Technology for Reducing the Power Consumption of

Mobile/Personal Handheld Terminals

Takeshi Hattori

NTT Telecommunication Network Laboratory Group, Musashino, Japan 180

Presently, with Fac. Sci. Eng., Jochi University

Shigeaki Ogose

NTT Wireless Systems Laboratories, Yokosuka, Japan 239

Shuji Kubota

NTT R&D Management Department, Tokyo, Japan 111

Kiyoshi Kobayashi

NTT Wireless Systems Laboratories, Yokosuka, Japan 239

SUMMARY

This paper discusses the reduction of power con-

sumption in handheld terminals from the viewpoint of

system configuration and hardware. The first part discusses

system development of portable and personal communica-

tion devices, as well as performance trends relating to size,

transmitter power, and standby time. We then focus on the

PHS (personal handy phone system) and discuss the system

technology that plays a basic role in power consumption

reduction, together with its effects. The configuration of the

handheld terminal from the viewpoint of the hardware as

well as the configuration techniques suited to the power

consumption reduction are compared and evaluated. The

effectiveness is discussed.

The future trends in terminal technology are also

discussed and a possible direction for research and devel-

opment is indicated. ©1998 Scripta Technica, Electron

Comm Jpn Pt 2, 81(3): 39�52, 1998

Key words: Handy phone; PHS; battery saving; in-

termittent control; microcell.

1. Introduction

Accompanying a paradime shift in recent life-style of

society, there is a rapidly increasing need for the communi-

cation at any time, any place, and with anybody. Commu-

nication, in transport or outside, not only satisfies the need

of business people to overcome the constraints of time and

space, but also leads to a new feeling of human relations as

a group in forming a new community. Portable/personal

communication technology in its wider sense provides the

basis for such activities.

CCC8756-663X/98/030039-14

© 1998 Scripta Technica

Electronics and Communications in Japan, Part 2, Vol. 81, No. 3, 1998Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J80-A, No. 5, May 1997, pp. 735�745

39

Portable/personal communication can be defined as

communication, in which each user has a palm-sized termi-

nal and can access the system at any time and from any

place. As an overall trend, there will be a development from

individual systems to system integration, from the tele-

phone and the data-dedicated system to the system integra-

tion, and from a terminal number to a personal number. At

present and in the near future, however, it is expected that

the main stream lies in telephone-based communication.

Development based on the telephone system can be largely

separated into development from the car (cellular) phone

and development from the cordless (microcellular) phone

[1]. Table 1 shows the features associated with these trends.

The cellular system uses centralized control as its

basic control architecture, to realize communication with

the high-speed transport. With the reduction in size of

handheld terminals, there was an increasing need for a

portable system, which resulted in car and personal phones.

Examples of such systems are provided by the PDC (per-

sonal digital cellular) system standardized in Japan [2], the

GSM (global system for mobile communications) system

standardized in Europe [3], and the IS-54 system stand-

ardized in the U.S.A. The communication method is TDMA

(time division multiple access) and FDD (frequency divi-

sion duplex), where different frequencies are used for the

down-link from the base station to the terminal and the

up-link in the other direction.

By introducing digital technology, the handheld ter-

minal in the system was reduced in size and power con-

sumption. At present, more than 80 percent of the demands

from a handheld phone are imposed by the car/handheld

phone system. In some of the systems, service is limited to

handheld phones. In the case of a handheld phone, the

transmitter power is 0.5 to 1 W, which is several times lower

than that for a car phone, but is larger by a factor of between

10 and 100 than the cordless phone system. This power

level is the result of a compromise between the battery

Table 1. System comparison between cellular and cordless systems

Cellular Cordless

moving speed high-speed handling (vehicular speed) from walking to moderate speed

service area wide continuous area mostly crowded area

transmitter of handheld terminal several W order of 10 mW

cell size 500 m ~ 10 km 200 ~ 500 m

control centralized control decentralized control

speech coding VSELP, CELP ADPCM

network independent network PSTN/ISDN

examples · GSM/DCS-1800 (Europe) · DECT (Europe)

· PDC (Japan) · CT-2 (U.K.)

· IS-54, IS-95 (U.S.A.) · PHS (Japan)

ADPCM: Adaptive Differential Pulse Code Modulation (adaptive differential pulse code modulation)

VSELP: Vector Sum Exited Linear Prediction (vector-sum linear prediction coding)

CELP: Code Exited Linear Prediction (code excited prediction coding)

GSM: Global System for Mobile Communications

PDC: Personal Digital Cellular

CT-2: Cordless Telephone Second Generation

DCS: Digital Cellular System

DECT: Digital Enhanced Cordless Telephone

PHS: Personal Handyphone System

IS-54: Interim Standard (TDMA standardized in U.S.A.)

IS-95: Interim Standard (CDMA standardized in U.S.A.)

40

supplying the power, the constraint on thermal radiation,

and the support of the high mobility.

From the viewpoint of speech coding, there is a

requirement for robustness against error as well as a narrow

band transmission. Three speech coding systems have been

developed that are unique to cellular systems. A high-speed

processing performance of several tens MIPS is required,

for which a dedicated DSP (digital signal processor) is used.

As a performance measure for handheld terminals, 40 min-

utes of continuous talk and 8 hours of continuous standby

between battery chargings were typical figures in the initial

stages. The transmission time as well as the standby time

were greatly improved by later developments, which fur-

ther increased the demands for personal phones.

Another system development is based on the cordless

phone. The system architecture is based on decentralized

control and portability has been emphasized from the out-

set. It is a system for which the moving speed is from

walking to moderate speeds [4]. Examples of such systems

are the PHS (personal handy phone system) standardized

in Japan [5], DECT (digital enhanced cordless telephone)

standardized in Europe, and CT-2 standardized in U.K.

CT-2 is a system that handles only outside transmissions

and cannot execute a hand-over. Consequently, it is going

to be abolished. Thus, future developments will be based

on PHS and DECT.

In both of these systems, access is by TDMA, but the

systems differ from the cellular system in that TDD (time

division duplex) is used. In this case the same frequency is

used for the up- and down-links and two-way communica-

tion is achieved by time division.

The transmitter power of a handheld terminal is 10

mW, which is as small as 1/60 to 1/100 compared to the

cellular system. For speech coding, ADPCM (adaptive dif-

ferential pulse code modulation), which was already stand-

ardized for communication through a fixed network, is

used. The bit-rate is 32 kbit/s, which is faster than the car

phone system by a factor of between 1.5 and 3. The proc-

essing power of several MIPS is sufficient, which helps to

achieve a system with low power consumption. A typical

performance for a handheld terminal with a terminal having

continuous communication of 7 hours and standby time of

500 hours has already been realized, and a terminal with a

higher performance is expected in the future.

The handheld terminal is to be carried, as its name

suggests. It is required to operate for a long period after

charging the battery, in addition to the requirement for small

size and light weight. The techniques for reducing power

consumption in handheld terminals separate into reduction

by the system configuration and reduction by hardware.

From the viewpoint of the system the consideration

is how can the necessary information be transmitted and

received efficiently in terms of power consumption. A

compromise is sought between the connection quality (i.e.,

the connection time for originating terminal and the con-

nection time for destination and the power consumption, as

well as between the communication quality and the power

consumption. The essential issue is to reduce the power, so

long as sufficient quality is secured, but avoid unduly high

quality. Examples of such refinements are provided by

intermittent reception, transmitter power control, and voice

activation.

Modifications of the hardware side, on the other

hand, are aimed at realization and implementation of the

hardware with low power consumption, the improvement

of battery energy density, a circuit configuration with low

power consumption, and intermittent operation at the cir-

cuit level. Some aspects of the component and material

technology, such as the ultra-microfabrication of LSI and

high energy density batteries, are not discussed in this

paper. It should be noted that the system and the hardware

are two wheels of a vehicle in the power consumption

reduction of handheld terminals, and an approach to the

problem from a comprehensive viewpoint is required.

Based on the above, this paper discusses the technol-

ogy for power consumption reduction in handheld termi-

nals, together with its effectiveness, from the viewpoints of

system and hardware. In section 2 we describe the perform-

ance trends of handheld terminals up to present, in terms of

the size, transmitter power, and standby time. In section 3

we discuss the basic system for power consumption reduc-

tion in PHS, together with the effectiveness of this ap-

proach. Section 4 discusses the hardware configuration of

PHS. Several proposals are presented for the role of this

configuration from a hardware viewpoint. The effective-

ness of these proposals is also discussed. Finally, we con-

sider the future development of handheld terminal and the

possibilities for technical development.

2. Macro-Trends in the Performance of

Handheld Terminals

This section concentrates on handheld terminals for

use in mobile communication, such as the portable phone

and the cordless phone, as well as mobile computing which

is based on mobile communication. Recent trends and

future development are discussed.

2.1. Trends in handheld terminals for mobile

communication

Major items in the design of handheld terminals for

use in mobile communication are weight, size (volume),

(continuous) standby time, and the (continuous) talk time.

Detailed technologies for achieving light weight, small

41

size, and low power consumption handheld terminals are

size reduction and high-density implementation techniques

for the components, such as the filter and LSI chip. We

discuss these aspects from a technical point of view in the

next section. In what follows, only the realized performance

of handheld terminals is discussed.

(1) Single mode terminal

Figure 1 shows the relation between the weight and

the volume of handheld terminals used in cellular systems,

i.e., cellular I (PDC) and cellular II (GSM and CDMA

(IS95) [6]), which are used in Japan, as well as PHS. As is

seen from the figure, the values are 100 to 250 g/120 to 300

cc for the cellular system and 70 to 120 g/65 to 130 cc for

PHS. The volume/weight of GSM/CDMA handheld termi-

nals is larger by 30 to 60 percent than that of the cellular

handheld terminal in Japan.

The slope in Fig. 1 gives the specific weight (i.e.,

weight/volume) of the handheld terminal. As can be seen

from the figure, the average specific weight of a conven-

tional handheld terminal is about 1.4. It is desirable that the

specific weight of a handheld terminal should be less than

1, since a specific weight less than unity offers a light-

weight feeling from the ergonomic point of view. Thus, it

is important that future handheld terminals achieve a spe-

cific weight less than unity.

As measures for low power consumption in the hand-

held terminals for mobile communication, the standby time

and the talk time are employed. The standby time is the time

after switching in the supply, after which the device is ready

to capture the control channel and prepare for the reception

of the call signal from the base station. It depends on the

frequencies of the position registration operation, as well as

the shift between in and out of service area due to motion

of the handheld terminal. The talk time is the period after

the start of a talk operation, for which the talk operation can

be continued.

Both measures greatly depend on the radio parame-

ters, such as the transmission/reception interval of the con-

trol signal, the transmission/reception interval of the speech

signal, and the transmitter power. The tendency differs

depending on the wireless communication scheme.

In order to cope with the rapid increase of traffic in

mobile communication, the microcell configuration is con-

sidered useful in system design. The microcell configura-

tion exerts an effect on the handheld terminal that reduces

transmitter power, which reduces the size of the terminal

and increases the operation time. In the case of the PHS

handheld terminal, the average transmitter power is as small

as 10 mW. Because of this, talk times of 5 to 6 hours and

standby times of 400 to 500 hours have been recently

attained. Those figures are achieved by system design and

hardware elaborations. Further improvements in operation

time are expected.

Figure 2 shows the relation between standby time and

talk time for practical handheld terminals. There is a posi-

tive correlation between the standby time and the talk time,

and the talk time is about 1/50 to 1/60 of the standby time.

When handheld terminals are classified as PDC,

GSM/CDMA, and PHS, there are clear tendencies depend-

ing on the class. It can be seen that a handheld terminal in

PHS has a longer battery life than other systems by the

factor of between 2 and 3.

It is easy to realize that the talk time and the standby

time very much depend on the capacity of the installed

Fig. 1. Relationship between volume and weight for

communication terminals. Fig. 2. Relationship between standby time and talk time.

42

battery. Figure 3 shows the talk time and the standby time,

normalized by the battery capacity. As is seen from the

figure, the normalized standby time in PHS handheld ter-

minal is 100 to 1100 (1/A) and the normalized talk time is

5 to 13 (1/A), these are three to four times better than for

cellular handheld terminals.

For the same battery capacity condition, the talk time

ratio is calculated by means of a simple formula for the case

of PDC, based on the transmitter power and the number of

TDMA slots (for the full-rate condition and peak transmit-

ter power 0.8 W). The ratio is 27. The difference between

this value and the above factor of three to four is due to the

power consumptions in the modulator, the baseband circuit,

and the control circuit, in addition to the power consump-

tion in the radio-frequency amplifier of the transmitter.

As is described below, the power consumption of the

whole PHS terminal is about 400 mW. Assuming that the

power efficiency in the high-frequency amplifier of the

transmitter is about 20 %, the power consumption in circuits

other than the high-frequency amplifier is 350 mW. Assum-

ing that the power efficiency of the high-frequency ampli-

fier as well as the power consumption in circuits other than

the high-frequency amplifier in PDC are the same as for in

PHS, the power consumption in the entire PDC terminal is

350 + 800/(3 ´ 0.3) = 1,240 mW. The ratio of the power

consumptions in PHS and PDC overall is then calculated to

be about three, which agrees with the value above.

Even if the size and the weight are reduced, it is a

problem, practically speaking, if the talk time or the standby

time is short. From this viewpoint, Fig. 4 shows the talk

time and the standby time, after normalizing by the weight

times the size. As can be seen from the figure, the normal-

ized standby time of PHS is 5 to 70 h/(g × cm3) and the

normalized talk time is 10 to 50 h/(g × cm3), which are seven

and six times longer, respectively, than the cellular hand-

held terminal.

Figure 5 shows an example of the change of the

weight and the size of the PHS handheld terminal. The data

for years prior to 1995, when the PHS service was started,

are those for terminals used in the field service experiments.

It can be seen from the figure that the size and the weight

have reduced over the years. In the case of the systems

commercialized in other countries, the handheld terminals

are usually large and heavy compared to those used in the

system in Japan. The reason for this may be the emphasis

Fig. 4. Relationship between standby time and talk time

normalized by the product of weight and volume.

Fig. 5. Evolution of PHS terminal.

Fig. 3. Relationship between standby time and talk time

normalized by battery capacity.

43

on operation and longer time of use, rather than the size, in

addition to the stout human body.

(2) Multi-mode handheld terminal

In the mobile communication up to the present time,

when multiple kinds of services are desired, such as wire-

less call services and cellular phone services, the user must

carry more than one handheld terminal corresponding to

those services. Thus, there was room for improvement from

the viewpoint of convenience. The multi-mode handheld

terminal offers an effective means to improve the situation.

In Japan, the dual mode handheld terminal, called P ´ P (P

by P) where the PHS terminal and the wireless call receiver

are contained in one case, is used in practice.

A phone set is now being considered that can be used

as a cordless phone for home use (low tier), and as a cellular

phone for outdoor use (high tier). Some examples of this

development are found in the DECT/GSM dual mode hand-

held terminal in Europe, and the PHS/GSM dual mode

terminal used in Japan and Southeast Asia. In such dual

mode terminals, size and power consumption can be re-

duced by sharing circuit blocks and optimally designing the

operation mode (optimal mode selection function).

2.2. Trends in handheld information terminals

Handheld information terminals in a wider sense can

be classified as PDA (personal digital assistance) and PC

(personal computer). Electronic Notebook and various

dedicated terminals with information processing/informa-

tion/communication functions belong to the former. The

latter is the result of size reduction from the viewpoint of

portability. The personal note computer and the personal

subnote computer belong to this category.

The functions to be provided by an information ter-

minal, as well as technical problems concerning the con-

struction of the terminal, are discussed below. Technical

progress that can be expected in terminal construction

techniques, are inclusion of speech/character recognition

and liquid crystal displays with an I/O function. Another

aspect concerns authentification and cryptography at the

terminal, since electronic commerce using handheld infor-

mation terminals is to be expected in future.

As to the interface between equipment, the following

development is anticipated. Up to the present, the personal

computer and PDA terminal are connected to a communi-

cation terminal through the PCM-CIA card slot, in the same

way as a CD-ROM drive or a floppy disk drive. Recently,

the IrDA (infrared data association) interface has been

introduced for connecting a computer and PDA to periph-

eral devices. In this interface, the connection cable and the

I/O device are eliminated by using an infrared ray radiated

from a light-emitting diode. In this way users are freed from

cumbersome equipment connections. IrDA is expected to

be widely used in the future, and will achieve a mobile

computing environment that is rich in portability and op-

erationability.

From a practical viewpoint the technical problems for

mobile computing and mobile multimedia are as follows.

(1) Realization of a long-life battery for the hand-

held information terminal.

(2) Improvement in portability by reducing the size

and the weight of the handheld information terminal.

Another future trend lies in the integration of infor-

mation terminals with handheld communication equip-

ment. In other words, there will be a mainstream addition

of handheld communication functions to information ter-

minals. With this function, it will become possible for a user

to exchange information with a counterpart without consid-

ering the connection or the operation of a handheld termi-

nal. The advent of a PHS handheld terminal with a

large-scale liquid crystal display and the PC card mecha-

nism suggests a future direction based on handheld termi-

nals.

3. Reduction of Power Consumption by

System Techniques

In this section we discuss system techniques for

reducing handheld terminals. The reduction of power con-

sumption by system techniques is considered to be the

reduction of a required function and the operation time, by

elaborating the system specifications, such as multiple con-

nections and signal frame configuration (intermittent op-

eration, and so on), the modulation/demodulation method

(compromise among the modulation method, the spectrum

and the nonlinear operation-HPA efficiency), the speech

coding/data transmission scheme, the circuit design and the

transmitter power, and the control method.

The battery saving technique that should be consid-

ered first, from the viewpoint of the system configuration,

involves separating the mode in the communication phase

to a standby mode, prior to starting communication for

information exchange, and a talk mode, after starting the

communication. The techniques used for battery saving are;

to completely stop all operations with unnecessary circuits

during basic operation timings, to reduce the required trans-

mission time for the wireless signal, and to reduce the

required transmitter power for the wireless signal.

44

3.1. Battery saving technique for standby

mode (intermittent reception)

In the standby mode before starting the exchange of

information, an intermittent reception technique is applied

in order to save the power consumption in the handheld

terminal. This is a technique in which the terminal waits for

the reception signal with a very long interval compared to

the ordinary communication state. It is effective in greatly

reducing power consumption in the standby mode. The

handheld terminal establishes reception synchronization to

the control signal from the base station at the time that

supply and access to the system is switched. The standby

reception operation only for the required timing, is set based

on the control and other signals. The terminal then enters

the intermittent reception state in which the reception op-

eration is executed only for the set timing.

In the case of PHS, a typical communication frame

(i.e., the burst transmission/reception period) is 5 ms, but

reception of the control channel is executed in the standby

mode in the superframe period (240 frames) of 1.2 s. Figure

6 shows the configuration of the superframe as well as the

operation time chart of the main blocks of the handheld

terminal. In the sleep period (the period in which no signal

is received) during intermittent operation, all operations are

stopped except for a frame counter to maintain the synchro-

nization of the superframe and the circuits concerned with

the man-machine interface for the origination operation.

In this period, a highly precise bitwise synchroniza-

tion control is not needed and power consumption in the

sleep period can be reduced by switching the frame counter

to a low-speed clock. The main CPU required for the call

control process, and so on can also be essentially stopped.

Those circuits that are stopped in the sleep period can be

started again by the superframe synchronization counter or

the circuit concerned with the man-machine interface.

The superframe period in the standby mode is set by

considering the reception success probability in standby

mode and the allowable call-setup time. The average call-

setup time is given by

where Pmiss is the probability for reception failure, and TFis the super-frame period.

The average power consumption in the intermittent

reception state is given by

where Ptotal is the average power consumption in intermit-

tent operation, Pa is the power consumption in reception

operation, Pb is the power consumption in sleep state, and

TR is the handheld terminal operation time for signal recep-

tion.

Using Eqs. (1) and (2), as well as typical parameters

for a PHS handheld terminal, the relation between the

average call-setup time and the standby time can be calcu-

lated. Figure 7 shows the result. It can be seen that the

standby time of a handheld terminal can be increased in

general by allowing a longer call-setup time, but the effect

of the call-setup on the standby time is diminished when

TR is decreased.

Figure 8 shows an example of the power consumption

measurements made for a PHS handheld terminal in the

standby mode. With the intermittent operation, power con-

sumption in the standby mode is greatly reduced, compared

to the communication state for speech and other signals. In

the case of PHS, a standby time 20 to 100 times longer than

the speech time can be achieved.

3.2. Battery saving techniques in the

communication state

(1) Operation in TDMA scheme

In a system based on TDMA, it is possible to disable

part of the circuit during the period when the terminal is not

actually transmitting or receiving a signal. The synchroni-

(2)

Fig. 6. Super-frame structure and operation time chart

of main blocks.

(1)

45

zation part, such as the frame counter, continues to maintain

frame synchronization. When only a wireless signal is sent,

the receiver part, i.e., the receiver RF circuits, such as LNA

(low-noise amplifier) and the down-converter, as well as the

demodulator, can also be stopped. Conversely, when only

the wireless signal is received, the transmitter part, i.e., the

transmitter RF circuits, such as the up-converter and the

modulator, can be stopped.

Figure 9 shows the frame configuration and the op-

eration time chart for the main blocks of a TDMA-TDD

PHS handheld terminal while in the talk mode. In those

parts executing relatively high-speed processings, the op-

eration chart must allow for the build-up time for stabiliza-

tion required from a stop to the start of the circuit. For

example, the synthesizer for the up- and down-converter

contained in the transmitter/receiver RF circuit requires a

longer acquisition time when starting the operation, and a

corresponding pre-operation control is required. The re-

quired acquisition time depends on whether the frequency

is fixed or hopped in the operation. Figure 10 is an example

of the measurements made for power consumption in a PHS

handheld terminal operating in talk mode.

(2) Battery saving by reducing the transmission

time

The required power consumption can be reduced by

reducing the transmission time for a wireless signal. Exam-

ples of battery saving are described below, where the trans-

mission time for a wireless signal is reduced considering

the kind and the properties of the information being trans-

mitted.

(i) Half-rate transmission

In the case of the speech communication, the opera-

tion time required for transmission/reception depends on

the required transmission rate (bit-rate) of the voice codec.

In the case of a digital handy phone, for example, a half-rate

codec of 5.6 kbit/s is employed, in contrast to the full-rate

codec of 11.2 kbit/s at the initial stage. Using this refine-

ment, the system capacity can be doubled. In addition, the

required operation time of a handheld terminal in the talk

mode is halved, which helps to reduce the power consump-

Fig. 8. An example of power consumption

characteristic in stand-by mode (PHS).

Fig. 9. Operation time chart for the main blocks in

talking mode.

Fig. 10. An example of power consumption

characteristic in talking mode (PHS).

Fig. 7. Stand-by time vs. average call-setup time.

46

tion. Figure 11 shows an example of the operation time in

the full- and the half-rate codecs. In the half-rate voice

codec, there are several crucial issues such as voice fidelity,

as well as the reduction in power consumption of the voice

codec itself by suppressing the required number of opera-

tion steps.

(ii) Effect of VOX processing

In the voice communication, there are cases where

the transmission of a wireless signal can be stopped while

the speaker is not actually speaking (voiceless period). This

stop of the wireless signal is called VOX (voice-operated

transmission) processing, or voice activation. This scheme

is used in both CDMA and TDMA. In the system, when

transmission is stopped, during the voiceless period, system

capacity increases in the sense that interference with other

communication channels is reduced. At the same time, there

is a battery saving effect, due to reduction in the operation

rate of the transmitter circuits, including the high-power

amplifier.

Figure 12 shows an example of the operation time

under VOX processing. Several points must be considered

in VOX processing, such as the prevention of an unnatural

feeling due to the omission of the head of the speech

(protection by discriminating the voiced/unvoiced period),

and the prevention of unnaturalness in the voiceless period

(such as an increase of background noise).

(3) Battery saving by transmitter power control

In wireless communication using portable equip-

ment, the transmitter power needed to satisfy the required

channel quality differs, depending on the distance to the

counterpart base station or mobile equipment. By control-

ling the transmitter power to an optimal value (required

minimum), interference with other communication chan-

nels can be reduced, and power consumption in the high-

power amplifier can be reduced, enhancing the battery

saving effect. In the CDMA system, where the different

channels share the same frequency and time period by using

code division, power control is necessary. In other words, a

battery saving effect is essentially included.

3.3. Access scheme and power consumption

reduction

TDMA and CDMA are considered to be promising

as access schemes for future handheld terminals. As was

already discussed, the transmitter power can be reduced in

CDMA by VOX (voice activation) and transmitter power

control. In this case, however, high-speed signal processing

such as the spreading/despreading is required, which tends

to increase power consumption in transmitter/receiver cir-

cuits. In TDMA, on the other hand, signal processing

requires a high-speed clock, corresponding to the multiplic-

ity. Power can be reduced by using a sleep operation, as with

CDMA. Power consumption can be reduced by VOX and

transmitter power control, as with CDMA.

4. Reduction in Power Consumption by

Hardware Techniques

This section discusses reductions in power consump-

tion of handheld terminals by means of hardware tech-

niques. These techniques are largely of two types.

(I) Power consumption reduction achieved by com-

posing circuits at a function level.

(II) Power consumption reduction achieved by im-

proving device performance.

In (I) power consumption is reduced in the circuit

design stage when realizing the functional specifications

for a handheld terminal. In other words, even if the same

function is realized, power consumption can be reduced by

choice of the circuit used. In (II), on the other hand, power

consumption is reduced by a choice of the function and the

performance of the device when designing the circuit con-

figuration. We discuss here only power consumption reduc-

tion techniques of type (I).

Fig. 11. Operation time chart with full/half rate

transmission.

Fig. 12. VOX operation.

47

4.1. Power consumption reduction by circuit

configuration at the function level

A PHS handheld terminal is used as an example in

the following discussion. Figure 13 is a block diagram that

shows the internal structure of a handheld terminal, sepa-

rated into functional blocks. Figure 14 shows an estimate

of the power consumption for each functional block (talk

mode). As can be seen from Fig. 13, the handheld terminal

is composed of the functional blocks, such as the RF (radio

frequency)/IF (intermediate frequency) block, the base-

band processing block, MMI (man-machine interface)

block, and the supply block.

Power consumption reduction by circuit configura-

tion at function level suggests that power consumption is

reduced by optimally selecting the circuit configuration or

the signal processing procedure when designing the func-

tion required for each functional block. Optimization of the

circuit configuration is essentially based on the following

points.

(i) Selection of a circuit configuration that mini-

mizes the required number of components.

(ii) Choice between the analog and digital signal

processing.

(iii) Selection between software processing by

CPU (central processing unit) or DSP (digital signal proc-

essor), and hardware processing by the individual circuits.

We use the composition of the baseband block and the

modulator/demodulator in the RF/IF block as examples of

applying (i) to (iii).

(1) Modulator

It is important in the composition of the modulator to

select a circuit configuration that minimizes the required

number of components and the division of functional roles

between analog and digital signal processing. Figure 15

shows a typical example of the modulator configuration [7].

In (a), processes up to the generation of the orthogonal

baseband signals, composed of a pair of band-limited in-

phase channel and quadrature channel, are executed by

Fig. 13. Block diagram of a PHS handheld terminal.

Fig. 14. Power consumption budget for a PHS handheld

terminal.

Fig. 15. Comparison of modulator configurations.

48

digital signal processing. Then, RF direct modulation is

applied by the orthogonal modulation of the carrier in the

RF band by the orthogonal baseband signal after D-A

conversion. IF modulation is shown in (b), where the or-

thogonal baseband signal is similarly generated by the

digital signal processing. After the D-A conversion, the

carrier in the IF band is orthogonal modulated, and (c) is

the case of the baseband modulation, where the processes

up to the generation of the orthogonal baseband signal and

the orthogonal modulation are executed by digital signal

processing, and the modulation signal after the D-A con-

version is frequency converted to the RF band.

Table 2 shows the number of components and the

operation frequency for each configuration. In RF direct

modulation, the circuit can be constructed with the smallest

number of components. At present, however, it is difficult

to realize a highly accurate 90 deg. phase shifter in the RF

band (especially, for the high-frequency band of several

GHz used in PHS). This creates the problem that large

power is required to realize such a phase shifter. This

problem does not arise in IF modulation and the baseband

modulation.

The differences between those two modulation

schemes are in operation speed and circuit scale of the

digital signal processing block, as well as the number of

D-A converters and mixers. As is estimated from the calcu-

lation in [7], power consumption in the digital signal proc-

essing block affects the whole system less. Consequently,

the baseband modulation configuration is the most advan-

tageous from the viewpoint of power consumption, since

the number of D-A converters and mixers is smallest.

Baseband modulation also has an advantage in that adjust-

ments for amplitude and offset of the orthogonal baseband

signal, as well as the phase adjustment of the 90 deg. phase

shifter, are eliminated.

(2) Demodulator

The demodulator must have a circuit configuration

corresponding to the modulator. In general, the demodula-

tor requires more complex processes than the modulator.

Thus, the application of the software signal processing by

DSP can offer a promising choice, depending on the trans-

mission rate. Figure 16 shows a typical configuration for

the demodulator. A configuration in which the RF band

signal is directly converted to the baseband is not consid-

ered in this paper, since it is very disadvantageous at the

present stage from the viewpoint of power consumption and

the realization of amplitude control.

Figure 16 shows three configurations that differ in the

method used to digitize the received modulated signal,

input as an analog waveform, for digital signal processing.

Table 2. Required number of components vs. modulator configuration

D/A MIX 90° divider SYNTH OSC

RF direct modulation 2 (768 kHz) 2 (1.9 GHz) 1 (1.9 GHz) 1 (1.9 GHz) 0

IF modulation 2 (768 kHz) 3 (200 MHz ´ 2)

(1.9 GHz ´ 1)

1 (200 MHz) 1 (1.7 GHz) 1 (200 MHz)

Baseband modulation 1 (1.92 MHz) 2 (200 MHz ´ 2)

(1.9 GHz ´ 1)

0 1 (1.7 GHz) 1 (200 MHz)

Fig. 16. Comparison of demodulator configurations.

49

Figure 16(a) shows the method in which the IF signal is

frequency-converted to baseband by orthogonal carrier sig-

nals with nearly the same frequency as the input (quasi-co-

herent detection), and is then processed after the A-D

conversion. Figure 16(b) shows the method in which the IF

signal is quantized to 1 bit by a converter, and the resulting

signal is quasi-coherent detected by digital signal process-

ing [8]. In this method, the A-D converter and the analog

orthogonal detector can be eliminated. However, the oper-

ating clock frequency of the pulse counter must be raised

to about 100 times the symbol frequency.

Figure 16(c) shows the method in which the IF signal

is quantized to 1 bit as in (b), and the digital phase informa-

tion is directly obtained by phase comparison using a

high-speed pulse [9]. In this method also, the A-D converter

can be eliminated and the operating clock frequency of the

phase comparator must be raised above the quantization

level of the digital phase information.

Among these configurations, the circuit configura-

tion that is best suited to low power consumption depends

on the bit-rate of the modulated signal, the quantization

steps, and the available devices. For the modulation speed

of PHS and PDC, configurations (b) or (c) are advanta-

geous.

In order to reduce power consumption in the digital

block of the demodulator, it is very effective to use process-

ing based on the phase information. Figure 17 compares the

circuit scale when the same function (delayed differential

detector in this case) is achieved in two ways; using com-

plex number operations on orthogonal baseband signals,

and using the scalor operations with the phase information.

In the case of the complex number operations, 4 multipliers,

2 adders, and 2 D-latches are required. In the scalor opera-

tion, an adder and a D-latch are sufficient. It is seen that for

8-bit operations, the latter circuit scale (the number of

gates) is about 1/27 of the former.

The demodulator configuration utilizing phase infor-

mation is discussed in Refs. 10�12. In order to apply these

methods, it is necessary to extract the digital phase infor-

mation from the received signal. Using the configuration of

Fig. 16(c), the digital phase information can be obtained

directly. In the configuration of Fig. 16(b), the digital

information can easily be extracted by applying the arc-tan-

gent function to the orthogonal baseband signals (for this

purpose, a conversion using a ROM table is generally used).

(3) Relation between system parameters and power

consumption of handheld terminal

Figure 18 shows the effects of the techniques and

parameters discussed up to this point on the power con-

sumption of a handheld terminal. The figure shows an

estimate of the power consumption in the PHS handheld

terminal shown in Table 2, when the time reduction ratio

RS of the synthesizer acquisition time, the voice period ratio

in VOX RV and the reduction ratio of the codec RC are

specified independently. The figure does not include the

increase in power consumption of the synthesizer and the

codec due to high-speed pull-in of the synthesizer and the

Fig. 17. Comparison of differential detector

configurations.

Fig. 18. Reduction of power consumption using power

saving methods in talking mode.

50

reduction of the codec rate. In other words only the effect

due to the block reduction operation is evaluated.

The reason that the various reduction effects depend

on the technique used is mainly due to the difference

between blocks that can be stopped in various methods.

Among these, the effect of the codec rate reduction is

largest, since the operation ratio of the codec can be reduced

both in transmission and reception. In the case of VOX, the

down-link signal must be received even during a voiceless

period. In PHS, the burst transmission is required once in

four frames, which suppresses the effectiveness, compared

to the case where the codec rate is reduced.

5. Conclusions

This paper has discussed reduction in the power

consumption of handheld terminals from the viewpoint of

the general trends, as well as the system and the hardware

configurations. PHS is especially considered, and the tech-

nical elaborations and their effectiveness are discussed. In

future it is expected that handheld terminals will integrate

telephone and computer terminals, resulting in more so-

phisticated functions.

Power consumption will increase as the functions

become more sophisticated. Thus, we can expect two dif-

ferent approaches; an approach that retains the single func-

tion and aims to reduce size and power consumption, and

an approach that attempts to realize composite and sophis-

ticated functions. In either case, reduction of power con-

sumption is a problem for posterity, together with the

reduction in size and weight. Progress will be made by a

collaboration of system configuration design and hardware

techniques, as is discussed in this paper. Further technical

development of this kind is desired for the future.

REFERENCES

1. Miyatsu (ed.). Personal Communication System and

Its Development. Sci. Press (1996).

2. RCR. Standard specifications for digital car tele-

phone systems. RCR STD-27B (April 1991).

3. ETSI. European digital cellular telecommunication

system (Phase 2): General description of a GSM

public land mobile network (PLMN). ETR-099

GSM01.02 (Oct. 1995).

4. T. Hattori et al. Personal communication�Concept

and architecture. Proc. IEEE ICC�90, pp. 1351�1357

(May).

5. RCR. Standard specifications for second-generation

cordless phone system. RCR STD-28 (Dec. 1993).

6. TIA/EIA. Mobile station-base station compatibility

standard for dual-mode wideband spread spectrum

cellular system. TIA/EIA/IS-95 (July 1993).

7. T. Sakata, K. Seki, S. Kubota, and S. Kato. A new

fully-digitalized p/4-shift QPSK modulator for per-

sonal communication terminals. I.E.I.C.E. Trans.

Commun., E77-B, No. 7, pp. 921�926 (July 1994).

8. Y. Matsumoto, S. Kubota, and S. Kato. A low power

consumption LSI for personal communication�

High-performance coherent detection demodulator.

Tech. Rep. I.E.I.C.E.J., RCS 94-9 (May 1994).

9. S. Saito, H. Yamamoto, and Y. Yamao. Totally digital

ACT coherent detector circuit. Tech. Rep.

I.E.I.C.E.J., RCS 89-64 (March 1990).

10. Y. Matsumoto, S. Kubota, and S. Kato. A new burst

coherent demodulator for microcellular

TDMA/TDD systems. IEICE Trans. Commun., E77-

B, No. 7, pp. 927�933 (July 1994).

11. Y. Yamamoto, K. Kunieda, K. Takahashi, and H.

Ohnishi. Configuration and performance of 384 kbps

p/4-shift QPSK burst demodulator. Tech. Rep.

I.E.I.C.E.J., RCS 92-100 (Jan. 1993).

12. H. Furukawa et al. A p/4-shifted DQPSK demodula-

tor for personal mobile communications system.

PIMRC�92, pp. 618�622 (Oct.).

51

AUTHORS (from left to right)

Takeshi Hattori (member) graduated in 1969 from Dept. Electrical Eng., Fac. Eng., Univ. Tokyo. Completed Master�s

Program in 1971 and Doctor�s Program in 1974 Grad. Sch. Affiliated with NTT Yokosuka Elect. Comm. Lab. Engaged from

1974 to 1984 in R&D of 800 MHz car telephone system, large-capacity mobile communication system, new cordless phone

system, high-speed wireless call system and new marine communication system. Engaged from 1984 to 1986 in R&D Center

in SE in research planning and development strategy. Planning Director 1991 Wireless System Lab. Res. Director 1992�1996

Personal Comm. Lab. Since 1990, engaged in R&D promotion in personal communication system such as PHS. Encour. Award

IEICEJ, 1979. Paper Award 1981 IEEE VTS. President Award 1984 Inv. Adv. Soc. Member, Imag. Elect. Soc., IEEE

Communication Society, IEEE Vehicular Technology Society. Authors of �Car phone system� (coauthor), �Basis of mobile

communication� (coauthor), �Personal communication technology� (coauthor), �Personal communication system and its

development� (coauthors) and other books. Presently, with Dept. Elect. Elect., Fac. Sci. Eng., Jochi Univ.

Shigeaki Ogose (member) graduated in 1975 from Dept. Electronic Eng., Fac. Eng., Hiroshima Univ. Completed Master�s

Program in 1977 Grad. Sch. Dr. Eng. 1986 (Kyoto Univ.). Affiliated with NTT in 1977. Engaged mostly in R&D of personal

communication technology, such as digital mobile communication, high-speed paging and the second-generation cordless phone

(PHS). Presently, Senior Research Engineer, NTT Wireless Systems Lab. Gr. Leader. Dr. Eng. Member, IEEE.

Shuji Kubota (member) graduated in 1980 from Dept. Radio Comm., Fac. Elect. Comm., Univ. Electrcomm. Dr. ( Osaka

Univ.) in 1995. Affiliated with NTT in 1980. Engaged mostly in research and practical applications of modulation/demodulation

methods in satellite communication and personal communication, as well as error-correction system. Visiting Researcher 1991

Univ. Calif. Davis. Presently, Head of R&D Prom. Section. Dr. Eng. Paper Award IEICEJ 1986. Shinohara Encour. Award 1991.

Member, IEEE.

Kiyoshi Kobayashi (member) graduated in 1987 from Dept. Electrical Eng., Fac. Eng., Sci. Univ. Tokyo. Completed

Master�s Program in 1989 Grad. Sch., and affiliated with NTT Wireless Systems Lab. Engaged in researches of modulation/de-

modulation methods in satellite communication, satellite mobile communication and personal communication, as well as

diversity system and TDMA synchronization control method. Presently, Res. Engineer, NTT Wireless Systems Lab. Member,

IEEE.

52