Fernando H. Gregorio

Nonlinear Amplifier Distortion in Cooperative Amplify-and-Forward OFDM Systems

This paper studies receiver techniques for nonlinear amplifier distortion compensation in an OFDM relay-assisted cooperative communication system. The system model includes a nonlinear amplifier at the amplify-and-forward relay, modeled as a solid state power amplifier (SSPA). A maximum ratio combiner (MRC) is introduced that includes the effects of the amplifier distortions. The MRC is obtained by a proper modeling of the nonlinear distortion noise. Furthermore, we introduce a power amplifier nonlinearity cancellation (PANC) technique that is suitable for cooperative systems. Our simulation results confirm that the new MRC and PANC techniques offer substantial performance improvements if the relayed signal is subject to nonlinear distortion.

Power amplifier linearization technique with IQ imbalance and crosstalk compensation for broadband MIMO-OFDM transmitters

The design of predistortion techniques for broadband multiple input multiple output-OFDM (MIMO-OFDM) systems raises several implementation challenges. First, the large bandwidth of the OFDM signal requires the introduction of memory effects in the PD model. In addition, it is usual to consider an imbalanced in-phase and quadrature (IQ) modulator to translate the predistorted baseband signal to RF. Furthermore, the coupling effects, which occur when the MIMO paths are implemented in the same reduced size chipset, cannot be avoided in MIMO transceivers structures. This study proposes a MIMO-PD system that linearizes the power amplifier response and compensates nonlinear crosstalk and IQ imbalance effects for each branch of the multiantenna system. Efficient recursive algorithms are presented to estimate the complete MIMO-PD coefficients. The algorithms avoid the high computational complexity in previous solutions based on least squares estimation. The performance of the proposed MIMO-PD structure is validated by simulations using a two-transmitter antenna MIMO system. Error vector magnitude and adjacent channel power ratio are evaluated showing significant improvement compared with conventional MIMO-PD systems.

Receiver-side nonlinearities mitigation using an extended iterative decision-based technique

This work presents an iterative receiver cancellation technique for mitigating the inband distortion introduced by a nonlinear wideband transmitter power amplifier (PA). The proposed decision-based technique employs a Wiener-Hammerstein model that accounts for the nonlinear transfer function and memory of the PA as well as for the wireless propagation channel. As such, the mitigation technique can be seen as a generalization of existing iterative decision-based techniques assuming memoryless PA nonlinearities. For successful distortion mitigation, the iterative technique requires an estimate of the nonlinear model that characterizes the PA. We propose to perform this model identification at the receiver, embedded in an iterative decision-based scheme, avoiding the nonideal analog-to-digital feedback loop associated with transmitter-based model identification. A stochastic algorithm is proposed for the model identification providing the necessary PA model parameters required for symbol detection. In addition, we analyze the convergence properties of the proposed technique. Simulation results confirm that the proposed mitigation technique provides distortion cancellation at almost the same level to the case of perfect knowledge of the PA model. These results enable us to employ power amplifiers with more relaxed linearity requirement, moving the operation point to a region with improved power efficiency while reducing the system overall degradation.

PAPR reduction in FBMC using an ACE-based linear programming optimization

This paper presents four novel techniques for peak-to-average power ratio (PAPR) reduction in filter bank multicarrier (FBMC) modulation systems. The approach extends on current PAPR reduction active constellation extension (ACE) methods, as used in orthogonal frequency division multiplexing (OFDM), to an FBMC implementation as the main contribution.The four techniques introduced can be split up into two: linear programming optimization ACE-based techniques and smart gradient-project (SGP) ACE techniques. The linear programming (LP)-based techniques compensate for the symbol overlaps by utilizing a frame-based approach and provide a theoretical upper bound on achievable performance for the overlapping ACE techniques. The overlapping ACE techniques on the other hand can handle symbol by symbol processing. Furthermore, as a result of FBMC properties, the proposed techniques do not require side information transmission. The PAPR performance of the techniques is shown to match, or in some cases improve, on current PAPR techniques for FBMC. Initial analysis of the computational complexity of the SGP techniques indicates that the complexity issues with PAPR reduction in FBMC implementations can be addressed.The out-of-band interference introduced by the techniques is investigated. As a result, it is shown that the interference can be compensated for, whilst still maintaining decent PAPR performance. Additional results are also provided by means of a study of the PAPR reduction of the proposed techniques at a fixed clipping probability. The bit error rate (BER) degradation is investigated to ensure that the trade-off in terms of BER degradation is not too severe.As illustrated by exhaustive simulations, the SGP ACE-based technique proposed are ideal candidates for practical implementation in systems employing the low-complexity polyphase implementation of FBMC modulators. The methods are shown to offer significant PAPR reduction and increase the feasibility of FBMC as a replacement modulation system for OFDM.

PAPR reduction in FBMC systems using a smart gradient-project active constellation extension method

The filter bank multicarrier (FBMC) modulation scheme has recently seen renewed interest and is being considered as a viable alternative to orthogonal frequency division multiplexing (OFDM). FBMC, however suffers from the same high peak-to-average power ratio (PAPR) drawback as OFDM systems. Conventional OFDM PAPR reduction techniques cannot be directly applied to FBMC due to the overlapping nature of FBMC symbols. A novel technique is proposed based on an evolution of the smart-gradient project active constellation extension (SGP-ACE) PAPR reduction method used for OFDM systems, namely the FBMC SGP-ACE method. The proposed method is applied to a set of contiguous FBMC symbols, thereby compensating for the overlapping nature of FBMC modulation. The proposed method requires less iterations than a projection-onto-convex-sets (POCS) ACE approach to converge to a lower PAPR and significant reduction in complexity can be achieved as opposed to current FBMC PAPR reduction techniques which tend to require the addition of advanced signal processing. The proposed FBMC SGP-ACE method outperforms a conventional FBMC POCS-ACE method by 2.6dB in PAPR reduction at a clip probability of 10-4 on the 1st iteration.

Compensation of IQ imbalance and transmitter nonlinearities in broadband MIMO-OFDM

Predistortion techniques are implemented in OFDM systems to improve power amplifier efficiency while keeping in-band and out-of-band distortion at acceptable levels. Predistorter (PD) design for broadband MIMO-OFDM systems introduces several implementation challenges. Coupling effects cannot be avoided in MIMO transceivers where the paths are implemented on the same chipset. Moreover, baseband PDs are affected by imbalances in the IQ modulator. This paper proposes a MIMO-PD that linearizes the power amplifier (PA) response and compensates for both crosstalk and IQ imbalance. Error vector magnitude (EVM) and adjacent channel power ratio (ACPR) results show a significant improvement compared with conventional PD systems.

OFDM receivers with iterative nonlinear distortion cancellation

We propose two practical schemes for the cancellation of nonlinear distortions caused by the power amplifiers (PA) in Orthogonal Frequency Division Multiplexing (OFDM) systems. The algorithms are iterative and located at the receiver. They utilize a soft channel decoder to improve the capabilities of the distortion estimation and relay all the computational complexity to the receiver. Simulation results show that the algorithm can improve the coded bit error rate (BER) to levels close to the linear system case.

Uplink CFO compensation for FBMC multiple access and OFDMA in a high mobility scenario

Abstract We study in this work CFO compensation methods for two multicarrier multiple access techniques in a high mobility scenario. In particular, we consider orthogonal frequency division multiple access (OFDMA) and filter bank multicarrier multiple access (FBMC-MA). The main motivation for this study is not only the different sensitivity these multicarrier techniques have to CFO but also the different methods they use to reduce CFO effect. In a high mobility scenario the CFO is re-estimated to follow its variation. We show that the frequency at which the CFO is re-estimated has a strong influence in the performance and the complexity of the proposed compensation methods. Additionally, we present a low-complexity CFO compensation method for OFDMA that employs a better approximation of the intercarrier interference than previous approaches. Regarding FBMC-MA, we introduce an extension of a CFO-compensation method that allows to consider a multitap channel equalizer. Finally, using simulations, we compare the performance of the compensation methods over several channel and time-varying CFO conditions.

Data-aided CFO estimators based on the averaged cyclic autocorrelation

Wireless communication systems typically employ a repetitive preamble in each slot which is used for parameter acquisition. The repetitive preamble is useful for estimating the carrier frequency offset (CFO), usually based on the autocorrelation of the received signal. In this paper, we derive a family of novel data-aided CFO estimators. The proposed estimators are based on a new autocorrelation function which is defined using cyclostationary properties of the repetitive preamble. In contrast to previous approaches, the new estimators make use of high-order noise terms leading to an improved performance. We present a detailed analysis of the proposed estimators and provide closed-form expressions for the variance of the estimators. The new estimators are shown to outperform the existing estimators obtaining a moderate improvement at high signal to noise ratio (SNR) and a considerable improvement at low SNR, by means of a reasonable increase in computational complexity.

Patch antenna design for full-duplex transceivers

We present the design, implementation and validation (by measurements) of a two-element antenna array for inband full-duplex (FD) applications. The antenna array is designed to provide large isolation between its transmission and reception ports by changing the polarization of the channels and by designing a suitable radiation pattern for the transmission antennas. Especially, the design of the antenna array is developed by tuning the characteristic S-parameters of the antenna elements so that isolation levels larger than 55 dB are achieved without degrading the far-field transmission pattern. Simulation and measurement results show that the proposed antenna design could be a promising solution for full-duplex applications, i.e., FD access points and/or FD sensing nodes in cognitive radio systems.