Gustavo J. González

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.

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.

Full-duplex amplify-and-forward relays with optimized transmission power under imperfect transceiver electronics

In-band full-duplex (FD) relays are useful for extending coverage areas and increasing overall throughput in wireless networks. The main technical difficulty hindering their implementation and use is their inherent self-interference (SI), generated due to simultaneous in-band reception and forwarding. Efficient SI mitigation is a practical necessity, and the imperfections in transceiver electronics, from which power amplifier (PA) non-linearity is one of the most serious phenomena, have to be taken into account in order to not limit the performance of such techniques. The magnitude of the distortion introduced by the PA depends on the relay input back-off (IBO) whose optimization for alleviating the effect of PA non-linearity is the main research objective in this paper. In particular, although plain signal-to-noise ratio (SNR) at the destination obviously increases when the IBO decreases, increased transmit power also strengthens the non-linear distortion leading to decreasing overall signal-to-interference-plus-noise ratio (SINR). We develop expressions for bounding the optimal IBO setting that maximizes the SINR at the destination, considering all relevant hardware impairments and SI cancellation with I/Q imbalance compensation. We provide closed-form solutions for the soft-limiter PA model and numerical results for more general PA models. Finally, the derived IBO bounds are compared with the numerical maximization of the SINR and the minimization of the bit-error rate (BER) to demonstrate that the theoretical bound settings provide good approximations to the optimal one.

Blind self-interference cancellation for full-duplex relays

Full-duplex relays increase the spectral efficiency and diversity, especially for the amplify and forward mode. However, transmission and reception in the same frequency band produce coupling between the transmitted and received signals that needs to be reduced. The cyclic prefix introduced in modern modulations produces a periodic autocorrelation that is perturbed by the relay coupling. We propose a blind adaptive algorithm to compensate for the coupling, that employs the known autocorrelation of the source signal as a reference. We study the proposed criterion and analyze the stationary points of the adaptive algorithm. Finally, we confirm the canceler performance using simulations.

Compensation of ADC-induced distortion in broadband full-duplex transceivers

The performance of full-duplex transceivers is highly dependent on their ability to remove the self-interference (SI) that is generated by the simultaneous transmission and reception in the same frequency band. Even after passive isolation and RF cancellation, the magnitude of the residual SI is usually considerably higher than the signal of interest. The resolution of the analog-to-digital converter (ADC) must be rather high to accommodate both the residual SI and the intended signal to allow the digital SI cancellation. Adding to this technical challenge, 5G systems will occupy large signal bandwidths of hundreds of MHz. Thereby, high-speed and high resolution ADCs are required. In order to obtain a reasonable compromise between performance and massive production cost, a time-interleaved ADC (TI ADC) structure is often used. In this paper, we analyze the TI-ADC induced nonlinear distortion on the performance of a full-duplex transceiver. In particular, time-mismatch errors are considered, and in addition, we apply a digital post-processing to mitigate ADC imperfections. Simulation results show that even a slight mismatch in the TI ADC array can severely deteriorate the performance of the whole system. On the other hand, we show that included ADC compensation can restore adequate performance.

Predistortion for power amplifier linearization in full-duplex transceivers without extra RF chain

Genuine full-duplex operation requires effective mitigation of self-interference (SI) due to simultaneous transmission and reception at the same frequency band. In addition to its well-known harmful effect on signals of high peak-to-average power ratio, nonlinear behavior of a power amplifier (PA) complicates SI cancellation and induces spectral regrowth. We introduce a digital predistortion architecture that linearizes the response of the PA in a full-duplex transceiver. Specifically, we propose a two-step procedure to estimate the predistorter parameters and the SI canceller coefficients. Different from a direct application of conventional predistorters, the proposed architecture does not need an extra RF chain to estimate the PA response and exploits the inherent SI signal instead. Finally, simulation results show that the proposed scheme is able to increase significantly the signal-to-interference-plus-noise ratio at the transceiver output and to reduce out-of-band emissions when compared to linear and nonlinear cancellation without predistortion.

Analysis of the CFO Successive Interference Cancellation for the OFDMA Uplink

The uplink of orthogonal frequency division multiple access or single-carrier frequency division multiple access suffers multiple access interference when carrier frequency offset (CFO) is not properly estimated and compensated. In particular, multicarrier uplink CFO compensation is highly complex due to the multiuser context. Successive interference cancellation algorithms are effectively employed to compensate for the CFO, where the interference produced by each user is handled sequentially through a series of iterations. The main contribution of this work is the analysis of the CFO compensation performance of efficient successive cancellation algorithms. We study the mean square symbol error, and derive a useful upper-bound of the compensation technique performance at convergence. This result extends the general convergence results for the space-alternating generalized expectation-maximization algorithm in the CFO compensation scenario. Finally, we validate the analysis with numerical simulations.