Mika Lasanen

Adaptive Time Misalignment Compensation in Envelope Tracking Amplifiers

This paper considers the compensation of time misalignment between RF and envelope signals of envelope tracking (ET) amplifier. In particular, we propose methods to compensate the time misalignment and compare them to a previously presented method. Our simulation results indicate that the proposed methods give very accurate timing estimates. In the presence of optimal predistorter, we achieve with 90 percent probability adjacent channel power (ACP) level of -80 dB. With adaptive predistorters, we cannot see any degradation of ACP comparing to case of ideal timing. On the other hand, the proposed methods give roughly 10 dB better ACP than the previously presented method.

Predistortion of Radio Over Fiber Links: Algorithms, Implementation, and Measurements

Distributed antenna systems (DASs) have been proposed for use in high-data-rate wireless communications. Connecting the central unit to remote antenna units, radio over fiber (RoF) links become essential parts of the DAS. One of the major problems in the RoF links is the nonlinear distortion experienced in its optoelectronic devices. This paper considers the adaptive compensation of the nonlinear RoF link. In particular, we present an extension to the conventional least-mean-square algorithm for memory-polynomial-based predistortion. In addition, we study different combinations of the algorithms for the predistortion. Moreover, we implement the adaptive predistorter in hardware, build a real RoF link, and verify the performance by measurements. The results demonstrate that distortion of the RoF link can be considerably diminished with a low complexity design.

Analysis of and compensation for non-ideal RoF links in DAS [Coordinated and Distributed MIMO]

Distributed antenna systems have been found to be an elegant solution for the problems arising in high-data-rate wireless communication, particularly in large service areas. This article considers radio over fiber links as an essential part of the DAS, connecting the central unit with the remote antenna units. In particular, we analyze and discuss delays and nonlinearities stemming from the RoF links. In addition, we study the compensation for these impairments. Our studies indicate that the RoF links are a viable and cost-effective solution for implementing the DAS, although some of the RoF link non-idealities require compensation.

Performance of adaptive algorithms for compensation of radio over fiber links

This paper considers the adaptive predistortion of the nonlinear distortions in a Radio over Fiber (RoF) link. In particular, we modify and compare two adaptive algorithms developed originally for the compensation of the linear systems, namely LMS and variable step size normalized LMS (VS NLMS). A recursive least squares (RLS) solution is used as a reference. Our simulation results indicate that over 40 dB improvement of adjacent channel power ratios can be achieved via the predistortion. Furthermore, we show that in the compensation of the nonlinear RoF link, the LMS can be used in such a way that its performance is comparable to more complex RLS.

Architectures for Joint Compensation of RoF and PA with Nonideal Feedback

A high capacity wireless communication system requires careful design to minimize interference and distortion effects. This paper considers the adaptive predistortion of the nonlinear distortions induced by a Radio over Fiber (RoF) link and power amplifier (PA) connected in series. In particular, we study the architectures for the joint compensation of the nonlinearities in the presence of nonideal feedback. From the adaptive algorithm point of view, we study different combinations of algorithms for the predistortion. Our simulation results indicate that combined use of least mean squares (LMS) and recursive least squares (RLS) gives the best trade-off between complexity and performance. In addition, we show that the use of nonideal feedback causes a collapse in the performance of the predistortion. However, when using a compensated RoF link for feedback, the degradation of the adjacent channel power is very small compared to the case of the ideal feedback.

Adaptive Predistortion Architecture for Nonideal Radio Transmitter

A nonideal radio transmitter both distorts the desired signal and generates spectral regrowth causing interference to adjacent channels. Predistortion techniques can be used to minimize these effects. The joint effect of different types of nonidealities is not straightforward to analyze and has not yet been studied in detail in the literature. In this paper, we demonstrate how to adaptively predistort a nonlinear high power amplifier and an I/Q modulator, and to compensate the frequency response of a radio frequency filter. The study considers the spatial order of adaptive predistorters and the temporal sequence of adaptation steps. In this way, both the system capacity and the power efficiency are improved.

Review of Latest Advances in 3GPP Standardization: D2D Communication in 5G Systems and Its Energy Consumption Models

Device-to-device (D2D) communication is an essential part of the future fifth generation (5G) system that can be seen as a “network of networks,” consisting of multiple seamlessly-integrated radio access technologies (RATs). Public safety communications, autonomous driving, socially-aware networking, and infotainment services are example use cases of D2D technology. High data rate communications and use of several active air interfaces in the described network create energy consumption challenges for both base stations and the end user devices. In this paper, we review the status of 3rd Generation Partnership Project (3GPP) standardization, which is the most important standardization body for 5G systems. We define a set of application scenarios for D2D communications in 5G networks. We use the recent models of 3GPP long term evolution (LTE) and WiFi interfaces in analyzing the power consumption from both the infrastructure and user device perspectives. The results indicate that with the latest radio interfaces, the best option for energy saving is the minimization of active interfaces and sending the data with the best possible data rate. Multiple recommendations on how to exploit the results in future networks are given.

Compensation of linear and nonlinear distortions in envelope tracking amplifier

This paper considers the compensation of linear and nonlinear distortions which are present in RF part of envelope tracking (ET) amplifier. In particular, we consider the compensation of magnitude and phase responses of analog filters, amplifier nonlinearity, and possible time misalignment between signals in RF and envelope branches. We propose a new adaptive predistorter architecture for ET amplifiers. Our analytical results indicate that the proposed architecture is a more general than the state-of-the-art architecture and is guaranteed to work with any ET amplifier. Furthermore, our simulations show that in order to keep adjacent channel power ratio below -60 dB, at least 9th order polynomial predistorter is needed. A comparable performance is obtained with look-up-table based predistorter having only 16 entries.

Compensation of time misalignment between input signals in envelope-tracking amplifiers

This paper considers a problem of time-misalignment between envelope and RF signals in envelope-tracking amplifiers. We propose two novel time-misalignment compensation methods which are based on the concepts of self-tuning control and model reference control from adaptive control theory. The performance of proposed methods, which we measure by the achievable adjacent-channel power ratio, is compared to the performance of the state-of-the-art method. Simulation results suggest that the time-misalignment compensation which is based on self-tuning structure offers a comparable performance as the state-of-the-art method. On the other hand, the time-misalignment compensation which is based on model reference structure outperforms the state-of-the-art method by 20 dB on the average. Furthermore, compensation with model reference structure appears to be very robust to changes in system parameters.

The Distribution of Link Distances in Distributed Multiple-Input Multiple-Output Cellular Systems

The probability density function (pdf) of the distance between randomly located user equipment (UE) and its nth closest base station (BS) is studied in this paper. The knowledge of these pdfs is essential in the analysis of cellular distributed multiple-input multiple-output (MIMO) systems where N BSs cooperate in transmission or reception. We show that earlier results on ordered distance distributions in regular point patterns can be applied to the analysis of distributed MIMO systems when the UE distribution is uniform and the BS locations form a regular lattice. We present previously unpublished pdfs of the distance between a cell edge UE, whose distance to the closest BS is at least r, and four closest BSs in the hexagonal cell topology. The pdfs are verified by simulated histograms. As an example on the application of the results, we show how the signal-to-noise ratio (SNR) gain from uplink cooperative reception increases as the transmitting UE moves further from the cell center. The results from this paper can be applied to the analysis of received and transmitted (in case of power adaptive transmission) power in distributed MIMO systems.