Chapter 1 Wireless Network. Chapter 1 - Wireless Network 2 Introduction FIRST-GENERATION (1G) In the late 1970s. Aim of providing voice telephony

Chapter 1Wireless 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.


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).


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.


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.


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.


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.


Intelligent Network

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

Services are detached from switching equipment.


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 Part (MTP) – handles the physical layer, data link layer and network layer.

Signaling Connection Control Part (SCCP) – provides both connectionless-oriented and connection-oriented message transport and enables addressing capabilities for message routing.

Transaction Capabilities Application Part (TCAP) – responsible for providing procedures for real-time transaction control.

Intelligent Network Application Protocol (INAP) – defines the necessary operations between the various IN elements.


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).


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).


Physical entities and functional entities in an IN


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.



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.



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).


The OSI/ISO Reference Model


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.



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).


SS7 and the corresponding OSI layers



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).


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 SS7

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.


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 DialingThis 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.


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.


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.


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).

3G systems - IMT2000 - are already entirely based and described in terms of

the IN architecture.


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.



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.



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 NetworkThe external network constitutes the ISDN, CSPDN (Circuit-Switched Public Data Network), PSPDN (Packet-Switched Public Data Network) and PSTN (Public-Switched Telephone Network).


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 LayerThe 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.


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.


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.


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.


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.


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.


Coherence Bandwidth

The coherence bandwidth, BC, 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, TC, is defined as the time interval during which the

fading characteristics of the channel remain approximately unchanged.

where fm 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

12CB T

12C mT f


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.


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).


Multiple Access

Frequency Division Multiple AccessFDMA 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 AccessTDMA 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.


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.


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.


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).


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.


Summary

Wireless network are multiuser systems in which information is

conveyed by radio waves. Modern wireless networks have evolved

through different generations:

1G systems, based on analog technology, aimed at providing voice

telephony service;

2G systems, based on digital technology, aimed at providing a better

spectral efficiency, a more robust communication, voice privacy, and

authentication capabilities;

2.5G systems, based on 2G systems, aimed at providing the 2G systems with

a better data rate capability;

and 3G 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 Rb=1/Tb

second, the modulated signal is used to modulate a wideband function with rate

Rc=1/Tc .

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


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 Rx,y() is given by

If x(t) and y(t) are periodic waveforms, with period T, then

The autocorrelation Rz() for either type of waveform z(t) is defined as

Correlation

dttytxR yx

,

dttytxT

RT

yx 0

,

1

dttztzRR zzz

,


Assume that z(t) is a binary waveform defined as

where Zk {+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, RX,Y(k) is the cross-correlation function of the two periodic binary sequences Xk and Yk, and is the nonperiodic cross-correlation function for the basic waveforms and . The cross-correlation between Xk and Yk is defined as

Correlation

W

ktZZtz

kk

ˆ

kYXYXyx W

kRkR

K

WR ˆ,ˆ,,

YX

R ˆ,ˆ

tX tY

1

0,

K

ikiiYX YXkR

Z t


1

)(ˆ tZ

(a)

W

1

W

1

(b)

)(ˆ ZR

W

1

The autocorrelation function RZ(k) of the sequence Zi is defined as

The autocorrelation function for z(t) is

Correlation

1

0,

K

ikiiZZZ ZZkRkR

kZZzzz W

kRkR

K

WRR ˆ,

Figure C.1A rectangular pulse (a) and its autocorrelation (b).


Now, for a sequence of K binary symbols, in which the number of +1s and -1s differ by one, the autocorrelation is K, for k=0 and -1 for k0.

Two real-valued waveforms are said to be orthogonal if Rx,y(0)=0 .

Correlation

K

-1

W

K

W

K

)(ˆ ZR

W

1

W

1

Figure C.2Autocorrelation function of a real signal waveform


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 2n-1 is known as maximal length sequence

or m-sequence or PN sequence. The following main properties characterize

the m-sequence:

Balance PropertyBalance Property. Within a complete period of sequence, the number of

1s and 0s differs from each other by at most 1.

Correlation PropertyCorrelation 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


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 example of

the Hadamard matrix for n=2 is shown as follows:

Walsh Codes

11

11

0 1

nn

nnn HH

HHH

H

1111

1111

1111

1111

2H


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

11

11

1

0,10,1

0,10,1

1,2

0,2

0,1

CC

CC

C

C

C


Orthogonal Variable Spreading Factor Codes

C 1,0 = (1)

C 2,0 = (1 ,1)

C 2,1 = (1 ,-1)

C 4,0 = (1 ,1,1,1)

C 4,1 = (1 ,1,-1,-1)

C 4,2 = (1 ,-1,1,-1)

C 4,3 = (1 ,-1,-1,1)

12,212,2

12,212,2

1,21,2

1,21,2

0,20,2

0,20,2

12,2

22,2

3,2

2,2

1,2

0,2

1!

1!

!

!

!

!

nnnn

nnnn

nn

nn

nn

nn

nn

nn

n

n

n

n

CC

CC

CC

CC

CC

CC

C

C

C

C

C

C

Figure C.3OVSF code tree.


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.

R eceiverFront End

C orre lator C orre lator C orre lator C orre lator

O ptim al C om bining

To dem odulator

W

1

W

1

W

1

Figure C.4Basic structure of a Rake receiver.


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 (SNRo, SNR of the spread information) and the

input signal-to-noise ratio (SNRi, SNR of unspread information), i.e.,

Processing Gain

w

WG

io SNRSNRG log10


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 replica of PN codes is used to unspread the

received signal.

Direct Sequence Spread Spectrum


Direct Sequence Spread Spectrum

xm (t) m (t)c(t) Balanced

M odulator

PN C odeG enerator

c(t)p(t)

C arrier

D ig ita lIn form ation

m (t)c(t)p(t)

x D em odulator

PN C odeG enerator

c(t)p(t)

C arrier

[m (t)c(t)p(t)]c(t)O utput

D S/ SS m odulation

IFBPF

u(t)

m (t)c(t)p(t)

Figure C.5.A simplified model of DS/SS system.


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

fIF.

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


Frequency Hopping Spread Spectrum

x M odulator

F requencySynthesizer

D ig ita lIn form ation

FH / SS m odulation

Band PassF ilter

PN -codeG enerator

W ideband M ixer

C arrierf1, f2, ..., fN

x

FrequencySynthesizer

Band PassF ilter

PN -codeG enerator

W ideband M ixer

f1+ f IF, f2+ f IF, ..., fN+ f IF

D em odulator O utput

Figure C.5.A simplified model of FH/SS system.

Documents

Chapter 1 Wireless Network. Chapter 1 - Wireless Network 2 Introduction FIRST-GENERATION (1G) In the late 1970s. Aim of providing voice telephony