-selective measurement channel from the transmitter I/Q imbalance and attain the exact amplitude and phase imbalances.In the initial stage,a blockbased blind method,which fully exploits the available second order signal statistics,is proposed.Then,owing to the WLAN standard-compliant training sequences,coarse I/Q imbalance estimation is performed jointly with channel equalization.In the next stage,the initially recovered symbols in DATA field of standardized OFDM systems are re-calibrated using a standard decision-directed scheme,which facilitates a fine I/Q imbalance estimation.The performance of the proposed I/Q imbalance measurement method is assessed through numerical simulations and in a real world experimental setup.Performance advantages over previous methods for OFDM-based WLAN transmission systems are observed,especially when higher-order modulation and coding schemes are employed.A blind frequency-dependent I/Q imbalance compensation method in OFDM/OQAM receivers is proposed.We examine and justify that the properness of OFDM/OQAM signals is dependent on the phase shift on adjacent subchannels.For proper OFDM/OQAM,the Newton's zero search method in can be straightforwardly applied,while for improper OFDM/OQAM,a cyclic blind adaptive compensation method is proposed by making use of the hidden properness nature in the cyclostationary improper second-order statistics.This time-domain blind compensation scheme can circumvent the intensified intrinsic interference that is faced with by frequency-domain compensation schemes,and provide satisfactory image interference suppression performance without dedicated pilot symbols.The proposed method can also be viewed as a preliminary image interference elimination process.Numerical simulations are presented to illustrate the improvement of bit error rate.An augmented nonlinear complex least mean square(ANCLMS)for digital self-interference cancellation in full-duplex direct-conversion transceivers is proposed.We establish that,for shared-antenna-based direct-conversion transceivers,the underlying widely linear signal model is not adequate for strong transmit signals,and the suboptimality of the conventional augmented complex least mean square(ACLMS)based SI canceller is analyzed.In order to achieve a sufficient signal-to-interference-plus-noise ratio(SINR)when the nonlinear SI components are not negligible,we propose an ANCLMS based SI canceller for a joint cancellation of both the linear and nonlinear SI components by virtue of a widely nonlinear model fit.A rigorous mean and mean square performance evaluation is conducted to justify the performance advantages of the proposed scheme over the ACLMS solution.Simulations on OFDM-based WLAN standard compliant waveforms support the analysis.A cost-effective dual-channel nonlinear self-interference canceller in full-duplex directconversion transceivers.By exploiting the duality between complex-valued signals and their bivariate-real counterparts in capturing the equivalent second order signal statistics in fullduplex transceiver SI cancellation task,a dual channel nonlinear complex least-mean-square(DC-NCLMS)canceller is proposed.This is achieved by splitting the mean square error function used in the ANCLMS SI canceller,and performing two independent optimisation processes.Comprehensive convergence and steady-state performance evaluation is next conducted,addressing its second order optimality and low computation complexity in the widely-nonlinear modelling full-duplex self-interference cancellation context.Simulations in practical full-duplex self-interference transceiver settings validate the theoretical results.