Tsoulos George (ред.) MIMO System Technology for Wireless Communications
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264 MIMO System Technology for Wireless Communications
words, RF MEMS actuators are used as part of the physical structure of the
antenna, owing to the monolithic integration capability of the processing
technique, providing a large degree of structural reconfigurability.
MRSA architecture is built on a number of printed rectangular-shaped
metal strips interconnected by RF MEMS actuators on a microwave laminate
PCB substrate, RO4003-FR4 (J
r
= 3.38, tanI = 0.002). Shown in Figure 9.9a
are two adjacent strips interconnected by a RF-MEMS actuator, which is
made of a metallic movable membrane, suspended over a metal stub pro-
truding from an adjacent strip, fixed to both ends of the strip through metallic
posts. The optimized height of these posts was found to be 8 µm for a good
tradeoff between up position switch coupling and actuation voltage. Metal
stubs are covered by silicon-nitride (SiN
x
) film to prevent metallic membrane
from sticking onto the stub upon contact. This film also provides a capacitive
contact for the actuator down state isolating RF signal from DC. A DC bias
FIGURE 9.8
Measured co-pol radiation patterns at 5GHz of each antenna element in the x-y plane when
the three switches are sequentially activated.
180
90 270
0
−10
−20
−20 −10
(a) (b)
180
90 270
0
−10
−20
−30
−30
−10
A3
(c)
180
90 270
0
−10
−20
−20 −10
−30
4190_book.fm Page 264 Tuesday, February 7, 2006 12:44 PM
Multifunctional Reconfigurable Microelectromechanical Systems 265
voltage of approximately 40 V applied between the membrane and the stub
causes an electrostatic force that pulls the suspended membrane on top of
the stub (actuator down state or actuator on, see Figure 9.9c), and the actuator
connects the strips; otherwise the strips are disconnected (actuator up state
or actuator off; see Figure 9.9b). Judicious activation of interconnecting actu-
ators, i.e., by keeping some of the actuators in the up position (zero bias)
while activating the rest of them by applying DC bias voltages, allows the
reconfigurable spiral to configure its architecture into single arm rectangular
spirals with opposite winding sense of the spiral, left or right senses (see
Figures 9.10a and b). Accordingly, right- and left-hand circularly polarized
(RHCP and LHCP) radiation is achieved. In Figures 9.10a and b, for the
clarity of illustration, each configured geometry is depicted separately and
actuators in the up state are shown without metallic membrane. The antenna
is fed by a single coaxial probe, as shown in Figure 9.10c. The supply voltage
is connected to the proper locations on the antenna segments through resistive
FIGURE 9.9
RF MEMS actuator interconnecting two adjacent metallic strips (a) top view; width of metal
strip, w
1
= 800 µm; width of stub, w
2
= 100 µm; width of membrane, w
3
= 150 µm; (b) side view
(up position); thickness of nitride, t
1
= 0.2 µm; thickness of membrane, t
2
= 0.5 µm; air gap, g =
7.8 µm; (c) side view (down position); thickness of electroplated copper, h
1
= 8 µm; thickness
of PCB copper, h
2
= 16 µm.
A A‘
W
2
W
3
W
2
W
1
A
A‘
t
2
g
t
1
PCB
h
1
h
2
(a)
(b)
(c)
Aluminum
Electroplated copper
PCB copper
Substrate
SiN
x
4190_book.fm Page 265 Tuesday, February 7, 2006 12:44 PM
266 MIMO System Technology for Wireless Communications
bias lines so as to prevent the RF signal from being shorted by the DC power
supply.
MSRA is designed to radiate an axial beam of RHCP and LHCP fields. It
is known that a single-arm rectangular spiral antenna with outermost arm
peripheral length (circumference) of C,
(9.4)
excites only the first radiation mode, giving rise to an axial beam of circular
polarization [23], where Q
eff
= Q
0
/[(J
r
+ 1)/2]
1/2
is the effective wavelength of
the current traveling on the spiral. The number and size of the strips are
optimized so that circumference of the antenna, C = 42 mm = 1.04 Q
eff
, satisfies
Equation 9.4 and a minimum number of actuators with associated bias
circuitries are needed.
9.4.2.2 Microfabrication and Results
The MRSA is microfabricated on a microwave laminate substrate RO4003-
FR4, which is conductor backed to ensure that the antenna radiates broadside
FIGURE 9.10
Schematics of the single arm rectangular spiral antennas, which are reconfigured from the
MRSA architecture by judicious activation of the interconnecting RF MEMS actuators, for
(a) left-hand circular polarization, (b) right-hand circular polarization, (c) side view of the
antenna. The outermost dimensions of the antenna are 9 h 12 (mm), the spiral line width is
0.8 mm.
50 V
50 V
50 V
0 V
0 V
0 V
50 V
50 V
0 V
0 V
0 V
50 V
Coax
feeding
point
Actuator
in the up
state
Actuator
in the
down state
z (coax-feed)
(a)
(b)
H
ε
r
= 3.38
Coaxial cable
(c)
12QQ
eff eff
C<<
4190_book.fm Page 266 Tuesday, February 7, 2006 12:44 PM
Multifunctional Reconfigurable Microelectromechanical Systems 267
to the printed spiral surface. Substrate thickness is chosen to be 7.6 mm,
which is one of the standard thicknesses for PCB family substrates, the closest
one to the quarter-wavelength at a center design frequency of 5 GHz. The-
oretical characterization of the antenna structure is conducted by a full-wave
analysis tool based on a finite element method, which takes into account the
edge effects due to finite size dielectric and conducting plane of the antenna.
A brief fabrication sequence for monolithic integration of RF MEMS actuators
with rectangular-shaped strip segments of the MRSA is given in Figure 9.11.
Details of the fabrication process can be found in references [2,3,20]. The
fabrication begins with RO4003 laminate with copper layers of 16 µm on
both sides. First, the segments of the antenna and planar part of the bias
circuitry are formed by wet etching copper layer. Vertical vias for bias cir-
cuitry and coax feed are created by standard PCB processes. After this step,
a thin layer of high-density inductively coupled chemical vapor deposition
(HDICP CVD) SiN
x
[20] is deposited and etched by reactive ion etching such
that the SiN
x
covers only the tips of the metal stubs protruding from the
antenna segments (see Figure 9.9a). We continue fabricating RF MEMS actua-
tors following the process flow shown in Figures 9.11b–d without affecting
the antenna structure.
Figure 9.12 shows the return loss of the MRSA with counterclockwise sense
of winding corresponding to the RHCP radiation. The simulated result is
FIGURE 9.11
Fabrication sequence for monolithic integration of RF MEMS actuators with rectangular-shaped
strip segments in constructing MRSA (a) Antenna pattern, DC bias path and via formation (b)
Dielectric layer deposition and sacrificial layer planarization, (c) Aluminum membrane depo-
sition, (d) Final release.
Spiral segments
DC bias path
(a)
(b)
(c)
(d)
Coax inner
conductor
RF MEMS
actuator
4190_book.fm Page 267 Tuesday, February 7, 2006 12:44 PM
268 MIMO System Technology for Wireless Communications
also validated by comparison with experimental data in this figure. Due to
the symmetry between two antenna configurations, the RHCP and LHCP
spirals exhibited almost identical return loss with a VSWR of less than two
covering the frequency band of 4.3–5.4 GHz. Measured and calculated radi-
ation patterns at 5 GHz in two different planes of K = 0° and 90° are shown
in Figures 9.13a–d for RHCP and LHCP configurations. As seen from these
figures, the MRSA radiates circularly polarized wave slightly off broadside
to the plane of spiral, forming an almost axial beam pattern. This slight tilt
from the z-axis is due to the asymmetry of the antenna structure with respect
to the z-axis. The measured average half-power beamwidth (HPBW) is
approximately 105°. The antenna radiates almost entirely circular wave in
the z-axis with an axial ratio value of 0.9 dB. The gain at this direction is 5.3
dB. Variations of axial ratio and gain in the z-direction with respect to
frequency are shown in Figure 9.14. The circular polarization bandwidth
over which the axial ratio is less than 3 dB is approximately 11%. Gain of
the antenna with average value of 4.9 dB shows small variation over this
bandwidth. The difference in performance characteristics between the RF
MEMS integrated spiral antenna and conventional single-arm rectangular
spiral antenna was observed to be negligible.
9.5 Concluding Remarks
Wireless applications that are increasingly bandwidth- and mobility-inten-
sive have driven MIMO research to challenge the physical limits of coding
and signaling. The multifunctional reconfigurable antenna technology pre-
sented in this chapter greatly impacts adaptive MIMO performance through
FIGURE 9.12
Return loss of the MRSA for RHCP radiation.
0
3 3.5 4 4.5
Frequency(GHz)
5
Calculated
Measured
Return loss
(
dB)
5.5 6
−5
−10
−15
−20
−25
−30
4190_book.fm Page 268 Tuesday, February 7, 2006 12:44 PM
Multifunctional Reconfigurable Microelectromechanical Systems 269
the capability to change its performance properties (i.e., radiation and
impedance characteristics). The reconfigurable antenna properties integrated
with signaling schemes (i.e., spatial multiplexing, space–time coding, beam-
forming) to the propagation environment provides an additional degree of
freedom in adaptive optimization; thus the gap between theoretical MIMO
performance and practice is closed. RF MEMS technology compatible with
microwave laminates enables very large-scale monolithic integration of
antenna and circuit components on a substrate that best meets the antenna
performance characteristics. MEMS integrated antennas; a diversity antenna
(three-element cactus) suitable for selection diversity; and a multifunction
reconfigurable spiral antenna have been fabricated by using this technology.
FIGURE 9.13
Radiation patterns for the RHCP configuration in (a) K = 0 plane and (b) K = 90 plane; radiation
patterns for the LHCP configuration (c) K = 0 plane and (d) K = 90 plane at 5 GHz.
(d) (c)
270
315
180
225
0
45
90
135
270
315
180
225
0
45
90
135
−10
−20
−20 −30 −10
−10
−20
−20 −30
−10
270
(a) (b)
315
180
225
0
45
90
135
270
315
180
225
0
45
90
135
−10
−20
−20 −30 −10
−10
−20
−20 −30 −10
Calculated
Measured
E
R
E
L
E
L
E
R
4190_book.fm Page 269 Tuesday, February 7, 2006 12:44 PM
270 MIMO System Technology for Wireless Communications
A long-awaited design space where an interplay between reconfigurable
antenna and adaptive signaling feed back to each other is likely to revolu-
tionize broadband MIMO system design methodology.
References
1. Special Issue on Multifunctional RF Systems. 2005. IEEE Trans. Microwave The-
ory and Techniques, Vol. 53, No. 3.
2. H.P. Chang et al. 2003. “Low cost RF MEMS switches fabricated on microwave
laminate PCBs,” IEEE Electron Device Lett., Vol. 24, No. 4, pp. 227–229.
3. B.A. Cetiner et al. 2003. “Monolithic integration of RF MEMS switches with a
diversity antenna on PCB substrate.” IEEE Trans. Microwave Theory and Tech-
niques, Vol. 51, No.1, pp. 332–335.
4. B.A. Cetiner et al. 2004. “Multifunctional reconfigurable MEMS integrated
antennas for adaptive MIMO systems,” IEEE Communications Magazine, Vol. 42,
No. 12, pp. 62–70.
5. S. Catreux et al. 2002. “Adaptive modulation and MIMO coding for broadband
wireless data networks,” IEEE Communications Magazine, Vol. 40, No. 6,
pp. 108–115.
6. L. Jofre, B.A. Cetiner, and F. De Flaviis. 2002. “Miniature multi-element antenna
for wireless communications,” IEEE Trans. on Antennas and Propagat., Vol. 50,
No. 5, pp. 658–669.
7. V. Tarokh, H. Jafarkhani, and A.R. Calderbank. 0999. “Space-time block coding
from orthogonal design,” IEEE Trans. on Inform. Theory, Vol. 48, pp. 611–627.
8. H. Jafarkhani. 2001. “A quasi-orthogonal space-time block code,” IEEE Trans.
Commun., Vol. 49, pp. 1–4.
9. M.A. Jensen and J.W. Wallace. 2003. “Antenna selection for MIMO systems
based on information theoretic considerations,” 2003 IEEE AP-S International
Symposium Digest, Vol. 2, pp. 515–518.
FIGURE 9.14
Frequency responses of axial ratio and gain in the z-direction for MRSA.
12
Axial ratio and gain
(
dB)
9
6
3
0
4.6 4.8 5
Frequency(GHz)
5.2 5.4
Calculated axial ratio
Calculated gain
Measured axial ratio
Measured gain
5.6
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Multifunctional Reconfigurable Microelectromechanical Systems 271
10. P. Kyritsi et al. 2003. “Effect of antenna polarization on the capacity of a multiple
element system in an indoor environment,” IEEE JSAC, Vol. 20, No. 6,
pp. 1227–1239.
11. D. Chizhik, et al. 2002. “Keyhole, correlations, and capacities of multielement
transmit and receive antennas,” IEEE Trans. on Wireless Comm., Vol. 1, No. 2,
pp. 361–368.
12. R.U. Nabar et al. 2002. “Performance of multiantenna signaling techniques in
the presence of polarization diversity,” IEEE Trans. on Signal Processing, Vol. 50,
No. 10, pp. 2553–2562.
13. L. Liu and H. Jafarkhani. 2005. “Application of quasi-orthogonal space-time
block codes in beamforming,” IEEE Trans. on Signal Processing, Vol. 53, No. 1,
pp. 54–63.
14. G. Jongren, M. Skoglund, and B. Ottersten. 2002. “Combining beamforming
and orthogonal space-time block coding,” IEEE Trans. on Inform. Theory, Vol. 48,
pp. 611–627.
15. Z.J. Yao et al. 1999. “Micromachined low-loss microwave switches,” J. Micro-
electromech. Syst., Vol. 8, No. 2, pp. 129–134.
16. J. Papapolymerou et al. 2003. ”Reconfigurable double-stub tuners using MEMS
switches for intelligent RF front-ends,” IEEE Trans. Microwave Theory and Tech-
niques, Vol. 51, No. 1, pp. 271–278.
17. N.S. Barker and G.M. Rebeiz. 1998. “Distributed MEMS true-time delay phase
shifters and wide-band switches,” IEEE Trans. Microwave Theory and Techniques,
Vol. MTT-46, No. 11, pp. 1881–1890.
18. C.L. Goldsmith et al. 1999. “RF MEMS variable capacitors for tunable filters,”
Int. J. of RF and Microwave Computer-Aided Eng., Vol. 9, No. 4, pp. 362–374.
19. E.R. Brown. 1998. “RF-MEMS switches for reconfigurable integrated circuits,”
IEEE Trans. Microwave Theory and Techniques, Vol. MTT-46, No. 11, pp. 1868–1880.
20. C.H. Chang et al. 2002. “RF MEMS capacitive switches fabricated with HDICP
CVD SiN
x
,” IEEE MTT-S Dig., pp. 231–234.
21. R. Garg et al. 2001. Microstrip Antenna Design Handbook, Norwood, MA: Artech
House, chap. 7.
22. Rogers Inc., Chandler, AZ, Microwave Products Tech. Information, 2001.
23. H. Nakano et al. 2002. “Tilted- and axial-beam formation by a single-arm
rectangular spiral antenna with compact dielectric substrate and conducting
plane,” IEEE Trans. Antennas and Propagation, Vol. 50, No. 1, pp. 17–23.
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273
10
Multi-Antenna Testbeds for
Wireless Communications
Raghu Rao, Christian Oberli, Stephan Lang, David Browne, Weijun Zhu,
Mike Fitz, and Babak Daneshrad
CONTENTS
10.1 Introduction................................................................................................274
10.2 Testbed Classification................................................................................276
10.3 Elements of a Successful Testbed............................................................278
10.4 Hardware Calibration ...............................................................................280
10.4.1 Design Tradeoffs............................................................................281
10.4.2 I/Q Mismatch ................................................................................282
10.4.2.1 A Simulation Model for I/Q Mismatch......................284
10.4.3 Phase Noise ....................................................................................285
10.4.3.1 Design of Low Phase Noise Local Oscillators...........286
10.4.3.2 Phase Noise Cancellation in the Demodulator
by Signal Processing.......................................................288
10.4.3.3 A Discrete-Time Simulation Model for Phase Noise...289
10.4.4 Benchtop Calibration of the 5.25 GHz 2 ×
2 MIMO Testbed....290
10.4.5 Radio Receiver Distortion............................................................293
10.4.6 Frequency Offset Calibration ......................................................295
10.4.7 Frequency Response Calibration ................................................297
10.4.7.1 Calibration of Transmitters ...........................................298
10.4.7.2 Calibration of Receivers.................................................299
10.5 Field Test Administration.........................................................................299
10.5.1 Automated Field Measurements ................................................300
10.5.2 Graphical User Interface Design ................................................303
10.6 Field Test Results .......................................................................................304
10.6.1 Measuring the Quality of the MIMO Channel ........................304
10.6.2 Effective Degrees of Freedom (EDOF) ......................................306
10.6.3 Analysis of Field Test Measurements........................................306
10.6.4 Channel Impact on Space–Time Coding...................................308
References.............................................................................................................312
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