Middleton W.M. (ed.) Reference Data for Engineers: Radio, Electronics, Computer and Communications

46-6

REFERENCE

DATA

FOR

ENGINEERS

TABLE

2.3G HARMONIZATION AGREEMENT

Legend

FDD: Frequency Division Duplex

TDD: Time Division Duplex

CDM: Code Division Multiplexing

TDM: Time Division Multiplexing

IMT-2000

(3G)

Spread Multican-ier

1.25

MHz

1.6

MHz

Common

Pilot

Dedicated Pilot

transmitted per channel). The data link can also carry

PCM voice

(64

kb/s per voice channel). There

are

gen-

erally three means of implementing the data links:

1)

use TI-carrier wire lines;

2)

use microwave radio links;

and

3)

use optical

fibers.

In digital systems, the functional architecture and

principal interfaces

are

shown in Fig.

2.

The

HLR

(home location register) is a permanent database that

contains subscriber information and is related to the

subscriber’s current location. VLR (visitor location

register) is linking to one

or

more MSC’s temporarily

storing subscription data. OAM (operation, adminis-

tration, and maintenance) is collecting the operational

A-bis interface Radio

interface

A

Interface

(or

Air Interface)

Fig.

2.

Functional

archirectum

and

principal interfaces.

data for troubleshooting and statistical analysis for

performance. The architecture shown in Fig.

2

is

mainly for voice communication using circuit switches

which provide dedicated lines. The general packet

radio system (GPRS) is called 2.5G and is used for

packet transmission. Every packet has its sending

address for delivery. The packet switch would not hold

the line until the call termination. The packet transmis-

sion link is called the virtue connection

or

the connec-

tionless transmission.

In

3G systems, there

are

two lagacy networks, GSM

MAP and ANSI

4,

shown in Fig. 3. In order to have

network-to-network interface (NNI) between two net-

works, the standards bodies (3GPP and 3GPP2)

are

developing new standards for hooks and extensions, as

shown in Fig 3. The hooks provide the specified func-

tionality. The extensions provide the additional func-

tions needed to meet the specified requirements. Also,

in 3G system networks, two industry alliances were

formed, 3GIP and MWiF (mobile wireless

IP

forum).

These two forums

are

developing the wireless Internet

network. The

P

(Internet protocol) network can

be

the

future 3G network, which is migrated from today’s

lagacy network.

Enhancers and Converters

The enhancer is a repeater with amplification. It

operates at the same cellular bandwidth. The signal

range can be extended by the enhancer.

CELLULAR TELECOMMUNICATIONS SYSTEMS

I

L1

(WCDMA)

1

Hook

ANSI

41

(cdrna2000)

Direct Spread

Fig.

3.

Hooks

and

extensions.

If the converter changes

the

cellular frequency to a

microwave frequency, it is called a microwave con-

verter. If it converts the cellular frequency to an optical

frequency, it is called an optical converter. If it down-

converts to baseband frequency, it is called a baseband

converter. The converted signal can

be

delivered to any

destination without interference, then converted back

to the cellular frequency at the destination.

Mobile Subscribers’ Units

The mobile subscribers’ units consist of three parts:

the transceiver, the logic unit (handles signal pro-

cesses), and the control unit (indicators). The mobile

subscribers’ units can be classified into two kinds,

mobile units and handsets. Mobile units

are

mounted

in vehicles; handsets are carried by the users.

OPERATION

OF

CELLULAR

SYSTEMS

The cellular systems does not require the user to

touch any adjustment buttons during a call. The

opera-

tion can be described in seven different stages, plus a

handoff procedure.

Mobile-Unit Initialization

In analog systems, when the mobile unit is turned

on, the receiver scans the

21

setup channels, selects the

strongest, and

locks

on for a certain time. Since each

cell site is assigned a different setup channel, locking

onto

the

strongest one means choosing the nearest cell

site. This scheme is called a self-location scheme and

is used at the idle stage. Every

60

seconds, the self-

location procedure is repeated. This repetition is nec-

essary due to the motion of the vehicle. A new nearest

cell site can

be

determined after

60

seconds.

rl

ANSI

41

Fl

(WCDMA)

L3

l(cdma2000~

1

Hooks1

L3

RRC (cdrna2000)

I

L2

(cdrna2000)

L1

(cdrna2000)

Multi-Carrier

46-7

In digital systems,

For

TDMA

SystemsThe mobile station

(MS)

guides the FCCH burst, then looks

for

an SCH burst on

the same frequency to achieve synchronization. The

MS

then receives BCCH on several time slots and selects a

proper

cell, remaining for a

period

in the idle mode. The

random-access channel (RACH) is

the

only common

uplink channel that

the

MS chooses to access the calls.

For

CDMA

Systems-This allows

the

MS

to

acquire

the

timing of the forward CDMA channel,

pro-

vides a phase reference for coherent demodulation, and

provides a means for signal strength comparisons

between base stations for determining when to hand off.

Mobile-Originated Call

In

analog systems, when the user wants to make a

call, a number is placed into an originating register in

the mobile unit. When the “send” button is pushed, a

request

for

service is sent on a preselected setup chan-

nel. The receiver has to check whether the setup chan-

nel is idle

or

not by examining the “busy” or “idle” bit

in the forward setup channel. The cell site receives the

request and in turn sends a request to the

MTSO

via a

data link. The

MTSO

selects an appropriate voice

channel from a group of allocated voice channels and

notifies the cell site; then the cell site informs the

mobile unit. At the same time, the

MTSO

also con-

nects the wire-line telephone party through the tele-

phone company central office.

In digital systems, the base station receives the sig-

nal from the RACH, an assigned traffic channel to the

MS, informing

MS

through the PAGCH (paging ch.).

Network-Originated Call

In analog systems, a land-line party dials a mobile-

unit number. The telephone company recognizes the

mobile number and forwards it to the MTSO. The

MTSO then sends a paging message to certain cell

sites depending on the mobile number. Paging is

needed at the cell site because the MTSO does not

know where the mobile unit is due to the self-location

scheme being used by the mobile unit. Each cell site

transmits the paging signal on its own assigned setup

channel. The mobile unit recognizes its number on the

strongest setup channel during the idle stage and

responds to the particular cell site. The mobile unit

also follows the message to tune to an assigned voice

channel, at this time, the mobile unit alerts the user.

In

digital systems, PAGCH is used

to

page the

MS,

and the MS will respond to the PAGCH message

through RACH. As soon as the base station receives

the response, it assigns a traffic channel to MS through

PAGCH.

Ca

I

Termination

When the analog mobile user terminates a call, a

special signal tone

(10

kHz)

is transmitted to the cell

site to free the voice channel. The mobile unit resumes

its initialization stage.

In

the digital mobile system, the

functions are handled by different information bits.

Call Blocking

If during a mobile-originated call stage all the voice

channels assigned to the nearest cell site are busy, then

the analog mobile-originated call is repeated every

100

ms. After ten tries, a busy tone is turned on to inform

the user. In digital systems, the operations can be

changed dynamically depending on the demand.

Call Completion

After the mobile-originated call or network-origi-

nated call is completed, the voice channel is assigned

to make a call connection. This process is called “call

completion.”

Call Drops

During a conversation, because of interference or

weak-signal spots in certain areas, if the analog system

cell site cannot receive an assigned SAT (generated at

the cell site and sent back by the mobile unit) in

5

sec-

onds, the cell site will

turn

off the transceiver. In digi-

tal systems, no SAT tone is needed. There

are

256

digital color codes to assign

to

different cells and sec-

tors. The call drops can be reduced.

Handoff Procedure

When a mobile unit moves out of the range of one

cell and enters the range of another cell during a con-

versation on a voice channel, that voice channel has to

change from one frequency channel to another. The

system can make this handoff procedure without either

interrupting the call or alerting the user. The handoff

process is a unique function in cellular-system opera-

tion. The signal threshold level setting for a handoff

request cannot be too high, or there will be many

unnecessary handoffs. However, if the threshold level

is set too low, this will cause dropped calls.

In

digital systems, TDMA is using hard handoff

(break before make) but with a

MAHO

(mobile assisted

handoff). The level

of

signal strengths of neighboring

cells, measured by MS, will send back to the home base

station for it

to

make the handoff decision.

CDMA is using soft handoff (make before break).

In

the handoff region, two cells (one home cell, one

new cell) will simultaneously receive the

MS

signals.

Also, many other potential cells will be considered (up

to six cell fingers) by their pilot signal, to choose

dynamically the new candidate cell in the soft handoff

process.

MOBILE RADIO

ENVIRONMENT

Uniqueness

of

the Mobile

Rad io Environment

Effect

of

the Natural Terrain Configuration-

Due to the strong ground-reflected wave at the vehicle,

both the direct wave and the reflected wave will be

received. Since the ground-reflected wave has a

180”

phase shift after reflection from the ground, the two

waves tend to cancel and result in an excessive signal

loss that is dependent on the natural terrain configura-

tion.

Effect of Man-Made StructuresBecause the

mobile antenna is lower than most man-made struc-

tures, the multipath phenomenon prevails. Multipath

waves arriving at the mobile unit cause signal fading

called multipath fading or Rayleigh fading.

Effect of Man-Made Noise-Due to automobile

ignition noise on the cellular frequency, the noise floor

is much higher in the city than in a suburban area. Due

to unintentional interference from signal sources such

as government or military communications equipment

or industrial machines, the noise floor can also be dif-

ferent

in

different areas.

Effect of Vehicle Motion-Different speeds

of

the

vehicle cause different signal fading characteristics.

When the speed is high, the fading is rapid when the

speed is low, the fading becomes slow.

Also,

the dura-

tion of the fading is short when the speed is high and is

long when the speed is slow.

A Model

of

a Mobile-Radio

Path Loss

A mobile-radio signal can be artificially separated

into two parts as shown in Fig.

4.

The first part is the

radio-path propagation loss due to the terrain configu-

CELLULAR TELECOMMUNICATIONS SYSTEMS

46-9

MULTIPATH

FADING

\

\.

\

J#

#/

I

II

.

Fig.

4.

Two

parts

of

a mobile-radio environment:

(A)

propagation

loss,

(B)

multipath fading.

ration, and the second part is the multipath fading due

to the man-made structures.

data is available for that location.

This

model com-

bines the effects

of

man-made structures. which are

different

in

different cities, and the natural terrain con-

expressed

as follows~

Radio-Path Propagation

LOssThe

path-1oss

figmation. The received signal in decibels can

&

curve follows a log-normal distribution with a standard

deviation of

&

8

dB. In theory:

P,

=

received signal (due to the man-made struc-

ture

+

due

to

the natural terrain configuration)

hp

=

40

log

(d,/d,,)

(A

40

dB/decade path loss)

(Eq.

1)

+

correction factor

AG

=

20

log

(hl’/h1)

(A

base-station antenna-height

The received predicted signal

P,

in dBm can be

obtained as follows for the indicated conditions.

gain of

6

dB/octave)

For a nonobstructive path:

P,.

=

Pro

-

y

log

(rho)

+

20

log

(he%,)

+

(Y

(Eq.

2)

where

AI’

is the difference in two received signal

strengths based on the two different path lengths

d,

and

do,

and

AG

is the difference in two received signal

and

h,‘.

(Eq.

3a)

For

an

obstructive

path:

strengths based

on

the two different antenna heights

h,

P,

2

Pro

-

log

(Y/Y~)

-

L

+

(Y

(Eq.

3b)

For land-to-mobile path over water:

Local Mean Prediction from a Prediction

Tool-

The Lee model* can predict the local mean on each

individual road provided that the geographical terrain

Use free-space formula.

For land-to-boat path on the water:

*

Reference

1.

Use

open-area formula.

46-1

0

REFERENCE

DATA

FOR ENGINEERS

In

equations 3a and 3b,

r

is the distance between the base and the mobile

he'

is the effective antenna height shown in Fig.

5,

P,

is the received signal at a reference distance

r,;

a

is a correction factor which will be described

unit in miles or kilometers,

f,

L

is the shadow loss shown in Fig.

6.

u,

is usually equal to

1

mile

(1.6

km),

later.

(A) Moblle

unlf

on

hlll

Typical values

of

P,,

and

y

are listed in Table 3. If

the area

of

interest is similar to an area listed in the

table, the corresponding data from the table can be

used for prediction purposes. The suburban data can be

used generally for most areas. The values

of

P,,

and

y

TABLE 3.

TYPICAL

VALUES

OF

8,

hTD

y

,r,

'

Pro

(dBm)

Area

1

mi

1

km

y(dB/dec)

Free Space

45

-40

20

Open Area

-49

-40

43.5

Suburban

41.7 -54 38.4

Philadelphia

-70 -62.5 36.8

Newark

(Urban)

44 -5 5 43.1

Tokyo

-84 -78 30.5

(6)

Base

stnflon

on

hlll

New York

-77 -67 48

Fig.

5.

Effective antenna heights,

h:,

for two cases.

i'

r1

'I'

'2

-1

SCALE

IN

MILES

Fig.

6A.

Shadow-loss prediction. (A) Prediction model.

CELLULAR TELECOMMUNICATIONS SYSTEMS

46-1

I

also can be obtained very easily by taking the average

values

of

five or more local mean data points at

1

mile

and

10

miles. The averaging process attempts

to

make

a real nonflat area into an imaginary flat area. Since the

area becomes flat, the different average values of

Pro

and

y

shown for each city are caused solely by the

man-made structures. Then the additional changes of

the signal strength due to the variation

of

the terrain

configuration in each area can be added back to the

first predicted signal obtained from a flat man-made

area. For mobile units traveling along paths in a non-

flat area with the terrain contour information provided,

the additional effect due to the terrain variation can be

predicted according to one

of

the four conditions listed

above. Therefore, in equations 3a and 3b, the first and

second parts are due to the man-made structures, and

THEORETICAL CURVE

ABOVE

WE

OF

SIGHT

L

=

MAGNITUDE

OF

RELATIW

STRENGTH

WITH

RESPECT

TO

NONOBSTRUCTION

SIGNAL IN

AREA

1

2

3

4

v-

3.83

0

-0.9

3

#

m

-1.95

L.1

-3

4.5

-6

3.4

3.3

$

3

0.2

5

x

-1

0.1

0

L-20

log

(-0.22WV)

FOR

Vc-2.4

Fig

6B.

Shadow-loss prediction.

(B)

Prediction curve.

46-1

2

REFERENCE

DATA

FOR ENGINEERS

the third part is due to the terrain-contour variation.

This model can be

run

on

a computer.

Standard

Condition:

This model can predict

the

power level in dBm that compares directly with exper-

imental data. For achieving this capability, a standard

condition has to be set, as follows:

Frequencyf,

=

900 MHz

Base-station antenna height

(h,)

=

30.48 m (100

ft)

Base-station power at the antenna

=

10

watts (40

Base-station antenna gain G,

=

6 dB above dipole

Mobile-unit antenna height

=

3 m

(10

ft)

r,

=

1.6

km

(or

1

mi)

Mobile-unit antenna gain G,

=

0

dB above dipole

Correction Factor: For a condition in an actual envi-

ronment that is different from the standard condition,

the following correction factor is used:

gain (6 dBd)

gain (0 dBd)

5

a=&

i=l

where

a,

=

20 log (h,'/30,48)

or,

=

10

log (h,'/3)

a2

=

10

log

(P,/lO)

a4

=

Base-station antenna gain in dBd, G,

-

6

a5

=

Mobile-unit antenna gain in dBd, G,

(P,

is the transmit power)

In

the above equations, the heights are in meters, and

the power is in watts.

General Formula

for

Mobile Radio Propagation

Path

Loss:

Based

on

the suburban-area formula with

correction factors inserted, the general formula can be

expressed as

P,

=

P,

-

134.4

-

38.4 logr,

+

20 log

h,

+

10 log

h,

+G,+G, (Eq. 4)

where P,. and P,

are

in decibels above

1

mW,

r1

is in

kilometers,

h,

and

h,

are in meters, and G, and G,

are

in

decibels.

Characteristics

of

Mu

Iti

pat

h

and

§elective Fading

Cumulative Distribution Function (CDF)-The

CDF of a multipath fading signal follows the Rayleigh

statistics shown in Fig.

7

when no line-of-sight (LOS)

path condition is present and follows Rician statistics

if an LOS path condition is present. Both conditions

are shown in Fig.

8.

The normalized amplitude of an

LOS

path is defined as

a"

-

amplitude of

LOS

path

&

rms of the received signal

--

The Rayleigh fading condition occurs when a.

=

0,

i.e., no

LOS

is present.

Level-Crossing Rate (LCR) and Average Dura-

tion

of

Fades (ADF)-These two statistical parame-

ters are second-order statistics. They involve time,

which is a function of the vehicle speed. When the

vehicle travels fast, more level crossing and shorter

average duration of fades can be seen in the received

signal plot.

The LCR curve shown in Fig.

9

normalizes to

nR

with the factor.

no

=

&(v/A)

where

Vis the vehicle speed in meters/second,

h

is the wavelength in meters.

Therefore, after normalization the curve shown

in

Fig.

9 is independent of the frequency and the vehicle

speed. For example, at a level of

-10

dB,

nR

=

0.3 from

Fig. 9. Assume that a signal of

850

MHz is received at

a mobile unit with a vehicle speed of 24

km/h

(15

mi/

h). Then

no

=

50. The LCR

E

equals

E(-10

dB)

=

0.3

x

50

=

15 crossings/second

The average duration of fades is shown in Fig. 10,

For example, at a level of -10 dB,

tR

=

0.33, With the

same assumption as was used in calculating LCR

above, to

=

l/no, and

ADF,

f

is:

t(-10 dB)

=

0.33/50

=

0.0066

s

=

6.6 ms

Distribution

of

Duration

of

Fades-The distribu-

tion of duration of fades

F,

(U,

R) is shown

in

Fig. 11,

where

U

=

r/t.

The distribution function F,.

(U,

R) can

be interpreted as the probability P(t

>

7,

R(t)

<

R)' that

R(t)

<

R for an interval lasting longer than

7.

The aver-

age duration of fades is

t,

and R is the envelope with

respect to its

rms

values. Furthermore, the vehicle

speed

V

can be related to

t,

which is a normalized fac-

tor for

U.

The probability P(t)

>

r,

R(t)

i

R is shown in

Fig.

11

as a family of curves for different values of

envelope levels. From the curves, we can find that the

probability of occurrence of a fade whose duration is

greater than twice that of an average duration

of

fades

(ADF)

at a level

of

3

dF3

below its

rms

value,

is

10%.

Selective Fading and Time Delay SpreadSelec-

tive fading

is

due to the time-dispersive medium.

When an impulse (short pulse) is sent out from the

base-station antenna, due to man-made structures the

reflected waves travel in different paths and arrive at

the mobile-unit antenna at different times. They form a

signal e(t). The delay spread

(A)

can be obtained from

e(t) as follows:

In

general, the delay spread is measured from the stron-

gest received level to the range which is at its -20-dB

CELLULAR TELECOMMUNICATIONS SYSTEMS

46-1

3

PERCENT

PRO&ABMTY

THAT

AMPLITUDE

<

ABSCISSA

Fig.

7.

Cumulative distribution function.

46-1

4

REFERENCE

DATA

FOR ENGINEERS

SIGNAL

LEVU.

IN

DEClBELS

amplitude

of

direct path

rms

value

Fig. 8.

CPD

of

Rician distribution.

CELLULAR TELECOMMUNICATIONS SYSTEMS

1.0

0.5

10-1

100

c

46-1

5

SIGNAL LEVEL

IN

dB

WITH

RESPECT

TO

rrns

VALUES

Fig.

9.

Level-crossing

rate.

Middleton W.M. (ed.) Reference Data for Engineers: Radio, Electronics, Computer and Communications

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