Paging Indicator in Fading Environment 01264288

8/9/2019 Paging Indicator in Fading Environment 01264288

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The 14* IEEE 2003 International Symposium on Persona1,lndoor and Mobile Radio Communication Proceedings

Performance of WCDMA Downlink Access and Paging Indicators in

Multipath Rayleigh Fading Channels

V i er i V angh i and S and i p S a r ka r

QUALCOMM Incorporated,

5775 Morehouse Drive, San Diego, CA

92 12 1

E-mai

I: {

vvanghi,ssarkar] 6ilqualcomm..com

Abstract - n WCDM.4 [ I ] , downlink channel indicators are

iised in (I varieiy ojprocedirr-es related to access

arid

paging. 4

binar,v

indicator is sent

on

the Paging Indicator Channel PICH)

when n

iiiobile

station (MS)

is

paged.

A

ternary indicator is sent on

the .Access

Indicator Channel AICH) when

the

MS attempts to access

(he i i e n v o i k While iinderlyirig inotivations for iising paging and

nccess indicators nre ddferent, t1ie.v both share same key

perjorniuiice nspects: detection

reliabilit?

at the cel l edge and

dowiilit ik capacity consiiinption. This paper details a finmework for

PICI-I ai ic l ICH p e

forniance

antilysis.

Results

are derived that are

of

pructicul interesl to the network operrrtor f o r downlink transinit

power b

id,yet ng.

1 Introduction

I n

UMTS radio access networks (UTRAN), the use of

indicator channels is key to the enhanced performance of the

paging and access procedures [ ] - in particular, for mobile

stations, stand-by time and uplink random access latency.

MS

stand-by time increases when battery consumption in idle

state is reduced. When idle, the M S must perform periodic

supervision procedures that require powering on its circuitry,

e.g., cell reselection, monitoring control channels to obtain

updated overhead information, and monitoring paging channel

to receive a call. When performing these periodic tasks, the

M S is awake and its circuitry is partly enabled.

In

between

such periods, the

M S

goes asleep and most of its circuitry is

disabled. When awake, the current drawn is significantly

larger (100 times or larger) than when asleep, so that it is

desirable to keep the awake period to

a

minimum. One

method is the use of paging indicators. The paging indicators

are binary and are sent periodically once per slot cycle on the

paging indicator channel

(PICH).

If set to

ON,

the MS is to

demodulate the next paging channel slot. Otherwise, the MS

can immediately go to sleep, dramatically reducing its battery

consumption [2].

Indicators are also used to support access procedures. When

attempting to access the radio network, the MS sends probes

on the uplink. Within each probe interval, the M S waits for

an

acknowledgment from the serving cell. If the acknowledgment

is received within a predetermined timeout period, the

M S

proceeds with sending the Layer

3

message on the random

access channel, otherwise it transmits another probe at higher

power. If the probe and the acknowledgment were based on

Layer

3

signaling, the access latency may be large

as a

result

of message processing, queuing, and transmission times.

Obviously. the access time can be reduced if the probe

duration and the base station

(BS)

turnaround time

in

sending

the acknowledgment are reduced. The solution adopted by

UTRAN is the use of access channel indicators that are sent on

the access indicator channel (AICH) by the

BS

in response to

a probe detected on the uplink. The indicators are very short in

duration and are sent within a few milliseconds of the receipt

of the probe, resulting in very fast uplink access protocol.

The caveat is that these channels must be transmitted at

a

rather high power, as the BS does not know the MS location

and no mechanism for open loop power control exists. The

transinit power must be set to

a

level high enough to guarantee

re1 iable detection at the cell edg e, thereb y consuming

downlink capacity.

The scope of this paper is to estimate the detection

performance of the paging and ac cess indicators in fading

channels and to assess the transmit power needed for these

channels. The paper is organized

as

follows. In Section 2 the

structure o f the PICH and the A ICH are outlined.

In

Section

3,

the received signal to noise ratio required to achieve target

probabilities of false alarm and detection are computed.

Section 4 provides a transmit power budget for the PICH and

AICH. Conclusions are drawn

in

Section 5.

2 The

Indicator Channels

This section describes the two indicator channels of our

interest used on the WCDMA downlink. For more details, the

reader is referred to

[ I ]

Note that SF means Spreading Factor,

i.e. the number of chips per bit.

2.1

The PICH

is

a fixed rate

(SF

= 256) physical channel used to

carry paging indicators (PI). Figure illustrates the structure

of the PICH. One IO-ms PlCH frame consists of

300

bits (bo,

b , , ... b299).Of these, 288 bits

bo,

b, ,

...

bZ8,) are used

to

carry paging indicators, and 12 bits are not transmitted.

The Paging Indicator Channel (PICH)

12 bi t s

( i iansmissiun

orn

88

b i ts fo r

paging

ind ica t ion

4

- -

b

bo bi h a 7 b i a a

b o

m

One

r a d io f r a m e ( I O ms)

Figure 1: Structure

of

PlCH

In each PlCH frame, N paging indicators {Po,

...,

PNp- , )are

transmitted, where Np=1 8,

36,

72, or 144. T he

PI

to be used by

a certain M S, is associated to the paging indicator P,

[ I ] .

If a

0-7803-7822-9/03/ 17.00

003

IEEE.

331 *
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EEE 2003 International Symposium on Persona1 lndoor and Mobile Radio Communication Proceedings

freedom and non-equal variances. When the paths energies are

identical,

The received indicator's

SNR

expressed as in Eq.(9) is a

central Chi-square random variable with 2L degrees of

freedom, whose distribution is known [4] and is

mathematically tractable.

r

(12)

The detector attempts to achieve a given target

PFA

by

decreasing the parameter 5 as the SNR increases, and vice-

versa. Note that the range of

5

is selected such

that qI, 5 = ) =

f/-

{ = -1) = 0.5 for any given S NR.

PICH

4

wo-palhs. Pf

= 10%

I --.-

wo-paths. Pf 20

- - - - - - - - - - - - - - _ - - - - - -

...

...

.. ,.,

i __ __

~

I

.. *..

,

F i gur e

3:

P e r f o r m a n c e

of

the

PICH

in

Fading Channels

Figure

3

illustrates the performance of the PICH

corresponding to PFA equal to 10% or

20%. In

order to,

achieve a reasonably low PM ay,

I%,

the required received

indicator's SNR is between 5.5 to

I O

dB, for the single path

and two-path channels, respectively.

3.2

The Acquisition Indicator Channel

Hereafter, w e use similar mode l as that used for the PICH, but

we account for the AICH temary decision as the AICH is

either gated-off or is BPSK modulated. The AICH filter is

matched to the signature used in the probe's preamble.

Assuming a noiseless phase reference for coherent

demodulation, the matched filter sampled outputs after de-

spreading, de-scrambling, and maxim um ratio comb ining give

rise to the same decision variable as in Eq.(l), where the

indicator

b

can now take one of three values,

+ I , -1,

or

0,

corresponding to a positive acknowledgment, negative

acknowledgment, or no transmission respectively. The

detector must select on e of three hypotheses:

HI

: r = Ebcai v

L

k=l

H , :

r = v 13)

H - ,

: r = - E ~ Z ~ :

V

A

biased MAP detector can be used that compares the

matched filter output with two thresho lds given by

z ( , , E b c a i 0

I ,,

1

z A

-( ,E, ,cai 0I

/

1

Like the PICH, the AICH detection threshold is proportional

to the estimated received

SNR,

which is obtained from the

estimated received CPICH energy. The detector selects

hypotheses H I

i f r > z,,, Ho i fz , 5 r

IT

r H.1 i f r

z/l16

= 01=

e 5, )

(15)

p+I/-I C A ,

rb)

= Pr[r >

b

= I] = e[&%('

{ h ) ) (16)

Th e subscript x / y stands for hypotheses x being selected when

y is the correct one. The e rror events 0/+1 and

- I / + ]

are nearly

equivalent in terms of cost (in both cases the

M S

misses a

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The

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valid oppoi-tunity to transmit the Layer 3 message) and

therefore the PM s defined as

PF,.\ can be defined a s the event +1/ 0on ly, as the cost

associated with even t -1/O is relatively small, i.e.,

f

P+O t>Y,) (22)

Com paring Eq.(2 1)-(22) with (5)-(6), the AICH detection

performance is

6

dB worse than that of the PICH. That can

also be seen by letting(,

= 4,

=

0.5

and

6 =

0,

in

which case,

er r ( 0.5.y,,)=p, 5,)

= 0 . 5 , Y , ) = Q ( m ) or the AICH,

and

e),( =

0,y,

= f 4 = 0, y , , ) =

e(&) for the PICH.

Like the PICH, the A ICH detector attempts to achieve a given

PI:-\ by varying

4,

as the SN R varies.

AICH

10 15 20

'

SNR [ d B ]

Figure

4:

Performance of the AICH

in

Fading Channels

Performance of the AICH

i s

depicted

in Figure

4 for PFA

equal to

5%

or 10%. I n order to achieve PMof 5 and PFA

=

WO,he received indicator's SNR must be

9.5

to 12 dB for the

single path and two-path channels, respectively.

4

Transmit

Power

Budg et at BS

The indicator channels need to be transmitted at a constant

power level such that they can be reliably demodulated at the

cell edge. Let i e power spectral density received at the

M S

from the cell where the M S is located. The interference power

spectral density due other cells' interference is denoted by

1

.

The ratio

i,,,

I,, is referred to as cell geometry

[ 5 ] .

The

indicator channel (either PICH or AIC H) received SN R, x s

related to the CPICH chip transmit en ergy, E, ; he total cell's

transmit power spectral density,

I,,,

; he cell geometry and the

processing gain, G,, as

in

Y*

= PG,,

l o r

(23)

where pis the indicator channel transmit power relative to the

common pilot power, or indicator channel's offset.

Note

that

the ratio CPlCH E, / l , )?s a measu re of downlin k cell loading.

The ratio is at its minimum, typically -10 dB, for a fully

loaded cell. Solving for the required indicator c hannel's offset,

p = y ,

/ '+I--

G , (24)

,,r ;)

cpl;,H 1

and

CPICH

Combining

Eq.(23)

with

(25)

and solving for

b,

P =

(25)

Eq.(26)

represents the transmit power budget used to

configure the cell. It represents the minimum required

indicator channel's offset to achieve the target SNR with a

confidence level represented by the minimum

received CPICH( E,

/ I , ,

) typically expected at the cell edge.

4.1 The Paging Indicator Channel

The PICH processing gain depends on the number of paging

indicators per fram e, N,,, the paging indicator bit rate,

Rl, =

30

kbps, the spreading chip

rate, R,,

as in

R,

16.18

G =--

I Rh N,'

Using the results from Section 3.1, the required PICH offset

resulting from Eq.(26), is plotted i n Figure 5.

s,

CPlCH

EcIlor

= -10

dB.

Eb/Nt = 5.5

dB

s , CPlCH Ec/lor= -4 d B, Eb/Nt = 5 5 dB

~-

.- . .

- -

. . .

8 -

,

7

-

- 1 0 -

-12

-14

Figure 5: PICH offset vs. CPICH E f l o for N , = 18

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The two-path channel is more benign than the single path

channel. as the diversity gain outweighs the partial

loss

of

orthogonality among downlink code channels. For a

given C131~1- 1 ( ~c/,,), a smaller offset is required for a fully

loaded cell than for a lightly loaded cell. Setting the PlCH

offset to -7 dB allows the BS to obtain same coverage

provided that N,,=

18

andC PiCI-I(L '//,) is no less than

approximately -16 dB. When N,, is varied. the required PICH

offset changes linearly i n dB's.

Results i n Figure

5

show that reliable detection cannot be

guaranteed for very low cell geometry unless a

disproportionate amount of power is allocated to the indicator

channel. The WCDMA standard prescribes that the MS

demodulate the common pilot as weak as -20 dB, but that

would require an indicator's offset equal to -3 d B (more than

5 of total downlink ca pacity). A practical solution is to

'erase' the indicator channel bit whenever the common pilot,

strength falls below a threshold [6]. When the PlCH is erased,

the M S always demodulates the assigned paging channel slot,

thus negating the battery power saving advantages provided

by the PICH. But this i s preferable to missing valid page

messages. The optimum setting of the erasure threshold,

A,,,,,,,,.

can be derived by solving Eq.(26) for the minimum

ci'lcl-l(E~//(,):

'

I ) L

Although the MS is unaware of the cell loading, the error

caused by the approximation on the right hand side of

Eq.(28)

is only a fraction of a dB. Eq.(28) is also of practical use, as

the resulting erasure threshold does not depend on the

multipath profile.

4.2 Acquisition Indicator Channel

Using similar analysis like the PICH, using

Eq.(26)

the results

for the required AICH offset ar e plotted

in Figure 6 .

I 1

m a t h ,

CPICH Ec/lor

= -10 dB.

E b / h

12 dB

I - pat h .

CPICH Ec/lor =

B,

Eb/Nt

= 12 dB

2-paths,

CPICH Ecilor =

-10dB,

Eb/Nt =

9

5

dB

2-paths, CPICH Ec/lor = B. Eb/Nt = 9.5 dB

- - ~ - /- ~ - - - - - .. . l-

- _ ~ _ _

- 2 ,

.

c j

i

- 4

~ _ _ _ _ _ ~ _

-6;.

.. I 1 I I

CPICH Ec/lo [ d B]

I n case of weak coininon pilot, the AlCH can also be erased.

When the AlCH is erased, the MS should proceed with

transmission of another preamble unless the preamble is the

last

in

the sequence, in which case the MS proceeds with

AlCH detection. The AlCH erasure threshold can be

determined using Eq.(28) The processing gain for the AlCH

is 36.12 dB as there are 4,096 chips per AICH slot. That IS

exactly 3 dB larger than the processing gain of the PlCH

operated with N,, = 18. The AICH detection performance is 6

d B worse than that of the PICH as the decision is ternary

rather than binary. However, the AICH can tolerate a slightly

larger probability of erro r, as the cost incurred is smaller than

that for the PICH. Then, differences above tend to mutually

cancel and the required A lCH offset is similar to that of the

PICH. Unlike the PICH. however, the AlCH is operated in

DTX mode, and therefore its contribution to downlink

capacity consumption is smaller. A nominal AlCH offset

value could then be -6 dB.

I

5

Conclusions

Typically, the PICH is operated to achieve

PM

=

1

and PI =

10%. This can be obtained when the indicat6r's received SNR

is I O dB for

a

single path, and 5.5 dB f or two' equal strength

paths respectively. Perform ance o f an ideal AICH detecto r,

due to its ternary symbol source, is 6 dB worse than that of the

PICH. Typical performance targets are Pbl =

5%

and PTA=

5 . This can be achieved when the indicator's received

SN R

is 12 dB for a single path, and 9.5 dB for two equal strength

paths respectively.

Based on the above, the operator should budget the cell 's

downlink transmit pow er by setting the offset

of

the PlCH and

the AlCH to be -7 dB and -6 dB relative

to

the common pilot

respectively. This provides adequate performance at or near

the cell edge, provided the received corninon pilot strength IS

no less than -16 dB, corresponding to most of the cell area for

a typical network layout [7]. When the pilot

IS

weaker than

that threshold, the

M S

i s aware that it cannot meet the

reliability requirement and therefore inay disregard, or 'era se',

the indicator. The

M S

can determine the erasure threshold as a

function of known UTRAN configuration parameters.

. ,

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1.

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hdio Access

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Channel

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of

transpor t cllaniiels. . I990

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vol. 48.

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6, pp 938-950, June 2000.

S. Sarkar, B. K . Butler and E. G. Tiedemann Jr . , Phone Standby

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cdma2000: Tlie Quick Paging Cliannel

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J. G. Proakis, Digital Cortirllzn?icat~or?s.

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G .

Tiedemann

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Generation Wireless Systems . Journal

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G . Tiedemanp Jr.. The Paging

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Figure 6:

AICH

offset vs.

CPICH

Ec/Io

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Paging Indicator in Fading Environment 01264288