Transcript Network Layer

Chapter 1
Wireless Network
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.
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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).
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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.
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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.
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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.
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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.
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Intelligent Network
 Rapid creation and deployment of enhanced services and new
features are substantially eased.
 Services are detached from switching equipment.
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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.
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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).
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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).
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Physical entities and functional entities in an IN
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Intelligent Network
 The communication between the several PEs relies on out-ofband 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.
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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
14
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
15
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
16
Open Systems Interconnection
 The OSI/ISO Reference Model
Appendix A - Open Systems Interconnection
17
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
18
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
19
Signaling System Number 7
 SS7 and the corresponding OSI layers
Appendix B - Signaling System Number 7
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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).
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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.
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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.
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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.
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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.
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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.
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Network Architecture
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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Narrowband and Wideband Systems
 Coherence Bandwidth
The coherence bandwidth, BC, is defined as the frequency components
are equally affected by fading due to multipath propagation
phenomena.
1
BC   2 T 
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.
TC   2 f m 
1
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.
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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.
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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).
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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.
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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.
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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.
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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).
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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.
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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.
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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 .
Appendix C - Spread Spectrum
Spread Spectrum
 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.
Appendix C - Spread Spectrum
47
Correlation
 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
Rx, y   

 xt yt   dt

 If x(t) and y(t) are periodic waveforms, with period T, then
1
Rx , y   
T
T
 xt  y t    dt
0
 The autocorrelation Rz() for either type of waveform z(t) is defined as
Rz    Rz , z   

 zt zt   dt

Appendix C - Spread Spectrum
48
Correlation
 Assume that z(t) is a binary waveform defined as

k 

ˆ
z t    Z k Z  t  
 W
k  
where Zk  {+1,-1},Zˆ  t is the pulse shape, and 1/W is the duration of
the pulse. Then
W
Rx , y   
K

k 

RX ,Y k RXˆ ,Yˆ   

W

k  
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 RXˆ ,Yˆ   is the nonperiodic crosscorrelation function for the basic waveforms Xˆ t  and Yˆ t  . The crosscorrelation between Xk and Yk is defined as
K 1
RX ,Y k    X iYi k
i 0
Appendix C - Spread Spectrum
49
Correlation
 The autocorrelation function RZ(k) of the sequence Zi is defined as
K 1
RZ k   RZ ,Z k    Z i Z i k
i 0
 The autocorrelation function for z(t) is
W




Rz   Rz , z  
K




R
k
R
 Z Zˆ  

k  
k 

W
RZˆ ( )
Zˆ (t )
1
1
W

(a)
1
W
1
W

(b)
Figure C.1
A rectangular pulse (a) and its autocorrelation (b).
Appendix C - Spread Spectrum
50
Correlation
 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 k0.
RZˆ ( )
K

K
W
1

W
-1
1
W

K
W
Figure C.2
Autocorrelation function of a real signal waveform
 Two real-valued waveforms are said to be orthogonal if Rx,y(0)=0 .
Appendix C - Spread Spectrum
51
Pseudonoise Sequences
 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 Property. Within a complete period of sequence, the number of
1s and 0s 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.
Appendix C - Spread Spectrum
52
Walsh Codes
 The Walsh sequence can be generated by means of
Rademacher functions or by the Hadamard matrices.
 The Hadamard matrix is defined as
H 0  1
 H n 1
Hn  
 H n 1
H n 1 
 H n 1 
 The Walsh sequences are indexed by the row of matrix. An example of
the Hadamard matrix for n=2 is shown as follows:
1 1 1 1 
1  1 1  1

H2  
1 1  1  1


1

1

1
1


Appendix C - Spread Spectrum
53
Orthogonal Variable Spreading Factor Codes
 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
C1,0  1
C2,0  C1,0


C2,1  C1,0
C1,0  1 1 


 C1,0  1  1
Appendix C - Spread Spectrum
54
Orthogonal Variable Spreading Factor Codes
C4,0 = (1,1,1,1)
C2,0 = (1,1)
C4,1 = (1,1,-1,-1)
C1,0 = (1)
C4,2 = (1,-1,1,-1)
C2,1 = (1,-1)
C4,3 = (1,-1,-1,1)
C2n ! , 0  C n
C2 n , 0


  2 ,0

C2n ! ,1
 C2n ,0  C2n ,0


 

C2n ! , 2  C2n ,1 C2n ,1

C
  C n
 C2n ,1

n !
2 ,1
2
,
3

 

 
  



 

C2n ! , 2n1  2  C2n , 2n 1 C2n , 2n 1 

 C n n
 C2n , 2n 1 
C
2 , 2 1
n  ! n1

 2 , 2 1 
Figure C.3
OVSF code tree.
Appendix C - Spread Spectrum
55
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.
Receiver
Front End
1
W
Correlator
1
W
Correlator
1
W
Correlator
Correlator
Optimal Combining
Figure C.4
Basic structure of a Rake receiver.
To demodulator
Appendix C - Spread Spectrum
56
Processing Gain
 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.,
W
G
w
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.,
10logG  SNRo  SNRi
Appendix C - Spread Spectrum
57
Direct Sequence Spread Spectrum
 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.
Appendix C - Spread Spectrum
58
Direct Sequence Spread Spectrum
m(t)
Digital
Information
x
m(t)c(t)
Balanced
Modulator
m(t)c(t)p(t)
c(t)
p(t)
PN Code
Generator
Carrier
DS/ SS modulation
m(t)c(t)p(t)
x
[m(t)c(t)p(t)]c(t)
IF
BPF
u(t)
Demodulator
Output
c(t)
p(t)
PN Code
Generator
Carrier
Figure C.5
.A simplified model of DS/SS system.
Appendix C - Spread Spectrum
59
Frequency Hopping Spread Spectrum
 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.
Appendix C - Spread Spectrum
60
Frequency Hopping Spread Spectrum
Wideband Mixer
Digital
Information
x
Modulator
Carrier
Band Pass
Filter
Frequency
Synthesizer
f1, f2, ..., fN
PN-code
Generator
FH/ SS modulation
Wideband Mixer
x
Band Pass
Filter
Frequency
Synthesizer
Demodulator
Output
PN-code
Generator
f1+fIF, f2+fIF, ..., fN+fIF
Figure C.5
.A simplified model of FH/SS system.
Appendix C - Spread Spectrum
61