Virtual antenna array based MIMO radio channel measurement system at 10 GHz

In this thesis, a 10 GHz multiple-input multiple-output radio channel measurement system using four port vector network analyzer and virtual antenna arrays in both transmitter and receiver ends is presented. The channel measurement system measures each single antenna channel separately. The radio propagation environment is assumed to be static during the recordings. As an antenna element, a dual polarized patch antenna with two feeding ports is used. Linear stages and programmable stepper motors are utilized to build an XY-gantry to move the antenna element in a plane. The stepper motors and the vector network analyzer are controlled by the same measurement control software. The basic principles of the control software are also presented along the measurement system. Three channel measurement scenarios and their initial results are presented to verify the system operation and to demonstrate the system in different cases. A verification measurement is performed in an anechoic chamber to verify that the system does not cause internal spurious responses to the results. The next measurement is performed in a classroom to demonstrate the multipath propagation environment. Furthermore, an indoor wall penetration loss measurement from the classroom to another is made to show that the measurement system can also be applied for the measurements requiring an accurate antenna shifting between the measurement points. The results measured with this setup can be applied angular domain algorithms to estimate the direction of arrival and departure, respectively.Tämä diplomityö esittelee moniantenniradiokanavan mittausjärjestelmän, jossa mittauslaitteena käytetään vektoripiirianalysaattoria ja kahta virtuaalista tasoantenniryhmää. Järjestelmä mittaa jokaisen antennielementin välisen kanavan erikseen. Etenemisympäristö on oletettu staattiseksi radiokanavan tallennuksen aikana. Antennielementtinä käytetään kaksoispolaroitua mikroliuska-antennia, jossa on omat syöttöportit molemmille ortogonaalisille lineaarisille polarisaatioille. Antennielementtiä siirretään tasossa käyttämällä ohjelmoitavia askelmoottoreita ja lineaariyksiköitä. Kaikkia mittausjärjestelmän laitteita ohjataan samalla ohjausohjelmistolla, jonka toimintaperiaate on myös esitetty tässä työssä. Mittausjärjestelmän toiminta varmistetaan ja sitä demonstroidaan suorittamalla kanavamittauksia erilaisissa etenemisympäristöissä. Varmennusmittaukset suoritetaan kaiuttomassa huoneessa, jotta voidaan varmistua siitä, ettei järjestelmä tuota sisäisiä harhatoistoja, jotka vaikuttaisivat mittaustuloksiin. Monitie-etenemisympäristöä demonstroidaan kanavamittauksilla luokkahuoneessa. Myös luokkahuoneiden välistä etenemisvaimennusta mitataan. Mittaustuloksia voidaan käyttää muodostettaessa erilaisia radiokanavamalleja ja niitä voidaan soveltaa myös aallon tulo- ja lähtökulman estimointiin käyttämällä siihen tarkoitettuja algoritmeja

Concepts for radiated nonlinear distortion and spatial linearization in millimeter-wave phased arrays

Abstract This thesis presents concepts for understanding and utilizing the spatial behavior of nonlinear distortion in a millimeter-wave (mmW) multi-antenna transmitter. The work begins with an approach for radio frequency (RF) link budget and performance evaluation in a multiple-input multiple-output (MIMO) system. In the approach, the radio channel is decomposed into independent MIMO channels whose link budgets are analyzed independently using power dependent nonideal models of RF transceivers. The nonlinear distortion caused by RF power amplifiers (PAs) is studied in phased array systems that use analog beamforming. In particular, the focus is on the scenarios in which also the amplitudes of individual paths are controlled to shape the beam pattern. The differences in the nonlinear behavior of the individual paths make the beams of the signal and the distortion differ from each other. This observation is important to consider when specifying the transmitter error vector magnitude (EVM) and adjacent channel power ratio (ACPR) with over-the-air measurements. In phased array systems, multiple PAs share the same digital input and hence the PA linearization by digital predistortion (DPD) turns into an array linearization problem. The linearization of such an array can be performed by modeling, emulating, or measuring the transmitter radiated response in various ways. In particular, this thesis discusses different conductive and radiated feedback receiver approaches and proposes procedures for training the DPD objective based on the measurements. Linearizing the array in the main lobe does not mean linearization of individual PAs if the transmit paths differ from each other. In such scenarios, it is shown that linearizing the signal in the main lobe creates a notch in the training direction that is a result of OTA combining different nonlinear components. This is shown to make a tradeoff between the EVM characterized in the steering angle and ACPR integrated over the sphere. It is proposed that such a problem can be tackled by modifying the nonlinearity of individual paths by PA bias control to make the distortion of individual paths more similar. This is shown to simplify the DPD trained in a certain steering angle to be effective also in other directions thus linearizing both EVM and total radiated ACPR of the array while achieving decent power efficiency.Tiivistelmä Tässä väitöskirjatyössä esitetään menetelmiä korkean taajuuden moniantennijärjestelmän epälineaarisen vääristymän tilatason käyttäytymisen ymmärtämiseksi ja hyödyntämiseksi. Työn alkuosa käsittelee radiojärjestelmän linkkibudjettia tavalla, joka yhdistää monikeilalähetyksen ja radiokanavamallin radiolähettimen ja -vastaanottimen käyttäytymismalleihin. Loppuosa työstä keskittyy tarkastelemaan tehovahvistimista aiheutuvaa epälineaarista häiriötä vaiheistetuissa keilanmuodostusjärjestelmissä. Työssä havaitaan erojen lähetinhaarojen epälineaarisessa käyttäytymisessä luovan eroja halutun lineaarisen signaalin keilan sekä häiriön keilan välille. Tämän johdosta häiriö voi antennien avulla säteillessään muodostaa erilaisen säteilykuvion kuin lähetetty signaali. Tämä havainto on erityisen tärkeä ymmärtää, kun muodostetaan säteilevän viereisen kaistan epälineaarisen häiriön sekä signaalin virhevektorin järjestelmätason vaatimuksia moniantennilähettimille. Ilmiötä voidaan myös hyödyntää lähettimen linearisoinnissa. Vaiheistetuissa ryhmissä useat antennikohtaiset tehovahvistimet jakavat yhden yhteisen tulon, jota voidaan digitaalisesti esivääristää. Tällaisissa ryhmissä lähettimen digitaalinen esivääristys (DPD) muuttuu säteilevän keilan esivääristämiseksi, joka sisältää myös antennit sekä niiden tilatason summausvaikutuksen. Ryhmän linearisointi voidaan suorittaa käsittelemällä koko lähetinryhmää yhtenä epälineaarisena komponenttina. Työssä esitetään erilaisia havainnointivastaanotinstrategioita sekä menetelmiä DPD-objektin muodostamiseksi hyödyntäen ryhmästä eri suuntiin säteilevän aaltomuodon mallinnusta ja mittauksia. Työssä huomataan, että lähettimen linearisointi tietyssä suunnassa ei takaa linearisointia muissa suunnissa, jos haarojen epälineaariset käyttäytymiset eroavat toisistaan. Tällöin yhden suunnan linearisointi muodostaa minimin häiriölle halutussa suunnassa summaamalla epälineaarisia komponentteja tilatasossa. Tällaisissa tapauksissa haarojen epälineaarisuutta voidaan analogisesti muokata esimerkiksi tehovahvistimien toimintapisteitä muuttamalla siten, että niiden epälineaarisuus muuttuu samankaltaiseksi. Työssä näytetään simuloinnein ja mittauksin, että tällöin yhden suunnan linearisoinnilla voidaan saavuttaa lineaarinen lähtösignaali kaikissa suunnissa hyvällä hyötysuhteella tapauksissa missä tehovahvistimien lähtöamplitudeja halutaan kontrolloida

Analyzing 5G RF system performance and relation to link budget for directive MIMO

Abstract Wideband fifth generation (5G) systems utilizing high carrier frequency and multiple-input multiple-output (MIMO) raise major challenges for the system design. Wave propagation and practical hardware tradeoffs at higher frequency ranges provide new boundary conditions for the implementation. This paper addresses system performance boundaries and the analysis method towards multibeam communications at mmW. We combine analysis from antennas and propagation to the RF transceiver specifications and beamforming requirements. Realistic propagation model and antenna implementation is used to generate beam-specific path gains and provide a wide variety of user scenarios. Using this approach, system level interdependencies and RF performance boundaries can be verified with different antenna configurations in various propagation environments. As an example, we present MIMO link budget analysis targeting 10 Gbits/s for multiple devices in the office scenario at 27 GHz

Inter-beam interference reduction in hybrid mmW beamforming transceivers

Abstract Practical radio frequency beamforming can suffer from high sidelobe levels which cause inter-beam interference (IBI) in multibeam transceivers. IBI can be reduced by shaping the amplitude and phase excitation over the individual antenna elements. However, such methods do not exploit the available power efficiently in practical arrays, where each antenna is driven with a dedicated power amplifier. In this paper, we show a simplified approach for hybrid beam synthesis in subarray-based beamforming architecture and propose a two-stage beamforming method for cancelling the IBI. The proposed technique cancels the interference between the subarrays while it maximizes the effective isotropic radiated powers in the desired directions. Simulation show typically over 40 dB IBI rejection for randomly spread beams and more than 4 dB improvement for radiated power compared to amplitude tapering

Statistical linearization of phased arrays using power adaptive power amplifier model

Abstract Phased arrays used in millimeter-wave systems challenge the concept of power amplifier (PA) linearization by digital predistortion (DPD). This is due to the shared digital path and inaccuracies in analog beamforming and other component variations. However, the group behavior of multiple parallel nonlinear branches can be expected to be more predictable due to averaging effect compared to a single branch behavior. In this paper, we use a power adaptive nonlinear model to mimic the average behavior of a single PA and utilize the probability distribution of the input power of each individual PA to approximate the expected nonlinear behavior of the array over-the-air. The approximated array response is used for the DPD training. The simulation results indicate that the proposed approach provides good linearization performance for large arrays that have varying amplitude and phase weights

Average linearization of phased array transmitters under random amplitude and phase variations

Abstract We study the impact of amplitude and phase differences between the parallel power amplifier (PA) branches in a phased array and their impact on the performance of the digital predistortion (DPD). The DPD coefficients are estimated from the array response in the far-field. The DPD coefficients need to be updated for changes in the nonlinear behavior of the PAs due to amplitude and phase variations. We present a training mechanism which makes the DPD robust to branch specific amplitude and phase weights and can tolerate these variations without the need of adapting to individual changes in the nonlinear behavior of the PA branches. The DPD is trained for a set of random amplitude and phase weights following normal distribution and the resultant mean DPD coefficients are used for predistortion. The simulation results show that the mean DPD can achieve the same average linearity performance as the continuously trained reference DPD for 32 elements uniform array

Out-of-band interference in 5G mmW multi-antenna transceivers:co-existence scenarios

Abstract Large antenna arrays used for compensating the millimeter wave (mmW) path loss gives interesting perspectives for considering out-of-band emissions and reception signals to be direction dependent. In addition, co-existence of lower frequency and mm W systems set special requirements for both conventional cellular and mm W systems. This paper presents discussion on these two cases as the most important out-of-band interference scenarios in fifth generation systems. Effects of inter modulation and adjacent channel leakage are discussed in both mm W beamforming transceiver and receiver, respectively. It is concluded that distortion is behaving differently in multi-antenna transmitter that at the receiver. Over-the-Air (OTA) measured and simulated examples of the nonlinear behavior of an array are given for receiver and transmitter, respectively. Furthermore, smart array linearization scheme given as a reference provides new perspective to consider the direction dependent out-of-band distortion as a part of the future cellular standards

Impact of the asymmetric signal routing on the wideband spatial behavior of large modular phased arrays

Abstract Phased arrays have typically equal lengths for all antenna paths. This paper compares three phased array architectures for 5/6G applications. The first two arrays present unequal-length feeding networks, while the third array is the corporate-fed one. The first array is fed from the center, while the second is fed from the side. The impacts of the unsymmetrical feeding network on the bandwidths are analyzed and compared to the corporate-fed array. We show that side-fed arrays present higher bandwidths for higher steering angles, while center-fed arrays work better in the broadside region. Moreover, side-fed arrays can also be better than corporate-fed ones near the end-fire region, making them a good alternative to reduce the footprint of the feeding networks. Finally, we illustrate that if the feeding point can be varied as a function of the desired steering angle, the broadband performance of the array can be optimized for different steering angles

Linearity measurement of 6G receiver with one transmission frequency extender operating at 330 GHz

Abstract The future sixth-generation (6G) is envisioned to support data rates up to 1 Tbps. The operational frequencies of the 6G system will be expanded towards the sub-mmW and THz regions. The 6G systems will utilize directive beams, as well, to compensate increased signal attenuation between link ends. The linearity of a receiver (Rx) is one of the most significant parameters for any radio system. Traditional Rx linearity measurement relies on a two-tone measurement technique, which requires two dedicated RF signals and combining them to the test signal. The generation of two independent RF signals at a 300 GHz frequency band leads to a costly and bulky solution. This paper proposes a linearity measurement method for 6G Rx, which uses only one continuous wave transmission frequency extender. A method is proposed where the RF input signal of frequency extender is narrowband amplitude modulated (AM), generating side tones around continuous wave carrier. The carrier frequency and first side tones are used as test signals, and the linearity test is like a traditional two-tone test with unequal signals. It is shown that the carrier level can be modified by back-offing the RF input power in the frequency extender input. By varying the AM modulation index, the side tones’ levels can be varied, enabling the sweep of the tone input power to perform Rx linearity measurements

Quality analysis of antenna reflection coefficient in massive MIMO antenna array module

Abstract Number of antennas per an antenna module will increase in upcoming 5G millimeter wave (mmW) radio products. The usage of antenna arrays compensates the increased propagation loss of a radio signal at mmW frequencies. The worst performing antenna defines the quality level MIMO antenna module. This paper presents an analysis and a relationship between the variation of the antenna resonance frequency and the reflection coefficient. A probability density function (PDF) of the antenna reflection coefficient at the specification limit is a non-linearly scaled mirrored version of the PDF of the variation of the antenna resonance. We measured the PDF of antenna reflection coefficient from manufactured prototypes and there is a good correlation between the measured and the simulated PDFs