Finding Structural Information of RF Power Amplifiers using an Orthogonal Non-Parametric Kernel Smoothing Estimator

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.Comment: Matlab sample code (15 MB): https://dl.dropboxusercontent.com/u/106958743/SampleMatlabKernel.zi

A Digital Predistortion Scheme Exploiting Degrees-of-Freedom for Massive MIMO Systems

The primary source of nonlinear distortion in wireless transmitters is the power amplifier (PA). Conventional digital predistortion (DPD) schemes use high-order polynomials to accurately approximate and compensate for the nonlinearity of the PA. This is not practical for scaling to tens or hundreds of PAs in massive multiple-input multiple-output (MIMO) systems. There is more than one candidate precoding matrix in a massive MIMO system because of the excess degrees-of-freedom (DoFs), and each precoding matrix requires a different DPD polynomial order to compensate for the PA nonlinearity. This paper proposes a low-order DPD method achieved by exploiting massive DoFs of next-generation front ends. We propose a novel indirect learning structure which adapts the channel and PA distortion iteratively by cascading adaptive zero forcing precoding and DPD. Our solution uses a 3rd order polynomial to achieve the same performance as the conventional DPD using an 11th order polynomial for a 100x10 massive MIMO configuration. Experimental results show a 70% reduction in computational complexity, enabling ultra-low latency communications.Comment: IEEE International Conference on Communications 201

Impact of Spatial Filtering on Distortion from Low-Noise Amplifiers in Massive MIMO Base Stations

In massive MIMO base stations, power consumption and cost of the low-noise amplifiers (LNAs) can be substantial because of the many antennas. We investigate the feasibility of inexpensive, power efficient LNAs, which inherently are less linear. A polynomial model is used to characterize the nonlinear LNAs and to derive the second-order statistics and spatial correlation of the distortion. We show that, with spatial matched filtering (maximum-ratio combining) at the receiver, some distortion terms combine coherently, and that the SINR of the symbol estimates therefore is limited by the linearity of the LNAs. Furthermore, it is studied how the power from a blocker in the adjacent frequency band leaks into the main band and creates distortion. The distortion term that scales cubically with the power received from the blocker has a spatial correlation that can be filtered out by spatial processing and only the coherent term that scales quadratically with the power remains. When the blocker is in free-space line-of-sight and the LNAs are identical, this quadratic term has the same spatial direction as the desired signal, and hence cannot be removed by linear receiver processing

Nonlinearity and Noise Effects in Multi-level Signal Millimeter-Wave over Fiber Transmission using Single- and Dual-Wavelength Modulation

We transmit multilevel quadrature amplitude modulation (QAM) data-IEEE 802.16 schemes-at 20 MSps and an orthogonal frequency-division multiplexing (OFDM) 802.11 g signal (54 Mbps) with a 25 GHz millimeter-wave over fiber system, which employs a dual wavelength source, over 20 km of single mode fiber. Downlink data transmission is successfully demonstrated over both optical and wireless (up to 12 m) paths with good error vector magnitude. An analysis of two different schemes, in which data is applied to one (single) and both (dual) of the wavelengths of a dual wavelength source, is carried out. The system performance is analyzed through simulation and a good match with experimental results is obtained. The analysis investigates the impact of Mach-Zehnder modulator (MZM) and RF amplifier nonlinearity and various noise sources, such as laser relative intensity noise, amplified spontaneous emission, thermal, and shot noise. A comparison of single carrier QAM IEEE 802.16 and OFDM in terms of their sensitivity to the distortions from MZM and RF amplifier nonlinearity is also presented

Modeling and Efficient Cancellation of Nonlinear Self-Interference in MIMO Full-Duplex Transceivers

This paper addresses the modeling and digital cancellation of self-interference in in-band full-duplex (FD) transceivers with multiple transmit and receive antennas. The self-interference modeling and the proposed nonlinear spatio-temporal digital canceller structure takes into account, by design, the effects of I/Q modulator imbalances and power amplifier (PA) nonlinearities with memory, in addition to the multipath self-interference propagation channels and the analog RF cancellation stage. The proposed solution is the first cancellation technique in the literature which can handle such a self-interference scenario. It is shown by comprehensive simulations with realistic RF component parameters and with two different PA models to clearly outperform the current state-of-the-art digital self-interference cancellers, and to clearly extend the usable transmit power range.Comment: 7 pages, 5 figures. To be presented in the 2014 International Workshop on Emerging Technologies for 5G Wireless Cellular Network

ワイヤレス通信のための先進的な信号処理技術を用いた非線形補償法の研究

The inherit nonlinearity in analogue front-ends of transmitters and receivers have had primary impact on the overall performance of the wireless communication systems, as it gives arise of substantial distortion when transmitting and processing signals with such circuits. Therefore, the nonlinear compensation (linearization) techniques become essential to suppress the distortion to an acceptable extent in order to ensure sufficient low bit error rate. Furthermore, the increasing demands on higher data rate and ubiquitous interoperability between various multi-coverage protocols are two of the most important features of the contemporary communication system. The former demand pushes the communication system to use wider bandwidth and the latter one brings up severe coexistence problems. Having fully considered the problems raised above, the work in this Ph.D. thesis carries out extensive researches on the nonlinear compensations utilizing advanced digital signal processing techniques. The motivation behind this is to push more processing tasks to the digital domain, as it can potentially cut down the bill of materials (BOM) costs paid for the off-chip devices and reduce practical implementation difficulties. The work here is carried out using three approaches: numerical analysis & computer simulations; experimental tests using commercial instruments; actual implementation with FPGA. The primary contributions for this thesis are summarized as the following three points: 1) An adaptive digital predistortion (DPD) with fast convergence rate and low complexity for multi-carrier GSM system is presented. Albeit a legacy system, the GSM, however, has a very strict requirement on the out-of-band emission, thus it represents a much more difficult hurdle for DPD application. It is successfully implemented in an FPGA without using any other auxiliary processor. A simplified multiplier-free NLMS algorithm, especially suitable for FPGA implementation, for fast adapting the LUT is proposed. Many design methodologies and practical implementation issues are discussed in details. Experimental results have shown that the DPD performed robustly when it is involved in the multichannel transmitter. 2) The next generation system (5G) will unquestionably use wider bandwidth to support higher throughput, which poses stringent needs for using high-speed data converters. Herein the analog-to-digital converter (ADC) tends to be the most expensive single device in the whole transmitter/receiver systems. Therefore, conventional DPD utilizing high-speed ADC becomes unaffordable, especially for small base stations (micro, pico and femto). A digital predistortion technique utilizing spectral extrapolation is proposed in this thesis, wherein with band-limited feedback signal, the requirement on ADC speed can be significantly released. Experimental results have validated the feasibility of the proposed technique for coping with band-limited feedback signal. It has been shown that adequate linearization performance can be achieved even if the acquisition bandwidth is less than the original signal bandwidth. The experimental results obtained by using LTE-Advanced signal of 320 MHz bandwidth are quite satisfactory, and to the authors’ knowledge, this is the first high-performance wideband DPD ever been reported. 3) To address the predicament that mobile operators do not have enough contiguous usable bandwidth, carrier aggregation (CA) technique is developed and imported into 4G LTE-Advanced. This pushes the utilization of concurrent dual-band transmitter/receiver, which reduces the hardware expense by using a single front-end. Compensation techniques for the respective concurrent dual-band transmitter and receiver front-ends are proposed to combat the inter-band modulation distortion, and simultaneously reduce the distortion for the both lower-side band and upper-side band signals.電気通信大学201

Design and demonstration of digital pre-distortion using software defined radio

Abstract. High data rates for large number of users set tight requirements for signal quality measured in terms of error vector magnitude (EVM). In radio transmitters, nonlinear distortion dominated by power amplifiers (PAs) often limits the achievable EVM. However, the linearity can be improved by linearization techniques. Digital pre-distortion (DPD) is one of these widely used linearization techniques for an effective distortion reduction over a wide bandwidth. In DPD, the nonlinearity of the transmitter is pre-compensated in the digital domain to achieve linear output. Moreover, DPD is used to enable PAs to operate in the power-efficient region with a decent linearity. As we are moving towards millimetre-wave frequencies to enable the wideband communications, the design of the DPD algorithm must be optimized in terms of performance and power consumption. Moreover, continuous development of wireless infrastructure motivates to make research on programmable and reconfigurable platforms in order to decrease the demonstration cost and time, especially for the demonstration purposes. This thesis illustrates and presents how software defined radio (SDR) platforms can be used to demonstrate DPD. Universal software defined peripheral (USRP) X300 is a commercial software defined radio (SDR) platform. The chosen model, X300, has two independent channels equipped with individual transceiver cards. SIMULINK is used to communicate with the device and the two channels of X300 are used as transmitter and receiver simultaneously in full-duplex mode. Hence, a single USRP device is acting as an operational transmitter and feedback receiver, simultaneously. The implemented USRP design consists of SIMULINK based transceiver design and lookup table based DPD in which the coefficients are calculated in MATLAB offline. An external PA, i.e. ZFL-2000+ together with a directional coupler and attenuator are connected between the TX/RX port and RX2 port to measure the nonlinearity. The nonlinearity transceiver is measured with and without the external PA. The experimental results show decent performance for linearization by using the USRP platform. However, the results differ widely due to the used USRP transceiver parameterization and PA operational point. The 16 QAM test signal with 500 kHz bandwidth is fed to the USRP transmit chain. As an example, the DPD algorithm improves the EVM from 7.6% to 2.1% and also the ACPR is reduced around 10 dB with the 16 QAM input signal where approximately + 2.2 dBm input power applied to the external PA

Behavioral modeling techniques for power amplifier digital pre-distortion

Abstract. The dramatic increase in the capacity of telecommunication networks has increased the requirements of the devices, one example of which is the continuously widening bandwidth. Using broadband signals requires often high linearity from the transmitter — and especially from the power amplifier at the end of the transmitter chain — but at the same time, it should operate as efficiently as possible. The power amplifier is the most power consuming component in the transmitter and inherently nonlinear, so its linearization is an essential part of the overall system performance. Of the current linearization techniques, digital pre-distortion has established itself as the most common tool for providing better linearity and efficiency in a power amplifier and transmitter. In this thesis, the performance of power amplifier models used in digital pre-distortion was investigated and their differences compared to the more complex reference model used in the actual base station product. The aim of this thesis was to create a behavioral model that corresponds the physical component as accurately as the reference model. This behavioral model could be used for example to design and optimize a power amplifier and linearization algorithm without the need for the secret model for the product. This, in its turn, would result in more efficient work with third parties such as component vendors and reduce the linearization time. The performance parameters of the behavioral models were introduced at the beginning of the thesis. These were also used in later parts to analyse the measurement results. Power amplifier linearization measurements were performed under laboratory conditions on a MATLAB® test bench. From the results, it was found that the use of a simple memoryless behavioral model is not enough to describe the physical nonlinear component with sufficient accuracy. The results also showed that the complexity of the model reduces its accuracy if the model coefficients are not correctly positioned. In this thesis, we succeeded in creating a memory model that describes the reference model sufficiently accurately on several meters, taking into account also the memory effects of the power amplifier. This thesis thus provides a good basis for further development of the actual modeling tool for product projects.Tehovahvistimen käyttäytymistason mallinnustekniikat esisärötyksen tueksi. Tiivistelmä. Tietoliikenneverkkojen kapasiteetin räjähdysmäinen kasvu on lisännyt laitteiden vaatimuksia, joista yhtenä esimerkkinä on jatkuvasti suureneva kaistanleveys. Laajakaistaiset signaalit vaativat usein lähettimeltä — ja etenkin lähettimen loppupäässä olevalta tehovahvistimelta — korkeaa lineaarisuutta, mutta samalla sen on toimittava mahdollisimman tehokkaasti. Tehovahvistin on lähettimen eniten tehoa kuluttava komponentti ja luonnostaan epälineaarinen, joten sen linearisointi on oleellinen osa koko systeemin suorituskykyä. Nykyisistä linearisointitekniikoista digitaalinen esisärötys on vakiinnuttanut paikkansa yleisimpänä työkaluna paremman lineaarisuuden ja tehokkuuden saavuttamiseksi tehovahvistimessa. Tässä diplomityössä tutkittiin digitaalisessa esisärötyksessä käytettävien käyttäytymistason tehovahvistinmallien suorituskykyjä ja niiden eroja varsinaisessa tukiasematuotteessa käytettävään, monimutkaisempaan referenssimalliin verrattuna. Työn tavoitteena oli luoda käyttäytymismalli, jolla voidaan kuvata fyysistä komponenttia yhtä tarkasti kuin referenssimallilla. Mallia voitaisiin käyttää esimerkiksi uuden tehovahvistimen suunnittelussa ilman, että kaupalliseen tuotteeseen tulevaa, salaista referenssimallia on tarve käyttää. Näin voitaisiin tehostaa työskentelyä ulkopuolisten tahojen, kuten komponenttitoimittajien kanssa ja vähentää linearisointiin käytettävää aikaa. Työn alussa esiteltiin käyttäytymismallien suorituskykyparametrit, joita käytettiin mittaustulosten analysointiin. Tehovahvistimien linearisointimittaukset suoritettiin laboratorio-olosuhteissa MATLAB®-testipenkissä. Mittauksissa todettiin, että yksinkertainen, muistiton käyttäytymismalli ei riitä kuvaamaan fyysistä komponenttia riittävän tarkasti. Tuloksista pääteltiin myös, että liian kompleksinen malli heikentää sen tarkkuutta. Työssä onnistuttiin luomaan muistillinen käyttäytymismalli, joka kuvaa referenssimallia riittävän tarkasti usealla eri mittarilla tarkastellen — huomioiden osaltaan myös tehovahvistimen muistiefektejä. Tämä opinnäytetyö tarjoaa siis hyvän pohjan varsinaisen mallinnustyökalun jatkokehitykselle tuoteprojekteihin