Measurement Aspects of Mobile Terminal Antennas Overview
HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory Antennas Sept. 5-9, 2005 SMARAD IDC / Radio Laboratory Clemens Icheln Clemens Icheln Antennas Sept. 5-9, 2005 Overview
Measurement Aspects • Introduction of Mobile Terminal Antennas • Small-antenna characteristics • Standard measurement methods Clemens Icheln • Other characterisation methods SMARAD IDC / Radio Laboratory • Specific error sources and solutions [email protected]
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HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Mobile communications antennas Antenna Characteristics The most important small-antenna characteristics are: • Trend: Increasing number of communication systems • input impedance / bandwidth • small(est) mobile terminals • 3-D radiation pattern complexity complex antenna structures and multi-element antennas • gain / directivity / radiation efficiency • User effect, Specific Absorption Rate (SAR) • simulation-based design and evaluation, but prototyping and Multi-element: Diversity and MIMO performance measurements usually needed
KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Radiation Efficiency Wheeler Cap Method (WCM) (1/2) • Measurement of S11 in free space is contrasted with measurement of S when antenna is placed inside a metal cap • The radiation efficient of an antenna is the ratio of the 11 total radiated power and the (net) power accepted by Difference in reflected power represents the radiated power in free space the antenna: Antenna • Cap eliminates radiation and thus also cap element ηr = Prad,tot / Pin eliminates radiation resistance Rr • Distinction between (physical) radiation efficiency η – size and shape of the cap not significant, r but cavity resonances must be avoided including only internal losses inside antenna structure, • Loss resistance Rl remains ~unchanged and total antenna efficiency, which is ηr reduced by mismatch losses (at the antenna connector) Wheeler Cap
• Efficiency is also given by ηr = Gain/Directivity
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HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 Wheeler Cap Method (cont’d) Stirred mode chamber • Under these conditions the radiation efficiency can be determined with two measurements by first measuring the input resistance of the AUT • Stirred mode chamber [1]: rotating blades without the cap (Rfs = Rr + Rl), and then with the cap (Rc = Rl + Rs) – Shielded chamber with metal walls – Lots of resonant modes can exist • The radiation efficiency ηr can then be calculated as – Moving metallic stirrers make all P R RRfs− c AUT η =r ×100% = r × 100% = ×100% resonant modes equally strong r P RR+ R in r l fs – Time-average of field is homogeneous field everywhere in the chamber probe • In practice, the current distribution and thus also the resonant total radiated power is obtained frequency of the antenna under test inside cap change slightly by probing e.g. E-field in one point some uncertainty (several percents) in the measurement
• Acceptable accuracy - yet, better results can be obtained by using so- [1] P. Corona, G. Ferrara, M. Migliaccio, “Reverberating chambers as sources of stochastic electromagnetic fields”, called Improved Wheeler Cap Method (IWCM): IEEE Transactions on Electromagnetic Compatibility, Vol. 38 No. 3, Aug. 1996 , pp. 348 –356.
KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Scattered field chamber measurement Pattern integration method • by integrating the 3-D gain pattern*, the total radiated power is obtained – Goal: simulate a realistic propagation environment • Typically, in 3-D pattern measurements a reference antenna is needed – shielded chamber with (here: usually fixed) reflectors – as gain reference, or – measurement antenna has no line of sight to AUT – as efficiency reference
– rotation of the AUT • The directivity D0 can be obtained from any 3-D pattern with: Rayleigh distribution
– if a gain reference was used: ηAUT = Gmax/D0 Ptot,, rad AUT – if an efficiency reference was used: ηη AUT= ref.. ant Ptot,,.. rad ref ant
(* for each direction, the received power in phi- and theta-polarisations needs to be summed up first) KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course
HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Anechoic Chambers Absorber lining • Sharp tips smooth change of impedance • Need: well-defined field strength at measurement position • Area filled with absorbing material increases • Means: placing the field source in a reflection-free environment • Absorber height > λ/2 of lowest frequency all walls need to be completely covered by RF absorbing material: e.g. fmin=900 MHz habs ~ 30 cm (better: h > 45 cm)
For ’flat’ incidence waves, wedges give better absorption
KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Effect of non-ideal absorber lining Measurement setup
Erefl. Option A: Measurement antenna Direct field / Amplitude Phase moves on an arch, AUT is rotated AUT E Etotal refl. field error/ripple error direct around vertical axis easy cabling 0 dB +6 … −∞ dB ±180° (static), complex construction for meas. • Random reflections antenna 10 dB +2.4…−3.3 dB ±18° measurement antenna (or array) cause interference pattern 20 dB +0.8…−0.9 dB ±5.7° Option B: Measurement antenna fixed, AUT is rotated around two axes • In good chambers 30 dB ± 0.3 dB ±1.8° AUT complicated cabling (rotary reflections level is 40 dB ±0.09 dB ±0.57° joints), easy installation of -25 dB or better measurement antenna(s)
KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course
HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 Small anechoic chamber Example: Dual axis 3 D pattern measurement antenna under test in the small anechoic chamber
AUT (embedded movie) two axes Dual-polarised measurement antenna distance ~ 1.3 m measurement antenna light-weight positioner
TKK/Radiolab’s Anechoic Room KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Novel measurement systems Satimo Stargate 64
Typical problems in far-field-pattern measurements with - circular array of 64 dual-polarised field probes
mobile terminals: - Perturbation technique gives amplitude and • Long measurement time for 3-D patterns due to phase of the incident field at each probe mechanical movement of antennas (either probes or AUT) - far-field radiation pattern obtained through spherical wave expansion • Only amplitudes of radiated fields of active mobile phones obtainable - the phase is needed e.g. for diversity or - Usable frequency range: 0.8 − 3.2 GHz MIMO evaluation - Ø = 4m user can also be in the setup - Full 3-D far-field measurement within minutes
Source: http://www.satimo.com
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HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 RAMS: Rapid antenna Body phantoms measurement system • Test-person effect important, variations up to 10 dB • Simultaneous use of 32 dual polarised antennas located on a • standardised head phantoms to model the handset user sphere around the AUT (possible • liquids for 900 MHz / 1800 MHz to model brain tissue to include a user) Effect of user on radiation characteristics / efficiency • Measurement distance about 1 m Standardised handset position ’cheek’ on SAM: • Phase retrieval network uses one of the measurement channels as phase reference • Spherical Wave Expansion yields full 3 D (complex) far fields SAM = Specific • Measurement time only 3 sec (per Anthropomorphic wideband Vivaldi- Mannequin frequency point) type antenna KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Antenna prototype with head phantom: Specific Absorption Rate
• Power dissipated in the user’s head in W/kg
• Limits according to standards such as EN50360: – EU: 2 W/kg for 10-g volume-averaged SAR – US: 1.6 W/kg for 1-g volume-averaged SAR • Representative head phantom: SAM phantom • Frequency-specific fluid (see IEEE P1528 specifications) Small isotropic E-field probe is moved inside liquid, while phone is placed in typical position(s) at ear
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HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 SAR measurements SAR measurements User effect
• Automated system required user’s • Standard EN50361 applies mobile torso • For terminals that terminal • Commercial DASY4 system: feature Web & Video user’s arms functionality, new user phantom(s) are needed to evaluate the effect a user has on the terminal
performance (e.g. by styrofoam base phone shadowing)
Simple test set-up for • Alternative: SAR prediction ”browsing” position [1] with EM-field simulations [1] J. Krogerus, “Phantoms for Terminal Antenna Performance Testing”, COST273 TD(02)154, Lisbon, Portugal, September 2002, 6 p. KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 Radio-channel sounder Example: Urban spherical array • Radio-channel sounder measurements measurement (@2.1GHz) – Fast-switched (~ms) multi-antenna arrays at both ends (BS and MS) – In multi-antenna systems, all antennas can be simultaneously measured sounder => diversity/MIMO performance directly available MS • Radio-channel characterisation (mobile – Multi-path environment station) – Spherical multi-element antenna array base station used as base- and mobile station – With beamforming methods the directions of departure (DOD) and directions of arrival (DOA) is obtained A 5.3 GHz General channel models for 16 element different environment categories antenna array KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course
HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Models of urban environment Example 2: Elevation power distribution and θ- and φ-polarized powers in another macrocell route (transmitter at rooftop level):
Line of sight−60 Measured elevation power Theta−polarization distribution at 2.1 GHz in an −65 Phi−polarization urban macrocell environment, −70 and two simplified models: −75 −80
−85
Received power [dB] −90
−95
Source: Kalliola, K. et al: “Angular power −100 0 40 80 120 160 200 240 distribution and mean effective gain of mobile MS location [m] antenna in different propagation environments”, IEEE Transactions on Vehicular Technology, • Incident signal power arrives mainly from the directions just above the Vol. 51, Issue 5, 9/02, pp: 823 838 elevation azimuth plane - especially true in macrocell environments
KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Multi-element terminal antennas MEasurement Based Antenna Testbed Spherical antenna Reference point • MEBAT is HUT/Radiolab’s novel multi- • Simplest case: Diversity (e.g. dual-polarised terminal antenna) antenna system evaluation tool Signals • Most complex case: MIMO (multi-element arrays at both ends) • Computational performance evaluation of a multi-antenna configuration already in the • Static or separate single-channel measurements not sufficient early phase of the design process: • Average performance affected by time- and place-dependent Signals characteristics of the (dynamic) radio propagation environment Spherical antenna MEBAT • Dynamic MS measurements in real propagation array radio channel DoA estimate of the incident Measured/simulated environments are time consuming and expensive measurements complex signals (beamforming) radiation patterns
(P. Suvikunnas et al: Evaluation of Σ Σ Σ ×Σ ××× Received signals performance of multi antenna Simulated or measured complex of the diversity terminals using two approaches, 3 D radiation patterns of a multi branches IMTC/04 proceedings, Como, Italy, element mobile terminal antenna KTH Stockholm, Sept ‘05 ACE MIMO course 05/04,pp. 1091 1096.)KTH Stockholm, Sept ‘05 ACE MIMO course
HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 Mutual coupling Mutual coupling (cont’d) • mobile antennas show mostly resistive mutual impedance • when measuring a combination of two • normalised mutual resistance is approximately equal to the closely spaced antennas such as a correlation coefficient: mobile-handset diversity antenna, mutual coupling may have an effect Envelope Normalised Attenuation Coupling correlation mutual res. of received betw. loads power [dB] S [dB] • in typical measurement arrangement both 21 antennas connected to matched loads: 0 0 0 - inf. ideal case 0.3 0.55 -0.4 -10.6 due to mutual coupling dissipation in both real cases 0.7 0.84 -1.6 -5.9 antennas (+ mismatch) => possibly less dual element antenna on power received than with single antenna head phantom (”SAM”) 1 1 -3.5 0 worst case Source: R. Vaughan and J. Bach Andersen, “Antenna diversity in mobile communications”, IEEE Transactions on Vehicular Technology, Vol. 36, No.4, November 1987, pp. 149 172.
KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Measurement errors Phase centre alignment
• General accuracies of RF signal source, the VNA, • Typical assumption is that the phase centre is at the feed point cables losses, connectors, wall reflections of the antenna – true only for simple structures • But for complex structures such as handsets the location of the • Positioning of antenna(s) phase centre varies as a function of frequency! – Choice of range (minimum distance) • In the presence of a head phantom, the location of the (effective) – Phase center alignment phase center may move outside the handset perimiter • Effect of RF cable shield Measured field-strength uncertainty when rotating off the phase centre: – Reflection and re-radiation misalignment d = 5 m d = 1 m d =0.5 m – Parasitic radiation = 10 mm <0.1 dB ±0.1 dB ±0.2 dB = 30 mm <0.1 dB ±0.3 dB ±0.5 dB
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HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005 Effect of RF cable Current chokes handset EM waves ferrite bead (lossy, wideband), cap/balun (resonant, narrowband)
• With an external signal Example: measured magnetic fields along RF cable (900MHz): source (e.g. VNA), we need an additional RF H-field Case M4 dBmA/m cable
no free-floating AUT AUT + feed cable cap • Antenna prototypes are unwanted mostly measured without an transmission independent RF transmitter ferrites cap + ferrites
there are several solutions: signal-feed cable KTH Stockholm, Sept ‘05 ACE MIMO course KTH Stockholm, Sept ‘05 ACE MIMO course HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005 Clemens Icheln Antennas Sept. 5-9, 2005
Current chokes (cont’d) Optical RF links Comparison of θ pol. yz plane gain patterns at 920 MHz for an AUT set up with bare RF feed cable only • ideal: no RF cables from/to AUT and a set up including a balun and ferrites ——. • still not many commercial products • power consumption, size • typically phase information is lost
• no information obtained about S11
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HELSINKI UNIVERSITY OF TECHNOLOGY MIMO Communication Systems and SMARAD IDC / Radio Laboratory Clemens Icheln Antennas Sept. 5-9, 2005
Select reading - IEEE Standard Test Procedures for Antennas, ANSI/IEEE Std. 149-1979, IEEE Press, New York, NY, 1980, 143 pages John D. Kraus (ed.), Antennas (3rd ed.), McGraw Hill, Chapter 24 Hiroyuki Arai, Measurement of mobile antenna systems, Artech House, 214p. Clemens Icheln, Methods for measuring RF radiation properties of small antennas, Doctoral dissertation, http://lib.hut.fi/Diss/2001/isbn9512256886 A. Lehto, A. Räisänen, Mikroaaltomittaustekniikka, 3. painos, Espoo, Otatieto Oy, 1995, 215 sivut K. Hirasawa, M. Haneishi, Analysis, Design, and Measurement of Small and Low-Profile Antennas, Artech House, 1992 K. Fujimoto, J.R. James, Mobile Antenna System Handbook, Artech House, 2001
KTH Stockholm, Sept ‘05 ACE MIMO course