The design of linear precoders or decoders for multiuser multiple-input multiple-output filterbank multicarrier (FBMC) modulations in the case of a strong channel frequency selectivity is presented. The users and the base station (BS) communicate using space division multiple access. The low complexity proposed solution is based on a single tap per-subcarrier precoding/decoding matrix at the BS in the downlink/uplink. As opposed to classical approaches that assume flat channel frequency selectivity at the subcarrier level, the BS does not make this assumption and takes into account the distortion caused by channel frequency selectivity. The expression of the FBMC asymptotic mean squared error (MSE) in the case of strong channel selectivity derived in earlier works is developed and extended. The linear precoders and decoders are found by optimizing the MSE formula under two design criteria, namely zero forcing or minimum MSE. Finally, simulation results demonstrate the performance of the optimized design. As long as the number of BS antennas is larger than the number of users, it is shown that those extra degrees of freedom can be used to compensate for the channel frequency selectivity.

Single-Tap Precoders and Decoders for Multi-User MIMO FBMC-OQAM under Strong Channel Frequency Selectivity

In a recent paper, Michalowicz et al. provide an extension of Isserlis theorem to the case of a Bernoulli location mixture of a Gaussian vector. We extend here this result to the case of any location mixture of Gaussian vector; we also provide an example of the Isserlis theorem for a "scale location" mixture of Gaussian, namely the d-dimensional generalized hyperbolic distribution.

A generalized Isserlis theorem for location mixtures of Gaussian random vectors

The 5G and beyond systems require the highly efficient linear power amplifier (PA) at the base station to deliver promised data rate in a power efficient manner. To facilitate higher data rates in future 5G communication, the demands for the wideband signals are continuously increasing, which creates adversity in employing digital predistortion (DPD) for the linearization of RF PA in ultra-wideband systems. System bandwidth constraint of the conventional DPD does not provide sufficient linearization of wideband signals. In this paper, we present two types of predistorter, first is RFin–RFout analog predistorter, in which linearization is performed by RF components in the analog domain, and the second is hybrid RF predistorter which takes the advantage of advanced DSP platform for improving the performance of RFin–RFout analog predistorter. The proposed predistorters eliminate the constraint on the system bandwidth of the conventional DPD. The requirements of data converters and reconstruction filters are relaxed in the analog predistorter (APD) architecture, whereas hybrid digitally controlled (HDC) APD eliminates the need of analog components by compensating the delay digitally. In order to validate this concept, a ZX60V-63+ PA from minicircuits is tested with 160-MHz eight-component carrier-long term evolution signal. The proposed APD architecture is able to deliver an adjacent channel power ratio (ACPR) of −46.4 dBc with an ACPR improvement of 13.4 dB at 1-dB back-off. Furthermore, with digital intervention, modified model, HDC-APD further provides a significant improvement in the linearization performance and delivers an ACPR of −53.5 dBc with an ACPR improvement of 20.5 dB.

Low-Cost RF in –RF out Predistorter Linearizer for High-Power Amplifiers and Ultra-Wideband Signals

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Rf Power Amplifier Behavioral Modeling

In this paper, the nonlinear effects of amplified dual-band orthogonal frequency-division multiplexing (OFDM) signals are analyzed. Furthermore, a spectrum model of the intermodulation and cross-modulation for dual-band OFDM signals in terms of third-order intercept points ( ${IP}_{3}$ ) is derived. The experimental measurements match the analytical predictions, which is useful to the design and test of concurrent dual-band RF transmitters for RF engineers.

Spectrum Modeling of Cross-Modulation for Concurrent Dual-Band RF Power Amplifiers in OFDM Modulation

Effects of even-order nonlinear terms on dual-band power amplifier modeling

Dual-band RF amplifiers play increasingly important roles in next-generation mobile communication systems including 5G, and the out-of-band intermodulation products are often not negligible since they generate interference to adjacent channels. In this article, following our previous modeling of cross-modulation for amplified dual-band signals, an analytical expression of out-of-band intermodulation for dual-band orthogonal frequency-division multiplexing signals is derived using the third-order intercept points I P 3 . The experimental measurement results validate the proposed analytical expression.

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Spectrum Modeling of Out-of-Band Intermodulation for Dual-Band RF Amplifiers in OFDM Modulation

The recent research on mobile communication systems has triggered increasing interest in multi-band transmitters. Our recent work advanced the spectrum modeling to cross-modulation of dual-band amplified orthogonal frequency-division multiplexing (OFDM) signals. However, it was limited to an assumption that the signal bandwidth in one band cannot be more than twice the signal bandwidth in the other band. This assumption may not always be valid; for example, the bandwidth for one long-term evolution signal could be multiple times wider than the bandwidth for the adjacent channel. In this paper, we extend the amplified spectrum model beyond that assumption. Together with the earlier work, this comprehensive model allows RF designers to predict the amplified dual-band spectrum of OFDM signals with no bandwidth restriction. The experimental measurements match well the analytic predictions.

Bandwidth Extension on Spectrum Modeling of Cross-Modulation for Dual-Band Amplification of OFDM Signals

/pdf/on-ip2-impact-to-nonlinear-distortion-of-rf-amplifiers-17d2zeujq8.pdf

On IP2 impact to nonlinear distortion of RF amplifiers

Orthogonal frequency-division multiplexing (OFDM) has been developed into a popular modulation scheme for wireless communication systems, used in applications such as LTE and 5G. In wireless communication systems, nonlinearity caused by radio frequency (RF) amplifiers will generate distortions to both passband and adjacent channels such that the transmission quality is degraded. The study of this article aims to predict the power spectrum for OFDM based signals at the output of an RF amplifier due to the nonlinearity. In this article, based on Taylor polynomial coefficients, a power spectrum expression for amplified OFDM signals in terms of intercept points (up to n th-order) is derived. This model is useful to RF engineers in choosing and testing RF amplifiers with appropriate specifications, such as intercept points and gain, to meet the requirements of wireless standards. Measurements are carried out to confirm the results of the proposed model. 

Xianzhen Yang

Papers

Effects of even-order nonlinear terms on dual-band power amplifier modeling

Spectrum Modeling of Out-of-Band Intermodulation for Dual-Band RF Amplifiers in OFDM Modulation

Bandwidth Extension on Spectrum Modeling of Cross-Modulation for Dual-Band Amplification of OFDM Signals

On IP2 impact to nonlinear distortion of RF amplifiers

Predicting the Power Spectrum of Amplified OFDM Signals Using Higher‐Order Intercept Points