Mobile assistive technologies for the visually impaired

This paper proposes a new technique to identify the parameters of a digital predistorter based on iterative learning control (ILC). ILC is a well-established control theory technique that can obtain the inverse of a system. Instead of focusing on identifying the predistorter parameters, the technique proposed here first uses an iterative learning algorithm to identify the optimal power amplifier (PA) input signal that drives the PA to the desired linear output response. Once the optimal PA input signal is identified, the parameters of the predistorter are estimated using standard modeling approaches, e.g., least squares. To this end, in this paper, we present a complete derivation of an ILC scheme suitable for the linearization of PAs, which includes convergence conditions and the derivation of two learning algorithms. The proposed ILC scheme and parameter identification technique were demonstrated experimentally and compared with the indirect learning architecture (ILA) and direct learning architecture (DLA). The experimental results show that, even for the most difficult cases, the proposed ILC scheme can successfully linearize the PA. The experimental results also indicate that the proposed parameter identification technique is more robust to measurement noise than ILA and can provide better linearity performance when the PA nonlinearities are strong. In addition, the proposed parameter identification technique can achieve similar or better linearity performance than DLA but with a simpler identification process.

Iterative Learning Control for RF Power Amplifier Linearization

This paper proposes three novel models for behavioral modeling and digital pre-distortion (DPD) of nonlinear 2 × 2 multiple-input multiple-output (MIMO) transmitters in the presence of crosstalk. The proposed models are extensions of the single-input single-output generalized memory polynomial model. Three types of crosstalk effects were studied and characterized as linear, nonlinear, and nonlinear & linear crosstalk. A comparative study was performed with previously published models for the linearization of crosstalk in a nonlinear 2 × 2 MIMO transmitter. The experiments indicate that, depending on the type of crosstalk, the selection of the correct model in the transmitter is necessary for behavioral modeling and sufficient DPD performance. The effects of coherent and partially noncoherent signal generation on the performance of DPD were also studied. For crosstalk levels of -30 dB, the difference in the normalized mean square error and adjacent channel power ratio was found to be 3-4 dB between coherent and partially noncoherent signal generation.

Behavioral Modeling and Linearization of Crosstalk and Memory Effects in RF MIMO Transmitters

The rate regions of half- and full-duplex links using the orthogonal frequency division multiplexing (OFDM) technique are analyzed after taking into account the non-ideality of practical transceivers. The non-ideality is quantified by a measure named error vector magnitude (EVM) level in practical systems. It is approximated as a Gaussian noise added to the original signal by the transmitter. The assumed full-duplex transceiver suppresses the self-interference via a three-stage process. The stages are antenna isolation, RF cancellation, and digital baseband cancellation. The self-interference caused by the EVM noise and the original signal are suppressed differently in the three-stage process. The optimal power allocation algorithms are developed to maximize the rates of the half- and full-duplex OFDM links under two different strategies. The first one aims at a low complexity design, where each node uniformly allocates the power over sub-carriers, whereas the second one adaptively allocates the power over sub-carriers to achieve the largest rate. Using the developed algorithms, the achieved rate regions under frequency-flat and frequency-selective environments are compared.

On Rate Region Analysis Of Half- and Full-Duplex OFDM Communication Links

This article reports on a survey of the use of buses in Santa Barbara, California, by 55 persons who are visually impaired (including those who are blind and those who have low vision). Findings on users’ frustrations, potential use of technological aids for travel, and perceptions of and attitudes toward the characteristics of bus services are presented. In addition, differences in the responses of those to whom household cars were and were not available are analyzed, and suggestions for dealing with the participants’ major concerns are provided.

Attitudes of Visually Impaired Persons Toward the Use of Public Transportation

In this paper, we present the results for system level convergence of indirect learning architecture (ILA) and direct learning architecture (DLA) for digital predistortion. We show that best performance with ILA and DLA can only be obtained if the system level identification of the power amplifier and predistorter is done iteratively. Results are demonstrated in terms of improvement in adjacent channel power ratio (ACPR) and error vector magnitude (EVM) at the output of power amplifier (PA) with each system level iteration for both the architectures when a Long Term Evolution-Advanced (LTE-Advanced) signal is applied at the input. We also show that predistorter identification with DLA is more robust compared to ILA in presence of additive white Gaussian noise (AWGN).

On the system level convergence of ILA and DLA for digital predistortion

The generalized memory polynomial (GMP) model is one of the most commonly used models in digital predistortion to compensate for the nonlinearities and memory effects of power amplifiers (PAs). A very difficult, yet crucial, aspect of behavioral modeling is model dimensioning, i.e., determining nonlinear orders and memory depths. In this paper, we propose an algorithm based on hill-climbing (HC) heuristic to search for a GMP model structure, which provides the best tradeoff between modeling accuracy and complexity with different search criteria. Algorithm performance and convergence are evaluated for two different search criteria. Some optimization methods of the algorithms are then proposed to reduce the execution time by controlling its search path. The algorithm has been evaluated using real measurements from two different Doherty PAs. The solutions of the algorithm are proved robust in linearization of PA on the test bench. The results show that the proposed algorithm allows us to decrease the searching time by a factor typically greater than $10^4$ compared with exhaustive search.

A Novel Algorithm for Determining the Structure of Digital Predistortion Models

This letter unfolds a digital predistortion (DPD) technique that improves the linearity of limited range mobile front haul links for the contemporary long-term evolution (LTE) and future (5G) networks. In particular, the proposed technique is applied to radio-over-fiber links based on single-mode (SM) vertical cavity surface emitting lasers emitting at 850 nm and standard SM fibers. Both the memory and generalized memory polynomial models are exploited for the predistorter and identified by utilizing indirect learning architecture. The impact of the DPD technique is observed by the link performance improvement in terms of normalized mean square error and adjacent channel power ratio, referring to complete LTE frames of 10 ms occupying 5-MHz bandwidth and having 64-quadrature amplitude modulation format. Furthermore, the effectiveness of the DPD approach, when varying input power levels, is investigated. The experimental results demonstrate the capability of the proposed DPD technique to achieve promising linearization performance.

Digital Predistortion for Linearity Improvement of VCSEL-SSMF-Based Radio-Over-Fiber Links

Voice channel in mobile communication systems has high priority and is almost always available. Its use as a bearer for data transmission allows to take advantage of this availability. But medium rate speech coders and associated VAD (Voice Activity Detector) and DTX (Discontinuous Transmission) which are well suited to voice signals, can strongly degrade other types of signals such as data modulated signals. In this paper we propose and evaluate a system for data transmission over mobile voice channel based on M-FSK modulation with optimized parameters. The parameters of the M-FSK modulation are tuned using real mobile communications voice channel in order to minimize the degradation introduced by the speech coder and by the discontinuous transmission (DTX) Our experiments show that it is possible to find a set of parameters for the M-FSK modulation which allows to achieve a moderate bit error rate (ie <; 10-2) without error-correcting code.

Data transmission over mobile voice channel based on M-FSK modulation

A R T I C L E A B S T R A C T Article history: Received:30 April, 2019 Accepted:18 May, 2019 Online: 10 June, 2019 The article proposes a Digital Predistortion (DPD) methodology that substantially meliorates the linearity of limited range Mobile Front Haul links for the extant Long-Term Evolution (LTE) and future (5G) networks. Specifically, the DPD is employed to Radio over Fiber links that contrive of Vertical Cavity Surface Emitting Lasers (VCSELs) working at 850 nm. Both, Memory and Generalized Memory Polynomial models are implied to Single Mode (SM) and Multi-Mode (MM) VCSELs respectively. The effectiveness of the proposed DPD methodology is analyzed in terms of Normalized Mean Square Error, Normalized Magnitude, Normalized phase and Adjacent Channel Power Ratio. The demonstration has been carried out with a complete (Long Term Evolution) LTE frame of 10 ms having 5 MHz bandwidth with 64-QAM modulation configuration. Additionally, the effectuality of the proposed DPD technique is evaluated for varying levels of input power and link lengths. The experimental outcomes signify the novel capability of the implied DPD methodology for different link lengths to achieve higher system linearization.

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Olivier Venard

Papers

On the system level convergence of ILA and DLA for digital predistortion

A Novel Algorithm for Determining the Structure of Digital Predistortion Models

Digital Predistortion for Linearity Improvement of VCSEL-SSMF-Based Radio-Over-Fiber Links

Data transmission over mobile voice channel based on M-FSK modulation

Linearity improvement of VCSELs based radio over fiber systems utilizing digital predistortion