Fundamentals. Senior Project Manager / AEO Taiwan. Philip Chang

Transcription

1 mmwave OTA Fundamentals Senior Project Manager / AEO Taiwan Philip Chang

2 L A R G E LY D R I V E N B Y N E W W I R E L E S S T E C H N O L O G I E S A N D F R E Q U E N C Y B A N D S 1. Highly integrated radios, no cabled connection points 2. Massive MIMO, requiring many antenna elements. 3. Need to test end-end performance in a controlled, repeatable manner 4. Drive to lower costs will eliminate multiple (expensive) coaxial antenna connectors 5. Phased arrays for multi-beam operation and spatial diversity with beamsteering 6. Millimeter-wave frequencies for broadband connectivity, high-capacity mobile data, backhaul, satellite and radar 2

3 E X A M P L E : B L O C K D I A G R A M O F A MMW R A D I O S Y S T E M Antenna element block Block containing transmit/receive amplifiers and gain/phase control to multiple elements No cabled access at this interface 3

4 D is the largest D antenna dimension Reactive Near-Field Radiating Near-Field Fresnel Region R = 2D2 λ Radiating Far-Field Fraunhofer Region The (Fraunhofer) Far Field distance R = 0.62 D3 λ Additional requirements to satisfy the Far Field Condition: Most mmwave OTA measurement systems operate in these two regions R D R This Far Field Condition holds for electricallylarge antennas where D > 4

5 Direct Far Field (DFF) test system is based on combination of the antenna aperture (D) and the operating frequency/wavelength (λ) to measure a true far field distance defined as FF= 2D 2 /λ. Indirect Far Field (IFF) Compact Test Range (CATR) uses reflectors to focus the RF energy into a plane wave within a much shorter distance than would normally be required to achieve radiated far field measurements. Near-field test system measures the energy in the radiating near-field region and converts those measurements by a Fourier transform into the farfield result. AUT AUT AUT 2D 2 /λ Source Horn Near Field Probing Source Horn Copyright 2018 Keysight Technologies Spherical Cylindrical Planar 5

6 3 G P P T S V ( ) The objective of this study is to define the over the air (OTA) testing methodology for UE RF, UE RRM, and UE demodulation requirements for New Radio frequencies above 6 GHz Direct far field (DFF) Direct far field (DFF) setup simplification for centre of beam measurements Indirect far field (IFF) method Near field to far field transform (NFTF) 6

7 D I R E C T F A R F I E L D ( D F F ) Place DUT at distance of minimum 2D 2 /λ from Source: Phase variation is 22.5 o Have to move well beyond minimal 2D 2 /l to get phase variation much less than 22.5 o Amplitude (Power) decreases as distance from source increases Lost Power Plane wave Lost Power DUT Far Field Test Range 24 cm 29 cm FF Distance Source Feed Horn Copyright 2018 Keysight Technologies 7

8 I N D I R E C T F A R F I E L D ( I F F ) The IFF method creates the far field environment using a transformation with a parabolic reflector. This is also known as the compact antenna test range (CATR). Indirect Far field of CATR as the one used in [9] with quiet zone diameter at least D. A positioning system such that the angle between the dual-polarized measurement antenna and the DUT has at least two axes of freedom and maintains a polarization reference For setups intended for measurements of UE RF characteristics in NSA mode with 1UL configuration, an LTE link antenna is used to provide the LTE link to the DUT. The LTE link antenna provides a stable LTE signal without precise path loss or polarization control Copyright 2018 Keysight Technologies 8

9 . new N E A R F I E L D T O F A R F I E L D T R A N S F O R M ( N F T F ) Key aspects of the Near Field test range Radiated Near Field UE beam pattern are measured and based on the NFtoFF mathematical transform. A positioning system has at least two axes of freedom and maintains a polarization reference The LTE link antenna provides a stable LTE signal without precise path loss or polarization control Applicability criteria of the NFTF setup The DUT radiating aperture is D 5 cm Manufacturer declares antenna array size EIRP, TRP, and spurious emissions metrics can be tested. Typical NFTF measurement setup of EIRP/TRP measurements 9

10 I N D O O R A N E C H O I C Chamber D U T Measurement Distance = far field 2D2 λ Absorber Probe Antenna Real-world DUT environment Antenna Beam pattern characterization EIRP/TRP and EIS measurements Beamforming/Beamsteering Validation RF Parametric Tests (if S/N high enough) Can fit blocking sources Can be very large Large chambers can be very expensive (construction/installation) High to very high path loss Phase variation is 22.5 o 10

11 L O N G E R F A R F I E L D A N D H I G H E R PAT H L O S S But to use this requires manufacturer declaration of antenna positions interface to signal what array is being used repositioning during testing preclusion of using more than one array at a time For these reasons RAN4 concluded in Aug. only a black box approach is allowed* Friis Transmission Equation Path Loss Proportional to R 2 Far-Field Distance (m) Coupling? Head/Hand? Black/White Box Testing? D (mm) 28 GHz 39 GHz 60 GHz Copyright 2018 Keysight Technologies 11

12 PAT H L O S S E X A M P L E Recall the Friis Transmission Equation: And the Free-Space Path Loss Equation: P r P t = c 4πRf 2 G t G r FSPL(dB) = 20log 10 4πR λ OTA signal attenuation requires careful system design to ensure measurements can be made. Signal loss increases with distance and frequency If the Device Under Test (DUT) has a maximum diameter D of 15cm at 28GHz, then the Far Field distance is 4.2m (minimum). FSPL = 73.9dB Chamber D U T Measurement Distance = 4.2m Absorber Assume DUT Gain = 20dB, Probe Antenna Gain = 15dB Actual signal loss (attenuation) is 73.9 ( ) db = 38.9dB Note that this calculation has to assume one can measure the conducted power of the DUT, then add the actual signal loss. In many cases one starts with EIRP (of the DUT), subtracting the FSPL and adding the antenna gain to obtain the signal level at the probe antenna port. 12

13 C O M PA C T A N T E N N A T E S T R A N G E S ( C AT R S ) Smaller footprint than Far-Field Lower path loss Antenna Beam pattern characterization EIRP/TRP and EIS measurements Beamforming/Beamsteering Validation RF Parametric Tests Reasonable speed of test Large chambers can be very expensive (construction/installation) Can t fit blocking sources 13

14 Reflector Probe Feed Quiet Zone Simulation of signal transmitted from Probe Feed, showing parallel phase fronts in Quiet Zone CATRs are reciprocal so that a beam transmitted from the DUT in the Quiet Zone is focused back to the probe feed. In a CATR system, a diverging beam from the probe antenna illuminates the parabolic reflector from the focal point. The reflector collimates the beam and directs it to the DUT. The collimated beam has a nearly uniform amplitude and phase across its extent; it provides a nominally ideal plane-wave illumination to the DUT. The reflector allows the DUT to be tested under far-field plane wave conditions at a shorter distance than 2D 2 Τλ (the far field distance), resulting in a system with potentially a much smaller footprint and lower path loss than the equivalent direct far-field method. 14

15 CATR specifications begin with determining Quiet Zone size, frequency range and performance parameters (e.g. Quiet Zone specification) The Quiet Zone normally encloses the entire active array of the DUT Typical Quiet Zone specifications are shown below these are a contributing component in determining the accuracy of measurements Example Quiet Zone Size 80cm x 80cm Quiet Zone Shape Circular Cylindrical Amplitude Taper (db) 1 Amplitude Ripple (db) ±0.5 Total Phase Variation ( ) ±10 Cross Polarization (db) 30 Minimum diameter Quiet Zone enclosing all of the active array, or multiple subarrays. 15

16 Rolled-edge reflector Often used for smaller mmwave ranges. Serrated-edge reflector More common for large and/or low-frequency reflectors 16

17 In a CATR, the path loss for both transmit and receive measurements at a particular frequency is fixed, and is determined by the focal length (distance from parabolic reflector to probe feed.) Take the previous example: DUT has a maximum diameter D of 15cm at 28GHz The Far Field distance is 4.2m (minimum) Free-Space path loss is 73.9dB Let s say we design a CATR with a 30cm Quiet Zone (twice as large as needed for this DUT. The reflector for this CATR is 50cm x 50cm The focal length is 76cm (we use this distance to calculate FSPL) Free-Space Path Loss = 59.0dB DUTs whose dimensions dictate different far field distances can be measured in a CATR with fixed dimensions and fixed path loss 17

18 Both far-field and CATR methods can provide comparable far-field measurements. These plots show 28 GHz antenna pattern measurements of a standard gain horn performed in two different chambers. The blue far-field measurements were performed in a >3 m direct far-field chamber. The red CATR measurements were obtained using a small CATR chamber. The results are in excellent agreement, where the direct far-field results have sufficient dynamic range for comparison. 18

19 These feedhorns measure gain for other antennas by comparing the signal level of a test antenna to the standard gain horn, then adding this difference to the calibrated gain of the standard gain horn at the test frequency. They can also be used as reference sources in dual-channel antenna test receivers, or as receiver horns for radiation monitoring, or simply as a general purpose antenna. Standard Gain Horns are available in either pyramidal or conical forms. Conical standard gain horns are best suited for applications requiring a small, inexpensive antenna capable of polarization diversity. The conical standard gain horns can support horizontal, vertical, left- and right-hand circular polarization when used with a polarizer. Typical beamwidth is 16ºand typical midband gain is 21 dbi. Pyramidal horns are linearly polarized, having a single-mode waveguide flange. They have a typical beamwidth of 25 and a typical midband gain of 24 dbi. Conical Rectangular Source: Millitech/Smiths Interconnect 19

20 Corrugated feedhorns are more expensive to fabricate than Standard Gain feedhorns, but offer several advantages: Beamwidth is the same in both E and H planes (unlike Standard Gain Horns) Corrugations Lower sidelobes Low VSWR Gaussian beam profile suitable for launching into quasi-optical systems Cross-section 20

21 Feeds (feedhorns) for use in Compact Antenna Test Ranges are generally designed specifically to work with the reflector and chamber design to ensure that the Quiet Zone specifications are achieved. Available in dual-polarized or linear (single polarized) Dual-polarized CATR feeds (NSI-MI) Wide beamwidth (e.g deg) to ensure correct illumination of the CATR reflector Gain around 10dB Cross polar isolation typ ~35dB (dual-pol) Linear (L) and Dual-polarized (R) CATR feeds (MVG) 21

22 Attribute Direct Far-Field Compact Antenna Test Range Chamber size Determined by dimension of antenna required far-field distance Example: 150mm diameter array at 28 GHz has far-field distance 4.2m, and chamber length of at least 5.5m Determined by required quiet zone diameter Example: 150mm diameter array at 28 GHz. Chamber length approximately 2m Path loss Determined by distance between DUT and probe antenna. Example, for a far-field distance of 4.2m at 28 GHz, freespace path loss 74dB Determined by focal length of reflector. Example, 300 mm diameter QZ, free-space path loss 59 db Cross-polarization isolation Antenna Pattern Measurements Position of DUT relative to probe antenna beam High Measurement accuracy of side lobes and nulls is better at larger distances between DUT and probe antenna Center of radiation and/or geometric center of DUT antenna array must be at center of quiet zone. ~30dB (curved reflector generates cross-polar component) Antenna pattern measurements are equivalent to those measured in direct far-field, measurement accuracy of nulls and side lobes depends on quiet zone flatness All DUT antenna arrays must be contained within the quiet zone Cost Scales with chamber size Cost of precision reflector offset by smaller chamber 22

23 E X A M P L E F R O M V E R I Z O N W I R E L E S S 5 G L A B mmw Connection 5 G N E T W O R K E M U L AT I O N S O L U T I O N IP Connection Positioner Controller T E S T S Y S T E M P C Video Source: 23

24 Near-field measurement systems sample the phase and amplitude of the electrical field in the radiated near-field region (the Fresnel region), over a two-dimensional surface using high-precision positioners. This surface is either planar, cylindrical or spherical. The far-field antenna pattern is then calculated using Fourier transform algorithms. Near-field scanning images in planar (left), cylindrical (middle) and spherical (right) surfaces. Power and phase are measured at each scan point Choice of Near Field range depends on the nature of the antenna being measured. Near Field systems can be realized using relatively small test chambers, and are often less expensive than indoor far field ranges Vertical Portable Planar Near Field system from NSI-MI. The DUT is the blue mmwave module on the left. The x-y positioner holds the probe feed and makes a series of linear scans across a surface in front of the DUT Linear Y stage Rotating azimuth stage Arch over azimuth system. DUT (in this case, a feed horn) is mounted on a rotating stage 24

25 C O N V E N T I O N A L U S E Near Field scanning systems using very high precision positioners are conventionally used to measure the pattern of an antenna system in transmission Smaller DUT to probe distances result in lower system (path) losses The requirement to measure phase and amplitude over the scan surface implies using a CW signal and a stimulus/response system such as a vector network analyzer or similar Post-processing transformations are used to calculated the Far Field antenna pattern, and can also be used to calculate fields on the surface of the antenna ( back transformation or microwave holography, useful for fault diagnosis) Near Field antenna pattern, 8x8 phased array Far Field antenna pattern, 8x8 phased array 25

26 Many mmwave devices are single-ended (integrated systems), so a conventional fast Vector Network Analyzer cannot be used. Different techniques must be used to provide a phase reference so that the far field transformations can be performed. In this example, a fixed probe antenna is positioned close to the DUT to provide a phase reference during a spherical scan Probe antenna for near field measurement Probe antenna for phase recovery 28GHz Transmit Antenna Simulated wideband 28GHz phased array 26

27 W H AT I S T H E P H A S E O F A N M H Z W I D E S I G N A L AT 2 8 G H Z? Near Field to Far Field transformation becomes problematic when a wideband modulated signal is transmitted, since the phase can vary across the signal. Why does this matter? Consider that a wideband modulated signal can be though of as a series of CW signal superimposed and transmitted simultaneously. Modulated-signal antenna patterns show differences in sidelobe level (higher than CW signals) and null depth (less deep than CW). What is the phase of an 800MHz wide signal at 28GHz? Measuring modulated signal antenna patterns could require a Far Field chamber, or a series of transformations performed on each subcarrier 27

28 The single probe antenna in a Near Field scanning environment will transmit a signal with an expanding phase front - looks something like a point source. If the phase front geometry of the probe antenna is known, it is possible to calibrate the receiver phase settings to simulate receiving a plane wave. Generation of known real-time spatial fields generally requires multiple probes, such as this 4G Multiprobe Anechoic Chamber (MPAC). (Such systems currently operate in the Far Field, not Near Field.) 28

29 Chambers with multiple probes fall into two main categories depending on use case: 1. End to end throughput testing (demodulation) for 4G UE s. a) Creates a known 2D spatial field b) Operates in Far-Field at frequencies below 6GHz c) Test zone becomes really small as frequencies increase d) Currently has no mmwave equivalent for 3D spatial fields MPAC at Aalborg University 2. Fast mmwave antenna testing using electronically scanned array of probes a) Operates in Near Field at mmwave b) Similar to spherical scanning but with speed advantages c) Reduced mechanical movement d) Performs all antenna measurements and TRP, TIS, EIRP and EIS MVG StarLab 50GHz 29

30 Feeds for use in Near Field Ranges are chosen to present a small cross-sectional area in order to minimize reflections back towards the DUT. Typically these are open ended waveguide probes with RF absorbing collars. Linear (rectangular waveguide) probes most common, aligned with waveguide bands Family of open-ended waveguide probes (NSI-MI) Dual-polarized designs also available Tapered edges to minimize diffraction Gain around 7.5dB-10dB 3-D beam pattern (MVG) 30

31 Broadband here means broader than normal waveguide bands. Open and closed quad ridge horns (dual polarized) Very wide bandwidth avoid changing antennas in automated sweeps Gain varies with frequency Closed boundary quad ridge horn (MVG) 10-40GHz 12-19dBi Open boundary quad ridge horn (MVG) 4-40GHz 5-16dBi Planar Vivaldi antennas (linearly polarized) Easier to fabricate for higher frequency operation e.g GHz Gain and efficiency are flat with frequency Vivaldi Antenna S11, Gain and Efficiency of Vivaldi Antenna 31

32 Far Field Setup CATR Setup Near Field Setup Very large dimensions due to black box approach Both Antenna & RF parametric measurements can be done Large path loss -> low measurement dynamic range 3GPP compliant High measurement uncertainty in case of making measurements of offset DUT placement from phase center of Quiet Zone Measurements in farfield conditions in a compact footprint Both Antenna & RF parametric measurements can be done Path loss only dependent on focal length. -> Good dynamic range 3GPP compliance work ongoing. Awaiting Beamlock mechanism definition to be standardized in Rel15. Low measurement uncertainty if the device can be placed within the Quiet Zone. Search algorithm can help identify the location of the radiating region to reposition the DUT Most compact setup Suitable only for antenna measurements. Question for RF parametric measurements Verylow path loss ->Best dynamic range 3GPP compliant in V High measurement uncertainty in case of making measurements of offset DUT placement from phase center of Quiet Zone Copyright 2018 Keysight Technologies 32

33 3GPP TS V2.1.0 ( ) DUT category Category 1 Category 2 Category 3 Description Maximum one antenna panel with D 5 cm illuminated by test signal at any one time More than one antenna panel D 5 cm without phase coherency between panels illuminated at any one time Any phase coherent antenna panel of any size (e.g. sparse array) Category? 1Tx 2Rx DUT category Direct Far Field (DFF) Indirect Far Field (IFF) Near Field to far field transform (NFTF) TBD Category 1 Yes Yes Yes TBD Category 2 Yes Yes Yes TBD Category 3 No Yes No TBD NOTE: A positive indication means that applicability exists for at least one RF test cases for the given UE category new 33

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