Ultra-high Efficiency Phased Arrays for Astronomy and Satellite Communications

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1 Ultra-high Efficiency Phased Arrays for Astronomy and Satellite Communications Karl F. Warnick Department of Electrical and Computer Engineering Brigham Young University, Provo, UT, USA Collaborators: Partners and Sponsors Brian D. Jeffs, Junming Diao, Zhenchao Yang, Kyle Browning, and Matt Morin, Department of Electrical and Computer Engineering, Brigham Young University, Provo, UT, USA J. Richard Fisher, Roger Norrod, Anish Roshi, and Bob Simon National Radio Astronomy Observatory, Green Bank, West Virginia, USA Peter Russer, Technische Universität München, Germany Leo Belostotski, University of Calgary, Canada November 2014

2 Brigham Young University Location: Provo, Utah, USA Students: 34,000 #10 in U.S. in number of graduates who go on to earn PhDs

3 Brigham Young University

4 Radio Astronomy Pulsars Cosmic jets Gravitational lenses Galactic center Black holes Astrochemistry Age of the universe Cosmology About 96% of the stuff in the universe isn t the protons, photons, etc. we know about it s dark matter and dark energy! Images courtesy of NRAO/AUI

5 Astronomical Instruments Last 75 years: Single-pixel large dish antennas Last 50 years: Sparse aperture synthesis arrays Last 5 years: Multi-pixel cluster feeds (moderately sparse) Present: Dense aperture phased arrays and phased array feeds Images courtesy of Neil Roddis, SKA PDO

6 Types of Dense Phased Arrays Aperture arrays - direct view to sky LOFAR Low frequency array, Northern Europe SKA Core Phased array feeds (PAFs) Large reflectors GBT, Arecibo, Westerbork Small reflectors SKA

7 Phased Array Feed Applications Large single-dish radio telescopes GBT, Arecibo, China FAST, etc. High cost, low quantity (one) High sensitivity Ultra-low noise, cryogenic Digital beamforming 300 MHz bandwidth or more Mid-size synthesis array telescopes ASKAP and other SKA pathfinders Moderate to high quantity (tens to thousands) High sensitivity Uncooled (ambient temp.) or cryogenic Digital beamforming 300 MHz bandwidth or more Small dish applications Satellite communications, direct broadcast satellite (DBS) or very small aperture terminals (VSAT) High quantity to very high quantity ( ,000+) Low noise, uncooled Analog beamforming 1 GHz bandwidth, 50 MHZ instantaneous

8 Current Research on Active Receiving Arrays Multipixel L band phased array feed on Arecibo Radio Telescope Digitally beamformed phased array receivers -FPGA implementations -Array calibration -Multiple simultaneous beams Cryogenic array feed on National Radio Astronomy Observatory 20-Meter Dish World-record sensitivity for a phased array antenna

9 Current Research on Active Receiving Arrays Satellite Communications Terminals Magnetic resonance imaging (MRI) coil arrays Near Field MIMO arrays

10 What do all these applications have in common? The key figure of merit in all cases is. Higher SNR = more customers, more science, better quality of service, more revenue, better image quality, longer range, lower power usage, less bandwidth. SNR is the money parameter for billion dollar satellite communications networks, cellular systems, astronomical instruments, and deep space networks.

11 Noise Considerations are Critical For terrestrial communications applications, the thermal noise environment is ~290 K or the channel is interference limited improving antenna radiation efficiency and reducing receiver noise leads to only a modest SNR improvement The microwave sky is much cooler than ambient temperature (~4 K at L band, K at K band) radiation efficiency and receiver noise are dominant Radio astronomy and satellite communications: When the signal comes from the sky, high radiation efficiency and low noise electronics are critical Key question: can phased arrays achieve noise performance and efficiency comparable to horn feeds?

12 What s going on right now in antenna theory? Basic antenna design is a mature field Multiband antennas, ultrawideband antennas, small antennas are well understood and widely used in industry Current hot topics include active phased arrays, ultra-high sensitivity array receivers, array feeds reconfigurable antennas, cognitive radio, multiple input multiple output (MIMO) These are mostly practical applications Are there any open theory questions? Lets go back to basics to figure that out

13 What is this quantity? Antenna efficiency, not aperture efficiency!

14 Aperture Efficiency How is aperture efficiency actually defined in the IEEE Standard Definition of Terms for Antennas? antenna [aperture] illumination efficiency: The ratio, usually expressed in percent, of the maximum directivity of an antenna [aperture] to its standard directivity. Syn: normalized directivity; See: standard [reference] directivity. standard [reference] directivity: The maximum directivity from a planar aperture of area A, or from a line source of length L, when excited with a uniform-amplitude, equiphase distribution. NOTE 1 For planar apertures in which A >> λ 2, the value of the standard directivity is 4πA/λ 2, with λ the wavelength and with radiation confined to a half space. [IEEE Standard Definition of Terms for Antennas, IEEE Std ] For most aperture antennas, antenna efficiency is the product of radiation efficiency and aperture efficiency.

15 Other Non-standard Antenna Terms Does gain include the effect of losses due to impedance mismatch between a driving amplifier and an antenna? No, but realized gain does Total gain (not defined in the IEEE standard) Total efficiency, overall efficiency (also not defined in the standard) Multiple element antenna efficiency for MIMO Absorption efficiency for receiving antennas Decoupling efficiency for array antennas Many others

16 IEEE Standard for Antenna Terms The IEEE Standard had a rigorous, complete, elegant system of figures of merit that was worked out many years ago Approximately 50% of common antenna textbooks get concepts like efficiency stuff wrong, and none agree completely on basic antenna terms! (Credit to Wim Van Capellen, ASTRON, The Netherlands.) So, not only are the open areas in antenna research practical rather than theoretical, we ve actually forgotten some of what the giants of antenna theory knew in the 1900s! Does this mean there are no meaningful problems left to solve in basic antenna theory?

17 Let s think deeper!

18 A few basic concepts from microwave network noise theory In microwave networks, noise at the system output is often referred to an equivalent power at the system input Since the signal and equivalent noise experience the same gain scale factors in the system, minimizing equivalent noise referred to the input actually maximizes SNR This is why noise theory is really important Noise power at the output can be converted to an equivalent noise temperature in Kelvin at the input using where B is the system bandwidth and k B is Boltzmann s constant.

19 What is the gain of this antenna? Array LNAs Receivers Digital Beamforming Beam output power:

20 Directivity Directivity is easy! Measure the power at the receiver output for a plane wave coming in from a bunch of angles, integrate the total power, divide... but we still don t know the gain, since that requires knowing something about losses in the antenna. What is the radiation loss for an antenna that includes amplifiers? Downconverters? Digital signal processing? One issue is that we normally extend gain and directivity to receivers using reciprocity, but complex active array receivers aren t reciprocal (how do you input a signal into a digital beamformer with analog to digital converters, amplifiers, etc. and get it to come out of the array? you can t!)

21 Gain Somehow, the antenna performance should be worse if the array elements are lossy than if they are lossless. How/why does it get worse? Hmmm..that s a tantalizing clue! Before we go there, are there other figures of merit already available for this active antenna in the IEEE standard or in the literature?

22 Existing Active Array Figures of Merit Receiving pattern directivity Solid-beam efficiency: ratio of the power received over a specified solid angle when illuminated isotropically by uncorrelated and unpolarized waves to the total received power (in the IEEE Standard, but rarely used) Embedded element efficiency: measures the efficiency of a radiating element in a large array, taking into account mutual coupling (used in the classical array antenna literature) Array gain or SNR gain: ratio of array output SNR to SNR of a single sensor (commonly used by the array signal processing community, and a very important concept) Array efficiency : array gain divided by standard directivity [Jacobs, A figure of merit for signal processing reflector antennas, TAP, 1985] Important but obscure paper, cited only four times in Google scholar (and three of the citations are in my papers.) This helps a bit but still raises lots of questions is array gain equal to antenna gain? What if we want the efficiency of the whole antenna and not an embedded element efficiency? Why aren t solid-beam efficiency or array efficiency used very often? What if we just want the plain old gain of the array antenna?

23 Simple Example Passive antenna followed by an amplifier: What is the gain of this antenna? We could break the connection, but what if we don t want to or can t? Is the antenna gain arbitrary? Does it increase if we increase the amplifier gain?

24 Radiation Efficiency Gain is directivity multiplied by radiation efficiency. Since we can get the directivity of this antenna, what we really need is the radiation efficiency. But, we can t put a signal into the amplifier output to see how much power is radiated. There s no way to get Prad/Pin!

25 What do antenna losses really do to a receiver? Super lossy antenna What happens at the output? The signal is attenuated But we could amplify the signal. The output SNR is low, because the lossy antenna adds noise. No way to fix that. Noise is the key!

26 Basic Antenna Noise Theory For a passive antenna in a thermal environment with brightness temperature T 0 in Kelvin, the available power at the antenna port is where B is the system bandwidth and k B is Boltzmann s constant.

27 Radiation Efficiency for an Active Antenna Let s try this: (External thermal noise divided by total thermal noise) measures noise added by antenna losses

28 What is the thermal noise due to antenna loss?

29 Radiation Efficiency for an Active Antenna Back to our attempt at radiation efficiency: This is the radiation efficiency of the antenna if it were disconnected from the amplifier and used as a transmitter! If the antenna system is so complex we can t break it apart and isolate the antenna losses, maybe we should call this receiving efficiency instead of radiation efficiency

30 Noise Theory This gives us a really nice clue as to how to define radiation efficiency, gain, etc. for active antennas: Use received noise instead of radiated power The beauty of this idea is that it s equivalent to the usual definitions for passive, reciprocal antennas, but can also be applied to nonreciprocal antenna systems that can t be disconnected and used as a transmitter More importantly for modern antenna applications, we can handle antenna systems that include digital processing! This includes MIMO systems, phased arrays, active array feeds, etc.

31 Can we use this idea to answer other questions?

32 Where do we put mismatch between the antenna and amplifier? What does mismatch do to a receiver? Does it reduce the directivity? No the directivity is only a function of the receiving pattern. Does it reduce the gain? No gain is directivity reduced by the radiation (or receiving) efficiency So, what does it do? For a transmitter, it reduces the realized gain, but it s not clear how that applies to active receivers. Answer: For receivers, mismatch increases the amplifier noise figure There s noise again. Can we use noise theory to create a new receiver figure of merit that captures mismatch effects?

33 Classical Amplifier Noise Matching Goal: Maximize SNR at LNA output Matching Network LNA Optimal source admittance When the source admittance is equal to the amplifier s optimal source admittance parameter, there is an optimal compromise between signal power transfer and amplifier noise minimization, and SNR at the output is maximized

34 Noise Matching Efficiency Let s define a new receiver figure of merit, noise matching efficiency Noise matching efficiency is the receiver noise at the antenna system output with all amplifiers ideally noise matched to the antenna, divided by the actual receiver noise Measures noise increase due to impedance mismatches Analogous to receiving efficiency (thermal noise without antenna losses divided by thermal noise) Measures noise increase due to losses Works for active antennas, active arrays, systems with any number of noisy amplifiers or active components

35 Process for New Standards This all seems pretty cool. Now, how do we get these ideas into the IEEE Standard? Antenna Definitions Working Group Antenna Standards Committee IEEE Standards Association IEEE Antennas and Propagation Society

36 Process for New Standards The real process.

37 Latest Version of the IEEE Standard for Antennas

38 New IEEE Standard Antenna Terms for Active Arrays isotropic noise response. For a receiving active array antenna, the noise power at the output of a formed beam with a noiseless receiver when in an environment with brightness temperature distribution that is independent of direction and in thermal equilibrium with the antenna. active antenna available gain. For a receiving active array antenna, the ratio of the isotropic noise response to the available power at the terminals of any passive antenna over the same bandwidth and in the same isotropic noise environment. New terms: Isotropic noise response Active antenna available gain Active antenna available power active antenna available power. For a receiving active array antenna, the power at the output of a formed beam divided by the active antenna available gain noise temperature of an antenna. The temperature of a resistor having an available thermal noise power per unit bandwidth equal to that at the antenna output at a specified frequency. Receiving efficiency NOTES Noise matching efficiency 1 Noise temperature of an antenna depends on its coupling to all noise sources in its environment, as well as noise generated within the antenna. Updated terms: 2 For an active antenna, the temperature of an isotropic thermal noise environment such that the isotropic noise response is equal to the noise power at the antenna output per unit bandwidth at a specified frequency. Noise temperature of an antenna Effective area effective area (of an antenna) (in a given direction). In a given direction, the ratio of the available power at the terminals of a receiving antenna to the power flux density of a plane wave incident on the antenna from that direction, the wave being polarization matched to the antenna. See: polarization match. NOTES 1 If the direction is not specified, the direction of maximum radiation intensity is implied. 2 The effective area of an antenna in a given direction is equal to the square of the operating wavelength times its gain in that direction divided by 4pi. 3 For an active antenna, available power is the active antenna available power. Receiving efficiency. For a receiving active array antenna, the ratio of the isotropic noise response with noiseless antenna to the isotropic noise response, per unit bandwidth and at a specified frequency. NOTE Equivalent to radiation efficiency for a passive, reciprocal antenna. Noise matching efficiency. For a receiving active array antenna, the ratio of the noise power contributed by receiver electronics at the output of a formed beam, with receivers impedance matched to the array elements for minimum noise, to the actual receiver electronics noise power at the formed beam output, per unit bandwidth and at a specified frequency. (K. F. Warnick, M. V. Ivashina, R. Maaskant, B. Woestenburg, Unified Definitions of Efficiencies and System Noise Temperature for Receiving Antenna Arrays, IEEE Antennas and Wireless Propagation Letters, 2009)

39 New Antenna Terms Isotropic noise response Active antenna available gain Active antenna available power Receiving efficiency Effective area (for active arrays) Noise matching efficiency Noise temperature (for active arrays)

40 Receiver Sensitivity How does all of this relate to SNR? New efficiencies should always enter into the overall system performance in a known way. Few authors bother to do this, but it s very important, and helps to avoid all sorts of misunderstandings and mistakes!

41 Measurement Techniques All figures of merit require the isotropic noise response How can it be measured? Full receiving pattern measurement: gives external part of isotropic noise response Network analyzer: array mutual resistance matrix based on Twiss s theorem Free space Y factor method: gives external part of isotropic noise response Hot source: Cold source: R s are noise correlation matrices How can we realize a very cold isotropic noise field?

42 NRAO Green Bank Cold Sky/Warm Absorber Facility The sky is quite cool at microwave frequencies Ground shield blocks thermal radiation from warm ground

43 Can we go even deeper into the theory?

44 Network Theory and Signal Correlation Matrices We ve only just scratched the surface! If we break apart the array and look at noise in terms of microwave network theory and signal correlation matrices, we can develop a whole new framework for working with array antennas! Array Signal Processing Theory Antenna Theory Microwave Network Theory Analysis Framework for Array Antennas

45 Array Signal and Noise Model Array LNAs Receivers Digital Beamforming Beam output power: Array output signal and noise contributions before beamforming: Array output correlation matrix: External thermal noise Noise due to antenna losses Noise due to electronics

46 Fundamental Noise Theorem of Array Receivers By conservation of energy: Embedded element pattern overlap integral matrix Part of array mutual resistance matrix due to antenna losses Real part of array mutual impedance matrix Twiss s theorem: Isotropic noise response External noise contribution Loss noise contribution Array noise response: Relates array radiation properties (element patterns) and loss part of mutual impedance matrix to the array noise response

47 Correlated Receiver Noise Assuming that the front end amplifier noise correlation admittance parameter is zero, the receiver noise correlation matrix is Amplifier minimum equivalent noise temperature Transformation from antenna open circuit loaded voltages to receiver output voltages Amplifier optimal source resistance parameter Array mutual impedance matrix In signal processing analysis and research, noise is often taken to be a scaled identify matrix Correlated noise matters in most modern array applications

48 Optimal Noise Matching Active reflection coefficients: To maximize SNR at array output with respect to receiver noise: Design antenna elements so that active impedances are close to 50Ω, or Match front end amplifiers to the active impedances Active impedances depends on beamformer weights more complicated than single port antenna impedance

49 How do we use all of this to build better array antenna technologies?

50 Active Impedance Matching Strategies Ignore active impedance variation and match to self-impedances This may be adequate for communications systems, but is not an option for radio astronomy, satcom, and other noise-limited applications Add a decoupling network so the impedance matrix is diagonal Again, this is practical for communications, but the network would add more noise due to loss than the savings in improved matching Design LNAs so that the optimal source impedance is equal to the active impedance for one beam (boresight) Sensitivity decreases as the beam scans away from the matched beam Element port active impedances are different - more convenient if all LNAs are identical Active impedances can be outside the unit circle on the Smith chart Design LNAs so that the optimal source impedance is a compromise over the array field of view or scan range Requires LNAs matched to nonstandard impedance value Our strategy: Design the array to maximize sensitivity (G/T) Tune array elements to present active impedances as close as possible to 50Ω to the LNAs over the array field of view (low noise) Simultaneously tune array element radiation patterns to maximize aperture efficiency (high gain)

51 Design Optimization Process Computationally challenging! Single Element 7 x 2 Element Array 19 x 2 Element Array Infinite Array Unit Cell HFSS Sensitivity Cost Function (System Model - Reflector, LNAs, Receiver Chains, Beamforming Algorithm)

52 Wideband Dual-polarized Dipole Element for Green Bank Telescope PAF Similar structure to famous Goubau antenna Fully utilizes the space in the bounding box around the antenna (low aspect ratio) Goubau antenna (Single-pol) GBT antenna (Dual-pol) Fields evenly distribute on dipole arms Broad bandwidth Dual polarization High isolation Ultra low loss Unbalanced feed line (ideal for LNA design) ka Complex multiobjective design goals: Optimized jointly for the current cryo PAF element spacing and a future larger cryostat that will increase gain and tighten the feed pattern for the larger f/d and narrower opening angle of the GBT dish geometry (compared to 20 m telescope) The dish matters in the array feed element design! Sievenpiper, Daniel F., et al. Antennas and Propagation, IEEE Transactions on 60.1 (2012): 8-19.

53 BYU/NRAO Cryogenic PAF Development Reducing electronics noise yields huge gains in sensitivity cryocooled front end amplifiers (in the Cornell Arecibo cryo PAF design, the elements are also cooled) PAF development cryostat containing 38 SiGe low noise amplifiers for 19 dual-polarized antenna elements. Closed cycle refrigerator cools LNAs to 15K Thermal transition to element feed lines

54 First Test of Cryogenic PAF on 20-Meter Dish Mounted on Green Bank 20-Meter Telescope in early 2011 Measured sensitivity figure of merit: Lower is better Instrument Green Bank Telescope L-band Single Pixel Feed Modeled Tsys/Efficiency Measured Tsys/Efficiency 25 ±3 K Room Temp PAF 68 K 87 K Cryo PAF on 20-Meter (May 2011) K 49.6 K Highest demonstrated phased array sensitivity to date

55 First Test of Cryogenic PAF on GBT (Dec. 2013) GBT: 100m aperture diameter largest fully steerable antenna in the world Kite dipole element was used in 2013 experiment (two generations old) 38 channel data sampled with narrowband ADCs and streamed to disk (~300 khz bandwidth) Correlation, beamforming, and imaging done in postprocessing

56 Focal L-Band Array for GBT (FLAG) Full System in Development Front End (GBT) Back End (Jansky Lab) An t. LN A Cryostat I-Q mix, ADC, Serialize & Optical Xmit LO Array aperture, Antenna elements, LNAs, Cryo system, Down converters Ch. 1 Ch. 8 ( Front end Analog 40) Signal Transport: Optical fiber 8 Fiber Digital Optical Rcvr Card 8 Fiber Digital Optical Rcvr Card ROACH II FPGA Digitizers, Fiber Links, Ch. Subsystem 33 Polyphase Filterbank ROACH PAF, LNAs, Electronics (frequency II (BYU/NRAO) FPGA I-Q mix, ADC, channelization), LN Serialize & Ch. A Optical Xmit 40 An Packetization t. In Mezzanine I/F LO Slot (NRAO) ( 5) 4 x 10 Gbe I/F Card 4 x 10 Gbe I/F Card F Engine: DDL deserialization, boundary alignment, polyphase filter bank and 10 Gbe I/O 10 Gbe 40 Gbe 10 Gbe 48 X 10 Gbe port, 12 X 40 Gbe ports Ethernet Switch Melanox SX Gbe Rack Mount PC 12 TB SATA RAID 0 Disk System Array control and data storage (existing) CPU/GPU (Blade server + 2 nvidia GTX680) ( 5) CPU/GPU (Blade server + 2 nvidia GTX680) Correlation, Beamforming, XB Engine: Data Formatting, Correlator/Beamform Streaming er, Spectrometer to Disk (BYU/WVU) NRAO DDL System BYU Correlator Beamformer

57 Array Feeds for SatCom Motivation: Fine, fast target tracking for mechanically steered dishes Compensate for mount degradation, roof sag, mispointing Reduced total cost of terminal ownership Low profile feed geometry Opposite end of spectrum in terms of cost requirement ultra-cheap, ultra-small, and mass manufacturable yet noise performance and efficiency must be state-of-the-art Fabricated and demonstrated array feeds: Dielectric resonator antenna (DRA) array Passive arrays with patch type elements Single band and dual band (transmit/receive) Linear and circular polarization Active analog beamsteered array

58 Traditional Horn Feeds vs. Planar Array Feeds Conventional horn feeds Bulky size Heavy Complicate design especially for dual band dual polarization Planar array feeds Low cost Low profile Easily fabricated Integrated with circuits 58

59 Antenna Design Process Single Element 2 x 2 Dual Band Array Fabrication and Test Rx Feed Network The Competition: Tx Feed Network - Very high efficiency - Bulky, costly to build

60 Impact of radiation, spillover, and aperture efficiencies on G/T / 4 / Each curve shows the independent impact of the corresponding efficiency on the SNR improvement or degradation, compared with current design. ƞ rad ƞ ap ƞ sp 1 db efficiency change SNR Variation 2.4 db 1 db 1.25 db SNR Variation (db) Current Design Basis Radiation Efficiency Aperture Efficiency Spillover Efficiency Radiation efficiency is most critical. Various recently developed advanced antenna types are not useful for satcom due to low efficiency Efficiency (%) Efficiency can be optimized by careful choice of substrate dielectric constant and thickness and rigorous design optimization

61 Measurement Results Averaged measurement results from three methods Good agreement between measurement and simulation (only study of its kind that we know of) Best radiation efficiency 93% reported to date for 2x2 microstrip antenna array (better even than previously reported single elements!) Measurements courtesy of Christopher L. Holloway U.S. National Institute of Standards and Technology (NIST), Boulder, CO, USA

62 Array Feed Designs Stacked shorted annular patch element (SSAP) Individual element matched to dish illumination Ultra-high radiation efficiency Non-planar Hex feed, two variants Planar fabrication Multilayer PCB with feed network Edge-fire Vivaldi Array Square ring slot dual circular polarization antenna element Unsolved problem in antenna world! High isolation, low loss, good cross pol 4x4 Ku band beamformed array feed Planar fabrication Electronic beamsteering 62

63 Conclusions This work on array noise theory and sensitivity optimization applies to wide range of antennas Active arrays Nonreciprocal antennas Mutually coupled arrays Digitally beamformed arrays and applications: Astronomical array receivers L band through mm-wave SatCom phased arrays and array feeds MIMO antennas MRI coil receivers Near field communication (NFC) arrays

64 Conclusions The IEEE Standard for Antenna Terms offers an elegant, time-tested system of figures of merit It should be taught in classes and referred to in textbooks Software packages, books, and articles should use terms consistent with the standard! However, traditional antenna concepts are inadequate for modern antenna systems, particularly digitally beamformed receiving arrays Using noise theory, gain, radiation efficiency, and other antenna parameters can be extended to phased arrays and arbitrarily complicated receivers yet to be built For simple antennas, the new terms agree exactly with existing definitions Although the new terms apply to any type of antenna that can receive a signal, some of the new terms are given the qualifier active antenna in the IEEE Standard to highlight the motivation and logical link between them Don t introduce a new efficiency or antenna parameter without: Checking to see if one is already defined that does the job Making sure you know how it enters into the overall system performance measure (usually SNR) Measurable using a free space Y factor method, augmented by mutual impedance/s-parameter measurements See IEEE Standard !