Selected Papers

Active Integrated Antenna Technique
Tomohiro Seki†, Fusao Nuno, Kenjiro Nishikawa,
and Keizo Cho
Abstract
We clarified the mounting precision for the active integrated array antenna for 25-GHz-band wirelessaccess communication systems and confirmed that wirebonding is useful in constructing such antennas
for the quasi-millimeter-wave frequency band, resulting in low-cost antenna systems. We also present anactive integrated antenna configuration designed for broadband mobile wireless access systems using the
25-GHz band. The antenna and RF circuits are integrated into each side of a thick copper backing plate
and they are connected through microstrip line/slot transitions. The antenna exhibited output power of
14.6 dBm and a noise figure of less than 5 dB. A wireless system using this antenna showed a 6-dB
improvement in packet error rate compared with one using a passive antenna with the same array design.
Furthermore, we obtained the first license for an active integrated antenna for high-speed wireless communication systems in Japan.

1. Introduction
System designs and wireless equipment for broadband mobile wireless access systems have been proposed and developed to achieve seamless communications [1]-[4]. A broadband mobile wireless accesssystem with faster data transmission (over 100
Mbit/s) must use frequencies corresponding to quasimillimeter or millimeter wave frequencies and havecompact wireless terminals. This system must use awider frequency band to achieve faster data transmission, while a compact terminal is desirable for mobileuse. The use of high frequency also requires high output power due to the huge propagation loss and passive circuit losses. However, it is difficult to achieve
high power in a compact wireless terminal. Active
integrated array antenna technology that mounts theamplifier for transmitting and receiving without using
a connector and a cable has been proposed andapplied to broadband wireless access systems [5]-[8].
This antenna significantly suppresses the transmis† NTT Network Innovation Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: seki.tomohiro@lab.ntt.co.jp

36sion loss between the antenna module and the radio
frequency (RF) module [5]-[7], resulting in a big
improvement in the packet error rate (PER) performance [9]. However, the present active integrated
array antenna [9] employs phase shifters and attenuators to compensate for the mounting error of theactive integrated antenna to improve the yield rate.
Using this antenna results in expensive and large RF
systems. The mounting error is caused by deviationsin the devices, monolithic microwave integrated circuit (MMIC) mounting positions, and wirebonding.
Flip-chip technology is useful for suppressing themounting error, but it is still expensive. Therefore, itis necessary to clarify whether the conventional
mounting method can be used to make an active integrated antenna for the quasi-millimeter-wave frequency band without any adjustment.
This paper first clarifies the mounting precision foran 8 × 8 active integrated array antenna with four subarrays that is used in 25-GHz-band fixed wirelessaccess systems. Experimental results show that wirebonding is useful for making active integrated array
antennas. Next, this paper presents an active integrated antenna configuration designed for actual wirelesscommunication systems using the 25-GHz band. ThisNTT Technical Review

Selected Papers

es four 4 × 4 sub-arrays. Figure 1 shows a block diagram of the prototype. The mounting point for the Au
wirebonding to connect the MMIC to the microstrip
line is indicated in the figure. The calculated array
antenna characteristics are given in Fig. 2 when thefeeding conditions between two sub-arrays were varied. Here, the amplitude and the phase represent the0–0.5
Relative gain (dB)

active integrated antenna comprises a microstrip
antenna (MSA) array and RF front-end circuits using
spatial power combining schemes for lower power
consumption of the power amplifiers. Furthermore,
the antenna and RF circuits are integrated into eachside of a thick copper backing plate and are connected through microstrip line (MSL)/slot transitions.
The developed active integrated antenna showed output power of 14.6 dBm and a noise figure of less than
5 dB. Wireless systems using this antenna achieved a
6-dB improvement in the PER compared with one
using a passive antenna, which must use a connector
to connect the amplifier, with the same array design
as the active integrated antenna.
The rest of this paper is organized as follows. Section 2 clarifies that the precision of the wirebondingis adequate for mounting the active integrated antenna for 25 GHz. The proposed concept of the active
integrated antenna structure employing a thick heatsink plate is described in section 3. The design of the
MSL/slot transition, which is a key part for assembling this antenna, is presented in section 4. Section 5
describes the performance of a prototype antenna.
Finally, section 6 discusses the effects of using the
proposed antenna structure in an actual wireless system.

–1
0°
20°
40°
–1.5

60°
80°
100°

–2
0

2

4
6
Amplitude (dB)

2. Mounting precision

8

10

8

10

(a) Gain reduction characteristics

2.1 Requirements
We selected an 8 × 8 array structure with a 0.5 λo(free-space wavelength array spacing) for conventional fixed communications in which the directive
gain is approximately 17 dBi. The structure comprisAmplifier
Subarray(4 × 4 structure)

Wilkinson
divider
Input port

0°Sidelobe level (dB)

Mounting point

0
60°

20°

80°

40°

100°

–5

–10

Power
divider–150

Measuring point

Fig. 1. Block diagram of test circuit for dispersion
measurement.

Vol. 2 No. 1 Jan. 2004

2

4
6
Amplitude (dB)(b) Sidelobe level characteristics

Fig. 2. Calculated characteristics of active integrated
antenna.

37

Selected Papers

difference in the feeding conditions between the two
sub-array antennas. The moment method*1 was usedas the calculation method. Figure 2(a) shows the gain
characteristics and Fig. 2(b) shows the sidelobe level
characteristics. These figures suggest that variationsin the gain and sidelobe levels occur due to slight differences in the feeding conditions. For example,
when the variation in the feeding power is approximately 1.0 dB, the variation in the antenna gain is less

Gain after wire-bonding (dB)

5

2.5

0

±1 dB

–2.5

–5
–5

–2.5
0
2.5
Gain before wire-bonding (dB)5(a) Gain dispersion relationship before and after wire-bonding

2.2 Measurement results(a) Mounting precision
To determine the mounting error, we constructed
three active integrated antennas based on the same
design. The measured mounting characteristics in the
25-GHz band using an MMIC chip (CHA2093, United Monolithic Semiconductors, Orsay, France) are
shown in Fig. 3. Here, “before wirebonding” represents on-wafer measurements and “after wirebonding” represents the measured characteristics of theamplifier mounted on the antenna substrate. Figure3(a) shows the gain dispersion characteristics before
and after Au wirebonding, and Fig. 3(b) shows thephase dispersion characteristics before and after. In
these figures, the measurement point is the mounting
point indicated in Fig. 1. In Fig. 3, the dispersion of
the gain is less than ±1 dB, and the dispersion of thephase is less than ±20°. These results clarify that the
prototype antennas satisfy the necessary requirements.
(b) Total characteristics
The measured horizontal plane radiation pattern isshown in Fig. 4. We prepared three antennas to veri*1 A moment method is the most conventional electromagnetic
method of analyzing the packaging of the high-frequency circuit
and antennas. There is a lot of commercial software based on the
moment method.

25
0

0

±20 deg

Relative power (dB)Gain after wire-bonding (deg)50than 0.1 dB. Furthermore, the effect of the sidelobe
level is small, while the effect of the feeding phase islarge. Therefore, the mounting phase error must be
less than 40° to achieve a sidelobe level of less than
–10 dB.

–25

Calculated
No. 1
No. 2
No. 3

–10

–20

–30
–50
–50

–25
0
25
Gain before wire-bonding (deg)50(b) Phase dispersion relationship before and after wire-bonding

Fig. 3. Relationship between gain and phase dispersion
before and after wire-bonding.

38

–40
–90

–60

–30

0
Angle (deg)

30

60

90

Fig. 4. Horizontal-plane radiation pattern of tested active
integrated antennas.

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fy the dispersion characteristics. The main lobe andsidelobe shapes agree with the calculated values andthe radiation patterns of the three antennas are ingood agreement, so we conclude that wirebondingachieves an adequate level of precision for the active
integrated antenna at 25 GHz.
3. Design concept of active integrated antenna
for broadband mobile wireless access systems
The design specifications required for this system
are given in Table 1. Figure 5 is a block diagram ofthe prototype active integrated antenna. The array
antenna is divided into two sets of sub-arrays individually connected to each RF sub-circuit. In the transmission system, radiated powers from the sub-arrays
are spatially combined to achieve the designed radiation characteristics and the total transmission power.
In order to eliminate the degradation caused byinefficient power combination, the power amplifiersmust all have equivalent electrical properties. However, MMIC characteristics always vary individually,
Table 1. Specifications.
Tx
Frequency bands
Total gain

Rx

24.75 GHz – 25.25 GHz
Greater than 27 dBi

Directivity gain

Greater than 31 dBi

Greater than 17.0 dBi

Output power

12.0 dBm – 15.8 dBm—

Noise figure—

Less than 7 dB

Spuriouscharacteristics

Less than –26.0 dBm—

75 mm × 75 mm

Antenna size

Sub-arrayPhase
shifterVVA

PA Isolator
SPDT

Phase
shifterand even if the same MMICs are used, their electricalproperties will differ due to the conditions when they
are mounted onto the substrates. Therefore, the circuits have voltage variable attenuators (VVAs) andphase shifters to adjust the amplitudes and phases of
the two sub-arrays in the transmission system. However, there are no VVAs in the receiving systembecause their insertion loss is intrinsically large, andthe noise figure is deteriorated by these attenuators.
4. Design of prototype active integrated antenna
4.1 Antenna structure
Figure 6 shows a cross-section of the prototype
active integrated antenna for 25 GHz. The prototype
has two sub-arrays constructed with 6 × 3 elements,
giving it an adequate antenna-directivity margin of
17.0 dBi. Here, each antenna element is 3.64 mm ×3.64 mm, and the array spacing is 0.71 λo. We used a
40Ω microstrip line for the feeding circuit to establish
the whole feeding circuit with more than a 0.1-mm
linewidth to use the composite feeding circuit structure. Moreover, we used the –23 dB Chebysheff distribution*2 to achieve low sidelobe characteristics. RF
front-end circuits were etched on a single-layer
Diclad880 substrate (εr = 2.17, 0.127 mm thick,
ARLON Inc., Santa Ana, CA, USA). Each antenna
array is etched on a two-layer structure comprisingDiclad880 substrates (εr = 2.17, 0.254 mm thick).
The copper plate binds the RF front-end substrate andthe antenna substrate for heat sinking. The feed circuits are arranged in the inner layer for broadband
characteristics of the MSA element with a narrowfeed-line. We used the TGA1073G (TriQuint Semiconductor Inc., Texas, USA) as the power amplifierand TGA1319A-EPU (TriQuint Semiconductor Inc.,*2 Chebysheff distribution is one of the most commonly employed
distributions in filter circuit design and is suitable for setting thesidelobe to less than a desired level.

RF sub-circuit #1
LNA
Sub-arrayPhase
shifterVVA

PA Isolator
SPDT

MSA

Feed line

Non-radiative slot

Phase
shifter

Antenna array

RF sub-circuit #2

Copper plate
SPDT
RFin
&
RFout
SPDT: single-pole double throw

Fig. 5. Block diagram of active integrated antenna.

Vol. 2 No. 1 Jan. 2004

RF circuits

MMIC

Microstrip line

Fig. 6. Cross-sectional view of active integrated antenna.

39

Selected Papers

Texas, USA) as the low noise amplifier. The copper
backing plate is 69 mm × 75 mm × 3 mm in order tofunction sufficiently well as a power amplifier heatsink and to have sufficient mechanical strength. The
non-radiating slot that penetrates the copper backing
plate establishes a connection from one antenna arrayto the RF front-end circuits.
4.2 MSL/slot transition design
Figure 7 shows the structure of an MSL/slot transition. When the slot length L is approximately λ/2, theslot resonates. However, it is necessary to lengthen
the slot to achieve sufficient matching and suppression of the loss of the MSL/slot transition as the metal
thickness increases. Suppressing this power leak isalso important to minimize the transmission loss. The
input impedance is matched by adjusting stub lengths
d1 and d2 and slot width W. Figure 8 shows the relationship between the calculated minimum insertion
loss of the MSL/slot transition and the metal thickness and the value of the slot length when the inserW
Slot

d1

L

Feeding
MSL

Copper Dielectricplate substrated2a) Top viewb) Cross-sectional view

10

1

8

0.8

6

0.6

0.4

4
Slot length (mm)2

Insertion loss (dB)

Slot length (mm)Fig. 7. Structure of MSL/slot transition.

0.2

Insertion loss (dB)
0

0

1
2
3
Metal thickness of copper plate (mm)40Fig. 8. MSL/slot transition characteristics versus metal
thickness.

40

tion loss is minimum. Here, we use the finite element
method in the simulation. The slot width is 0.5 mm
determined by the minimum manufacturing process,and the stub lengths, d1 and d2, are optimized to minimize the loss of the MSL/slot transition. For metal
thicknesses of 0, 1, 2, 3, and 4 mm, the optimized values of d1/d2 are 1.50/1.80, 1.20/1.50, 1.15/1.20,
1.50/1.65, and 1.60/1.65 mm, respectively. In Fig. 8,
it is clear that the slot length is determined by themetal thickness, and the insertion loss increases linearly in proportion to it. We decided that the metal
thickness should be 3 mm to provide adequate heatsinking and to accommodate the assembly.
5. Performance of prototype active integrated
antenna
The prototype active integrated antenna is shown in
Fig. 9(a). It is 75 mm × 69 mm. The transmission test
system for it is shown in Fig. 9(b).
Figures 10(a) and 10(b) show the antenna’s transmission (Tx) and reception (Rx) radiation patterns at
25 GHz, respectively. Here, the solid line represents
results for the vertical plane and the dotted line represents those for the horizontal plane. These results
show good performance for the spatial power combining mechanism.
Table 2 gives the measured and estimated total
gains of the active integrated antenna at 25 GHz. For
transmission and reception, they were 29.0 and 35.4
dBi, respectively. The measured gains were higher
than the estimated values because the measured
antenna directivity was approximately 1.5 dB higher
than the estimated value.
Since it was difficult to measure the noise figure of
the active integrated antenna, we measured the noise
figure of the front-end circuit without the antenna or
the MSL/slot transition. The measured noise figure
was less than 3.4 dB. Accordingly, the total noise figure of the active integrated antenna is less than 5 dB
because the loss of the MSL/slot transition and the
MSL is approximately 1.6 dB.
To acquire a license for the experimental station of
the 25-GHz band wireless communication system,
we confirmed the characteristics of the manufactured
active integrated antenna. The specifications under
the license and the actual results for the active integrated antenna are given in Table 3. We also confirmed that the spurious characteristics, i.e., theundersirable frequency components, are less than
–35.0 dBm outside the 25-GHz band. The transmission output power is 14.6 dBm at the output of the
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Sub-array #1

69 mm

0

Sub-array #2

E-Plane6 × 6 antenna array

RF circuits

RF sub-circuit#1(a) 6 × 6 antenna and RF front-end circuits of active integrated
antenna

Relative power (dB)

75 mm

H-PlaneRF sub-circuit
#2

–10

–20

–30

Prototype active antenna
–40
–180

–90

Transmission test system

0Angle (deg)

90

180

(a) Tx radiation pattern at 25 GHz
0
E-Plane
H-PlaneRelative power (dB)

–10

–20

–30

(b) Transmission test system using active integrated antenna

Fig. 9. Photograph of prototype active integrated antenna
and transmission test system.

Table 2. Measured and estimated total gains.
Tx

Rx

–40
–180

–90

0
Angle (deg)

90

180

(b) Rx radiation pattern at 25 GHz

Fig. 10. Radiation characteristics of prototype active
integrated antenna.

Table 3. License specifications of experimental stationand measured results.
Specifications

Measured resuts

Measured total gain (dBi)

29.0

35.4

Estimated total gain (dBi)

28.15

32.55

Frequency deviation Less than 7539 kHz

22

19

Output power

12.0 dBm – 15.8 dBm

14.6 dBm–10.85–3.45

Less than –26 dBm

Less than –35 dBm

17

17

Spuriouscharacteristics
Occupied frequency
bandwidth

Less than 18 MHz

Less than 16.5 MHz

PA(Tx) & LNA(Rx) (dB)
Loss of RF front-end circuits (dB)Directivity gain (dBi)Vol. 2 No. 1 Jan. 2004

Less than 2 kHz

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Selected Papers

time division duplexing (TDD) switch circuit connected to the sub-array antenna. These results showthat this antenna satisfies the license specifications.
Furthermore, we confirmed that this antenna has sufficient thermal stability because the fluctuation of its
total gain over five hours was less than 0.5 dB.
6. PER improvement achieved using active
integrated antenna
It is necessary to evaluate the effect of using theprototype active integrated antenna in practical wireless systems. Accordingly, we examined theimprovement in PER characteristics using a transmission test system [4] that uses coded orthogonal frequency division multiplexing (OFDM) modulation
based on the MMAC-HiSWANb specification. The
measurements were performed in an anechoic chamber. The distance between the transmitting andreceiving antennas was 3.0 m. A programmableattenuator was used to control the variation in transmission power. The PER characteristics for the prototype active integrated antenna and a passive antenna are given in Fig. 11. We used 16QAM with coding
rate R = 9/16 for the modulation where the transmission rate was 27 Mbit/s. The passive antenna used thesame antenna and feeding circuit design. Moreover,
the outputs from the two sub-arrays were directly
combined at the RFin & RFout terminals without any
devices. The figure clearly shows that the PER results
were approximately 6 dB better than for the passive
1.0

1.0 E–1

6 dB

PER

1.0 E–2

Passiveantenna
1.0 E–3

Activeintegrated
antenna

1.0 E–4

1.0 E–5
–50

–45

–40

–35

–30

–25

antenna when the PER value was 1.0 × 10–2. Based onthese results, we clarified that the noise figure of theactive integrated antenna is less than 5 dB because thetotal noise figure of the transmission test systemusing a passive antenna is approximately 11 dB
including the loss of the coaxial cable used to connect
the transmission test system to the antenna terminal.
Accordingly, we conclude that using the active integrated antenna is a very effective way of extending
the transmission distance.
7. Conclusion
Since wirebonding, which is a typical method ofmounting MMICs, is used to reduce the mounting
cost, we investigated its mounting precision in theconstruction of active integrated antennas for the 25GHz band. We constructed four spatial power-combining array antennas for the 25-GHz band mounted
by wirebonding. The test results showed that thepower dispersion and phase dispersion of the mounting precision were ±1 dB and ±20°, respectively. The
results also showed that wirebonding is useful in constructing an active integrated antenna that forms afixed narrow beam in the quasi-millimeter-wave frequency band.
We proposed an active integrated antenna configuration designed for broadband mobile wireless accesssystems using the 25-GHz band and made a prototype
active integrated antenna, integrating a 6 × 6 antenna
array and RF front-end circuits on each side of a thickcopper backing plate for power amplifier heat sinking. The MSL/slot transition was designed taking into
account non-radiating slots and exhibited good performance. The prototype antenna achieved adequate
output characteristics when the power was 14.6 dBm
and the noise figure was less than 5 dB. We examined
the advantages of using the prototype active integrated antenna. The experimental results confirmed that
the PER was improved by approximately 6 dB compared with a passive antenna using the same antenna
design.
We also obtained the first license in Japan to use an
active integrated antenna that does not have any measuring points between the antenna and the front-end
circuit for high-speed wireless communication systems. We will verify the characteristics of the prototype active integrated antenna through field research.

Transmission power (dBm)

References
Fig. 11. Measured PER versus transmission power ofaccess point station (16QAM, R=9/16).

42

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Selected Papers

[2]

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Conf., Tokyo, Japan, B-5-156, Mar. 2002.
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2002.
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antennas,” IEEE Trans. on MTT, Vol. 50, No. 3, pp. 937-944, Mar.
2002.
T. Seki, K. Uehara, and K, Kagoshima, “A Three-Dimensional Active
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IEEE MTT-S Int. Microwave Symp., Denver, USA, Vol. 2, pp. 975978, Jun. 1997.
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Tomohiro Seki
Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from the Science University ofTokyo, Tokyo in 1991 and 1993, respectively. In
1993, he joined the NTT Radio Communication
Systems Laboratories (now NTT Network Innovation Laboratories), Yokosuka, Japan. He has
been engaged in the development of array antennas and active antennas for the millimeter-wave
and microwave bands. He received the Young
Engineer Award from the Institute of Electronics, Information and Communication Engineers(IEICE) Japan in 1999. He is a member of IEEEand IEICE.

Fusao Nuno
Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. degree in electrical engineering from Ehime University, Ehime in 1991
and the M.E. degree in electrical engineeringfrom Kumamoto University, Kumamoto in 1993.
In 1993, he joined the NTT Radio Communication Systems Laboratories (now NTT Network
Innovation Laboratories), Yokosuka, Japan. He
has been engaged in R&D of portable terminals
and base stations for broadband wireless accesssystems.

Kenjiro Nishikawa
Senior Research Engineer, Wireless Systems
Innovation Laboratory, NTT Network Innovation Laboratories.
He received the B.E. and M.E. degrees in welding engineering from Osaka University, Suita,
Osaka in 1989 and 1991, respectively. In 1991,
he joined the NTT Radio Communication Systems Laboratories (now NTT Network Innovation Laboratories), Yokosuka, Japan. He has
been engaged in R&D of three-dimensional anduniplanar MMICs on Si and GaAs and theirapplications. His current interests are millimeterwave MMIC design/packaging technologies andmicrowave/millimeter-wave communication
systems. He received the 1996 Young Engineer
Award from IEICE. He is a member of IEEE and
IEICE.

Keizo Cho
Director of Antenna and Propagation Group,
Wireless Link Development Department, NTT
DoCoMo.
He received the B.E. degree in electrical computer engineering from Yokohama National University, Yokohama in 1986, the M.E. degree in
information processing from Tokyo Institute ofTechnology, Tokyo in 1988, and the Dr.Eng.
degree in electrical engineering from Tokyo
Institute of Technology, Tokyo in 2002. In 1988,
he joined NTT Radio Communication Systems
Laboratories (now NTT Network Innovation
Laboratories), Yokosuka, Japan. He was a visiting researcher at the Communications ResearchLaboratory, McMaster University, Hamilton,
Ontario, Canada from 1994 to 1995. He moved
to the Wireless Link Development Department,
NTT DoCoMo, Yokosuka, Japan in 2003. He has
been engaged in R&D of base station and terminal antennas for mobile communication systems
and advanced wireless communication systems.
He received the Young Engineer Award fromIEICE in 1995. He became Secretary of the
IEICE Technical Group on Antennas and Propagation in 2003. He is a senior member of IEEE.

Vol. 2 No. 1 Jan. 2004

43