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Digital Compensation Techniques for Transmitters in Wireless Communications Networks
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences. KTH, Skolan för elektro- och systemteknik (EES), Signalbehandling.ORCID iD: 0000-0001-8460-6509
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Since they appeared, wireless technologies have deeply transformed our society. Today, wireless internet access and other wireless applications demandincreasingly more traffic. However, the continuous traffic increase can be unbearableand requires rethinking and redesigning the wireless technologies inmany different aspects. Aiming to respond to the increasing needs of wirelesstraffic, we are witnessing a rapidly evolving wireless technology scenario.This thesis addresses various aspects of the transmitters used in wireless communications.Transmitters present several hardware (HW) impairments thatcreate distortions, polluting the radio spectrum and decreasing the achievabletraffic in the network. Digital platforms are now flexible, robust and cheapenough to enable compensation of HW impairments at the digital base-bandsignal. This has been coined as ’dirty radio’. Dirty radio is expected in future transmitters where HW impairments may arise to reduce transmitter cost or to enhance power efficiency. This thesis covers the software (SW) compensation schemes of dirty radio developed for wireless transmitters. As describedin the thesis, these schemes can be further enhanced with knowledge of thespecific signal transmission or scenarios, e.g., developing cognitive digital compensationschemes. This can be valuable in today’s rapidly evolving scenarioswhere multiple signals may co-exist, sharing the resources at the same radiofrequency (RF) front-end. In the first part, this thesis focuses on the instrumentation challenges andHWimpairments encountered at the transmitter. A synthetic instrument (SI)that performs network analysis is designed to suit the instrumentation needs.Furthermore, how to perform nonlinear network analysis using the developedinstrument is discussed. Two transmitter HW impairments are studied: themeasurement noise and the load impedance mismatch at the transmitter, asis their coupling with the state-of-the-art digital compensation techniques.These two studied impairments are inherent to measurement systems and areexpected in future wireless transmitters. In the second part, the thesis surveys the area of behavioral modeling and digital compensation techniques for wireless transmitters. Emphasis is placed on low computational complexity techniques. The low complexity is motivated by a predicted increase in the number of transmitters deployed in the network, from base stations (BS), access points and hand-held devices. A modeling methodology is developed that allows modeling transmitters to achieve both reduced computational complexity and low modeling error. Finally, the thesis discusses the emerging architectures of multi-channel transmittersand describes their digital compensation techniques. It revises the MIMOVolterra series formulation to address the general modeling problem anddrafts possible solutions to tackle its dimensionality. In the framework of multi-channel transmitters, a technique to compensate nonlinear multi-carrier satellite transponders is presented. This technique is cognitive because it uses the frequency link planning and the pulse-shaping filters of the individual carriers. This technique shows enhanced compensation ability at reduced computational complexity compared to the state-of-the-art techniques and enables the efficient operation of satellite transponders.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xiii, 63 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:017
Keyword [en]
Digital compensation, MIMO, wireless communications, satellite, Volterra, Amplfiers, HW effects
National Category
Telecommunications Communication Systems
Research subject
Information and Communication Technology
Identifiers
URN: urn:nbn:se:hig:diva-19393ISBN: 978-91-7595-540-7 (print)OAI: oai:DiVA.org:hig-19393DiVA: diva2:814615
Public defence
2015-06-15, Sal 99131, Kungsbäcksvägen 47, Gävle, 10:00 (English)
Opponent
Supervisors
Available from: 2015-05-27 Created: 2015-05-27 Last updated: 2017-01-12Bibliographically approved
List of papers
1. Noise Impact on the Identification of DigitalPredistorter Parameters in the Indirect LearningArchitecture
Open this publication in new window or tab >>Noise Impact on the Identification of DigitalPredistorter Parameters in the Indirect LearningArchitecture
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2012 (English)In: 2012 Swedish Communication Technologies Workshop (Swe-CTW), IEEE conference proceedings, 2012, 36-39 p.Conference paper (Refereed)
Abstract [en]

The indirect learning architecture (ILA) is the mostused methodology for the identification of Digital Pre-distorter(DPD) functions for nonlinear systems, particularly for highpower amplifiers. The ILA principle works in black box modelingrelying on the inversion of input and output signals of thenonlinear system, such that the inverse is estimated. This paperpresents the impact of disturbances, such as noise in the DPDidentification. Experiments were performed with a state-of-artDoherty power amplifier intended for base station operationin current telecommunication wireless networks. As expected,a degradation in the performance of the DPD (measured innormalized mean square error (NMSE)) is found in our experiments.However, adjacent channel power ratio (ACPR) canbe a misleading figure of merit showing improvement in theperformance for wrongly estimated DPD functions.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2012
Keyword
Digital Predistortion, Noise Impact, Indirect learning architecture
National Category
Signal Processing Telecommunications
Identifiers
urn:nbn:se:hig:diva-13508 (URN)10.1109/Swe-CTW.2012.6376285 (DOI)2-s2.0-84871878986 (ScopusID)978-1-4673-4763-1 (ISBN)
Conference
2012 Swedish Communication Technologies Workshop (Swe-CTW)
Available from: 2013-01-24 Created: 2012-12-07 Last updated: 2016-08-17Bibliographically approved
2. Density estimation models for strong nonlinearities in RF power amplifiers
Open this publication in new window or tab >>Density estimation models for strong nonlinearities in RF power amplifiers
2014 (English)In: Proceedings of the Asia-Pacific Microwave Conference 2014, 2014, 116-118 p.Conference paper (Refereed)
Abstract [en]

The paper presents a method for modeling strongnonlinear effects in power amplifiers based on the principlesof density estimation. The static nonlinear transfer function isobtained by averaging measured data. The performance obtainedwith density estimation is similar to the one using high ordernonlinear static polynomial models. The benefit of consideredmethod over the ones using polynomial models is that the formerestimates blindly the structure of the transfer function and doesnot suffer from numerical instabilities.

Series
Asia Pacific Microwave Conference-Proceedings
Keyword
power amplifier, density estimation, power amplifier modeling, nonlinearities, memory polynomials
National Category
Communication Systems
Identifiers
urn:nbn:se:hig:diva-17718 (URN)000380417700393 ()2-s2.0-84988258886 (ScopusID)978-490233931-4 (ISBN)
Conference
Asia Pacific Microwave Conference (APMC) 2014, 4-7 November 2014, Sendai, Japan
Available from: 2014-10-23 Created: 2014-10-23 Last updated: 2016-12-07Bibliographically approved
3. Combating the Dimensionality of Nonlinear MIMO Amplifier Predistortion by Basis Pursuit
Open this publication in new window or tab >>Combating the Dimensionality of Nonlinear MIMO Amplifier Predistortion by Basis Pursuit
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2014 (English)In: Proceedings of the 44th European Microwave Conference, 2014, 833-836 p.Conference paper (Refereed)
Abstract [en]

A general description of nonlinear dynamic MIMO systems, given by Volterra series, has significantly larger complexity than SISO systems. Modeling and predistortion of MIMO amplifiers consequently become unfeasible due to the large number of basis functions. We have designed digital predistorters for a MIMO amplifier using a basis pursuit method for reducing model complexity. This method reduces the numerical problems that appear in MIMO Volterra predistorters due to the large number of basis functions. The number of basis functions was reduced from 1402 to 220 in a 2x2 MIMO amplifier and from 127 to 13 in the corresponding SISO case. Reducing the number of basis functions caused an increase of approximately 1 dB of model error and adjacent channel power ratio.

Keyword
Digital pre distortion (DPD), MIMO power ampli fiers, LASSO, basis pursuit, power amplifier linearization
National Category
Communication Systems Signal Processing
Identifiers
urn:nbn:se:hig:diva-17716 (URN)000392912200209 ()2-s2.0-84922721597 (ScopusID)
Conference
The 44th European Microwave Conference (EuMC 2014), Roma, Italy, October 5-10, 2014
Available from: 2014-10-23 Created: 2014-10-23 Last updated: 2017-04-03Bibliographically approved
4. A synthetic vector network analyzing measurement system
Open this publication in new window or tab >>A synthetic vector network analyzing measurement system
2011 (English)In: IEEE Transactions on Instrumentation and Measurement, ISSN 0018-9456, Vol. 60, no 6, 2154-2161 p.Article in journal (Refereed) Published
Abstract [en]

In this paper a synthetic vector network analyzing measurement system is presented. The system is based on a hardware set-up, including a signal generator and a vector signal analyzer, with the vector network analyzing functionality implemented in the software. The measurements of the proposed system demonstrated comparable performance in terms of accuracy and speed compared with a modern traditional vector network analyzer, but it is more flexible due to its inherent software implementation. The proposed system’s ability to measure nonlinear phenomena is addressed and discussed, and some preliminary results are given.

Place, publisher, year, edition, pages
Institution of Electrical Engineers (IEE), 2011
Keyword
vector network analyzer, S-parameters, nonlinear, virtual instrument, synthetic instrument
National Category
Control Engineering
Identifiers
urn:nbn:se:hig:diva-9735 (URN)10.1109/TIM.2011.2113132 (DOI)000290538700025 ()2-s2.0-79955971576 (ScopusID)
Available from: 2011-07-01 Created: 2011-07-01 Last updated: 2015-05-27Bibliographically approved
5. Pilot tone aided measurements to extend the bandwidth of radio frequency applications
Open this publication in new window or tab >>Pilot tone aided measurements to extend the bandwidth of radio frequency applications
2016 (English)In: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 90, 534-541 p.Article in journal (Refereed) Published
Abstract [en]

A technique to extend the effective measurement bandwidth of a non-coherent vector receiver is presented. This bandwidth extension technique relies on the use of a pilot signal (known a priori), which is added on the signal of interest and is measured in a single receiver. Compared to other bandwidth extension techniques referred as stitching techniques, the proposed approach avoids error propagation in the measurement bandwidth and simultaneously enables the measurement of signals that do not contain energy in certain spectral bands.

The pilot signal is created in digital stages, which tackles to large extent the requirement of the a priori knowledge of this signal. Further, the pilot signal is designed to minimize estimation errors of the proposed technique, providing enhanced performance. It is analytically shown that the error incurred by the proposed method is always lower than the error from the measurement noise.

Measurement results show the method functionality with an error in the range of −50 dB of the signal measured. Finally, the usefulness of the proposed technique is illustrated by measuring the input and output of an amplifier with dynamic range in excess of 80 dB over 290 MHz using an 18 MHz bandwidth receiver. This measurement could not have been performed by existing stitching techniques.

Keyword
Bandwidth extension, Digitally modulated signals, Noncoherent vector receivers, Vector signal analyzer (VSA), Multisine signal
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:hig:diva-18942 (URN)10.1016/j.measurement.2016.05.005 (DOI)000377389600062 ()2-s2.0-84969632452 (ScopusID)
Available from: 2013-02-04 Created: 2015-02-05 Last updated: 2017-01-12Bibliographically approved
6. Output impedance mismatch effects on the linearity performance of digitally predistorted power amplifiers
Open this publication in new window or tab >>Output impedance mismatch effects on the linearity performance of digitally predistorted power amplifiers
2015 (English)In: IEEE transactions on microwave theory and techniques, ISSN 0018-9480, E-ISSN 1557-9670, Vol. 63, no 2, 754-765 p.Article in journal (Refereed) Published
Abstract [en]

This paper analyzes the effects of load impedancemismatch in power amplifiers which linearity has been enhancedusing various digital predistortion (DPD) algorithms. Two different power amplifier architectures are considered: a class AB and a Doherty amplifier and three model structures for the DPD model are compared: memoryless polynomial (MLP), general memory polynomial (GMP) and Kautz-Volterra functions (KV). This paper provides a sensitivity analysis of the linearized amplifiers under load mismatch conditions and reports the performance when dynamic parameter identification for the DPD is used to compensate for the changes in the load impedance. In general,power amplifiers linearity is sensitive to load impedance mismatch. Linearity may degrade as much as 10 dB (in normalized mean square error) according to the magnitude and the phase of the reflection coefficient provided by the load impedance. However, depending on the amplifier design, the sensitivity toload impedance mismatch varies. While the Doherty amplifier studied show significant linearity degradations in the in-band and out-of-band distortions, the out-of-band distortions of the studied class AB were less sensitive to the load impedance mismatch. In adaptive DPD schemes, the performance obtained in the MLP model does not benefit from the updating scheme and the performance achieved is similar to a static case, where no updates are made. This stresses the memory requirements in the predistorter. When employing the GMP and the KV models in an adaptive DPD scheme, they tackle to a larger extent the linearity degradations due to load impedance mismatch.

Keyword
Digital predistortion, impedance mismatch, load impedance, power amplifiers
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:hig:diva-18709 (URN)10.1109/TMTT.2014.2387060 (DOI)000349623800019 ()2-s2.0-84922708343 (ScopusID)
Available from: 2015-01-09 Created: 2015-01-09 Last updated: 2017-01-12Bibliographically approved
7. Finding Structural Information about RF Power Amplifiers using an Orthogonal Nonparametric Kernel Smoothing Estimator
Open this publication in new window or tab >>Finding Structural Information about RF Power Amplifiers using an Orthogonal Nonparametric Kernel Smoothing Estimator
2016 (English)In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 65, no 5, 2883-2889 p., 7109926Article in journal (Refereed) Published
Abstract [en]

A non-parametric technique for modeling the behavior of power amplifiers is presented. The proposed technique relies on the principles of density estimation using the kernel method and is suited for use in power amplifier modeling. The proposed methodology transforms the input domain into an orthogonal memory domain. In this domain, non-parametric static functions are discovered using the kernel estimator. These orthogonal, non-parametric functions can be fitted with any desired mathematical structure, thus facilitating its implementation. Furthermore, due to the orthogonality, the non-parametric functions can be analyzed and discarded individually, which simplifies pruning basis functions and provides a tradeoff between complexity and performance. The results show that the methodology can be employed to model power amplifiers, therein yielding error performance similar to state-of-the-art parametric models. Furthermore, a parameter-efficient model structure with 6 coefficients was derived for a Doherty power amplifier, therein significantly reducing the deployment’s computational complexity. Finally, the methodology can also be well exploited in digital linearization techniques.

Keyword
Power amplifier, non-parametric model, kernel, basis functions, power amplifier linearization, Digital pre distortion.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:hig:diva-19395 (URN)10.1109/TVT.2015.2434497 (DOI)000376094500004 ()2-s2.0-84970016798 (ScopusID)
Available from: 2015-05-27 Created: 2015-05-27 Last updated: 2017-01-13Bibliographically approved
8. Low complexity predistortion and equalization in nonlinear multicarrier satellite communications
Open this publication in new window or tab >>Low complexity predistortion and equalization in nonlinear multicarrier satellite communications
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2015 (English)In: EURASIP Journal on Advances in Signal Processing, ISSN 1687-6172, E-ISSN 1687-6180, Vol. 2015, 30Article in journal (Refereed) Published
Abstract [en]

Aiming to reduce the power/mass requirements in satellite transponders and toreduce mission costs, joint amplification of multiple-carriers using a singleHigh-Power Amplifier (HPA) is being considered. In this scenario, a carefulinvestigation of the resulting power efficiency is essential as amplification isnonlinear, and multicarrier signals exhibit enlarged peak-to-average power ratio.Thus, operating the amplifier close to saturation vastly increases signal distortionresulting in a severe degradation of performance, especially for higher ordermodulations. This paper proposes a reduced-complexity digital pre-distortion(DPD) scheme at the transmitter and a corresponding equalizer (EQ) at thereceiver to mitigate these nonlinear effects. Scenarios include both the forward aswell as the return links. In particular, the paper exploits the MIMO Volterrarepresentation and builds on a basis pursuit approach using a LASSO (leastabsolute shrinkage and selection operator) algorithm to achieve an effienct basisrepresentation, avoiding large computational complexity, to describe the selectionof pre-distorter/ equalizer model. The work further compares and contrasts thetwo mitigation techniques taking various system aspects into consideration. Thegains, in performance and amplification efficiency, demonstrated by the use ofDPD/ EQ motivate their inclusion in next generation satellite systems.

Keyword
Satellite communications; multicarrier signal; MIMO systems; joint amplification; nonlinear distortions; predistortion DPD; equalization; LASSO
National Category
Signal Processing
Identifiers
urn:nbn:se:hig:diva-18910 (URN)10.1186/s13634-015-0215-0 (DOI)000360573500001 ()2-s2.0-84925867893 (ScopusID)
Available from: 2015-02-06 Created: 2015-02-03 Last updated: 2016-08-17Bibliographically approved
9. A MIMO Sympol Rate Signal Digital Predistorter for Nonlinear Multicarrier Satellite Channels
Open this publication in new window or tab >>A MIMO Sympol Rate Signal Digital Predistorter for Nonlinear Multicarrier Satellite Channels
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:hig:diva-19396 (URN)
Available from: 2015-05-26 Created: 2015-05-27 Last updated: 2016-08-17Bibliographically approved

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Citation style
  • apa
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