Chapter 1 Wireless Network

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Oct 23, 2013 (3 years and 8 months ago)

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Chapter 1

Wireless Network

Chapter 1
-

Wireless Network

2

Introduction


FIRST
-
GENERATION (1G)



In the late 1970s.


Aim of providing voice telephony services.


Analog frequency modulation (FM).


FDMA


Frequency Division Multiple Access as its
multiple access architecture.


One subscriber by physical channel .


Major Technologies:


AMPS
-

Advanced Mobile Phone Service in EUA.


TACS
-

Total Access Communication System, ETACS
-

European TACS and NMT


Nordic Mobile Telephone system
in Europe.


JTACS


Japan TACS and NTACS
-

Nippon TACS in Japan.



Chapter 1
-

Wireless Network

3

Introduction


SECOND
-
GENERATION (2G)



In the early 1990s.


Aim of providing:


Better spectral efficiency.


More robust communication.


Voicresponsiblepeed data services.


Voice privacy.


Authentication capabilities.


Based on digital transmission techniques.


Major technologies:


GSM


Global System for Mobile communications.


D
-
AMPS


Digital AMPS or TIA/EIA/IS
-
136 (IS
-
136).


TIA/EIA/IS
-
95A (IS
-
95A).








Chapter 1
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4

Introduction


GSM and IS
-
136 use TDMA


Time Division Multiple Access
whereas IS
-
95A uses CDMA


Code Division Multiple Access.


Data transmission capability is modest.


An evolution of the existing 2G systems to support data
transmission.


Major technologies:


GPRS


General Packet Radio Service.


IS
-
95B


an evolution of IS
-
95A.


HDR


High Date Rate.






Chapter 1
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Wireless Network

5

Introduction


THIRD
-
GENERATION (3G)



Is embodied by the IMT
-
2000 (International Mobile
Telecommunications).


IMT
-
2000


under the auspices of the ITU (International
Telecommunications Union.


Must provide for:


Multimedia services.


All user sectors.


Terrestrial
-
based and satellite
-
based networks.


Personal pocket, vehicle
-
mounted or any other special terminal.


Major transmission technologies:


UTRA


Universal Terrestrial Radio Access.


CDMA2000


CDMA Multi
-
Carrier radio interface.

Chapter 1
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Wireless Network

6

Introduction


WIRELESS NETWORKS



Is defined in terms of standards and specifications.


Standards and specifications vary for different technologies.


A common framework exists that characterizes the wireless
systems.


This chapter describes the wireless in terms of their common
features.


The mains concepts developed are based on an ITU
recommendation for IMT
-
2000.






Chapter 1
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Wireless Network

7

Intelligent Network


CONCEPT


“In an Intelligent Network (IN), the logic for controlling
telecommunications services migrates from the traditional
switching points to computer
-
based, service
-
independent
platforms.”




Services are separated from switching equipment.


Their implementation is based on the following steps:


Creation of separate service data in a centralized database
outside the switching node.


Separation of the service programs, or service logic, and definition
of a protocol that allows interaction between switching systems
and intelligent nodes containing the service logic and data.

Chapter 1
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Wireless Network

8

Intelligent Network



Rapid creation and deployment of enhanced services and new
features are substantially eased.


Services are detached from switching equipment.









Chapter 1
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Wireless Network

9

Intelligent Network



IN Protocol Architecture




The IN architecture is based on the Signaling System 7 (SS7)
and its protocol architecture.



The IN protocol contains the following elements:



Message Transfer P
a
rt (MTP)


handles the physical layer, data link layer
and network layer.


Signaling Connection Control P
a
rt (SCCP)


provides both
connectionless
-
oriented and connection
-
oriented message transport and
enables addressing capabilities for message routing.


Transaction Capabilities Application P
a
rt (TCAP)


responsible for
providing procedures for real
-
time transaction control.


Intelligent Network Application Protocol (INAP)


defines the necessary
operations between the various IN elements.

Chapter 1
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Wireless Network

10

Intelligent Network



IN Elements




In an IN, several physical entities (PEs), comprising functional
entities (FEs), are identified.



These PEs are represented by rectangles and their
corresponding FEs , represented by ellipses.



Description:



Service Switching Point

(SSP
). The following FEs are encompassed by
SSP: Call Control Function (CCF), Service Switching Function (SSF),
Specialized Resource Function (SRF) and Call Control Agent Function
(CCAF).



Service Control Point
(SCP). The following FEs are encompassed by
SCP: Service Control Function (SCF) and Service Data Function (SDF).


Chapter 1
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11

Intelligent Network



Intelligent Peripheral

(IP). The IP is described by the following FE:
Specialized Resource Function (SRF).



Service Management Point
(SMP). The SMP is described by the
following FEs: Service Management Function (SMF), Service
Management Access Function (SMAF).



Service Creation Environment Point

(SCEP). The SCEP is described by
the following FE: Service Creation Environment (SCE).



Service Data Point

(SDP). The SDP is described by the following FE:
Service Data Function (SDF).


Chapter 1
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12

Physical entities and functional entities in an IN

Chapter 1
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Wireless Network

13

Intelligent Network



The communication between the several PEs relies on out
-
of
-
band signaling or on SS7 protocols.



The SS7 protocols provide means to:



Place service logic and service data into network elements responsible for
handling control and connection remotely.



Enable the communication between intelligent applications and other
applications.



Access databases located in various parts of the network.

Appendix A
-

Open Systems Interconnection

14

Open Systems Interconnection



The Open System Interconnection (OSI) Reference
Model was formulated by the International
Standards Organization (ISO) in the early 1980s.



The model simplifies the design of complex
networks by means of the use of a modular and
structures approach.



It is partitioned into seven layers and protocols
implement the functionality assigned to each layer.




Appendix A
-

Open Systems Interconnection

15

Open Systems Interconnection



Each layer provides services to the layer above it
and uses the service from the layer bellow it.



At the transmitting side, each layer adds its own
header to the message received from the layer
above it and delivers the composite message to the
layer bellow it.



On the receiving side, each layer removes the
corresponding header from the message and
delivers I to the layer above it.


Appendix A
-

Open Systems Interconnection

16

Open Systems Interconnection



The seven layers of the OSI Reference Model are:



Physical Layer (Layer 1).


Data Link Layer (Layer 2).


Network Layer (Layer 3).


Transport Layer (Layer 4).


Session Layer (Layer 5).


Presentation Layer (Layer 6).


Application Layer (Layer 7).





Appendix A
-

Open Systems Interconnection

17



The OSI/ISO Reference Model

Open Systems Interconnection

Appendix B
-

Signaling System Number 7

18

Signaling System Number 7



Signaling System Number 7 (SS7) emerged as an
international standard and gained worldwide
acceptance.



SS7 conforms to a layered model that parallels the
OSI Reference Model.



SS7 is responsible for the control of the fixed
network as well as the mobile network.




Appendix B
-

Signaling System Number 7

19

Signaling System Number 7



SS7 parts:


Message Transfer Part Level 1 (MTP 1).


Message Transfer Part Level 2 (MTP 2).


Message Transfer Part Level 3 (MTP 3).


Telephone User Port (TUP).


ISDN TUP (ISUP).


Signaling Connection Control Part (SCCP).


Transaction Capabilities Application Part (TCAP).


Base Station System Management Applications Part
(BSSMAP).


Direct Transfer Application Part (DTAP).


BSSMAP and DTAP.


Mobile Application Part (MAP).

Appendix B
-

Signaling System Number 7

20



SS7 and the corresponding OSI layers





Signaling System Number 7

Chapter 1
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Wireless Network

21

Intelligent Network


Wireless

Service

Requirements

(
1
/
2
)


Roaming


Mobility,

a

feature

inherent

to

a

wireless

network,

creates

situations

in

which

subscribers

may

roam

out

of

their

local

calling

area

or

out

of

their

service

provider's

area
.



Carrier

Select


Carrier
-
select

services

allow

providers

to

select

the

network

to

be

used

to

handle

a

call
.

In

the

same

way,

they

allow

subscribers

to

route

their

calls

selectively

through

their

network

of

preference
.


Hands
-
Free

Operation


For

voice
-
activated

dialing

and

feature

activation,

voice

recognition

technology

must

be

available
.

In

such

a

case,

messages

or

voice

signals

must

be

collected,

translated

into

data,

and

routed

to

the

required

device,

the

so
-
called

intelligent

peripheral

(IP)
.


Chapter 1
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Wireless Network

22

Intelligent Network


Wireless Service Requirements (2/2)



Fee Structure


The

interaction

among

the

various

networks

involved

in

a

call,

both

wired

and

wireless,

renders

billing

a

difficult

task
.

IN

flags

can

be

used

to

facilitate

the

billing
.

They

can

be

included

into

the

call

record

so

that

billing

reflects

the

specific

call

handling

and

fees

can

be

processed

more

easily
.



Data
-
Service Capability


Wireless

phones

are

allowed

to

send

and

receive

messages

in

addition

to

making

or

taking

telephone

calls
.

SMS

works

like

a

pager,

and

requires

SS
7

messages

for

the

several

tasks

involved

in

its

implementation
:

access

to

database,

authentication,

message

encapsulation,

paging,

routing,

etc
.

IN

procedures

are

certainly

required

to

implement

SMS
.


Chapter 1
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Wireless Network

23

Intelligent Network


Wireless IN Service (1/3)


Voice
-
Based

User

Identification


The

service

employs

a

form

of

automatic

speech

recognition

to

validate

the

identify

of

the

speaker
.

Access

to

services

can

then

be

restricted

to

the

user

whose

voice

(phrase)

has

been

used

to

train

the

recognition

device
.



Voice
-
Based

Feature

Control



This

service

allows

the

authorized

user

to

specify

feature

operations,

which

can

be

carried

out

via

feature
-
control

string

by

means

of

spoken

commands
.



Voice
-
Control

Dialing


This

service

allows

the

subscriber

to

place

a

call

using

spoken

commands
.



Voice
-
Controlled

Services




This

feature

allows

the

subscriber

to

control

features

and

services

using

spoken

commands
.


Chapter 1
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Wireless Network

24

Intelligent Network


Wireless IN Service (2/3)


Incoming

Call
-
Restriction/Control



This

service

allows

user

to

impose

restrictions

to

an

incoming

call

as

follows
:

it

may

terminate

normally

to

the

subscriber
;

it

may

terminate

to

the

subscriber

with

normal

alerting
;

it

may

terminate

to

the

subscriber

with

special

alerting
;

it

may

be

forwarded

to

another

number
;

it

may

be

forwarded

to

voice

mail
;

it

may

be

routed

to

any

specific

announcement
;

or

it

may

be

blocked
.



Calling

Name

Presentation



This

service

provides

the

name

identification

of

the

calling

party

(personal

name,

company

name,

restricted,

not

a

available)

as

well

as

the

date

and

time

of

the

call
.



Password

Call

Acceptance



This

service

allows

the

subscriber

to

restrict

incoming

calls

only

to

those

callers

who

can

provide

valid

passwords
.


Chapter 1
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25

Intelligent Network


Wireless IN Service (3/3)


Selective

Call

Acceptance



This

service

allows

the

subscriber

to

restrict

incoming

calls

only

to

those

calling

parties

whose

numbers

are

in

the

restricted

list
.



Short

Message

Service



This

service

allows

the

short

message

entities

(SMEs)

-

the

short

message

users

-

to

receive

or

send

short

messages

(packet

of

data)
.



Speech
-
to
-
Text

Conversion



This

service

allows

the

user

to

create

a

short

alphanumeric

message

by

means

of

spoken

phrases
.



Prepaid

Phone



This

service

allows

the

user

to

pay

before

calling,

i
.
e
.
,

not

to

be

billed

(postpaid)
.

This

can

take

a

number

of

forms,

for

example,

a

debit

card,

a

connection

to

a

smart

card,

and

others
.



Chapter 1
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Wireless Network

26

Intelligent Network


IN

Standards



In

North

America,

the

movement

to

develop

a

wireless

intelligent

network

(WIN)

was

triggered

by

the

Cellular

Telecommunications

Industry

Association
.



In

Europe,

the

same

movement

for

GSM
-
based

network

was

carried

out

through

the

Customized

Applications

for

Mobile

Network

Enhanced

Logic

(CAMEL)
.



3
G

systems

-

IMT
2000

-

are

already

entirely

based

and

described

in

terms

of

the

IN

architecture
.


Chapter 1
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27



Network Architecture

Chapter 1
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Wireless Network

28

Network Architecture


The

main

Components

of

a

wireless

System

(
1
/
3
)


Mobile

Station

(MS)



It

incorporates

user

interface

functions,

radio

functions,

and

control

functions,

with

the

most

common

equipment

implemented

in

the

form

of

a

mobile

telephone
.



Base

Station

(BS)



Base

Transceiver

Station

(BTS)
.

The

BTS

consist

of

a

radio

equipment

(transmitter

and

receiver

-

transceiver)

and

provides

the

radio

coverage

for

a

given

cell

or

sector
.


Base

Station

Controller

(BSC)
.

The

BSC

incorporates

a

control

capability

to

manage

one

or

more

BTSs,

executing

the

interfacing

functions

between

BTSs

and

the

network
.



Mobile

Switching

Center

(MSC)



The

MSC

provides

an

automatic

switching

between

users

within

the

same

network

or

other

public

switched

networks,

coordinating

calls

and

routing

procedures
.


Chapter 1
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29

Network Architecture


The main Components of a wireless System (2/3)


Visitor Location Register (VLR)



The

VLR

is

a

database

containing

temporary

records

associated

with

subscribers

under

the

status

of

a

visitors
.

A

subscriber

is

considered

a

visitor

if

such

a

subscriber

is

in

a

roaming

condition
.


Home Location Register (HLR)


The

HLR

is

the

primary

database

for

the

home

subscriber
.

It

mantains

information

records

on

subscriber

current

location,

subscriber

identifications,

user

profile,

and

so

forth
.

An

HLR

usually

operates

on

a

centralized

basis

and

serves

many

MSCs
.



Gateway

(GTW)


The

GTW

serves

as

an

interface

between

the

wireless

network

and

the

external

network
.




Chapter 1
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Wireless Network

30

Network Architecture


The

main

Components

of

a

wireless

System

(
3
/
3
)


Service

Control

Point

(SCP)



The

SCP

provides

a

centralized

element

to

control

service

delivery

to

subscribers
.

It

is

responsible

for

higher
-
level

services

that

are

usually

carried

out

by

the

MSC

in

wireless

networks

not

using

IN

facilities
.



Service

Transfer

Point

(STP)



The

STP

is

a

packet

switch

device

that

handles

the

distribution

of

control

signals

between

different

elements

in

the

network
.



Intelligent

Peripheral

(IP)


The

IP

processes

the

information

of

subscribers

in

support

of

IN

services

within

a

wireless

network
.



External

Network


The

external

network

constitutes

the

ISDN,

CSPDN

(Circuit
-
Switched

Public

Data

Network),

PSPDN

(Packet
-
Switched

Public

Data

Network)

and

PSTN

(Public
-
Switched

Telephone

Network)
.


Chapter 1
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Wireless Network

31

Protocol Architecture


A

general

radio

protocol

contains

the

three

lowest

layers

of

the

OSI/ISO

Reference

Model,

as

follows
:



Physical

Layer



The

physical

layer

is

responsible

for

providing

a

radio

link

over

the

radio

interface
.

Such

a

radio

link

is

characterized

by

its

throughput

and

data

quality
.


Data

Link

Layer



Medium

Access

Control

(MAC)

sublayer
.

The

MAC

sublayer

is

responsible

for

controlling

the

physical

layer
.

It

performs

link

quality

control

and

mapping

of

data

flow

onto

this

radio

link
.

It

is

defined

for

the

BTS

and

for

the

MT
.



Link

Access

Control

(LAC)

sublayer
.

The

LAC

sublayer

is

responsible

for

performing

functions

essential

to

the

logical

link

connection

such

as

setup,

maintenance,

and

release

of

a

link
.

It

is

defined

for

BSC,

BTS,

MT,

and

control

functionalities

of

the

MS
.



Network

Layer


The

network

layer

contains

functions

dealing

with

call

control,

mobility

management,

and

radio

resource

management
.

It

is

mostly

independent

of

radio

transmission

technology
.

Such

a

layer

can

be

transparent

for

user

data

in

certain

user

services
.

It

is

defined

for

BSC,

BTS,

MT,

and

control

functionalities

of

the

MS
.

Chapter 1
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32

Channel Structure


A

channel

provides

means

of

conveying

information

between

two

network

elements



RF

Channel



An

RF

channel

is

defined

in

terms

of

a

carrier

frequency

centered

within

a

specified

bandwidth,

representing

a

portion

of

the

RF

spectrum
.

The

RF

channel

constitutes

the

means

of

carrying

information

over

the

radio

interface
.

It

can

be

shared

in

the

frequency

domain,

time

domain,

code

domain,

or

space

domain
.


Physical

Channel



A

physical

channel

corresponds

to

a

portion

of

one

or

more

RF

channels

used

to

convey

any

given

information
.

Such

a

portion

is

defined

in

terms

of

frequency,

time,

code,

space,

or

a

combination

of

these
.

A

physical

channel

may

be

partitioned

into

a

frame

structure,

with

the

specific

timing

defined

in

accordance

with

the

control

and

management

functions

to

be

performed
.

Chapter 1
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33

Channel Structure


Logical

Channel

(
1
/
3
)


A

logical

channel

is

defined

by

the

type

of

information

it

conveys
.

The

logical

channels

are

mapped

onto

one

or

more

physical

channels
.

Logic

channels

may

be

combined

by

means

of

a

multiplexing

process,

using

a

frame

structure
.

Chapter 1
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34

Channel Structure


Logical

Channel

(
2
/
3
)


Traffic

Channels


Dedicated

Traffic

Channel

(DTCH)
.

The

DTCH

conveys

user

information
.

It

may

be

defined

in

one

or

both

directions
.


Random

Traffic

Channel

(RTCH)
.

The

RTCH

conveys

packet
-
type

data

user

information
.

It

is

usually

defined

in

one

direction
.


Control

Channels



Dedicated

Control

Channels

(DCCH)
.

A

DCCH

is

a

point
-
to
-
point

channel

defined

in

both

directions
.

Two

DCCHs

are

specified
:


Associated

Control

Channel

(ACCH)
.

An

ACCH

is

always

allocated

with

a

traffic

channel

or

with

an

SDCCH
.


Stand
-
Alone

Dedicated

Control

Channel

(SDCCH)
.

An

SDCCH

is

allocated

independently

of

the

allocation

of

a

traffic

channel
.

Chapter 1
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35

Channel Structure


Common

Control

Channels

(CCCH)
.

A

CCCH

is

a

point
-
to
-
multipoint

or

multipoint
-
to
-
point

channel

used

to

convey

signaling

information

(connectionless

messages)

for

access

management

purposes
.

Four

types

of

CCCHs

are

specified
:


Broadcast

Control

Channel

(BCCH)
.

The

BCCH

is

a

downlink

channel

used

to

broadcast

system

information
.

It

is

a

point
-
to
-
multipoint

channel

listed

to

by

all

MSs,

from

which

information

is

obtained

before

any

access

attempt

is

made
.


Forward

Access

Channel

(FACH)
.

The

FACH

is

a

downlink

channel

conveying

a

number

of

system

management

messages,

including

enquiries

to

the

MS

and

radio
-
related

and

mobility
-
related

resource

assignment
.

It

may

also

convey

packet
-
type

user

data
.


Paging

Channel

(PCH)
.

The

PCH

is

downlink

channel

used

for

paging

MSs
.

A

page

is

defined

as

the

process

of

seeking

an

MS

in

the

event

that

an

incoming

call

is

addressed

to

that

MS
.


Random

Access

Channel

(RACH)
.

The

RACH

is

an

uplink

channel

used

to

convey

messages

related

to

call

establishment

request

and

responses

to

network
-
originated

inquiries
.


Chapter 1
-

Wireless Network

36

Narrowband and Wideband Systems



Narrowband

Systems


Narrowband

systems

support

low
-
bit
-
rate

transmission
;


Systems

operating

with

channels

substantially

narrower

than

the

coherence

bandwidth

are

known

as

narrowband

system
;



In

narrowband

systems,

all

the

components

of

signals

are

equally

influenced

by

multipath

propagation
;


Narrowband

systems

are

affected

by

selective

fading
.



Wideband

Systems


wideband

systems

support

high
-
bit
-
rate

transmission
;



Wideband

systems

operate

with

channels

substantially

wider

than

the

coherence

bandwidth
;


In

wideband

systems,

the

various

frequency

components

of

the

signal

may

be

differently

affected

by

fading
.



Chapter 1
-

Wireless Network

37


Coherence

Bandwidth


The

coherence

bandwidth,

B
C
,

is

defined

as

the

frequency

components

are

equally

affected

by

fading

due

to

multipath

propagation

phenomena
.




where

the

time

span

between

the

arrival

of

the

first

and

the

last

multipath

signals

that

can

be

sensed

by

the

receiver

is

known

as

delay

spread

(
T
)
.



Coherence

Time


The

coherence

time,

T
C
,

is

defined

as

the

time

interval

during

which

the

fading

characteristics

of

the

channel

remain

approximately

unchanged
.



where

f
m

is

the

maximum

Doppler

shift,

in

hertz,

is

given

as

v/

,

where

v
,

in

m/s,

is

the

speed

of

the

mobile

terminal

and


,

in

m,

is

the

wavelength

of

the

signal
.

Narrowband and Wideband Systems


Chapter 1
-

Wireless Network

38

Multiple Access


Wireless

networks

are

multiuser

systems

in

which

information

is

conveyed

by

means

of

radio

waves
.


In

a

multiuser

environment,

access

coordination

can

be

accomplished

via

several

mechanisms
:


by

insulating

the

various

signals

sharing

the

same

access

medium
;


by

allowing

the

signals

to

contend

for

the

access
;


or

by

combining

these

two

approaches
.


The

choice

for

the

appropriate

scheme

must

take

into

account

a

number

of

factors,

such

as
:



type

of

traffic

under

consideration
;


available

technology
;


cost

and

complexity
.

Chapter 1
-

Wireless Network

39

Multiple Access


Access

coordination

may

be

carried

out

in

different

domains
:


frequency

domain


time

domain


code

domain


space

domain
.



Four

main

multiple

access

technologies

are

used

by

the

wireless

networks
:


frequency

division

multiple

access

(FDMA)


time

division

multiple

access

(TDMA)


code

division

multiple

access

(CDMA)


space

division

multiple

access

(SDMA)
.

Chapter 1
-

Wireless Network

40

Multiple Access


Frequency

Division

Multiple

Access


FDMA

is

certainly

the

most

conventional

method

of

multiple

access

and

was

the

first

technique

to

be

employed

in

modern

wireless

application
.


The

channel

bandwidth

is

a

function

of

the

services

to

be

provided

and

of

the

available

technology

and

is

identified

by

its

center

frequency,

known

as

a

carrier
.



Time

Division

Multiple

Access


TDMA

is

another

widely

known

multiple
-
access

technique

and

succeeded

FDMA

in

modern

wireless

applications
.


In

TDMA,

the

entire

bandwidth

is

made

available

to

all

signals

but

on

a

time
-
sharing

basis
.


Transmission

then

occurs

within

a

time

interval

known

as

a

(time)

slot
.

Chapter 1
-

Wireless Network

41

Multiple Access


Code Division Multiple Access


CDMA

is

a

nonconventional

multiple
-
access

technique

that

immediately

found

wide

application

in

modern

wireless

systems
.


In

CDMA,

the

entire

bandwidth

is

made

available

simultaneously

to

all

signals
.


In

theory,

very

little

dynamic

coordination

is

required,

as

opposed

to

FDMA

and

TDMA

in

which

frequency

and

time

management

have

a

direct

impact

on

performance
.


To

accomplish

CDMA

systems,

spread
-
spectrum

techniques

are

used
.


In

CDMA,

signals

are

discriminated

by

means

of

code

sequences

or

signature

sequences
.


Each

pair

of

transmitter
-
receivers

is

allotted

one

code

sequence

with

which

a

communication

is

established
.

Chapter 1
-

Wireless Network

42

Multiple Access


Code Division Multiple Access


At

the

reception

side,

detection

is

carrier

out

by

means

of

a

correlation

operation
.


In

general,

CDMA

systems

operate

synchronously

in

the

forward

direction

and

asynchronously

in

the

reverse

direction
.


In

theory,

the

use

of

orthogonal

codes

eliminates

the

multiple
-
access

interference
.


In

practice,

however,

interference

still

occurs

in

synchronous

systems,

because

of

the

multipath

propagation

and

because

of

the

other
-
cell

signals
.


Channels

in

the

forward

link

are

identified

by

orthogonal

sequences
.


Base

stations

are

identified

by

pseudonoise

(PN)

sequences
.


Chapter 1
-

Wireless Network

43

Multiple Access


Code Division Multiple Access


Hence,

multiple

access

in

the

forward

link

is

accomplished

by

the

use

of

spreading

orthogonal

sequences
.


In general, the use of orthogonal codes in the reverse link finds no direct
application, because the reverse link is intrinsically asynchronous.


Some systems implement some sort of synchronous transmission on the reverse
link.


Several PN sequences are used in the various systems.


Two main orthogonal sequences used in all CDMA systems:


Walsh codes


Orthogonal variable spreading functions (OVSF).


Chapter 1
-

Wireless Network

44

Multiple Access


Space Division Multiple Access


SDMA

is

a

nonconventional

multiple
-
access

technique

that

finds

application

in

modern

wireless

systems

mainly

in

combination

with

other

multiple
-
access

techniques
.


In

SDMA,

the

entire

bandwidth

is

made

available

simultaneously

to

all

signals
.


Signals

are

discriminated

spatially,

and

the

communication

trajectory

constitutes

the

physical

channels
.


The

implementation

of

an

SDMA

architecture

is

based

strongly

on

antennas

technology

coupled

with

advanced

digital

signal

processing
.


The

antenna

beams

must

be

electronically

and

adaptively

directed

to

the

user

so

that
.


The

location

alone

is

enough

to

discriminate

the

user
.

Chapter 1
-

Wireless Network

45

Summary


Wireless

network

are

multiuser

systems

in

which

information

is

conveyed

by

radio

waves
.

Modern

wireless

networks

have

evolved

through

different

generations
:


1
G

systems,

based

on

analog

technology,

aimed

at

providing

voice

telephony

service
;


2
G

systems,

based

on

digital

technology,

aimed

at

providing

a

better

spectral

efficiency,

a

more

robust

communication,

voice

privacy,

and

authentication

capabilities
;


2
.
5
G

systems,

based

on

2
G

systems,

aimed

at

providing

the

2
G

systems

with

a

better

data

rate

capability
;


and

3
G

systems

that

aim

at

providing

for

multimedia

services

in

their

entirety
.

Appendix C
-

Spread Spectrum


Spread

Spectrum

is

defined

as

a

communication

technique

in

which

the

intend

signal

is

spread

over

a

bandwidth

in

excess

of

the

minimum

bandwidth

required

to

transmit

the

signal
.


This

is

accomplished

by

the

use

of

a

wideband

encoding

signal

at

the

transmitter,

which

operates

in

synchronism

with

the

receiver,

where

the

encoding

signal

is

also

known
.


Generating

a

spread

spectrum

signal

involves

two

steps
:


first,

the

carrier

is

modulated

by

the

baseband

digital

information

with

rate

R
b
=
1
/T
b



second,

the

modulated

signal

is

used

to

modulate

a

wideband

function

with

rate

R
c
=
1
/T
c

.

Spread Spectrum

Appendix C
-

Spread Spectrum

47


The

desired

wideband

signal

arrives

at

the

receiver

together

with

other

wideband

signals,

interference,

and

noise
.


Other

waveforms

are

not

correlated

and

will

be

spread,

appearing

a

noise

to

the

modulator
.


The

correlated

signal

is

then

a

band
-
pass

signal,

whereas

the

noise

component

is

a

wideband

signal
.


Two

main

spread

spectrum

techniques

are

used
:


direct

sequence

spread

spectrum
;


and

frequency

hopping

spread

spectrum
.

Spread Spectrum

Appendix C
-

Spread Spectrum

48


The

correlation

function

quantifies

the

degree

of

similarity

between

two

functions
.



Lets

x(t)

and

y(t)

be

two

nonperiodic

waveforms

with

finite

energy
.

The

cross
-
correlation

function

R
x,y
(

)

is

given

by




If

x(t)

and

y(t)

are

periodic

waveforms,

with

period

T
,

then




The

autocorrelation

R
z
(

)

for

either

type

of

waveform

z(t)

is

defined

as

Correlation

Appendix C
-

Spread Spectrum

49


Assume

that

z(t)

is

a

binary

waveform

defined

as




where

Z
k



{+
1
,
-
1
},

is

the

pulse

shape,

and

1
/
W

is

the

duration

of

the

pulse
.

Then





where

K

is

the

number

of

pulses

composing

the

period

of

the

sequence,

R
X,Y
(k)

is

the

cross
-
correlation

function

of

the

two

periodic

binary

sequences

X
k

and

Y
k
,

and

is

the

nonperiodic

cross
-
correlation

function

for

the

basic

waveforms

and


.

The

cross
-
correlation

between

X
k

and

Y
k

is

defined

as

Correlation

Appendix C
-

Spread Spectrum

50


The

autocorrelation

function

R
Z
(k)

of

the

sequence

Z
i

is

defined

as




The

autocorrelation

function

for

z(t)

is

Correlation

Figure C.1

A rectangular pulse (a) and its autocorrelation (b).

Appendix C
-

Spread Spectrum

51


Now,

for

a

sequence

of

K

binary

symbols,

in

which

the

number

of

+
1
s

and

-
1
s

differ

by

one,

the

autocorrelation

is

K
,

for

k=
0

and

-
1

for

k

0
.











Two

real
-
valued

waveforms

are

said

to

be

orthogonal

if

R
x,y
(
0
)=
0

.

Correlation

Figure C.2

Autocorrelation function of a real signal waveform

Appendix C
-

Spread Spectrum

52


Pseudonoise

(PN)

or

pseudorandom

sequences

are

used

for

two

main

purposes

data

scrambling

and

spread

spectrum

modulation
.


Note,

in

the

scrambling

operation

(as

well

as

in

the

modulation

operation),

that

both

transmitter

and

receiver

must

work

exactly

the

same

PN

sequence
.


A

sequence

with

a

period

equal

to

2
n
-
1

is

known

as

maximal

length

sequence

or

m
-
sequence

or

PN

sequence
.

The

following

main

properties

characterize

the

m
-
sequence
:


Balance

Property
.

Within

a

complete

period

of

sequence,

the

number

of

1
s

and

0
s

differs

from

each

other

by

at

most

1
.


Correlation

Property
.

By

comparing

a

complete

sequence

with

any

shifted

version

of

it,

within

the

sequence

period,

the

number

of

agreements

minus

the

number

of

disagreements

is

always

-
1
.

Pseudonoise Sequences

Appendix C
-

Spread Spectrum

53


The

Walsh

sequence

can

be

generated

by

means

of

Rademacher

functions

or

by

the

Hadamard

matrices
.



The

Hadamard

matrix

is

defined

as






The

Walsh

sequences

are

indexed

by

the

row

of

matrix
.

An

exampl e

of

the

Hadamard

matrix

for

n=
2

is

shown

as

follows
:

Walsh Codes

Appendix C
-

Spread Spectrum

54


Channelization

in

multirate

CDMA

systems

can

be

provided

by

orthogonal

variable

spreading

factor

(OVSF)

codes
.


Uplink

and

downlink

channels

make

use

of

OVSF

codes
.


The

OVSF

codes

preserve

the

orthogonality

between

channels

of

different

rates

and

spreading

factors
.


They

can

be

defined

as

Orthogonal Variable Spreading Factor Codes

Appendix C
-

Spread Spectrum

55

Orthogonal Variable Spreading Factor Codes

Figure C.3

OVSF code tree.

Appendix C
-

Spread Spectrum

56

Rake Receiver


In

a

multipath

propagation

environment,

the

received

signal

contains

replicas

of

attenuated

and

delayed

version

of

the

transmitted

signal
.


Assume

that

the

signal

is

pseudorandom

with

a

correlation

width

of

1
/
W
.

Figure C.4

Basic structure of a Rake receiver.

Appendix C
-

Spread Spectrum

57


Processing

gain

G

is

defined

as

the

ratio

between

the

bandwidth

W

of

the

spread

signal

and

the

bandwidth

w

of

the

unspread

signal,

i
.
e
.
,




which

represents

the

gain

achieved

by

processing

a

spread

spectrum

signal

over

an

unspread

signal
.


It

can

be

obtained

by

the

difference

in

decibels

between

the

output

signal
-
to
-
noise

ratio

(SNR
o
,

SNR

of

the

spread

information)

and

the

input

signal
-
to
-
noise

ratio

(SNR
i
,

SNR

of

unspread

information),

i
.
e
.
,

Processing Gain

Appendix C
-

Spread Spectrum

58


Direct

sequence

(DS)

spread

spectrum

(SS)

uses

PN

sequence

to

modulate

a

carrier
.


In

principle,

any

modulation

technique

such

as

AM

(pulse),

FM,

or

PM

can

be

used
.

However,

the

most

widespread

form

is

the

binary

phase

shift

keying

(BPSK)

modulation
.


At

the

receiver,

which

is

assumed

to

operate

in

synchronism

with

the

transmitter,

an

exact

r epl i ca

of

PN

codes

is

used

to

unspread

the

received

signal
.

Direct Sequence Spread Spectrum

Appendix C
-

Spread Spectrum

59

Direct Sequence Spread Spectrum

Figure C.5

.A simplified model of DS/SS system.

Appendix C
-

Spread Spectrum

60


Frequency

hopping

(FH)

is

a

spread

spectrum

(SS)

technique

in

which

the

carrier

is

allowed

to

hop

from

one

frequency

to

another

in

a

sequence

dictated

by

a

PN

code
.


At

the

receiver,

which

is

assumed

to

operate

in

synchronism

with

the

transmitter,

the

signal

is

mixed

with

a

locally

generated

replica

of

the

transmitter

frequency

sequence,

offset

by

the

intermediate

frequency

f
IF
.


The

two

basic

FH

systems
:

slow

FH

(SFH)

and

fast

FH

(FFH)
.


In

SFH

systems,

several

symbols

of

information

are

transmitted

on

each

frequency

hop,

where

each

symbol

is

a

chip
.


In

FFH,

several

hops

occur

during

the

transmission

of

one

symbol,

where

the

chip

is

characterized

by

hop
.

Frequency Hopping Spread Spectrum

Appendix C
-

Spread Spectrum

61

Frequency Hopping Spread Spectrum

Figure C.5

.A simplified model of FH/SS system.