Tsoulos George (ред.) MIMO System Technology for Wireless Communications

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254 MIMO System Technology for Wireless Communications

the bandwidth (BW)

, efﬁciency (M

and the volume of small size antennas

is given by

(9.2)

where Q

is the minimum radiation quality factor of the antenna, which

increases by the reduction in antenna volume. From this formula, one can

conclude that there is a compromise between the maximum realizable per-

formances, i.e., bandwidth and efﬁciency, and the given electrical dimensions

of small size antenna. The concept of RF MEMS integrated MRA is a revo-

lutionary approach to circumventing the performance limitations of small

size antennas by taking advantage of combined multiple functions in one

single antenna. An MRA combines multiple functions in one single antenna

architecture with the capability of altering its radiation, polarization, and

frequency characteristics. Thus, for given antenna performance characteris-

tics an MRA will occupy

only a small fraction of the space required by single-

function multiple antenna elements.

9.2.2 Links among Transmission Algorithms, Antenna Properties,

and Propagation Environment

In this section, we identify relationships among the transmission algorithms,

the radiation/polarization characteristics, and the conﬁguration of the recon-

ﬁgurable antenna, and the propagation environment. These relationships

FIGURE 9.2

Block diagrams of (a) conventional and (b) MRA arrays.

Antenna 1

Antenna 2

Antenna M

Output

signal

(a) (b)

Recon

ﬁ

gura

antenna 1

Reconﬁgurable

antenna 2

Reconﬁgurable

antenna M

Processing

unit

BW k a

× % ×

()

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Multifunctional Reconﬁgurable Microelectromechanical Systems

255

enable the joint adjustment of the characteristics of the reconﬁgurable

antenna array and the coding schemes over varying channel conditions to

optimize performances at all times.

1. Number of antenna elements:

The most basic relationship, which does

not require reconﬁgurability in the geometrical domain of the mul-

tiple antennas, relates the number of antenna elements to a speciﬁc

transmission algorithm. For multiple antennas, if the number of

transmit antennas is larger than two, then it is not possible to design

orthogonal space–time block codes (STBCs) [7]. In case of more than

two antennas, recently developed quasi-orthogonal STBCs are used

to achieve full rate and full diversity at the expense of slight increase

in decoding complexity [8]. For high SNRs and very large number

of antennas SM is favorable over STBCs, since the data rate of SM

increases linearly with an increasing number of antennas, while the

diversity gain of STBC will saturate.

2. Array conﬁguration and polarization:

Besides the number of antenna

elements, the subset of the elements selected in an array conﬁgura-

tion is an important factor to achieve the majority of the capacity

available in the channel [9]. This does not only improve performance

but also results in a less complex MIMO system as the number of

the transmit and receive RF chains are reduced. The performance

can be further enhanced if the polarizations of the elements are also

taken into account as the propagation of the electric ﬁeld for different

polarizations differs depending on the environment. It has been

shown experimentally that for a line-of-sight (LOS) indoor environ-

ment, vertically polarized systems achieve higher capacity than hor-

izontally polarized ones [10]. Moreover, an antenna array with

hybrid polarization (i.e., some elements are vertically polarized

while others are horizontally polarized) performs better than single

polarization systems for both LOS and non-LOS (NLOS) conditions.

In practical communication scenarios, degenerate channel phenomena

called the keyhole channel effect

may arise where the antenna elements both

at the receiver and the transmitter have very low correlation due to rich

scattering, but the channel matrix has a very low rank, resulting in a single

mode of communication [11]. This shows that low correlation itself is not a

guarantee for achieving high capacity. It is shown in [11] that in an outdoor

propagation scenario, the keyhole problem may be avoided by using a hor-

izontally oriented transmitter array instead of a vertically oriented array. As

a consequence, both the array conﬁguration and the polarization of each

individual element need to be adaptive in order to maintain the channel

performance over varying characteristics of the propagation environment.

A reconﬁgurable antenna array that can change its conﬁguration and polar-

ization has the characteristics necessary to adapt variable transmission/

receiving environments.

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256 MIMO System Technology for Wireless Communications

1. Spatial and polarization antenna diversity:

A compromise between data

rate maximization and diversity maximization (i.e., choosing

between SM and STC) is critical in realizing MIMO gains, since the

performance of these signaling strategies is strongly dependent on

time-varying channel characteristics. As is known, SM performs par-

ticularly well in high SNR regions, while STBC has better perfor-

mance in a low SNR region. It is shown in [12] that, while having

multiple linear polarization diversity antennas at both ends of the

link degrades the performance of STBC from that of spatial diversity,

signiﬁcant improvements in the symbol error rate for a SM scheme

are achieved in certain channel conditions such as in environments

with high scattering density and with a high K

-factor. This leads to

an important conclusion: A reconﬁgurable antenna array that can

readily switch between polarization and spatial diversity schemes

is needed to optimize an antenna performance for a given coding

scheme (i.e., SM or STBC) in a given channel environment.

2. Beamforming

, MIMO with STC

: When only the receiver knows the

channel, STC achieves the maximum diversity in a system with

multiple transmit antennas. On the other hand, if the transmitter

knows the channel perfectly, beamforming

is the optimal solution.

In some practical cases, the transmitter has some information about

the channel (e.g., the mean or variance) instead of a perfect knowl-

edge. When side information is available at the transmitter, it can

be exploited to enhance the performance. Even when the channel

information is based on poor channel estimation, its use improves

the performance of the system in combating fading. The improve-

ment can be achieved by combining STC and beamforming. Typi-

cally, when the quality of the channel feedback is high, the diversity

rank is less critical and the transmitter should lay most energy on

the “good” beam. On the other hand, when the feedback is unreli-

able, we should rely more on diversity and distribute energy evenly

among different beams. In the extreme case when the channel feed-

back quality is so poor it is entirely independent of the actual situa-

tion, the system becomes an open loop system and the beamforming

scheme should gradually fall back to nonbeamformed traditional

space–time coding. Therefore, the performance of the scheme should

be similar to that of the original space–time code. This requires the

design of an adaptive system that can utilize the partial available

channel information to change its behavior and provide the optimal

performance in all cases [13,14]. Such an adaptive system should

converge to space–time coding when the transmitter does not know

the channel at all and to beamforming when the transmitter knows the

channel perfectly. Performance can be improved further if the opti-

mal array design is employed simultaneously. While antenna elements

are closely spaced and correlated in beamforming arrays, MIMO

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Multifunctional Reconﬁgurable Microelectromechanical Systems

257

systems employing STCs require large antenna spacing for uncorre-

lated antennas. It is also important to note that beamforming is more

effective if the propagation environment has low scattering density

(i.e., LOS or near LOS) resulting in fewer multipaths. Weight selec-

tion algorithms can be more easily optimized for a few multipaths

than for many. In contrast, STCs take advantage of the multipath

richness by maximizing data rate or diversity. Variation in antenna

element separation is achieved by employing MRA instead of anten-

nas with ﬁxed properties.

9.3 RF MEMS Technology Compatible with Microwave

Laminates for Fabricating MRAs

As indicated earlier, MEMS technology is gaining a greater foothold in many

existing applications, such as RF wireless communications, biomedical, and

optical, to name a few; at the same time, it is ﬁnding new applications such

as fuel cells and power generators. In particular, RF MEMS have had a

signiﬁcant impact due to their potential to revolutionize RF and microwave

system implementation for the next generation of communication applica-

tions. One of the ﬁrst impacts of RF MEMS was single MEMS switches [15]

with their excellent switching characteristics — very low insertion loss, very

low power requirements, and high isolation —which cannot be attained by

semiconductor switches. The true potential of RF MEMS goes beyond a

single switch. The key advantage of RF MEMS can be realized by system-

level implementation through monolithic integration capability with other

circuit components. This capability is key, in particular, for creating multi-

functional reconﬁgurable antenna systems employing a large number of RF

MEMS components and/or operating at high frequencies. Although RF MEMS

devices on semiconductor substrates (tunable RF matching circuits/ﬁlters,

variable capacitors, phase shifters, switches) have been demonstrated

[16–19], realization of monolithically integrated RF MEMS antenna systems

requires common substrate housing of both MEMS and antennas. Microwave

laminate printed circuit boards (PCBs) are commonly used for the design of

RF circuits and antennas and represent an ideal substrate for RF MEMS

device integration. However, PCB processing has been historically difﬁcult

due to the process limitations imposed by PCBs, such as low temperatures

and non-planar surfaces. In order to overcome these problems, we recently

developed an RF MEMS technology compatible with microwave laminate

PCBs that overcomes the drawbacks of the silicon-based MEMS technology

in establishing reconﬁgurable antennas with low cost and high performance.

The details of PCB compatible RF MEMS technology and associated fabri-

cation processes are beyond the scope of this chapter and can be found in

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258 MIMO System Technology for Wireless Communications

references [2,3,20]. The main advantage of this technology lies in allowing

the monolithic integration of the antenna and the switches

on the same

substrate as part of the same lithographic process. This approach eliminates

the need for wire bonds to be interconnected to the RF MEMS switches, thus

simplifying the matching circuit as well, while reducing the switch parasitics.

Figures 9.3 and 9.4 compare two MRA systems realized by silicon-based and

microwave laminate compatible RF MEMS technologies in terms of com-

plexity, cost, and performance. It is obvious from these ﬁgures that the latter

technology provides higher performance, less complexity, and low cost com-

pared to the former technology.

9.4 RF MEMS Integrated Antennas

In this section, the design, microfabrication, and results of two different RF

MEMS integrated antennas will be presented. The ﬁrst design is a multi-

element selection diversity antenna (i.e., cactus antenna) in which RF MEMS

actuators are monolithically integrated with antenna feed lines to selectively

route the RF feed signal. In the second design, RF MEMS actuators are

integrated within the geometrical structure of the antenna to construct a

multifunctional reconﬁgurable spiral antenna capable of dynamically chang-

ing its polarization between right-hand and left-hand circular polarizations.

9.4.1 Three-Element Selection Diversity Antenna

We ﬁrst designed a single antenna, which will henceforth be referred to as

the cactus antenna. The geometry of the cactus is depicted in Figure 9.5. It

consists of two half-wavelength inductively coupled coplanar waveguide

(CPW) slots combined with the half-wavelength capacitively coupled slot

antenna, which is fed through a triangular-shaped half-wavelength CPW strip.

The inductively coupled slots are bent upward for compactness, necessary

in multi-element antenna systems. This structure produces three different

resonant frequencies, f

, f

, and f

, deﬁned by the lengths of the two slots

, L

) and the triangular-shaped CPW line (L

), respectively (see Figure 9.5).

These lengths are determined based on the half-wavelength resonance cri-

terion and are approximately calculated using Equation 9.3. By optimizing

, L

, and L

in conjunction with other design parameters (see Figure 9.5),

resonant frequencies of f

, f

, and f

are located such that the cactus antenna

possesses 42% impedance bandwidth for a voltage standing wave ratio

(VSWR) of 2.

; 0.8 f

1, i

= 1, 2, 3 (9.3)L

reff ri

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259

FIGURE 9.3

Process ﬂows for silicon-based RF MEMS technology for realizing MRA systems, (a) Fabrication

of MEMS switches on a semiconductor, (b) Packaging individual MEMS switches, (c) Dicing,

(d) Assembling MEMS and antenna elements through wire bonding (or ﬂip-chip) on PCB to

construct the MRA.

Semiconductor

substrate (Si, GaAs, etc.)

(a)

(b)

(c)

(d)

PCB substrate

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260 MIMO System Technology for Wireless Communications

In Equation 9.3, c

is the speed of light in free space and J

reff

represents the

effective dielectric constant of the dielectric supporting material. P

is a coef-

ﬁcient and ranges from 0.8 to 1 [21].

Next, a three-element selection diversity cactus antenna with RF MEMS

switches monolithically integrated on CPW feed network is fabricated on

RT/Duroid 5870 (see Figure 9.6). This material is particularly suitable for

antenna applications due to its low dielectric constant and low loss property

in the desired frequency range (J

= 2.33, tanI

= 0.0005 @ 5 GHz) [22]. The

switches used are capacitive and have a structure similar to those previously

published [15].

Three RF MEMS switches located on the CPW feed, single-pole three-

throw switch (SP3T), route the input to one of the branches in order to

maximize the signal-to-noise-ratio (SNR) of the diversity signal at the receiver

FIGURE 9.4

Process ﬂows for microwave laminate compatible RF MEMS technology for realizing MRA

systems, (a) Fabrication of monolithically integrated MEMS antenna on PCB, (b) System level

packaging.

(b)

(a)

PCB substrate

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Multifunctional Reconﬁgurable Microelectromechanical Systems

261

(see Figure 9.6b). The distances from the centers of switches to the cross-

junction are designed to be quarter-wavelength in the substrate at the center

design frequency of 5 GHz. Thus, at the center frequency, the diversity

branch with switch in the down position is transformed into an open circuit

at the cross-junction due to the quarter-wave transmission line. This mechanism

slightly narrows the impedance bandwidth of the antenna since the signal

propagating on the selected diversity branch sees two quarter-wavelength

open transmission lines of the two disconnected diversity branches at the

junction. High impedance quarter-wavelength lines using the same concept

are utilized to actuate the switches. In this case, the bandwidth degradation

is very small due to the combined effect of the quarter-wave line and high

impedance of the line. In the current design, a bias voltage of 40V is required

to actuate the switch. Three metal-insulator-metal (MIM) capacitors located

on the CPW line are employed to decouple the RF signal path from the bias

voltage. Air bridges are used to ensure the continuity in the CPW ground

planes and to suppress possible slotline modes excited at the junctions. The

fabrication of air bridges, as well as MIM capacitors, is compatible with the

monolithic process.

The fabrication procedures of the cactus antenna integrated with RF MEMS

switches and that of the MRA that will be presented in the next section are

very similar. Therefore, only a brief summary of the fabrication process is

FIGURE 9.5

Top view of the CPW-fed antenna geometry and dimensions in mm. L

= 25, W

= 3.5, L

28.6, W

= 4.8, L

= 18, L = 12, S = 0.15, G = 0.25, W = 2.6, D = 15, h = 1.52, F: orientation angle.

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262 MIMO System Technology for Wireless Communications

given here. The process starts with standard via hole formation for switch

biasing. Ease of via layout is an advantage of this process over existing

technologies, where deep vertical vias are normally difﬁcult to form. Via

holes through the substrate connect the central electrode of each switch to

the bias line placed on the backside of the antenna. This separation does not

only create a space for multiple antennas but also helps increase the isolation

FIGURE 9.6

(a) Photograph of the selection diversity cactus antenna integrated with RF MEMS switches;

S1, S2, S3 are RF MEMS switches; A1, A2, A3 are cactus antennas; (b) Schematic of the feeding

structure, single-pole-three-throw switch (SP3T).

MIM capacitor

Air bridge

Bias via hole

MIM capacitor

Bias line

(b)

(a)

Bias line

S1 S3

Bias via hole

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Multifunctional Reconﬁgurable Microelectromechanical Systems 263

between antenna elements and biasing circuit. This, in particular, is an impor-

tant advantage as the circuit complexity and the number of switches increase.

Next, antennas and all feed lines are formed by a simple wet etch process.

Finally, MEMS switches, MIM capacitors, and air bridges are monolithically

fabricated.

Results are presented in the following ﬁgures. Figure 9.7 shows the return

losses of each switched antenna. As mentioned earlier, the two quarter-

wavelength lines narrow the input impedance bandwidth of the cactus

antenna from its original 42% to 30%. However, the covered bandwidth is

still very large for most wireless local area network (WLAN) applications.

Figure 9.8 illustrates the measured x-y plane co-polar radiation patterns at

the center design frequency of 5 GHz, normalized with respect to the same

reference, corresponding to the sequential and individual activation of each

antenna. As a result of the inter-element 45° rotation angle, radiation patterns

with 45° rotation are clearly distinguished in the ﬁgure. Appropriate activa-

tion leads to angular or polarization discrimination of the signal of interest

in multipath scenarios. For example, the axis of the active element can be

oriented to the direction of strong interference.

9.4.2 Multifunctional Reconﬁgurable Spiral Antenna

9.4.2.1 Antenna Structure and Operational Mechanism

In the three-element cactus antenna design the role of RF MEMS switches,

which are located on antenna feed lines, are limited to selectively routing

the RF feed signal. In this design, a number of RF MEMS actuators are

monolithically integrated within the geometrical structure of the antenna to

construct a multifunctional reconﬁgurable spiral antenna (MRSA). In other

FIGURE 9.7

Measured return loss of individual selected cactus antenna.

3.5 4 4.5 5

Frequency(GHz)

5.5 6

Antenna 1

Antenna 2

Return loss(dB)

Antenna 3

6.5 7

−5

−10

−15

−20

−25

−35

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