antennas in cellular phones for mobile communications.pdf

INV ITEDP A P E R

Antennas in Cellular PhonesforMobile CommunicationsProgress in the design of mobile phone antennas during the past 15 years

is reviewed in this paper; a recent development on new antennas is

also described.

By Zhinong Ying, Senior Member IEEE

ABSTRACT | The mobile industry has experienced a dramatic

growth; it evolves from analog to digital 2G (GSM), then to high

date rate cellular wireless communication such as 3G (WCDMA),

and further to packet optimized 3.5G (HSPA) and 4G (LTE and

LTE advanced) systems. Today, the main design challenges of

mobile phone antenna are the requirements of small size, built-

in structure, and multisystems in multibands, including all cel-

lular 2G, 3G, 4G, and other noncellular radio-frequency (RF)

bands, and moreover the need for a nice appearance and

meeting all standards and requirements such as specific ab-

sorption rates (SARs), hearing aid compatibility (HAC), and over

the air (OTA). This paper gives an overview of some important

antenna designs and progress in mobile phones in the last

15 years, and presents the recent development on new antenna

technology for LTE and compact multiple-input–multiple-

output (MIMO) terminals.

KEYWORDS | Body effect; HAC; integrated antenna; LTE

antenna; MIMO antenna; mobile terminal antenna; multiband

small antenna; over-the-air (OTA) performance; SAR

I . INTRODUCTION

The cell mobile systems have evolved from analog systems,

called the first-generation (1G) systems, to digital systems,

called the second-generation (2G) systems, and further tothe third-generation (3G) systems, which are capable of

multimedia transmission. Now the 3G systems evolve to

the fourth-generation systems (LTE advanced) through

the, e.g., HSPA and LTE systems. In addition to the cellu-lar mobile phone systems, various wireless mobile systems

have been deployed and started services in various areas.

For example, multiband global navigation satellite system

(GNSS), multiband WiFi (2.4 and 5.2 GHz) and 60 GHz,

TV, FM radio, Bluetooth, near-field communications (NFC),

and ultrawideband (UWB) wireless systems. The operating

frequencies used by these systems range from kilohertz

regions to as high as gigahertz regions, depending on thesystem performance, complexity, transmitting media and

data, and so forth. The services of these systems range from

very short distances to intermediate distances, whereas

mobile phone systems provide the nationwide service. Va-

rious antenna systems have been developed for these mobile

systems and accordingly the antenna technology has made

progress along with the deployment of these systems.

The main challenges for the mobile terminal antennaare as follows: small size, built-in, multiband, and

coexistence of a multiradio system and a multiple-input–

multiple-output (MIMO) system. For example, a smart-

phone needs to support more than ten 4G and 3.5G

networks and all 2G and 3G networks for global coverage

and roaming, which means dozens of cellular radio-

frequency (RF) bands [see Fig. 1, third-generation

partnership project (3GPP) bands evolution] plus multi-antenna systems in some of the bands. There have been two

major trends in the antenna design. One is that antennas

for mobile terminal require small size, built-in, and

multiband operation with a nice appearance. Another is

that antennas for mobile terminals need to fulfill various

standardizations and requirements, depending on the func-

tion and complexity of the system, its service areas, the

quality and quantity of data to be transmitted, and so forth.For example, 3GPP and many of the mobile communication

network operators are introducing the requirements of the

RF over-the-air (OTA) performance of a mobile phone in

Manuscript received July 31, 2011; revised December 17, 2011; accepted

January 18, 2012. Date of publication March 16, 2012; date of current version

June 14, 2012.

The author is with the Network Research Laboratory, Research and Technology,

CTO Office, Sony Ericsson Mobile Communication AB, Lund 22188, Sweden

(e-mail: [email protected]).

Digital Object Identifier: 10.1109/JPROC.2012.2186214

2286 Proceedings of the IEEE | Vol. 100, No. 7, July 2012 0018-9219/$31.00 �2012 IEEE

certain conditions with impact of user head and hand in

order to improve the network operation performance.

Another important issue to be considered in designing

antennas for a mobile phone is the reduction of specific

absorption rate (SAR) values which could be caused by RF

radiation from multiple radio and antennas in the mobile

phone. The SAR should be as low as possible, especiallywhen in contact with the human brain. The handset with

multiple transmitters and antennas will increase the com-

plexity of the near-field problem. In addition, hearing aid

compatibility (HAC) is required for mobile phones in some

countries, which will also increase the complexity of the

antenna design. For a compact mobile terminal, environ-

mental conditions are also a serious issue to be considered

in the antenna design. Proximity effects due to materialsnear the antenna element, such as circuit components,

acoustic components, camera, print circuit board ground-

ing quality, may degrade antenna performance. In modern

antenna design, the proximity effect is treated with integ-

ration concept, in which nearby materials are included in

an antenna system as an integral part of the radiator.

For MIMO and diversity application in WiFi, 3.5G, and

4G systems, the multiantenna system requires low mutualcoupling loss and low pattern correlation between an-

tennas in order to realize good diversity or MIMO perfor-

mance. The channel impact is not only path loss, but also

signal transmission rate, bandwidth, delay spread, and

Doppler shift in the Rayleigh fading environments. These

parameters are particularly important in the digital modu-

lation systems used in high data-rate transmission.

In this paper, the author will only discuss some built-inmultiband cellular antenna designs in mobile communi-

cation handsets including SAR, human body effect issues,

and the recent compact MIMO antenna technology for

mobile terminals due to the limited space.

II . MULTIBAND BUILT-INANTENNA DESIGN

A. Antenna TypesSmall antenna types can be classified according to their

geometry: dipoles, slots, and cavities. More complex geom-

etries can be developed from these fundamental antenna

types. The simplest omnidirectional type of the antennais the dipole. The external antenna or the internal

antenna which is free from ground plane on a mobile

terminal can be considered as an unbalanced dipole.

Usually, we call it a monopole antenna, because the an-

tenna element is much smaller than the actual handset

chassis size. Slot antennas, also called magnetic dipoles,

can be seen from a long, narrow opening on a metallic

surface. The planar inverted-F antenna (PIFA) andinverted-F antenna (IFA) can be considered as a mixed

dipole and slot antenna. The cavity antenna in its simplest

cases can be a patch antenna, or a dielectric resonator

antenna (DRA) [1].

Before 1998, most of the mobile phones used the ex-

ternal antenna, which is simple and easy to reuse. The

most popular design was the dual-band nonuniform helical

antenna proposed by Ying [2]. The helix had quarterwavelengths at the low band functioning as a quarter wave

monopole, and it had a nonuniform pitch angle or diam-

eter to control the second resonance frequency band. The

antenna had a high efficiency and was cheap to manufac-

ture and it has been used in over a billion mobile phones

worldwide.

B. Multiband Internal Antenna DesignThe internal antenna can increase the mechanical

robustness of a mobile terminal, and the internal antenna

housing can also be used as an acoustic cavity to improve

the audio performance. It has followed the trend of the

mobile phone to become a multimedia mobile handset

since the late 1990s. These factors have led to a market

acceptance worldwide.

The main types of internal antennas for stick-typehandsets are the PIFA [3], the folded monopole antenna

[4], and the loop antenna [12], [13]. The PIFA is usually on

a ground plane, and may have a feeding pin and several

ground pins. The radiation pattern is affected by the

ground plane, and can be directive, especially in the high-

frequency range. Fig. 2 shows an example of a mobile

phone equipped with an internal PIFA in 2000. The an-

tenna is located at the top of the phone behind theprinted wire board (PWB). On the other hand, the folded

monopole antennas are usually used at the bottom of the

handset to have less head loss and risk of high SAR.

In 1997, Hall first proposed a PIFA composed of two

separate patches of different sizes to achieve dual-band

performance [5]. The initial idea of a common feed for a

dual-band PIFA was mentioned and later a lot of work was

done to realize it for cellular application. Between 1997and 1999, several dual-band PIFAs based on a slot cutting

patch were developed in the mobile phone industry, and a

lot of work was later published [6]–[9]. The antenna

bandwidth depends on the antenna size and ground plane

size. A PIFA on a bar phone has the maximum bandwidth

with 120 mm for the 900-MHz band and 80- or 140-mm

PCB length for the 1800-MHz band.

Fig. 1. 3GPP cellular RF bands with different standards evolution.

Ying: Antennas in Cellular Phones for Mobile Communications

Vol. 100, No. 7, July 2012 | Proceedings of the IEEE 2287

To meet the multiband application, some slot cutting

designs were developed. Some PIFA variants in the mobile

terminal applications are shown in Fig. 3. A PIFA with one

slot has dual resonance, mainly used in dual-band appli-

cations. A PIFA with two slots (one slot is between feeding

and grounding posts) can have an extra resonance at the

high band. A PIFA with a parasitic element can have anextra resonance at the high band [8]. All of those three

antennas have been very popular solutions in the handset

applications. The compact multiband phone in Fig. 2 was

equipped with the internal antenna design in Fig. 3(c).

The mentioned branched monopole antenna can be

built with a very low profile form. Then, it is possible to

make it integrated with the housing of a mobile terminal.

The design of the antenna is similar to the external mono-pole antenna and can be found in many references today

[6]–[9]. The bandwidth of the monopole is much larger

than a PIFA; the monopole can be capacitive, coupled, or

matched to the terminal housing to excite the chassis mode

to gain some extra bandwidth when the chassis length is

larger than a quarter wavelength. The first systematic ana-

lysis work about bandwidth enhancement by using the

chassis mode of the mobile handset was done byVainikainen’s group [10], [11]. When the height of the

low-profile monopole is too low, the antenna becomes ca-

pacitive, an extra ground pin is needed to form a matching

loop, and the antenna becomes a branch IFA or non-

grounding PIFA [6]–[9]. The monopole or ground-free

PIFA could be designed with very small thickness, and

could become a very popular antenna solution for slimmobile phones.

A loop antenna (usually one end is the feed and another

end is grounded) has a multiband feature due to the

multimode of the loop trace. It could be designed both on

ground or as a nongrounding type. The high-band reso-

nances could be merged to be able to cover a wide fre-

quency range. Due to the coupling between different

sections of the loop, the loop antenna has better bandwidththan PIFA and possibly less hand effect. It becomes a very

useful design in mobile phone applications [12], [13].

C. Bandwidth-Enhanced AntennasIt was discovered that the patch antenna bandwidth

can be enhanced by introducing a distributed capacitive

coupling feed in the antenna with an optimized design

[14]. The dual resonance feature was found in both the low

band and the high band by combining the feeding element

resonance and the resonances of passive element and

chassis. A dual-layer coupling fed multiband antenna wasinvented and used in mobile phone application in 2001

[15]. Later the design was developed further to a single-

layer design and LC loading design [16], [18]. High dielec-

tric material was introduced to the c-fed antenna to make it

compact and less sensitive to human body [17]. The c-fed

concept was applied to the ground-free-type antenna, and

the bandwidth was enhanced dramatically. Wong’s group

has conducted a lot of work, which attracted interests fromthe industry [18]–[22]. Fig. 4 shows an example of engi-

neering sample based on a c-fed ground-free antenna with

LC loading and the bandwidth performance of such an

antenna on a 46 � 96 mm2 small ground plane [22]. The c-fed antenna design concept built in PWB was sold by some

of the antenna vender as Bmetamaterial antenna[ [23]. In

that design, the c-fed structure is equivalent to one cell of

Fig. 2. A small handset with an internal multiband PIFA in 2000.

Fig. 3. Summary of slot cutting PIFAs. (a) A PIFA with one slot has

dual resonant. (b) A PIFA with multislots (one slot is between feeding

and grounding posts) can have extra resonant at high band. (c) A PIFA

with a parasitic element can have extra resonant at high band.

Fig. 4. The engineering sample (10 � 5 � 45 mm3) of the c-fed

monopole with LC loading [22], and the reflect coefficient of the

antenna in 110 � 50 mm2 phone chassis in free space and close to the

humanhead; it haswide band coverage (700–960 and 1710–2170MHz)

with a compact antenna size.


2288 Proceedings of the IEEE | Vol. 100, No. 7, July 2012

the structure of metamaterial, which results in a lot ofpublic interests. The c-fed design was extended to a com-

bination of a compact ultrawideband antenna and parasitic

element which is connected to a small ground plane. The

bandwidth of the antenna could cover from few hundred

megahertz to over 10 GHz [24], [25].

III . COMPACT MIMO ANTENNA DESIGNFOR HSPA AND LTE MOBILE TERMINALS

A. Some Important Characterizing ParametersRecently, HSPA (3.5G) and LTE (4G) mobile systems

have been deployed worldwide to support high-speed data

communication in the cellular system. The compact MIMO

and diversity antenna design is essential to such terminals.

To be able to characterize the performance of a compactMIMO antenna in a mobile environment, some parameters

were used or newly defined. The mean effective gain

(MEG) is a statistical measure of antenna gain in the mobile

environment. In MEG computation, the incoming wave

propagation model is defined to describe a specific mobile

environment. The correlation coefficient of the antennas is

another important parameter to describe the pattern

correlation between the antennas. These parameters canbe calculated from the 3-D far-field complex radiation pat-

terns of the antennas, which can be obtained from a nume-

rical method or an advanced measurement system. The

detailed definition could be found in the early literature

[26], [27].

According to the Shannon capacity theory, the capacity

is linearly proportional to the signal-to-noise ratio (SNR)

for low SNR cases and logarithmically proportional to theSNR for high SNR cases. So for a weaker signal and strong

fading case, diversity scheme is usually used. The effective-

ness of diversity is usually presented in terms of the diver-

sity gain (DG). The DG can be defined as the improvement

in time-averaged SNR of a combined signal from a diversity

antenna system, relative to the SNR from one single anten-

na in the system, preferably the best one. This definition is

conditioned by the probability that the SNR is above areference level. The detailed description can be found in

[28]. An effective diversity gain (EDG) is defined to include

the total antenna efficiency EDG ¼ DG � �ant, where �ant is

the antenna efficiency of the best antenna, including reflec-

tion losses, ohmic losses, and mutual coupling losses [28].

For a good signal and strong fading cases, MIMO

scheme is usually applied. The MIMO capacity is widely

accepted in communication domain to characterize theperformance of MIMO systems. They are not uniquely

defined. For example, capacity (in bits per second per

hertz) is calculated based on a reference SNR value (e.g.,

10 and 20 dB). The lack of universal reference values for

the outage probability level and SNR complicates the use

of these metrics for comparison between different MIMO

antennas. Recently, the multiplexing efficiency for MIMO

antenna has been defined [29]. The multiplexing efficiencyof a given M-element MIMO antenna can be defined as the

SNR required by the ideal M�M independent and iden-

tically distributed (i.i.d.) in the Rayleigh case to achieve an

ergodic capacity minus the required SNR to achieve the

same ergodic capacity for the MIMO antenna under test

[29]. For high SNR case, the multiplexing efficiency re-

duces to the closed form [29]

�mux ¼YMi¼1

�i

! 1M

detðRÞ ¼ �gdetðRÞ (1)

where �g is the geometric mean of the antenna efficien-

cies, �i is the total efficiency of the ith antenna element,

R has diagonal values of 1, and ½R�ij is the complex correla-

tion between the 3-D radiation patterns of antennas i and j.It turns out that, for two-element antennas, this metric has

been found to converge at a relatively modest SNR value of20 dB for the i.i.d. case. It is observed in (1) that �mux

consists of the geometric mean of the antenna efficiency, as

well as the loss of efficiency due to correlation between the

antennas. In the 2 � 2 MIMO case, the multiplexing

efficiency could be simply expressed as

�mux ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�1�2ð1 � �eÞ

p(2)

where �1 and �z are the total efficiency of antennas 1 and 2,

including reflection losses, ohmic losses, and mutual

coupling losses; and �e is the envelop correlation betweenantennas 1 and 2. From the equation, it was found that the

MIMO gain depends mainly on the total antenna effi-

ciency of the antenna elements when the envelop correla-

tion between the antenna elements is below 0.5, and the

correlation has significant impact on MIMO gain when the

correlation is high [29].

B. Decoupling and Decorrelation of CompactMIMO Antennas

To realize a good compact MIMO antenna, the antenna

system needs low coupling loss and low correlation ac-

cording to (2). Several different methods have been pro-

posed to reduce the coupling and pattern correlation

between antennas [30]–[46]. The most effective decou-

pling method is to design the orthogonal mode antenna,

e.g., the three-port DRA proposed by Ying [34], [35]. It hasmore than 15-dB isolation, over 80% efficiency, and very

low correlation, and has a very good diversity and MIMO

performance. Fig. 5 shows the comparison of the MIMO

throughput of WiFi 11n (which has similar frequency range

in LTE band 7) by using a typical three-port DRA antenna

and conventional three dipoles. The orthogonal mode

could also be realized by combining the electric dipole and



the magnetic dipole such as loop or slot antennas [37],

[38]. The bandwidth of the orthogonal mode antenna is

limited by the bandwidth of the antenna resonant modesand the antenna size. The principle was used even for

cellular low band between 700 and 960 MHz by using the

orthogonal property of the loop antenna and the dipole

antenna [36].

Another way to reduce coupling is to use the parasitic

scatter. The scatter could be realized by chokes, parasitic

stub, and modification of the ground plane by notches and

slots [39], [40]. With the scattering of the structure be-tween the MIMO antennas, the isolation could be im-

proved and correlation will be low. The bandwidth is

limited by the size of the MIMO antenna array [39]. The

engineering scatter solution by using decorrelation wave

traps for the LTE cellular phone was proposed [41].

Closed space antennas have strong scattering effects.

The scattering and coupling could be well controlled to

reduce the pattern correlation of the compact MIMO an-tenna and have good antenna efficiency and bandwidth. A

wide colocated MIMO antenna has been recently proposed

for the multiband LTE mobile handset based on this design

concept [42].

Neutralization line, hybrid coupler, and lumped LCmatching network are also effective ways to reduce mutual

coupling and correlation.

1) A neutralization line is a metal strip that connectsthe two antennas. The line allows currents to in-

teract between the antenna elements, thus result-

ing in reduced coupling at a certain frequency.

This technique has proven good with PIFA and

monopole antennas around 2 GHz by Luxey [30].

One of the advantages of this design is that the

decoupling element could be a part of the antenna.

The industry products based on this concept havebeen used in the high-band MIMO system [43].

The bandwidth of this design is limited by the size

of the antenna array. Recently, an investigation

has been done for the 700-MHz LTE terminal

based on this method [44]. The bandwidth was

very small.

2) The 90� hybrid coupler is a standard componentin the microwave design, used to both separate

and combine signals in RF applications. The sig-

nals at the output ports are 180� separated from

each other, but relative to the input port the phase

shift is �90 � (balanced output), which results in a

good isolation between the antennas [43].

3) The 180� coupler can be used in a couple of dif-

ferent ways, mainly for splitting or combiningsignals. The four-port network could realize the

input ports have one even mode and one odd

mode, thus the two ports are decoupled. In this

case, the two ports are not symmetric, the common

mode has nearly the same pattern as the single-

port antenna, and the differential mode has

narrowband, which depends on the size of the

array antenna [43].The miniaturization of impedance control circuit

methods mentioned above could be achieved by an equiv-

alent circuit of lumped elements. The topologies could be

found in the early literature. In [43], some comparison

works were performed. We took a simple model of two

monopoles of 0:1� spacing over a large ground plane, as

shown in Fig. 6(a). Due to the mutual coupling, the radia-

tion efficiency drops to about 55%. With the neutralizationline it could be improved to 75%. With the coupler and

lamped circuit network it could improve over 90% with a

limited bandwidth [43].

The bandwidths of different decoupling techniques

such as the neutralization line, the 90� hybrid coupler, the

180� coupler and their equivalent LC circuits, and

Fig. 5 Measured throughputs of the WiFi terminal with compact

MIMO DRA antenna and dipoles.

Fig. 6. The decoupling network based on the 180� coupler:

(a) two closed monopoles with 0:1�; (b) decoupling network;

and (c) S-parameters.



matching networks have been studied [43]. It shows thatthe bandwidth of the decoupling is mainly limited by the

size of the antenna array. Even though the coupler or the

network has a wide bandwidth, the compact MIMO an-

tenna has only limited uncoupled bandwidth due to the

physical constraint. However, for the 180� coupler case, it

has an even mode at port1 and an odd mode at port2. The

even mode ðS11Þ is observed to give significantly larger

bandwidth than the odd mode ðS22Þ. The coupling andpattern correlation is extremely low. Fig. 6(c) shows the S-

parameters by using the equivalent circuits of the 180�

coupler. The even mode ðS11Þ is observed to give signifi-

cantly larger bandwidth than the odd mode ðS22Þ, which

implies that the approach may be suitable for systems (e.g.,

LTE) that only require the MIMO operation in the down-

link. Unfortunately, the insertion loss of the lumped cir-

cuit in this case could be rather high [43].The microwave coupler decoupling has been investi-

gated recently for the multiband LTE mobile terminal in

the 700–960-MHz range [45]. It shows that it works well

within the limited bandwidth.

For the mobile phone case, good MIMO and diversity

performance could be achieved by pattern diversity when

the frequency band is around 2 GHz or above. But a do-

minant coupling utilization occurs when more than oneantenna element efficiently exploits the ground plane as a

radiator especially for the low bands between 700 and

960 MHz [46], [47]. A detailed parametric study has been

performed based on the insight gained from the charac-

teristic mode analysis and the results indicate that isolation

can be improved by optimizing both the antenna type and

the antenna location, i.e., only one of the antennas uses the

chassis mode and the other antenna uses the localizedmode. It is confirmed that both capacity and diversity

performance for a given bandwidth can be improved using

this simple approach. The localized mode antenna could be

realized by using more directive antennas such as patch,

notch, and balanced dipoles. One significant advantage of

this approach is that no additional (and inherently lossy)

lumped element or matching circuit(s) is required to

achieve better MIMO performance [46].

C. Antenna and MIMO ChannelThe mean effective gain of the mobile phone antenna

depends on both the antenna pattern and the propagation

channel. The antenna system performance in multiple

path environments depends on both the antenna perfor-

mance and channels.

1) For the diversity case, it was found that the EDG ismainly determined by the signal correlation,

signal imbalance, and diversity combining

method, while DG includes the effects of antenna

matching, losses, and mutual coupling. It was

found that the parameters such as correlation and

apparent DGs do not depend strongly on the

various average propagation models [48]. Thus,

the isotropic random environment seems to be a

good simplified scenario to evaluate the diversity

performance.

2) For the LTE MIMO case, the mean effective gain

and the correlation will determine the MIMOperformance. Inside a typical smartphone, the

antennas have quite low correlation for the fre-

quency bands around 2 GHz and above. So the

main effort to realize good MIMO performance is

to design highly efficient antennas with less cou-

pling loss at those frequency ranges. However, it is

quite a big challenge to design the low correlation

and low coupling antenna at low band, especiallyat the 700-MHz band. Some detailed mockups in

different correlation levels were built and tested

at the Sony Ericsson research laboratory and dur-

ing a field trial with Ericsson Research. Fig. 7

shows four different MIMO antenna designs for a

smartphone at the 700-MHz band. The design

shown in Fig. 7(a) has two monopoles at both

ends, where the chassis mode is strong and thecorrelation is rather high. The design in Fig. 7(b)

has one monopole and one notch. The antennas

are orthogonal to each other, and correlation be-

comes lower. The design in Fig. 7(c) has the colo-

cated loop antenna, where one antenna element is

fed by two port; it is a design for very high

correlation on purpose. The design in Fig. 7(d) has

one chassis mode monopole and one localizedmode patch antenna. The correlation is extremely

low. The mockups were tested and characterized

in the Sony Ericsson laboratory. The perfor-

mances are summarized in Table 1.

Comparing with the performance of reference

antenna orthogonal dipoles, it was found that the

mobile terminals with the compact MIMO

antenna arrangement could have fairly goodMIMO performance when the envelope correla-

tion is less than 0.5, even at the 700-MHz band.

Good antenna efficiency of both antennas is

Fig. 7. Different LTE antenna designs in a mobile phone at the

700-MHz bands. (a) Two monopoles. (b) Monopole and notch.

(c) Colocated loop. (d) Monopole and patch.



essential to cover larger service area of LTE with

good data communication quality [49], [50].

From the field trial, it was also found that the MIMO

performance depends on the local dynamic channels. Withadaptive matching and beamforming of the MIMO an-

tenna, the throughput performance could be improved; the

research on this topic is ongoing [51], [63].

D. Multiband and Multistandard AntennaFront–End Architectures

As mentioned in the introduction, the mobile phone

antenna has to meet multiband and multistandard radiorequirements. The antenna has to be wideband or multi-

band and has to be designed in different RF architectures

to meet different requirements, e.g., carrier aggregations.

It is obvious that a flexible and cognitive RF front–end

technology will become very important to a future mobile

phone. The discussion on this technology is out of the

scope of this paper.

IV. STANDARDS AND INDUSTRYREQUIREMENTS

A. SAR and HACSAR is a value measuring how much power is absorbed

in a biological tissue when the body is exposed to the

electromagnetic radiation. The units are watts per kilo-gram of tissue elements. The maximum SAR is specified as

applying to any 1- and 10-g tissue elements. Governments

around the world have agreed to define the guidelines

concerning SAR limits. In Europe, the International Com-

mission on Non-Ionizing Radiation Protection (ICNIRP)

organization has set the SAR limit as 2.0 W/kg over a 10-g

cube. In the United States, the limit is 1.6 W/kg over a 1-g

cube according to the Federal Communication Commis-sion (FCC) [52]. Spatial-peak SAR is defined as the maxi-

mum average SAR of a 10- or a 1-g cubic volume of tissue.

SAR is measured on a completely head or flat phantom

placed next to the mobile phone and the highest value

detected decides how large the SAR value is. The mea-

surements are made on a biology-simulating liquid, which

has a relative permittivity and a conductivity that depends

on the frequency. The detail standard could be found in the

literature [7], [8], [52].

Mobile phones interfere with hearing aids both by their

RF emission and their electromagnetic (EM) field. Modernhearing devices have an audio amplifier, which makes it

easy for hearing aid users to use an ordinary phone. If the

hearing aid is in a high level pulsed EM signal such as the

talking position of the mobile phone, the amplifier will

generate some unpleasant buzzing noise. HAC is defined

as the highest E-field and the highest H-field detected in

the near-field area. The details of the measurement

method can be found on the FCC’s website [53]. SAR andHAC can also be predicted by using numerical methods

such as finite differential time domain (FDTD) or finite

element method (FEM) [7], [8], [52].

It is observed that exposure to magnetic fields (electric

currents of the radiator) rather than electric fields leads to

a high value of SAR in the body. So the antenna type,

antenna position, phone style, material loading, and metal

grounding will influence the SAR values. In order torealize lower SAR in the brain tissue, in general the

ground-free antenna has to be placed at the bottom of the

mobile phone and on-ground antenna such as PIFA could

be placed at the back of the PWB of the phone to minimize

SAR. Since it is a near-field problem, the detailed solution

depends on the individual antenna design, grounding, and

nearby components [56].

In practical cases, SAR and HAC are caused by the nearfield which not only depends on the antenna type and po-

sition, antenna efficiency, and phone factor, but also on the

load pull of source impedance of the RF power amplifier [7],

[8], [56]. This will increase the uncertainty of the prediction.

Recently, the new challenges for the near-field issue in

a mobile phone handset were born from the RF multi-

transmitters in MIMO and multiband, such as how to ar-

range and control the antenna elements and how to designthe slim handset while still meeting safety limits. MIMO

such as LTE and WiFi will introduce multiradio transmit-

ters and multiantennas in a compact terminal. The body

tissue property such as the conductivity and permittivity

are frequency dependent, which increases the complexity

of the SAR problem. The research on minimizing SAR and

HAC such as the antenna types, the antenna arrangement,

Table 1 The Measured MIMO Antenna Performance of Four Different Mockups, and the Field Trial Test Results



and the smart adaptive control RF architecture in themobile terminal is ongoing [54], [55], and to shorten the

SAR evaluation for the handset with multiple transmitters

and antennas, FCC has proposed the simplified test

method [58].

B. System OTA Performance and Body Impact onOTA Performance

The 3GPP has been working on the system require-ments and regulations of a mobile terminal for several

years. The test regulations on different standards were de-

veloped or are ongoing. The Cellular Telecommunications

and Internet Association (CTIA)/The Wireless Association

is a United States-based international organization that

serves the interests of the wireless industry by lobbying

government agencies and assists with regulation settings. A

working group, including operators, mobile phone manu-facturers, and test equipment vendors, is developing a

detailed test plan that includes LTE OTA test. The most

recent release of this plan can be found in [60]. According

to the test plan, the total radiated power (TRP) and the total

isotropic sensitivity (TIS) are obtained by full sphere

radiated measurements in an anechoic chamber for 2G and

3G mobile terminals. The test setup includes a base station

emulator which is used to establish a call to the mobilephone inside the anechoic environment. For transmitting,

the mobile’s effective isotropic radiated power (EIRP) can

be recorded as a function of the direction of radiation using

a narrowband power measurement device. For receiving,

the base station emulator is used to record the receiver

sensitivity as a function of the angle of arrival. Integration

of the EIRP and the sensitivity over the full sphere yields

the TRP and TIS, respectively. The test conditions thatinclude a human head and hand phantom are also defined.

It was found that the radiation from a mobile terminal

is from both the antenna element and the terminal chassis.

It is very important to measure the final radiation perfor-

mance that influences the human body. The body absorp-

tion can be defined as head loss and hand loss when the

handset is in a talking or browsing position. CTIA has

proposed several body test cases for a mobile phone. Therequirements are under discussion for passive and active

modes. A phantom equivalent to the human body tissue is

usually used to do the test. A typical head phantom is the

specific anthropomorphic mannequin (SAM) phantom

which is defined for SAR measurement. The hand

phantoms are defined for talking and browsing modes in

evaluating the effects on different phone factors [57],

such as mono-block phone (width is less than 56 mm),clamshell phone, and personal digital assistant (PDA)

phone (width is larger than 56 mm). Fig. 8 shows the test

case for a typical mono-block phone which has 45 mm in

width and 12 mm in thickness. The body loss depends on

the antenna design, phone size and thickness, and antenna

arrangement. There is no simple solution to all the

problems.

Systematic studies of body loss of the mobile phones

with different phone size, different antenna types, and

locations have been performed at the Sony EricssonResearch Laboratory [59]. It was found that body losses

strongly depend on the antenna location, type, phone

factor size, etc. For example the bottom mount monopole

antenna and the top mount PIFA antenna were built in a

stick phone as shown in Fig. 8 and both cases were studied

individually. Fig. 9 shows the body loss analysis results of

cases (b)–(d) in the 900-MHz band. It was found that in

the low band and the talking position with the hand thePIFA antenna had advantages with 4-dB less body loss for

a large phone factor. Fig. 10 shows the body loss of

cases (b)–(d) in the 1900-MHz band. It was found that

in the high band and the talking position with the hand the

top PIFA antenna had advantages with 3-dB less body loss

in general. These results are only valid for a feature phone

(width less than 56 mm). It was found that the smartphone

(width more than 56 mm) with PDA hand had totallydifferent features, and the relevant study was performed

recently [62].

For LTE and diversity application in a handset, the

body will usually reduce the pattern correlation. The

Fig. 8. CTIA-defined four different test positions: (a) free space;

(b) talking position; (c) talking position with hand; (d) browsingmode.

Fig. 9. Body loss analysis of a bar phone with the top PIFA and

bottommonopole in the 900-MHz band.



performance will drop due to the body loss and mismatch-

ing [47], [62]. The work of adaptive matching and adaptive

control of distributed MIMO antennas is a promising solu-tion to overcome human body detuning and loading [51],

[63]. The OTA measurement technology for LTE is under

study and will be proposed in the near future [61].

V. CONCLUSION

In this paper, the major challenges of the mobile handset

antenna for communication were addressed. Some impor-tant multiband internal antenna technologies in mobile

industry were reviewed. The mobile handset antennas

need to be small in size; built-in to meet multiband,

MIMO, and multistandard RF coexistence requirements;and fulfilling all standards and industry requirements with

a nice appearance. The recent research work of compact

MIMO antennas includes different decoupling techniques

such as the orthogonal mode, the localized mode, parasitic

scatter, and impedance coupler, which were also dis-

cussed. The decoupling bandwidth is limited by size of the

MIMO antenna array. For the multiband decoupling, the

use of the optimal design of hybrid decoupling techniquesor reconfigurable structures is required. The newly defined

MIMO antenna multiplexing efficiency was described,

which simply showed the relation of the MIMO perfor-

mance and the efficiencies, and the correlation of the

antenna elements. The practical issues of the human body

impact, SAR, and HAC with single and multitransmitter

were described and discussed, and the related research is

ongoing. Due to the small size, integration, multiband, andmultistandard requirements and human body effects, the

antenna design of the mobile phone is always the art of

compromising between the size, the phone appearance,

and the performance. The future research will focus on the

optimization of the MIMO, multiband, and reconfigurable

multiantenna system to reduce the human body impact

and enhance communication performances. h

Acknowledgment

The author would like to thank Dr. B. K. Lau’s group at

the Lund University, Lund, Sweden, and Prof. S. He’s

group at the Royal Institute of Technology, Stockholm,Sweden, for collaborative work. He would also like to

thank Dr. P. Karlsson of Sony Ericsson Mobile Commu-

nications AB for his helpful comments.

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ABOUT THE AUTHOR

Zhining Ying (Senior Member, IEEE) received the

B.S. degree from Zhejiang Normal University,

Jinhua, China, in 1982, the M.S. degree in electrical

engineering from Beijing University of Posts and

Telecommunications, Beijing, China, in 1986, and

did Ph.D. study at Chalmers University of Tech-

nology, Chalmers, Sweden, during 1992–1995 and

got the Licentiate degree.

He is an expert of antenna technology at the

Network Research Laboratory, Sony Ericsson Mo-

bile Communication AB, Lund, Sweden. He joined Ericsson AB in 1995. He

became Senior Specialist in 1997 and Expert in 2003. He has been guest

professor at Zhejiang University, China, since 2002. His main research

interests are small antennas, broad and multiband antenna, multichannel

antenna (MIMO) system, near-field and human body effects, and mea-

surement techniques. He has authored and coauthored over 80 papers in

various journal, conference, and industry publications. He holds more

than 70 patents andmore are pending in the antenna andmobile terminal

areas. He contributed a book chapter to the well-known Mobile Antenna

Systems Handbook (Reading, MA: Artech House, H. Fujimoto, Ed., 3rd ed.).

He had invented and designed various types of multiband antennas and

compact MIMO antennas for the mobile industry. One of his contributions

in the 1990s is the development of nonuniform helical antenna. The

innovative designs are widely used in mobile terminal industry. His

patented designs have reached a commercial penetration of more than

several hundred million products worldwide.

Mr. Ying received the Best Invention Award at Ericsson Mobile in 1996

and the Key Performer Award at Sony Ericsson in 2002. He was

nominated for the President Award at Sony Ericsson in 2004 for his

innovative contributions. He served as the Technical Program Committee

(TPC) Co-Chairman at the 2007 International Symposium on Antenna

Technology (iWAT), and served as session organizer of several interna-

tional conferences including the IEEE Antennas and Propagation Society,

and a reviewer for several academic journals. He was a member of the

scientific board in the European 6th Framework Program of the Antenna

Centre of Excellence (ACE) from 2004 to 2007.



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