Wireless Communication Notes

Communication

Kashif

Laeeq Wasti

This is a complete document of subject Wireless Communication taught by Lecturer Kashif Laeeq to BS-8, Batch-5 of Department of Computer Science & I.T of Federal Urdu

University of Arts, Science & Technology’ Karachi during the session from September-December 2010.

Chapter#1

Introduction

Wireless Comes of Age

• Guglielmo Marconi invented the wireless telegraph in 1896. • In 1901, he sent telegraphic signals across the Atlantic Ocean a

distance of about 3200 km.

• Allowed two parties to communicate by sending each other alphanumeric characters encoded in an analog signal.

• Communications satellites were first launched in the 1960s. • Those first satellites could only handle 240 voice circuits.

• Today, satellites carry about one-third of the voice traffic and all of the television signals between countries.

Broadband

• Higher data rates obtainable with broadband wireless technology. • Broadband wireless service shares the same advantages of all wireless services:

convenience and reduced cost.

• Operators can deploy the service faster than a fixed service and without the cost of a cable plant. The service is also mobile and can be deployed almost anywhere.

Limitations & Difficulties of Wireless Technology

• Wireless is convenient and often less expensive to deploy than fixed services. • There are limitations, political and technical difficulties that may ultimately prevent

wireless technologies from reaching their full potential. • Incompatible standards and device limitations.

• Device limitations also restrict the free flow of data. The small display on a mobile telephone can only displaying more than a few lines of text.

Chapter#2

Transmission Fundamentals

Electromagnetic Signals

• An electromagnetic signal is a function of time or frequency. • The signal consists of components of different frequencies.

Time Domain Concepts

• An analog signal is one in which the signal intensity varies in a smooth fashion over time or there are no breaks or discontinuities in the signal.

• A digital signal is one in which the signal intensity maintains a constant level for some period of time and then changes to another constant level.

• A periodic signal, in which the same signal pattern repeats over time. S(t + T) = s(t) -∞ < t < +∞

Where T is the period of the signal.

• Aperiodic signal is an analog or a digital signal pattern that doesn't repeat over time

• The peak amplitude (A) is the maximum value or strength of the signal over time, typically, measured in volts.

• The frequency (f) is the rate [in cycles per second, or Hertz (Hz)] at which the signal repeats.

• The period (T) of a signal is the amount of time it takes for one repetition.

T = 1/f

• Phase (φ ) is a measure of the relative position in time within a single period of a signal.

• The wavelength (λ) of a signal is the distance occupied by a single cycle or the distance between two points of corresponding phase of two consecutive cycles.

Sine Wave Parameters

• The general sine wave can be written as

s( t) = A sin(2πft + φ )

• A function with the form of above Equation is known as a sinusoid.

• Figure 2.3 shows the effect of varying each of the three parameters (a) A = 1, f = 1 Hz, φ = 0; thus T = 1s

Time vs. Distance

• In Figure 2.3 when the horizontal axis is time; the graphs display the value of a signal at a given point in space as a function of time.

• With horizontal axis in space, the graphs display the value of a signal at a given point in time as a function of distance.

• For example, at a particular instant of time, the intensity of the signal varies as a function of distance from the source.

Frequency Domain Concepts

• When all of the frequency components of a signal are integer multiples of one frequency, it’s referred to as the Fundamental frequency. • The spectrum of a signal is the range of frequencies that it contains. • The absolute bandwidth of a signal is the width of the spectrum. • Many signals have an infinite bandwidth, but with most of the energy

contained in a relatively narrow band of frequencies. This band is referred to as the effective bandwidth, or just bandwidth.

• Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes,

frequencies, and phases.

• The period of the total signal is equal to the period of the fundamental frequency.

Relationship between Data Rate and Bandwidth

• The greater the bandwidth, the higher the information-carrying

capacity.

• Any digital waveform will have infinite bandwidth.

• The transmission system will limit the bandwidth that can be transmitted.

• For any given medium, the greater the bandwidth transmitted, the greater the cost.

Transmission is the communication of data by the propagation and processing of

signals.

Analog Signals

• An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency.

o Examples of media:

- Copper wire media (twisted pair and coaxial cable) - Fiber optic cable

- Atmosphere or space propagation

• Analog signals can propagate analog and digital data

Digital Signals

• A digital signal is a sequence of voltage pulses that may be transmitted over a copper wire medium.

o Advantages

- It is generally cheaper than analog signaling - Less susceptible to noise interference.

o Disadvantage

- Digital signals suffer more from attenuation. • Digital signals can propagate analog and digital data.

Analog Transmission

• Transmit analog signals without regard to their content.

• The analog signal will suffer attenuation that limits the length of the transmission link. • Cascaded amplifiers boost signal’s energy to achieve long distance, but the signal

becomes more and more distorted.

o For analog data, quite a bit of distortion can be tolerated.

o For Digital data transmitted as analog signals, introduces errors.

Digital Transmission

o Repeaters achieve greater distance

o Repeaters recover the signal and retransmit • Analog signal carrying Digital data

o The retransmission device recovers the digital data from the analog signal.

o And generates a new, clean analog signal.

Channel Capacity

• A variety of impairments (such as noise, limit data rate) can distort or corrupt a signal.

• The maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions is referred to as the channel capacity.

Concepts Related to Channel Capacity

• Data rate: This is the rate, in bits per second (bps), at which data can

be communicated.

• Bandwidth: This is the bandwidth of the transmitted signal as

constrained by the transmitter and the nature of the transmission medium, expressed in cycles per second, or Hertz.

• Noise: Average level of noise over the communications path.

• Error rate: This is the rate at which errors occur,

o Where an error is the reception of a 1 when a 0 was transmitted or the reception of a 0 when a 1 was transmitted.

Classifications of Transmission Media

• Transmission medium

o The physical path between transmitter and receiver.

• Guided media

o The waves are guided along a solid medium.

o Such as copper twisted pair, copper coaxial cable, or optical fiber.

• Unguided media

o Provide a means of transmitting electromagnetic signals but do not guide them.

o This form of transmission is usually referred to as wireless

transmission.

o Examples are: The atmosphere and outer space.

o Transmission and reception are achieved by means of an

antenna.

o Configurations for wireless transmission - Directional

General Frequency Ranges

• Microwave frequency range

o 1 GHz to 40 GHz

o Directional beams possible

o Suitable for point-to-point transmission o Used for satellite communications • Radio frequency range

o 30 MHz to 1 GHz

o Suitable for omnidirectional applications • Infrared frequency range

o Roughly, 3x1011 _{to 2x10}14 _Hz

o Useful in local point-to-point multipoint applications within confined areas.

Multiplexing

• Capacity of the transmission medium exceeds the capacity required for the transmission of a single signal

• Multiplexing - carrying multiple signals on a single medium.

Reasons for Widespread Use of Multiplexing

• The cost per kbps of the transmission facility declines with an increase in the data rate.

• The cost of transmission and receiving equipment, per kbps, declines with increasing data rate.

• Most individual data communicating devices require relatively modest data rate support.

Multiplexing Techniques

• Frequency Division Multiplexing (FDM)

o FDM takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal.

• Time Division Multiplexing (TDM)

o TDM takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal.

Chapter#3

Communication Networks

Types of Communication Networks

1. Traditional (Traditional LAN, Traditional WAN)

2. Higher Speed (High-speed LAN, High-speed WAN, MAN)

Characteristics of WANs

• WANs cover a large geographical area. • Circuits provided by a common carrier.

• A WAN consists of a number of interconnected switching nodes. • WANs have provided only relatively modest capacity to subscribers.

o data rates of 64,000 bps

o Business subscribers using T-1 service operates at 1.544 Mbps, being common

• Higher-speed WANs use optical fiber and transmission technique known as Asynchronous Transfer Mode (ATM)

o Provide user connections in the 10s and 100s of Mbps.

Characteristics of LANs

• A LAN is a communications network that interconnects a variety of devices and provides a means for information exchange among those devices.

• Traditional LANs provide data rates of 1 to 20 Mbps.

• High-speed LANS provide data rates of 100 Mbps to 1 Gbps

LAN

WAN

The scope of the LAN is small, typically interconnects devices within

a single building or a cluster of buildings.

WANs cover a large geographical area.

LAN is owned by the same organization that owns the attached

devices.

For WANs, most of network assets are not owned by same organization. The internal data rates of LANs are

Metropolitan Area Networks (MAN)

• A MAN occupies a middle ground between LANs and WANs.

• The traditional point-to-point and switched network techniques used in WANs may be inadequate for the growing needs of organizations.

• Need for high capacity at low costs over a large area. • MAN provides:

o Service to customers in metropolitan areas.

o Required capacity at lower cost and greater efficiency than obtaining an equivalent service from the local telephone company.

Switching Techniques

• Switching nodes

o Intermediate switching device that moves data.

o The switching nodes are not concerned with the content of the data.

• Stations

o The end devices that wish to communicate. o Each station is connected to a switching node.

o The stations may be computers, terminals, telephones, or other communicating devices.

• The collection of nodes is referred to as a communication network.

Techniques Used in Switched Networks

• Circuit switching

o A dedicated communication path between two stations.

o The most common example of circuit switching is the telephone network.

• Packet Switching

o Data are transmitted in blocks, called packets.

o Each node determines next leg of transmission for each packet.

Phases of Circuit Switching

• Circuit establishment - Before any signals can be transmitted, an

end-to-end (station-to-station) circuit must be established. • Information transfer

o Information transmitted through the network.

o Data may be analog voice, digitized voice, or binary data, depending on the nature of the network.

• Circuit disconnect

o After some period of information transfer, the connection is terminated.

Characteristics of Circuit Switching

• Circuit switching can be rather inefficient.

o Channel capacity is dedicated for the duration of a connection. o Utilization still does not approach 100%.

o A delay prior to signal transfer for call establishment.

• Once the circuit is established, the network is effectively transparent to the users.

• Information is transmitted at a fixed data rate with no delay other than the propagation delay.

Components of Public Telecommunications Network

• Subscribers

o The devices that attach to the network (mostly telephones). • Subscriber line

o The link between the subscriber and the network.

o The subscriber line is also known as a subscriber loop, or a

local loop.

• Exchanges

o The switching centers in the network.

o A switching center that directly supports subscribers is known as an end office.

• Trunks

o The branches between exchanges.

Working of Packet Switching

• Data are transmitted in blocks, called packets.

• Each node determines next leg of transmission for each packet. • Before sending, the message is broken into a series of packets

o Typical packet length is 1000 octets (bytes).

o Each packet consists of a portion of the data, plus a packet

header that contains control information.

o At each node en route, the packet is received, stored briefly, and passed on to the next node.

Advantages of Packet Switching

• Line efficiency is greater

o A single node-to-node link can be dynamically shared by many packets over time.

• A packet-switching network can carry out data-rate conversion. o Two stations of different data rates can exchange packets.

• When traffic becomes heavy on a circuit-switching network, some calls are blocked while on a packet-switching network, packets are still accepted, but delivery delay increases.

• Priorities can be used.

Disadvantages of Packet Switching

• Each time a packet passes through a switching node introduces a delay.

• The overall packet delay can vary substantially. o This phenomenon, called jitter.

o Caused by differing packet sizes, routes taken and varying delay in the switches.

• Each packet requires overhead information

o Includes address of the destination and sequencing information. o Reduces the communication capacity available for carrying user

data.

• More processing is involved in the transfer of information at each node.

Packet Switching Networks - Datagram

• Each packet is treated independently, with no reference to previous packets.

• Each node chooses the next node on a packet's path.

• The packets, with the same destination address, do not follow the same route and may arrive out of sequence.

• The exit node restores the packets to their original order before delivering them to the destination.

• Responsibility of exit node or destination to detect the loss of a packet and decide how to recover it.

• Advantages

o Call setup phase is avoided.

o Because it’s more primitive, it’s more flexible o Datagram delivery is more reliable

Packet Switching Networks – Virtual Circuit

• A preplanned route is established before any packets are sent. • All the packets between source and destination follow this route. • Routing decision not required by nodes for each packet

• Emulates a circuit in a circuit switching network but is not a dedicated path

o A packet is still buffered at each node and queued for output

over a line. • Advantages

o Packets arrive in original order. o Packets arrive correctly.

o Packets transmitted more rapidly without routing decisions made at each node.

Effect of Packet Size on Transmission Time

• Breaking up packets decreases transmission time because transmission is allowed to overlap

• Figure 3.9a

o Entire message (40 octets) + header information (3 octets) sent at once

o Transmission time: 129 octet-times • Figure 3.9b

o Message broken into 2 packets (20 octets) + header (3 octets) o Transmission time: 92 octet-times

• Figure 3.9c

o Message broken into 5 packets (8 octets) + header (3 octets) o Transmission time: 77 octet-times

• Figure 3.9d

o Making the packets too small, transmission time starts increases

o Each packet requires a fixed header; the more packets, the more

Asynchronous Transfer Mode (ATM)

• Also known as cell relay.

• Operates at high data rates

• Resembles like packet switching, i.e.

o Involves the transfer of data in discrete chunks.

o Allows multiple logical connections to be multiplexed over a single physical interface.

• The information flow on each logical connection is organized into fixed-size packets, called cells.

• Minimal error and flow control capabilities, this reduces the overhead of processing & size.

• Fixed-size cells simplify the processing required at each ATM node.

ATM terminology

• Virtual Channel Connections (VCCs)

o Logical connection in ATM.

o Basic unit of switching in ATM network.

o A VCC is analogous to a virtual circuit in a packet-switching network.

o Exchanges variable-rate, full-duplex flow of fixed-size cells.

• Virtual Path Connection (VPC)

o A bundle of VCCs that have the same endpoints. o Advantages of Virtual Path

 Simplified network architecture

 Increased network performance and reliability

 Reduced processing and short connection setup time  Enhanced network services.

Virtual Channel Connection Uses

• Between end users

o Can carry end-to-end user data or control signaling between two users

• Between an end user and a network entity o Used for user-to-network control signaling • Between two network entities

o Used for network traffic management and routing functions

Virtual Path/Virtual Channel Characteristics

• Quality of service

o Specified by parameters such as cell loss ratio and cell delay variation

• Switched and semi-permanent virtual channel connections • Cell sequence integrity

• Traffic parameter negotiation and usage monitoring • Virtual channel identifier restriction within a VPC

ATM Cell Header Format

• Generic flow control (GFC) – 4 bits, used only in user-network interface o Used to alleviate short-term overload conditions in network • Virtual path identifier (VPI) – 8 bits at the user-network interface, 12

bits at network-network interface o Routing field

• Virtual channel identifier (VCI) – 8 bits

o Used for routing to and from end user • Payload type (PT) – 3 bits

o Indicates type of information in information field • Cell loss priority (CLP) – 1 bit

o Provides guidance to network in the event of congestion • Header error control (HEC) – 8 bit

ATM Service Categories

• Real-time service

o Constant bit rate (CBR)

o Real-time variable bit rate (rt-VBR) • Non-real-time service

o Non-real-time variable bit rate (nrt-VBR) o Available bit rate (ABR)

o Unspecified bit rate (UBR)

Examples of CBR Applications

• Videoconferencing

• Interactive audio (e.g., telephony)

• Audio/video distribution (e.g., television, distance learning, pay-per-view)

• Audio/video retrieval (e.g., video-on-demand, audio library

Examples of UBR applications

• Text/data/image transfer, messaging, distribution, retrieval • Remote terminal (e.g., telecommuting)

Chapter#4

Protocols and the TCP/IP Suite

Features of a Protocol

• Syntax: Concerns the format of the data blocks

• Semantics: Includes control information for coordination and error

handling

• Timing: Includes speed matching and sequencing

Agents involved in Communication

• Applications

o Exchange data between computers (e.g., electronic mail) • Computers

o Connected to networks • Networks

o Transfers data from one computer to another

TCP/IP (Transport Control Protocol/Internet Protocol)

Layers

• Physical layer

• Network access layer • Internet layer

• Host-to-host, or transport layer • Application layer

• Physical Layer

• Covers the physical interface between a data transmission device and a transmission medium or network.

• It specifies:

o the characteristics of the transmission medium o the nature of the signals

o the data rate and o other related matters

• The specific software used at this layer depends on the type of network to be used;

o circuit switching

o packet switching (e.g., ATM) o LANs (e.g., Ethernet)

o Others

• Internet layer

• Uses internet protocol (IP)

• Provide the routing function across multiple networks. • Implemented in the end systems & routers.

• Host-to-host, or transport layer

• Commonly uses transmission control protocol (TCP) • Provides reliability during data exchange

o Completeness o Order

• Application layer

• Contains the logic needed to support the various user applications.

• Uses separate modules that are peculiar to each different type of application

TCP/IP Applications

• Simple mail transfer protocol (SMTP)

o Provides a basic electronic mail facility • File Transfer Protocol (FTP)

o Allows files to be sent from one system to another • TELNET

o Provides a remote logon capability.

OSI Model

TCP/IP

The OSI model however is a generic,

protocol or independent standard.

TCP/IP Protocols are considered to be standards around which the internet

has developed.

OSI model is unnecessarily complex. TCP/IP appears to be a simpler model and this is mainly due to the fact that

it has fewer layers. OSI separates presentation and

session layers.

TCP/IP combines the presentation and session layer issues into its

application layer. OSI has separate data link and

physical layers.

TCP/IP combines the OSI data link and physical layers into the network

access layer.

All packets are reliably delivered TCP reliably delivers packets, IP does not reliably deliver packets

Internetworking not supported TCP/IP supports Internetworking Strict layering Loosely layered

o Semantics of all fields

o Allowable sequence of PDUs • Service Definition

o Functional description that defines what services are provided, but not how the services are to be provided

• Addressing

o Entities are referenced by means of a service access point (SAP)

Internetworking

Internetworking is the practice of connecting a computer network with other networks.

Internetworking can be extremely complex because it generally involves connecting networks that use different protocols. Internetworking is accomplished with routers, bridges, and gateways.

Terms of Internetworking

• Communication Network

o A facility that provides a data transfer service among devices attached to the network

• Internet

o A collection of communication networks interconnected by bridges/routers

• Intranet

o An internet used by a single organization for internal purposes  Provides key Internet applications (e.g. World Wide Web)  Can exist as an isolated, self-contained internet

• End System (ES)

o A device used to support end-user applications or services • Intermediate System (IS)

o A device used to connect two networks. • Bridge

o An IS used to connect two LANs that use similar LAN protocols • Router

o An IS used to connect two networks that may or may not be

Functions of a Router

Internetworking among dissimilar sub networks is achieved by using routers. Essential functions of a router are:

• Provide a link between networks.

• Provide for the routing and delivery of data between processes on end systems attached to different networks.

• Provide these functions in such a way as not to require modifications of the networking architecture of any of the attached subnetworks.

Network Differences Routers Must Accommodate

• Addressing schemes

o Different schemes for assigning addresses • Maximum packet sizes

o Different maximum packet sizes requires segmentation • Interfaces

o Differing hardware and software interfaces • Reliability

Chapter#5

Antennas and Propagation

Antennas

• An antenna is an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy.

o Transmission - radiates electromagnetic energy into space o Reception - collects electromagnetic energy from space

• In two-way communication, the same antenna can be used for both transmission and reception.

Radiation Patterns

A common way to characterize the performance of an antenna is the radiation pattern, • Radiation pattern

o Graphical representation of radiation properties of an antenna o Depicted as two-dimensional cross section

• Beam width (or half-power beam width) o Measure of directivity of antenna • Reception pattern

o Receiving antenna’s equivalent to radiation pattern

Types of Antenna

• Isotropic antenna (idealized)

o A point in space that radiates power in all directions equally.

• Dipole antenna

o the half-wave dipole, or Hertz, antenna (π/2)

- A half-wave dipole has a uniform or omnidirectional radiation pattern in one dimension

o the quarter-wave vertical, or Marconi antenna (π/4)

- Commonly used for automobile radios and portable radios. • Parabolic Reflective Antenna

Antenna Gain

• Antenna gain is a measure of the directionality of an antenna.

• The power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna).

• Effective area of an antenna

o Related to the physical size and shape of the antenna.

• The relationship between antenna gain and effective area is

Where, G

=

Antenna gain Ae= Effective area f = Carrier frequency c = Speed of light (≈ 3 * 108 m/s) λ = carrier wavelength 2 2 2 4 4 c A f A G e π e λ π ₌ =

Problem

For a parabolic reflective antenna with a diameter of 2m , operating at 12GHz, what is the effective area & antenna gain?

Given

Antenna type = Parabolic Diameter = d = 2m Operating frequency = 12GHz To Find Effective Area = Ae =? Antenna Gain = G =? Solution: Area of Parabolic A= πr2 _---Eq-(1) For r r = d/2 = 2/2 = 1m Now, Eq-(1) becomes

A=π ---Eq-(2) Therefore, Ae = 0.56π (Reference: Table 5.2) As we know that λ = c/f = 3 * 108 _{/ 12 * 10}9 _{(where c = 3 * 10}8 _m/s) λ = 0.025m As we know that G = 7A / λ2

= 7π / (0.025)2 _{(where A=π and λ=0.025)} G = 35,200

In dB

Propagation Modes

A signal radiated from an antenna travels along one of three routes: 1. Ground-wave propagation

2. Sky-wave propagation

3. Line-of-sight propagation (LOS)

Ground Wave Propagation

• Follows the contour of theearth • Can propagate considerable distances, • Found in frequencies up to about 2 MHz

• The best-known example of ground wave communication is AM radio.

Sky Wave Propagation

• A signal froman earth-based antenna is reflected from the ionized layer of the atmosphere (ionosphere) back down to earth.

• A sky wave signal can travel through a number of hops, bouncing back andforth between the ionosphere and the earth's surface.

• Reflection effect caused by refraction.

• Sky wave propagation is used for amateur radio, CB radio, and international broadcasts.

Line-of-Sight Propagation

• The transmitting and receiving antennas must bewithin an effective line of sight of each other.

o For satellite communication, a signal above 30 MHz is not reflected by the ionosphere

o For ground-based communication, the transmitting and receiving antennas

must be within an effective line of sight of each other due to refraction. • Refraction - Bending of microwaves by the atmosphere

o the velocity of an electromagnetic wave is a function of the density of the medium

o When electromagnetic wave changes medium, its speed changes.

Optical and Radio Line of Sight

• The optical line of sight can be expressed as d = 3.57 h

• Effective, or radio, line of sight d = 3.57 Kh

Where,

d = distance between antenna and horizon (km) h = antenna height (m)

K = adjustment factor to account for refraction, rule of thumb K = 4/3 • The maximum distance between two antennas for LOS propagation Where, 3.57

(

Κh1 + Κh2

)

Problem

The maximum distance between two antennas for LOS transmission if one antenna is 100m high and the other is at ground level.

Given:

Height of antenna one = h1 = 100m

To Find

Maximum distance b/w two antennas = d =?

Solution: As we know that d = 3.57 Kh (For h1) d=3.57 (4/3)(100 ) (where K=4/3) d= 41km Now, d= 41= => 41 = = (41/3.57) - 1.33

h2= (7.84)2 / 1.33 (Simplify & Taking square both sides) h2 = 46.2 m

(

1 2

)

57 . 3 Κh + Κh

(

(4/3)(100) ( 2

)

57 . 3 + Kh

(

Κh2

)

(

1.33 ( 2

)

57 . 3 + Kh

LOS Wireless Transmission Impairments

• Attenuation and attenuation distortion • Free space loss

• Noise

• Atmospheric absorption • Multipath

• Refraction

Attenuation

• The strength of a signal falls off with distance over any transmission medium. • For guided media reduction in strength, or attenuation, is generally exponential and

thus is typically expressed as a constant number of decibels per unit distance. • Attenuation introduces three factors for unguided media:

o A received signal must have sufficient strength so that the electronic circuitry in the receiver can detect and interpret the signal.

o The signal must maintain a level sufficiently higher than noise to be received without error.

o Attenuation is greater at higher frequencies, causing distortion.

Free Space Loss

• A transmitted signal attenuates over distance because the signal is being spread over a larger and larger area. This form of attenuation is known as free space loss.

• For the ideal isotropic antenna, free space loss is Where,

Pt = signal power at the transmitting antenna Pr = signal power at the receiving antenna

λ = carrier wavelength f = carrier frequency

d = propagation distance between antennas c = speed of light (3 X 108 _m/s)

Where d and λ are in the same units (e.g., meters)

(

)

(

)

2 2 2 2 ₄ 4 c fd d P P r t π λ π ₌ =

This Free space Loss equation can be recast as:

Free space loss accounting for gain of other antennas:

Where,

Gt = gain of the transmitting antenna Gr = gain of the receiving antenna

At = effective area of the transmitting antenna Ar = effective area of the receiving antenna

Free space loss accounting for gain of other antennas can be recast as:

Noise

• Unwanted signals that are inserted somewhere between transmission and reception. • Noise may be divided into four categories:

o Thermal or White noise

- Thermal noise is due to thermal agitation of electrons.

- Thermal noise is uniformly distributed across the frequency spectrum and hence is often referred to as white noise.

o Intermodulation noise

- When signals at different frequencies share the same transmission medium.

o Crosstalk

- An unwanted coupling between signal paths. o Impulse noise

- Is non-continuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude.

      = = λ πd P P L r t dB 4 log 20 log 10

( )

20log

( )

21.98dB log 20 + + − = λ d

( )

20log

( )

147.56dB log 20 4 log 20 = + −      = f d c fd π

( ) ( ) ( )

( )

t r t r t r r t A A f cd A A d G G d P P 2 2 2 2 2 2 4 _{= =} = λ λ π

( )

( )

(

t r

)

dB d AA

L =20log λ +20log −10log

( )

20log

( )

10log

(

)

169.54dB log

20 + − + −

Atmospheric Absorption

• An additional loss between the transmitting and receiving antennas. • Water vapor and oxygen contribute most to attenuation.

• At frequency 22 GHz attenuation is on peak.

• At frequencies below 15 GHz, the attenuation is less.

Multipath

• The signal can be reflected by obstacles so that multiple copies of the signal with varying delays can be received.

Question: Describe Microwave line of sight multipath problem.

For wireless facilities where there is a relatively free choice of where antennas are to be located, they can be placed so that if there are no nearby interfering obstacles, there is a direct line-of-sight path from transmitter to receiver.

For fixed microwave, in addition to the direct line of sight, the signal may follow a curved path through the atmosphere due to refraction and the signal may also reflect from the ground.

Weaknesses

• Paths could be blocked by buildings • Spectral congestion

• Interception possible • Possible regulatory delays

• Sites could be difficult to maintain • Towers need periodic maintenance • Atmospheric fading

Refraction

Multipath Propagation

• Reflection

o Occurs when an electromagnetic signal encounters a surface that is large relative to the wavelength of the signal.

• Diffraction

o Occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave.

• Scattering

o Occurs when incoming signal hits an object whose size in the order of the

wavelength of the signal or less.

Effects of Multipath Propagation

• Multiple copies of a signal may arrive at different phases.

o If these phases add destructively, the signal level relative to noise declines,

making signal detection at the receiver more difficult. • InterSymbol Interference (ISI)

o One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit.

Fading & its Types

• The time variation of received signal power caused by changes in the transmission medium or path.

• Fading is affected by changes in atmospheric conditions, such as rainfall. • Types of Fading

o Fast fading

- Rapidly changes of amplitude as much as 20 or 30 dB over a short distance.

o Slow fading

- Arises when the coherence time of the channel is large relative to the delay constraint of the channel.

o Flat or Non-Selective fading

- All frequency components of the received signal fluctuate in the same proportions simultaneously.

o Selective fading

- Affects unequally the different spectral components of a radio signal. o Rayleigh fading

- Occurs when there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path.

o Rician fading

- Best characterizes a situation where there is a direct LOS path in addition to a number of indirect multipath signals.

Chapter#9

Satellite Communications

Satellite Parameters and Configurations

• Earth Station

o The antenna systems on or near the earth. • Uplink

o A transmission from an earth station to the satellite. • Downlink

o Transmissions from the satellite to the earth station. • Transponder

o The component in the satellite that takes an uplink signal and converts it to a

downlink signal.

Categories of Communications Satellites

There are a number of different ways of categorizing communications satellites: • Coverage Area

o Global, regional, or national. • Service Type

o Fixed service satellite (FSS), broadcast service satellite (BSS), and mobile

service satellite (MSS). • General Usage

o Commercial, military, amateur, experimental.

Classification of Satellite Orbits

Satellite orbits may be classified in a number of ways:

1) Circular and Elliptical Orbits

a. In circular, the center of the circle at the center of the earth,

b. In elliptical, the earth's center at one of the two foci of the ellipse.

2) Orbit around earth in different planes

a. An equatorial orbit is directly above the earth's equator.

b. A polar orbit passes over both poles.

3) Altitude of Satellites

a. Geostationary orbit (GEO)

b. Medium earth orbit (MEO) c. Low earth orbit (LEO)

Geometry Terms

• Elevation Angle (θ)

o The angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite.

• Minimum Elevation Angle

o The minimum elevation angle of the earth station's antenna be somewhat

greater than 0° • Coverage Angle (β)

o The coverage angle is a measure of the portion of the earth's surface visible to

the satellite taking into account the minimum elevation angle.

Reasons affecting minimum elevation angle of earth station’s antenna (>0o₎ • Buildings, trees, and other terrestrial objects block the line of sight • Atmospheric attenuation is greater at low elevation angles

• Electrical noise generated by the earth's heat near its surface adversely affects reception.

Geostationary Satellites (GEO)

If the satellite is in a circular orbit 35,863 km above the earth's surface and rotates in the equatorial plane of the earth, it will rotate at exactly the same angular speed as the earth and will remain above the same spot on the equator as the earth rotates.

Advantages

• There is no problem with frequency changes due to the relative motion of the satellite and antennas on earth (Doppler Effect).

• Tracking of the satellite by its earth stations is simplified. • High coverage area

Disadvantages

• The signal can get quite weak after traveling over 35,000 km. • The Polar Regions are poorly served by geostationary satellites. • Signal sending delay is substantial.

Low-earth-orbiting satellites (LEO)

Characteristics

• Circular/slightly elliptical orbit under 2000 km • Orbit period ranges from 1.5 to 2 hours

• Diameter of coverage is about 8000 km

• Round-trip signal propagation delay less than 20 ms • Maximum satellite visible time up to 20 min

• System must cope with large Doppler shifts • Atmospheric drag results in orbital deterioration

Categories

1) Little LEOs

• Frequencies below 1 GHz • 5MHz of bandwidth • Data rates up to 10 kbps

• Aimed at paging, tracking, and low-rate messaging

2) Big LEOs

• Frequencies above 1 GHz

• Support data rates up to a few megabits per sec

• Offer same services as little LEOs in addition to voice and positioning services

Medium-earth-orbiting satellites (MEOS)

Characteristics

• Circular orbit at an altitude in the range of 5000 to 12,000 km • Orbit period of 6 hours

• Diameter of coverage is 10,000 to 15,000 km • Round trip signal propagation delay less than 50 ms • Maximum satellite visible time is a few hours

Frequency Bands Available for Satellite Communications

Satellite Footprint

At microwave frequencies, which are used in satellite communications, highly directional antennas are used. Thus, the signal from a satellite is not isotropically broadcast but is aimed at a specific point on the earth, depending on which area of coverage is desired.

Chapter#10

Cellular Wireless Networks

Principles of Cellular Networks & its Organization

• The essence of a cellular network is the use of multiple low-power transmitters, on the order of 100W or less.

• Due to small range transmitters an area can be divided into cells, each one served by its own antenna.

• Each cell is allocated a band of frequencies and is served by a base station, consisting of transmitter, receiver, and control unit.

• Adjacent cells are assigned different frequencies to avoid interference or crosstalk. • If the width of a square cell is d, then a cell has four neighbors at a distance d and four

neighbors at a distance .

• For a cell radius R, the distance between the cell center and each adjacent cell center is

Frequency Reusability

• To use the same frequency band in multiple cells at some distance from one another. • This allows the same frequency band to be used for multiple simultaneous

conversations in different cells.

• In characterizing frequency reuse, the following parameters are commonly used: D = minimum distance between centers of cells that use the same frequency band (called co-channels )

R = radius of a cell

d = distance between centers of adjacent cells (d = )

N = number of cells in a repetitious pattern (each cell in the pattern uses a unique set of frequency bands), termed the reuse factor

Increasing Capacity

In time, as more customers use the system, traffic may build up so that there are not enough frequency bands assigned to a cell to handle its calls.

Following approaches have been used to cope with this situation: • Frequency borrowing

o Frequencies are taken from adjacent cells by congested cells. • Cell splitting

o Cells in areas of high usage can be split into smaller cells.

• Cell sectoring

o A cell is divided into a number of wedge-shaped sectors, each with its own set of channels, typically 3 or 6 sectors per cell.

• Microcells

o As cells become smaller, antennas move from the tops of tall buildings or

hills, and finally to lamp posts, where they form microcells.

o Microcells are useful in city streets in congested areas, along highways, and inside large public buildings.

Cellular Systems

• Center of each cell is a base station (BS).

• The BS includes an antenna, a controller, and a number of transceivers, for communicating on the channels assigned to that cell.

• The controller is used to handle the call process between the mobile unit and the rest of the network.

• Each BS is connected to a mobile telecommunications switching office (MTSO). • The link between an MTSO and a BS is by a wire line.

• Two types of channels are available between the mobile unit and the base station (BS):

1. Control channels

 Used to exchange information having to do with setting up and

Mobile Telecommunications Switching Office (MTSO)IMPORTANT

• The MTSO connects calls between mobile units.

• The MTSO is also connected to the telecommunications network and can make a connection between a fixed subscriber to the public network and a mobile subscriber to the cellular network.

• The following are the steps in a typical call between two mobile users within an area controlled by a single MTSO:

o Mobile unit initialization o Mobile-originated call o Paging o Call accepted o Ongoing call o Handoff • Other functions o Call blocking o Can termination o Call drop

GSM

(Global System for Mobile communications)

What is GSM?

GSM (Global System for Mobile communications) is the technology that underpins most of the world's mobile phone networks.

OR

A network which generally covers a fairly broad geographic area and which offers customized travel, financial, reference and commercial information to smart-phone subscribers.

The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber; the Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center, performs the switching of calls between the mobile and other fixed or mobile network users, as well as management of mobile services, such as authentication.

The boundaries at Um, Abis, and A refer to interfaces between functional elements that are standardized in the GSM documents.

GSM Interfaces

The different interfaces used in GSM are as follows:

1. Um (Radio Air) Interface – Interface between MS (Mobile Station) & BSS (Base

Station System). Um is used for providing circuit & packet data services over the radio interface to the MS.

2. Abis Interface - The interface between BTS (Base Transceiver Station) & BSC

(Base Station Controller). This is responsible for carrying traffic and maintenance data.

3. A Interface – This is the interface between a BSC & a MSC.

GSM Channels

GSM supports two types of channels:

1. Traffic Channels

These channels carry digitally encoded user speech or data.A GSM traffic channel (TCH) carries speech and data traffic. Of each 26-frame multiframe, 24 frames are used for traffic channels, one is used for the slow associated control channel (SACCH) and one is measuring signal strength.

2. Control Channels (D Channel)

Signaling and synchronizing commands between Base Station and Mobile Station are transmitted through these channels. There are three types of GSM control channels:

i. Broadcast channels - The GSM broadcast channels are used to provide

information to a mobile station about the network and timing information required for synchronization.

ii. Common control channels - The GSM common control channels are used

to inform mobile stations of incoming calls and to request and grant channels.

iii. Dedicated control channels - GSM dedicated control channels carry data

Mobile Station (MS)

A mobile station communicates across the Um interface, with a base station transceiver in the same cell in which the mobile unit is located.

A mobile station is a combination of terminal equipment and subscriber data. The terminal equipment as such is called ME (Mobile Equipment) and the subscriber’s data is stored in a separate module called SIM (Subscriber Identity Module).

The SIM is a portable device in the form of a smart card or plug-in module that stores the subscriber's identification number, the networks the subscriber is authorized to use, encryption keys, and other information specific to the subscriber.

Base Station SubSystem (BSS)

Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing operation between components made by different suppliers.

A base station subsystem (BSS) consists of a BSC (Base Station Controller) and one or more BTS (Base Transceiver Stations).

1. Base Transceiver Station (BTS) - The base transceiver station (BTS) handles the

radio interface to the mobile station. The base transceiver station is the radio equipment (transceivers and antennas).

2. Base Station Controller (BSC) - The BSC provides the control functions and

physical links between the MSC (Mobile Switching Center) and BTS. It handles radio channel setup, frequency hopping, and handovers. A number of BSCs are served by a MSC.

NS

Network SubSystem (NS)

The network subsystem provides the link between the cellular network and the public switched telecommunications networks. The NS controls handoffs between cells in different BSSs, authenticates users and validates their accounts, and includes functions for enabling worldwide roaming of mobile users.

The main part of network subsystem is MSC (Mobile Switching Center) which performs the switching of calls between the mobile & other fixed or mobile network users, as well as the management of mobile services such as authentication.

It has three main jobs:

1) Connects calls from sender to receiver

2) Collects details of the calls made and received

3) Supervises operation of the rest of the network components The switching system includes the following functional elements.

Mobile Switching Center (MSC)

The central component of the Network Subsystem is the MSC. The MSC performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others. Every MSC is identified by a unique ID. It is supported by four databases that it controls:

MSC

VLR AUC

EIR HLR

1. Home Location Register (HLR)

The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription in the form of SIM then all the information about this subscription is registered in the HLR of that operator.

2. Visitor Location Register (VLR)

The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

3. Authentication Center (AUC)

The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel. The AUC protects network operators from different types of fraud found in today's cellular world.

4. Equipment Identity Register (EIR)

The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS. An IMEI is marked as invalid if it has been reported stolen or is not type approved.

Basic Features Provided by GSM

• Call Waiting - Notification of an incoming call while on the handset

• Call Hold - Put a caller on hold to take another call

• Call Forwarding - Calls can be sent to various numbers defined by the user

• Multi Party Call Conferencing - Link multiple calls together

Advanced Features Provided by GSM

• Calling Line ID - Incoming telephone number displayed

• Closed User Group - Call by dialing last for numbers • Fax & Data - Virtual Office / Professional Office

• Roaming - Services and features can follow customer from market to market

• On-the-air privacy -The privacy is maintained by encryption of the digital data according to a specific secret cryptographic key that is known only to the cellular carrier and the key is changed with time.

Advantages of GSM

• Cleaner quieter calls

• Security against fraud and eavesdropping

• International roaming capability in over 100 countries • Improved battery life

• Efficient network design for less expensive system expansion • Advanced features such as short messaging and caller ID • A wide variety of handsets and accessories