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Using RF power detector in mobile stations

Posted: 01 Oct 2004     Print Version  Bookmark and Share

Keywords:POWER 

/ARTICLES/2004OCT/B/2004OCT01_RFD_POW_TA.pdf

Figure 2: Transmit radio architecture can reduce DC power consumption of the power amplifier.

By Barry Yuen

RF System Engineer

National Semiconductor Corp.

Since the commercialization of

CDMA IS-95 cellular network

in 1996, CDMA has been

proven to be the best wireless

technology for the advancing

cellularandpersonalcommuni-

cationsindustry.Alltheprevail-

ing 3G standards--cdma2000,

W-CDMA and TD-SCDMA--

have adopted CDMA as their

access method. The CDMA

methodisbasedonspreadspec-

trum modulation techniques

with a principle based on the

Shannon information theory.

The Shannon capacity law

states that the channel capacity

in an additive white Gaussian

noiseis:Csh=BRF*log2(1+SNR),

where Csh is the channel trans-

mission capacity in bits per sec-

ondandBRF channelbandwidth

in hertz.

To achieve the same capac-

ity Csh, systems with a wider

bandwidth require a lower SNR

than narrowband systems.

Meanwhile, a high SNR will

have a higher transmission ca-

pacityinthechannelforagiven

bandwidth BRF. This means you

can have more users if all the

users transmit the same

amount of data.

Power control

Since any mobile station is con-

sidered to be an interferer to

other users within the same

base station coverage and

neighbor cells, transmit power

control of the mobile station

becomes extremely important

for good performance and

higher system capacity in

CDMA operation. The greater

the number of active mobile

stations, the higher the inter-

ference level within the system.

Therefore, the lower the mo-

bile station transmission

power, the lower the total inter-

ference--for example, the

higher SNR within the RF car-

rier bandwidth BRF, the higher

the system capacity Csh.

The main idea of power con-

trolschemesistosetmobilesta-

tion transmission power to a

minimum level so that the base

station receives signals from all

mobile stations with a similar

level. Under this condition, the

SNR at the base station input is:

SNR =

1

M-1

where M is the total number of

users for this base station.

The main benefits of power

control in the CDMA are in-

crease of system capacity, mini-

mization of near-fare effect and

reduction of mobile station

power consumption.

The power control feature

manages the interference level

inthereverselinkbyestimating

the best transmission power

levelandreactingtopowercon-

trol directions sent by the net-

work or base station.

In CDMA IS-95 and cdma-

20001X,thebasestationdeter-

mines the power control; while

in cdma2000 EV-DO, the ac-

cess terminal executes power

control. Overall, the power

control schemes in both stan-

dards are similar, employing

open-loop and closed-loop

controls.

Open-loop

This method uses power level

at the mobile station receiver

PPRX to estimate the forward-

link path loss. It then specifies

theinitialtransmitpowerPTx of

the mobile station such that

the sum of forward and reverse

link powers is kept constant by

the user terminal's choice--for

example, cellular or PCS or 3G.

PTx+PRx=constant. The PRx

iscalculatedfromEb/Io,which

is measured by mobile sta-

tion's DSP.

After the initial PTx is ob-

tained, both the mobile station

and the base station will start

the closed-loop control. Base

station transmits an error sig-

nal to the mobile station in-

structing it to increase or de-

crease power by a step depend-

ing on which CDMA standard

is implemented.

Closed-loop

This power control consists of

two processes--outer loop (only

base station) and inner loop

(mobilestationandbasestation

Using RF power detector in mobile stations

simultaneously),whichachieve

up to 800Hz of power control

rates in IS-95 and CDMA 1X.

The closed-loop power

control's main purpose is to

minimize fast-fading effects

caused by signal multipath

propagation losses based on

measurementsmadeatthebase

station.

The use of combined closed-

loop power-control processes

(outer and inner loop) may lead

to fading compensations of

20dB to 35dB within a 20ms

frame-by-frame interval within

a dynamic range of up to 80dB.

Outer closed-loop

In the outer loop, base station

specifies a target Eb/Io (from

mobile station to base station)

at the receiver for each frame

every 20ms. This Eb/Io auto-

matically reduces in steps of

0.2dB to 0.3dB, or rises up

to 3dB to 5dB if frame error

occurs.

Thewholeouterclosed-loop

process only involves the base

station.

Inner closed-loop

In the inner loop, the base sta-

tion compares the Eb/Io of the

reverse traffic channel with the

target Eb/Io every 1.25ms and

then instructs the mobile sta-

tiontoreduceorincreasetrans-

mit power so that the targeted

Eb/Iowouldbeachieved.Power

steps vary from 10.25dB to

10.5dB for cdma2000, and

11dB for CDMA IS-95. Its up-

date rate is 800bps.

Figure 1: The output power level for linear power amplifier signal chain has to be adjusted

through an automatic gain control amplifier in CDMA mobile station.

AGC Linear PA

PA

RFout

RFin

R1

Enable (H)

Coupled power to LMV225

20log{R1/(50+R1)}

Out

RFIn /EN

Logic

Enable

Vdd

GND

B1

A2

B2

A1

LMV225

Murata CES30/CE04

National Semiconductor

high detection range

CDMA RF power detector

I/I

Hardware implementation

The CDMA IS-95 requires the

MS to adjust transmit power

level in a 11dB per step every

1.25ms, while the step size for

cdma2000 could be either

10.25dB to 10.5dB. Figure 1

shows a generic output power

control for linear power ampli-

fier signal chain for a handheld

device. Since high linearity is

required for the CDMA signal,

the output power amplifier is

usuallybiasedatfixedgain.The

output power level has to be ad-

justed through a gain control

linear driver amplifier, usually

an automatic gain control

(AGC) amplifier in CDMA mo-

bile station.

It was found that the trans-

mit radio architecture in Fig-

ure 2 can reduce DC power

consumption of the power am-

plifier, thanks to the use of iso-

lators, such as Murata CE04

and CES30, and RF power de-

tector LMV225. The isolator

provides close-to-perfect 50

load for the power amplifier's

output, while the LMV225 de-

tects accurate transmit power

level so that the mobile

station's DSP can smartly set

its output power just to the

right level as required by the

base station.

In this application circuit, a

resistor is used to divert RF sig-

nalfromthemainsignalpathto

the input of LMV225. About

100pF capacitor is needed to

DC block the control signal

from entering the main signal

path. This DC blocking capaci-

tor is necessary since we do not

want the DC voltage to go into

the output of power amplifier

nor the isolator. Since an isola-

tor has been in place, it is safe

to say that most of the diverted

RF energy is from the transmit

power amplifier. Reflection en-

ergy from the antenna will be

diverted to a built-in 50 load

inside the isolator and seldom

reaches the power amplifier's

output or LMV225. Therefore,

the coupled power to LMV225

can be estimated by 20log.

Figure3showsandexplains

the test results. The power

amplifier's distortion perfor-

manceof-40dBcadjacentchan-

nel power rejection (ACPR) at

500mA supply current can be

improved to -50dBc ACPR at

450mA supply current. The

current reduction in this case is

10 percent and distortion im-

provement is about 10dB.

Now we have shown that

LMV225 together with a

CES30 isolator can deliver bet-

ter performance in terms of

DC power consumption and

RF distortion in linear PA

applications for IS-95,

W-CDMA, cdma2000 and TD-

SCDMA air interfaces. In fact,

it is necessary for a cdma2000

mobile station or access termi-

nal to have LMV225 as a trans-

mit power detector to imple-

ment the stringent inner

closed-loop power control be-

cause of uncertainties in per-

formance of the transmit sig-

nal path components.

Transmit signal path

Thetransmitradioarchitecture

on Figure 2 can be used for

many different kinds of CDMA

chipset. One of the popular so-

lutions is MSM6500/RFT6120

from Qualcomm Inc. Figure 4

depicts a recommended LMV-

225 application block diagram

for transmit power detection in

cdma2000 1X or EV-DO single-

band handheld devices. In this

transmitter architecture, the

output power to antenna is

found to be:

R F o u t = P R F T - L S AW + G PA

-Lisolator-Lduplexer

where: RFout

is the RF power

to antenna (assume 50

matched); PRFT

is the output

power from RFT transmit chip;

LSAW

istheinsertionlossofSAW

filter; GPA

is the fixed gain of

CDMA power amplifier; Lisolator

is the insertion loss of isolator;

and Lduplexer

is the insertion loss

of the duplexer.

The insertion loss of the re-

sistive power divider formed by

R1 and LMV225 is negligible

since both R1 and LMV225

have formed a high impedance

shunt load to the signal path.

We can see that LSAW, GPA,

Lisolator, Lduplexer can be consid-

ered to be constant in room

temperature. The RF power to

antenna RFout will be adjusted

by PRFT, which is controlled by

the AGC in transmit chip. In

fact, the AGC amplifier typi-

cally supports about 80dB dy-

Figure 3: Smith chart for output impedance of power amplifier.

Figure 4: Single-band cdma2000 block diagram with LMV225/MSM6500.

RFout

Coupled power to LMV225

20 log{R1/(50+R1)}

RFIN/EN

R1

50

LogicEnable

B1

B2

A2

I/I

A1

LMV225

AGC Cellular band

RF amplifier

Simplified RFT6120

MSM6500

CDMA2000

7 1x

7 1xEV-DO

GPRS class 10

CDMA

power amplifier

Tx SAW

filter

Vout

VDD

Pin

To Rx

LSAW

GPA

PPAPRFT

Lduplexer

Lisolator

CES30/CE04

isolator

Duplexer

Modulator

10dB 10dB 10dB

Figure 6: Dual-band cdma2000 block diagram with LMV225/MSM6500.

namic range as per require-

ment from IS-95 or cdma2000.

It was also discovered that

CDMA-MS works in a medium

output power most of the time,

and the power control accuracy

is more important from me-

dium power level to high out-

put power. Inaccurate high-

output power level will reduce

MS talk time and generate

more interferences to other

network users.

LMV225 is designed and op-

timized to provide the best

power detection range in a

CDMA handheld device. Its de-

tection range is shown in Fig-

ure 5a and Figure 5b. As men-

tioned, it is more critical to

have accurate power control in

themedium-outputpowerlevel

than the high-output power

level. The coupling resistor R1

should be chosen so that

LMV225 can see RF signal rep-

resenting that critical range.

Suppose the AGC is trying to

set at the high/highest gain

level so that maximum output

level from the CDMA PA can be

achieved, say 28dBm. If crest

factor of the sent RF signal at

this time is 3dB, then instanta-

neous peak power level from

the CDMA PA will be 28dBm +

3dBm = 31dBm. If this is the

maximum reference point in

which LMV225 should see and

detect (for example, 0dBm to

RFin/enable of LMV225 when

the power amplifier's instanta-

neous output power is 31dBm),

a31dBmcouplingfactorshould

beused.Itisfoundthata1.8K

resistor for R1 could make a

31dB coupling factor in this

circuit.

Additional benefits

In Figure 5b and Figure 5b,

the LMV225 has a 30dB linear-

in-dB detection range. This

characteristic has reduced

complexity in production cali-

brationprocess.Automatictest

equipment is used to collect in-

formation regarding output

powerofmobilestationvs.con-

trol code/signal from small sig-

nal to strong signal. This infor-

mationisthenstoredinthemo-

bile station memory for field

uses. Anytime an output power

level is requested by the base

station, DSP of the mobile sta-

tion will go to the memory and

find out what control code/

signal should be used to

achieve the requested output

power level.

Some of the AGC in the mar-

ket may have exponential char-

acteristics between control sig-

nal and output gain. If this is

the kind of AGC used together

with the LMV225, the linear-

in-dB characteristic will not

maketheoriginalcontrolcurve

more complicated as the origi-

nal AGC characteristic curve of

other detection methods. How-

ever, if the AGC has a linear

control range, the linear-in-dB

characteristic will reduce the

calibration points from 20+ to

2 or so. This two-point calibra-

tion process is based on the

principle that only two differ-

ent points are needed to repre-

sent a first-order linear equa-

tion in a 2D plane. If y = mx + b

is the equation, the slope m and

intercept b could be found out

by two test coordinates: x1, y1

and x2, y2.

LMV225 in dual-band

Figure 6 is a recommended

block diagram for use in

cdma2000 handheld devices.

Although resistors R1 and R1'

may not be the same, it is pos-

sible for the users to optimize

both bands performance so

that both R1 and R1' have the

same value. Meanwhile, isola-

tion between low band and

high band should be accept-

able because only one of the

power amplifiers is on in real

application and the resistor

(R1 or R1') will typically pro-

vide 30dB isolation.

Finally,alogarithmicampli-

fier RF power detector

LMV225 has been presented

as critical component in

cdma2000 power control in the

reverse link. LMV225 not only

delivers the fundamental re-

quirements specified by the

cdma2000 air interface stan-

dard, but also saves DC power

consumption, improves distor-

tion in the transmit path like

ACPR, and saves production

calibration efforts and ease-

of-use in R&D stage.

Figure 5: LMV225 typical performance in cellular bands.

0

0.5

1

1.5

2

-50 -40 -30 -20 -10 0 10

Vout(V)

1.71GHz

2.05GHz

Detected voltage at cellular bands

on LMV225

Detected Vout at DCS/PCS/UMTS bands

on LMV225

0

0.5

1

1

2

-50 -40 -30 -20 -10 0 10

RF input power (dBm)

RF input power (dBm)

Vout(V)

824MHz

915MHz

(b)

(a)

RFout

to

duplexer

Coupled power to LMV225

20 log{R/(50+R)}

R=R1 or R 1

RFIN

/EN

VDD

50

10dB10dB

LogicEnable

B1

A2

B2

I / I

A1

LMV225

10dB

Modulator

AGC Cellular band

RF amplifier

Simplified RFT6100

MSM6500

CDMA2000

- 1X

- 1xEV-DO

GPRS Class 10

Tx SAW filter

CDMA

power amplifiers

Vout

Tx SAW Filter

PCS band

Cellular band

PCS band

RF amplifier

R 1

R1

PRFT PPA

Pin

`

`





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