Cellular and Mobile

Communications
P. Anvesh
Assistant Professor
ECE Dept
SNIST

Prerequisites: Analog & Digital Communications (II-II)

Upcoming Subjects: Microwave and Optical Communications (III-II)

Advanced Communications and Networking (IV-I)
 Course Outcomes
• CO1: Understand the working principle and limitations/advancements of
conventional mobile telephone systems, cellular mobile systems and Advanced
generations of cellular wireless systems

• CO2: Analyze Frequency reuse concept and avoidance of Co-channel

interference.

• CO3: Explore the concepts of adjacent channel interference, its effects and
avoidance mechanism.

• CO4: Analyze signal reflections, path loss, propagation delay/loss, near and
long distance propagation loss under different conditions, Merits of Lee model.

• CO5: Analyze frequency allocation of cellular systems

• CO6: Demonstrate the concept of handoff mechanism and dropped calls.

 Syllabus
Unit -I: Introduction to Cellular Mobile Radio Systems:
• Limitations of Conventional mobile telephone systems, Significance of 800MHz,
Basic cellular wireless systems; 1G, 2G, 2.5G, 3G, 4G, 5G cellular wireless
systems; Uniqueness of mobile radio environment – Long term fading, Factors
influencing short term fading, Parameters of mobile multi path fading: Time
dispersion parameters, Coherence bandwidth, Doppler spread and coherence time.
Types of small scale fading. Diversity techniques – time, space, frequency.

Unit-II: Fundamentals of Cellular Radio System Design:

• Concept of Frequency reuse, Co-channel Interference, Co-channel Interference
Reduction Factor, Desired C/I from a normal case in a omni directional Antenna
system, System capacity, Trunking and grade of service; Improving coverage and
capacity in cellular system – Cell splitting, Sectoring, Micro cell zone concept.
 Syllabus
Unit-III: Channel Interference:
• Measurement of real time Co-Channel Interference, Design of antenna system,
Antenna parameters and their effects; Diversity techniques- Space diversity,
Polarization diversity, Frequency diversity and Time Diversity. Non-co-channel
interference- Adjacent channel Interference, Near end and far end interference,
Cross talk, Effect on coverage and Interference by power decrease, antenna height
decrease, effect of cell site components, UHF TV interference
• Applications: Design of a cellular systems using frequency reuse factor (k=19)
for directional and Omni-directional antenna systems

Unit-IV: Cell Coverage for Signal and Traffic:

• Signal reflections in flat and hilly terrain, Effect of human made structures, Phase
difference between direct and reflected paths, Constant standard deviation, Straight
line path loss slope, General formula for mobile propagation over water and flat
open area, Near and long distance propagation, Path loss from a point to point
prediction model in different conditions, Merits-of-LEE-model.
 Syllabus
Unit-V: Frequency Management and Channel Assignment:
• Numbering and grouping, Setup access and paging channels, Channel assignments
to cell sites and mobile units, Channel sharing and borrowing, Sectorization,
Overlaid cells, Non fixed channel assignment.
Handoff, Dropped Calls:
• Handoff initiation, Types of Handoff, Delayed handoff, Advantages of handoffs,
Power difference handoff, Forced handoff, Mobile assigned handoff and Soft
handoff, Intersystem handoff. Introduction to dropped call rates and their
evaluation.

Unit-VI: Digital Cellular Networks:

• GSM architecture, GSM channels,
• Multiple access scheme, TDMA, FDMA, CDMA, WCDMA, SDMA, OFDM.
• Text Books:
1. Mobile Cellular Telecommunications – W.C.Y. Lee, Tata McGraw Hill,
2rd Edn., 2006.
2. Principles of Mobile Communications – Gordon L. Stuber, Springer
International 2nd Edition, 2007.

• Reference books:
1. Wireless Communications - Theodore. S. Rapport, Pearson education, 2nd Edn.,
2002.
2. Wireless and Mobile Communications – Lee McGraw Hills, 3rd Edition, 2006.
3. Wireless Communication and Networking – Jon W. Mark and WeihuaZhqung,
PHI, 2005.
4. Wireless Communication Technology – R. Blake, Thompson Asia Pvt. Ltd., 2004.
 Cellular vs Mobile
• Mobile phone - Phone that is not connected by any wires.
– Satellite phones, Wi-Fi phones and Cell phones
• Mobile phone is a portable device to make calls. Being portable, it provides
an opportunity to speak outside your home or any destination if you are
traveling.

• Cell Phone - mobile phone that works utilising radio cells (an area of radio
coverage).
– Cell phones are used while moving from one cell to another without losing coverage or
dropping the connection.
– Cell-based technologies associated with 2G, 3G, 4G AMPS, along with PCS.
• Cell phones are electronic devices to make calls, provided they have
cellular network within range.

• Mobile describes the quality of the phone. Cell is used to describe the
technology.
• Satellite phones are not cell phones, although they are mobile phones.
 Unit-I
Unit -I: Introduction to Cellular Mobile Radio Systems:
• Limitations of conventional mobile telephone systems,
• Significance of 800MHz,
• Basic cellular wireless systems;
• 1G, 2G, 2.5G, 3G, 4G, 5G cellular wireless systems;
• Uniqueness of mobile radio environment –
• Long term fading,
• Factors influencing short term fading,
• Parameters of mobile multi path fading:
• Time dispersion parameters,
• Coherence bandwidth,
• Doppler spread and coherence time.
• Types of small scale fading.
• Diversity techniques – time, space, frequency.
 Introduction to Cellular Mobile Radio Systems
• Mobile Communication
• It is the process of communication while moving around a wide geographic
area.
• Portable - hand-held devices used at walking speed.
• Stay connected in everywhere we go.
• Stay connected in many ways (e.g. Calls, video, etc)
• Communication facility between stationary and mobile or mobile and
mobile users (units)

• Concept of Mobile Communication

• A conventional mobile telephone system is usually designed by selecting
autonomous geographic zones. (50miles)
• One or more channels from a specific frequency allocation for use in the
geographic zones.
• High powered transmitters are used for coverage.
 Introduction
• Mobile Communication Operation
 Introduction
Examples of Mobile Communication Systems
• Pagers - Simplex
• Hand held Walkie-Talkies - Half duplex
• Cordless phones - Full duplex
• Cellular telephones - Full duplex

pager Walkie-Talkie Cordless phone Cellular telephone

 Introduction
• Classification of Wireless Systems
• Simplex - Communication is possible in only one direction. e.g. Paging systems
• Half Duplex - Two way communication but not simultaneous. e.g. Walkie-Talkies
• Full Duplex - Two way simultaneous communication. e.g. Cellular systems

• Duplexing:
• In wireless communication systems, it is often desirable to allow the user to
send simultaneously information to the base station while receiving
information from the base station.
• Duplexing is done either using frequency or time domain techniques:
– Frequency division duplexing (FDD)
– Time division duplexing (TDD)
• FDD - is more suitable for radio communication systems,
• TDD - is more suitable for fixed wireless systems
 Introduction
• Frequency division duplexing (FDD)
• Two bands of frequencies for every user
– Forward band
– Reverse band
• Frequency separation between forward band
and reverse band is constant.
• FDD - provides two simplex channels at the
same time.
• Channel 1 825.030 MHz (Reverse channel)
• 870.030 MHz (forward channel)
 Introduction
• Time division duplexing (TDD)
• Uses time for forward and reverse link
• Multiple users share a single radio channel
• Forward time slot
• Reverse time slot
• TDD - provides two simplex time slots on the
same frequency.
 FDD vs TDD
Frequency Division Duplex (FDD) Time Division Duplex (TDD)
• Simpler to implement • Implementation is complex
• Simultaneous downlink and • Only uplink (UL) or downlink
uplink transmission (DL) at any time
• No need for synchronisation • Need for synchronisation within
hence simpler implementation the whole network
• Needs paired spectrum. • No need for paired spectrum
• UL/DL ratio is fixed. • Number of UL/DL ratio is
changeable
 Limitations of Conventional Mobile
Telephone Systems
• One of many reasons for developing a cellular mobile
telephone system and deploying it in many cities is the
operational limitations of conventional mobile telephone
systems:
– Limited service capability,
– Poor service performance,
– Inefficient frequency spectrum utilization.
 Limited service capability
• Each area is allocated with one or more channels.
• Which is large autonomous geographic zone.
• The transmitted power should be as high as the federal
specification allows.

• In a conventional mobile system

• High power
• Larger cell
 Limited service capability
• The user who starts a call in one zone has to reinitiate the call
when moving into a new zone because the call will be
dropped.
– The handoff is a process of automatically changing frequencies as the
mobile unit moves into a different frequency zone so that the
conversation can be continued in a new frequency zone without
redialing.

• Another disadvantage of the conventional system is that the

number of active users is limited to the number of channels
assigned to a particular frequency zone.
 Poor Service Performance
• In the past, a total of 33 channels were all allocated to three
mobile telephone systems.
– Mobile Telephone Service (MTS) - 40MHz – 11 Channels
– Improved MTS (IMTS) MJ - 150MHz – 11 Channels
– Improved MTS (IMTS) MK - 450MHz – 12 Channels
• In 1976, New York City had
– 6 channels of MJ serving 320 customers, with another 2400 customers on a waiting list.
– 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list.

• The large number of subscribers created a high blocking

probability during busy hours.
• Although service performance was undesirable, the demand
was still great.
• A high-capacity system for mobile telephones was needed.
 Inefficient frequency spectrum utilization
• The frequency utilization measurement (Mo), is defined as the maximum
number of customers that could be served by one channel at the busy hour.
• Mo = Number of customers/channel (in busy Hour)
• Mo = 53 for MJ system
• 37 for MK system
• The offered load can then be obtained by
Avg calling time (min) x total customers x No. of channels
• A= (Erlangs)
60 min
 Inefficient frequency spectrum utilization
• Assume average calling time = 1.76 min.
▪ A1 = 1.76 x 53 x 6 / 60 = 9.33 Erlangs (MJ system)
▪ A2 = 1.76 x 37 x 6 / 60 = 6.51 Erlangs (MK system)
• If the number of channels is 6 and
▪ the offered loads are A1 = 9.33 and A2 = 6.51,
• then from the Erlang B model the blocking probabilities,
➢ B1 = 50 percent (MJ system)
➢ B2 =30 percent (MK system),
• It is likely that half the initiating calls will be blocked in the MJ system, a
very high blocking probability.

• To reduce blocking probability we must decrease Mo.

• The conventional system does not utilize the spectrum efficiently since
each channel can only serve one customer at a time in a whole area.
• Limitations of Conventional Mobile Telephone Systems
– Limited service capability,
– Poor service performance,
– Inefficient frequency spectrum utilization.

• Finally we need to increase the capacity(efficiency) of the

system by giving service to maximum number of customers
with available resources (frequency spectrum).

• To overcome the Limitations Cellular concept was introduced.

• Mobile communications → Cellular communications
 Cellular Concept
• Replacing a single, high power transmitter (large cell) with many low
power transmitters (small cells).
• Each providing coverage to only a small portion of the service area.
• Each base station is allocated a portion of the total number of channels
available to the entire system.
• All the available channels are assigned to a relatively small number of
neighboring base stations.
• Neighboring base stations are assigned different groups of channels.
• The interference between base stations (and the mobile users under their
control) is minimized.
• Frequency spectrum may be reused as many times by systematically
spacing base stations and their channel groups in a given geographic area.
Cellular Concept
 Cellular Concept
• Cell:
• Cell is the small geographic area covered by the base station.
• The area around an antenna where a specific frequency range is used.
• Cell is represented graphically as a hexagonal shape, but in reality it is
irregular in shape.

• Cluster:
• A cluster is a group of adjacent cells.
• No frequency reuse is done within a cluster.
 Why 800 MHz?
• The FCC’s (Federal Communications Commission) decision to choose
800MHz was made because of severe spectrum limitations at lower
frequency bands.

• FM broadcasting services operate in the vicinity of 100MHz.

• TV Broadcasting service starts at 41MHz and extends upto 960MHz.
• Air to Ground systems use 118 to 136 MHz.
• Military Aircraft use 225 to 400 MHz.

• It was Hard for FCC to allocate a spectrum in the lower portions of the 30
to 400 MHz band since the services of this band had become so crowded.

• Mobile radio transmission cannot be applied at 10GHz or above because of

severe propagation path loss, multipath fading, and rain activity.
 Why 800 MHz?
• Fortunately, 800MHz was allocated to Educational TV channels. Cable TV
service become a big factor in mid-70’s and shared the load of providing
TV channels. This situation opened up the 800MHz band to some extent.
• FCC allocated a 40-MHz system at 800MHz to Cellular Mobile Radio
Systems.

• Although 800MHz is not the ideal transmission medium for mobile radio,
Cellular Mobile Radio System that does not go beyond this frequency can
be deployed.
Spectrum
 Basic Cellular Wireless Systems
• A basic analog cellular system consists of three subsystems:
– a Mobile unit,
– a Cell site, and
– a Mobile Telephone Switching Office (MTSO),
 Basic Cellular System
Mobile units:
• A mobile telephone unit contains a control unit, a transceiver, and an
antenna system.
Cell site:
• The cell site provides interface between the MTSO and the mobile units.
• It has a control unit, radio cabinets, antennas, a power plant, and data
terminals.
MTSO:
• The switching office, the central coordinating element for all cell sites,
contains the cellular processor and cellular switch.
• It interfaces with telephone company zone offices, controls call processing,
provides operation and maintenance, and handles billing activities.
• The cellular switch, switches calls to connect mobile subscribers to other
mobile subscribers and to the nationwide telephone network.
• Its processor provides central coordination and cellular administration.
 Basic Cellular System
Connections:
• The radio and high-speed data links connect the three subsystems.
• Each mobile unit can only use one channel at a time for its communication
link.
• But the channel is not fixed; it can be any one in the entire band assigned
by the serving area, with each site having multichannel capabilities that can
connect simultaneously to many mobile units.
• Microwave link or T-carriers(wire line) are
used to carry both data and voice between
MTSO and the base-station.
• The voice-trunks are used to connect MTSO
with PSTN.
• For wideband data and information, optical
fibers can be used.
 Basic Cellular System
Communication Protocols:
• The protocols govern the process of call connection and disconnection at
the end of the conversation.
• Example : IS-54, IS-95, GSM, etc.

Operation of Cellular Systems:

1. Mobile Unit Initialization:
▪ Out of total radio channels (say 416) available for a cellular coverage area,
few channels (say 21) are designated for setting up connections, and are called
as set-up-channels.
▪ When a subscriber activates his mobile unit by switching on the power, its
receiver scans the set-up channels, selects the strongest one (nearest cell-site)
and locks on it for a certain time. This is known as self-location scheme.
▪ This scheme has the disadvantage that trace of idle mobile units does not
appear on cell site. This limitation can be removed by a process called
registration.
 Basic Cellular System
Operation of Cellular Systems:
2. Mobile Unit Originated Call :
▪ The mobile user enters the called number and presses the send button, his
request goes to cell-site through a set-up channel.
▪ The cell-site, sends the request via a high speed link to MTSO for allocating a
voice-channel.
▪ The MTSO allots a suitable free voice channel and cell-site links two
subscribers for conversation.
3. Land Line Originated Call:
▪ When a land-line subscriber dials a mobile unit number, the respective
telephone exchange transfers it to MTSO via voice grade trunk lines.
▪ The MTSO sends this information on relevant cell sites along with a search
algorithm.
▪ Each cell-site uses its setup channel to transmit information and mobile unit
recognizes it and locks into it.
4. Call Termination :
▪ When the mobile user switches off its transmitter, a signaling tone is sent to
the respective cell-site and both sides makes the voice channels free.
 Basic Cellular System
Spectrum Allocation Example:
• Ex. 2G GSM Mobile Communication
• Spectrum Allotted – 890 to 960MHz (70MHz)
• Service Provider Plan:
Up-link – 890 to 915MHz (25MHz)
Down-link – 936 to 960MHz (25MHz)
Guard Band – 916 to 935MHz (20MHz)
• Note:
• 2G Voice Call Bandwidth – 200KHz
• Hence, Number of Voice Channels available = 25MHz/200KHz
= 125 channels
• If we allocate, all the Bandwidth of frequency to the
single base station means, 125 users can make a call
to 125 users
 Basic Cellular System
Concepts Involved:
• Frequency Reuse Concept
• Channel Assignment Techniques
– Fixed Channel Assignment Techniques
– Dynamic Channel Assignment Techniques
• Hand-off
• Capacity Expansion
– Cell Splitting
– Cell Sectoring
– Microcell Zone Concept
• Interference
– Co-channel Interference (Same frequency)
– Adjacent Channel Interference (different frequency)
 History
• Morse code
– The first form of communication started after the invention of the Morse code by Samuel F
B Morse in the 1830s. via a wired medium.
• History of Wireless communication
• 1890s – First Wireless Communication - Wireless Telegraph
– by Italian engineer Guglielmo Marconi in 1896.
– SOS signals from the sinking Titanic was received by nearby ships and 700 peoples were
rescued.
• 1900s – First Radio Broadcast - December 1906
• 1920s – First Commercial Radio Broadcast - 15th June 1920 – UK
• 1930s – Radar technology, Television Broadcasting
– The first experimental radar system was built in England in 1935.
– German scientists also developed radar during World War II
– In 1950s, radar systems were developed for use in air traffic control and weather forecasting.
– The first commercial television broadcasting started in the US on July 2, 1928.
– Non- experimental TV broadcasted started in Germany in 1935 and one year later in England
by British Broadcasting Corporation (BBC).
– BBC is the first organization to start broadcasting signals using satellite on 25th June 1967
• 1940s – Quality Radio Services - FM Radio Broadcasting
– The first fully licensed commercial FM station started broadcasting on March 1st, 1941
from Nashville, Tennessee in the US.
– In the UK, FM broadcasting started after a decade by BBC on 2 May 1955.
• 1950s – First Satellites, the Space program
– The first-ever artificial satellite was Sputnik 1, launched by Russia in 1957.
• 1970s – Mobile networks, GPS
– The first cellular phone was developed in 1973 by Dr. Martin Cooper of Motorola.
– The first call was made on a prototype Motorola handset on April 3, 1973.
– The first GPS was launched in 1978.
• 1980s – First-generation mobile networks
– an analog technology - to make phone calls – Speeds of few kbps
• 1990s – Second-generation mobile networks, Wi-Fi
– text messaging - speeds of few hundred kbps.
– The first generation of wireless standards (IEEE 802.11) was finalized in 1997.
• 2000s – Satellite internet, 3G Networks
– The first satellite internet service for customers started service in September 2003.
– 3G mobile networks was digital technology - speeds of few Mbps.
• 2010s – 4G (LTE advanced), 5G network
– 4G mobile networks was launched in the 2010s. speeds of tens of Mbps and introduced
high-definition (HD) video streaming.
– The first 5G networks were deployed in 2019
Mobile Radio Systems around the world
 Evolution of Mobile Communication
• It is useful to think of cellular Network/telephony in terms of generations:
» 0G: Briefcase-size mobile radio telephones
» 1G: Analog cellular telephony
» 2G: Digital cellular telephony
» 2.5G: Packet based telephony
» 3G: High-speed digital cellular telephony (including video telephony)
» 4G: IP-based “anytime, anywhere” voice, data, and multimedia telephony
» 5G: sub-6 GHz and mmWave Technology
 0G Mobile Systems
• Briefcase-size mobile radio telephones
 1G Mobile Systems
• 1G or 1st generation mobile phones were the earliest cellular systems that
were developed in early 80’s.
• Analog cellular networks
• Voice communication (Speech Only).
• Very limited in capacity.
• Speed up to 2.4kbps. (almost no consideration of data services)
• Systems using 1G :
➢ Advanced Mobile Phone Service (AMPS)
➢ Total Access Communication System (TACS)
➢ Nordic Mobile Telephone System (NMTS)

• Dr. Martin Cooper of Motorola made the first

private handheld mobile-phone call on a larger
prototype model in 1973.
 1G Mobile Systems
• Systems using 1G
• Advanced Mobile Phone Service (AMPS)
– FDMA for control and FDD for two way transmission.
– 825 MHz to 890 MHz frequency range.
• 824 MHz-849 MHz (uplink), 869 MHz-894 MHz (downlink).
• Total Access Communication System (TACS)
– Sweden, Norway, Demark & Finland
– 890 MHz - 915 MHz –uplink & 935 MHz -960 MHz downlink.
• Nordic Mobile Telephone System (NMTS)
– Introduced to Europe in 1981.
– 450 MHz and 900 MHz
 1G Mobile Systems
• Features
– Frequency 800 MHz and 900 MHz
– Bandwidth: 10 MHz (666 duplex channels with bandwidth of 30 KHz)
– Technology: Analogue switching system
– Modulation: Frequency Modulation (FM)
– Mode of service: voice only
– Access technique: Frequency Division Multiple Access (FDMA)
– Limited local & regional coverage i.e. Low capacity.
– Phones were large in size.
• Limitations
– Supports only speech
– Limited number of users and cell coverage - Inefficient use of bandwidth
– Poor voice quality due to interference
– Long call setup time, low service quality
– Insecure transmission (calls could be decoded by an FM demodulator)
– Roaming was not possible between similar systems
 2G Mobile Communication
• Digital cellular technology - GSM
• Digital multiplexing - TDMA, CDMA
• Circuit Switched Data.
• Developed in Europe and the US
• Offer support for simple non-voice services
– SMS (short messaging service) and email services
• Speeds up to 64kbps
• Systems using 2G
➢ Global System for Mobile communication (GSM),
➢ TDMA IS-136
➢ CDMA IS-95
 2G Mobile Communication
• Circuit Switching

• Many paths are possible but only One path is selected per call.
• Once a call is established, all communication takes place on this path or circuit.
• A circuit is dedicated to this call for the duration of call.
 2G Mobile Communication
• Systems using 2G
• Systems with TDMA Technology are
➢ GSM (Global Systems for Mobile Communication)
➢ DCS 1800 (Digital Cellular System) in Europe
➢ IS-54 and IS-136 (Interim Standard-136) in North America,
➢ PDC (Personal Digital Cellular system) in Japan.
➢ IDEN (Integrated Digital Enhanced Network) in US and Canada.

• Systems with CDMA technology are

➢ IS-95 in South Korea, operates in the 800 MHz and 1900 MHz
frequency bands. also known as CDMA One.
 2G Mobile Communication
• Features
– Digital system,
– Better voice quality,
– Higher capacity,
– Lower power consumption.
– Short Messaging Service
• Advantages of 2G over 1G
– Increased capacity, better speech quality, Short Message Service (SMS) and
global roaming.

• Limitations
– Does not support Multimedia
– Symmetric UL & DL traffic
– Base station synchronization needed
 2.5G Mobile Communication
• 2.5G technology allow existing 2G equipment to be modified and
supplemented with new base station add-ons and subscriber unit software
upgrades to support higher data rate transmissions
• Packet Based Cellular that have been enhanced to provide advanced
communication applications

• Systems use 2.5G:

➢ GPRS (General Packet Radio service)
➢ EDGE (Enhanced Data rate for GSM Evolution)

• Features
– Wireless application protocol (WAP) access,
– Multimedia Messaging Service (MMS),
– Location based mobile services
– Internet communication services such E-mail and World Wide Web access.
– Camera Phones
– Speed up to 64-144kbps.
 2.5G Mobile Communication
• Packet Switching

• Many paths may be used for a single communication as individual packets are
routed to a destination.
• No fixed path is established. Packets are routed according to the best path available
at the time.
• Prior to transmission, each communication is broken into packets, which are
addressed and numbered.
• At the destination, packets may be reassembled into order according to their
sequence number.
 2.5G Mobile Communication
• GPRS and EDGE
• GPRS – General Packet Radio Services
– 2.5G protocol
– Involved only software changes to the GSM network.
– Used under utilized TDMA channels more effectively.
– Increased data rates to a max of 170 Kbps.
• EDGE – Enhanced Data rates for GSM Evolution.
– 2.75G protocol.
– Required minimal hardware changes
– Added a new encoding scheme that allowed for more bits to be added into each
time slice.
– Data can now be passed optimally at 384 Kbps.
• Both of these use TDMA over GSM
 3G Mobile Technology
• The basic data rates are low in 2G networks
• Could not satisfy multimedia services like real-time video, digital TV,
mobile web browsing, file transfer etc.
• To enhance the data rates of 2G networks and for supporting multimedia
services, 3G was introduced.
• 3rd Generation was introduced in 2000.
• The 3G mobile systems are also called as International Mobile
Telecommunications for the year 2000 (IMT-2000).
• Data speed up to 144kbps-2Mbps.
• Smart phones
• Systems using 3G:
➢ IMT 2000
➢ WCDMA
➢ CDMA 2000
 3G Mobile Technology
• Features:
– Providing faster communication.
– Send/receive large email messages
– High speed web, Video conferencing and 3D gaming
– TV streaming - Mobile TV
– Large Capacities and Broadband Capabilities
– Less time to download MP3 songs and data

• The data is sent through the technology called Packet Switching.

• Voice calls are interpreted through Circuit Switching.

• Limitations
– Expensive fees for 3G Licenses, Services
– It was challenge to build the infrastructure for 3G
– High Bandwidth Requirement
– Expensive 3G Phones.
– Large Cell Phones.
 3G Mobile Technology
• Systems use 3G:
• With technology enhancements, such as
– Cellular Digital Packet Data (CDPD) that operates over AMPS,
– High Speed Data (HSD) in IS-95,
– GPRS and HSCSD in GSM,
• 2G wireless systems evolved towards 3G.
• Several standardization bodies form 3rd Generation Partnership Project
(3GPP) in 1998 had defined radio interface standards, technical
specifications for 3G networks.
• WCDMA which is third generation radio interface deployed in Asia
including Korea, Japan and Europe.
• The 3G systems within the range of 3GPP are named as Universal Mobile
Telecommunication System (UMTS), and WCDMA is called as UMTS
Terrestrial Radio Access (UTRA).
 4G Mobile Technology
• Best aspect of 4G is the use of data services for everything
• Only packet switching.
• Cloud computing,
• Gaming Services.

• 4G (LTE)
• LTE stands for Long Term Evolution
• Next Generation mobile broadband technology
• Promises data transfer rates of 100 Mbps
• Based on UMTS 3G technology
• Optimized for All-IP traffic
 4G Mobile Technology
Major LTE Radio Technologies used in 4G:
• Uses Orthogonal Frequency Division Multiplexing (OFDM) for downlink.
• Uses Single Carrier Frequency Division Multiple Access (SC-FDMA) for
uplink.
• Uses Multi-input Multi-output(MIMO) for enhanced throughput.
• Reduced power consumption.
• Higher RF power amplifier efficiency (less battery power used by handsets)
 4G Mobile Technology
• Features
– Much higher data rate up to 1Gbps
– Enhanced security and mobility
– Reduced latency for mission critical applications
– High definition video streaming and gaming
– Voice over LTE network VoLTE (use IP packets for voice)

• Limitations
– Expensive hardware and infrastructure
– Costly spectrum (most countries, frequency bands are too expensive)
– High end mobile devices compatible with 4G technology required, which is
costly
– Wide deployment and upgrade is time consuming
 4G Mobile Technology
• Applications
– For Customers
• Video streaming, TV broadcast
• Video call , video clips news, music, sports
• Enhanced gaming, chat, location services…
– For Business
• High speed Tele working / VPN access
• Sales force automation
• Video conferencing
• Real time financial information
 5G Mobile Technology
• 5G is based on OFDM (Orthogonal frequency-division multiplexing)
• 5G uses 5G NR (New Radio)air interface alongside OFDM principles.
• 5G also uses wider bandwidth technologies such as sub-6 GHz and
mmWave.
• 5G can operate in both lower bands (e.g., sub-6 GHz) as well as mmWave
(e.g., 24 GHz and up), which will bring extreme capacity, multi-Gbps
throughput, and low latency.

• Key technologies:
– Massive MIMO ,
– Millimeter Wave Mobile Communications etc.

• 5G is used across three main types of connected services, including

Enhanced mobile broadband, Mission-critical communications, and the
Massive IoT.
 5G Mobile Technology
• Features:
– Ultra fast mobile internet up to 10Gbps
– Low latency in milliseconds (significant for mission critical applications)
– Total cost deduction for data
– Higher security and reliable network
– Uses technologies like small cells, beam forming to improve efficiency
– Forward compatibility network offers further enhancements in future
– Cloud based infrastructure offers power efficiency, easy maintenance and
upgrade of hardware
 Uniqueness of Mobile Radio Environment
• The Mobile radio channel places fundamental limitations on the
performance of wireless communication systems.
• The transmission paths can vary from simple line-of-sight to ones that are
severely obstructed by buildings, mountains, and foliage.
• Radio channels are extremely random and difficult to analyze.

• Mobile Radio Transmission Medium

– The Propagation Attenuation.
– Severe Fading.
• Model of Transmission Medium
• Direct Wave Path - a path clear from the terrain contour.
• Line-of-Sight Path - a path clear from buildings.
• Obstructive Path - when the terrain contour blocks the direct wave path.
• Amplifier Noise
• Long-Term Fading
Uniqueness of Mobile Radio Environment
 The Propagation Attenuation
• Consider the below Mobile Radio Transmission Model
• If the antenna height at the cell site is 30 to 100 m and at the mobile unit
about 3 m above the ground, and the distance between the cell site and the
mobile unit is usually 2 km or more,
• Then the incident angles of both the direct wave (θ1)and the reflected wave
(θ2) are very small.

• Path Loss:
• The difference between the Tx Power and Rx power
Path Loss = Tx Power – Rx Power
PL = Pt – Pr
𝑃𝑡
PL (dB) = 10 𝑙𝑜𝑔 𝑃𝑟
The Propagation Attenuation
 The Propagation Attenuation

• The propagation path loss increases not only with frequency but also with
distance.
• The propagation path loss would be 40 dB/dec,
– here “dec” is an abbreviation of decade, i.e., a period of 10.
• This means that a 40-dB loss at a signal receiver will be observed by the
mobile unit as it moves from 1 to 10 km.
• Therefore C is inversely proportional to R4.
C ∝ R−4 = α R−4
• where
– C = received Carrier Power
– R = distance measured from the transmitter to the receiver
– α = constant
 The Propagation Attenuation
• The difference in power reception at two different distances R1 and R2 will
result in

𝐶2 𝑅2 −4
=
𝐶1 𝑅1

• The decibel Expression is

∆C (in dB) = C2-C1 (in dB)
𝐶2 𝑅1
= 10 log = 40 log
𝐶1 𝑅2
• When R2=2R1, ∆C = -12dB;
• When R2=10R1, ∆C = -40dB;
• This 40 dB/dec is the general rule for the mobile radio environment and is
easy to remember.
• It is also easy to compare to the free-space propagation rule of 20 dB/dec.
• In a real mobile radio environment, the propagation path-loss slope varies
as,
C ∝ 𝑅−𝛾 = 𝛼 𝑅−𝛾
• γ usually lies between 2 and 5 depending on the actual conditions.
– Of course, γ cannot be lower than 2, which is the free-space condition.
• The decibel scale expression of 𝐶 = 𝛼 𝑅−𝛾 is,
C = 10 log α − 10γ log R dB

• where
– C = received Carrier Power
– R = distance measured from the transmitter to the receiver
– α = constant
 Electromagnetic Wave Propagation
• Radio wave propagation is affected by the following mechanisms:
i.e. Three Basic Propagation Mechanisms:
➢ Reflection - large obstacles
➢ Diffraction - edges
➢ Scattering - small obstacles
 Electromagnetic Wave Propagation
Reflection- large obstacles
➢ Reflection occurs when a radio wave collides with an object which has very
large dimensions compared to the wavelength of the propagating wave.
➢ Ex. the surface of the Earth, buildings, walls, etc.
Diffraction – edges
➢ Radio path between transmitter and receiver obstructed by surface with sharp
irregular edges, Waves bend around the obstacle, even when LOS (line of
sight) does not exist.
Scattering - small obstacles
➢ Scattering occurs when the radio wave travels through a medium consisting
of objects with dimensions that are small compared to the wave’s wavelength.
➢ Ex. foliage, street signs, lamp posts..

diffraction scattering
reflection
 Electromagnetic Wave Propagation
• Due to multiple reflections from various objects ,the electromagnetic waves
travel along different paths of varying lengths resulting in multipath
propagation.
• In multipath scenario, several copies of the same signal arrive at the
receiver with different path lengths at different times and with varying
amplitudes and phases.
• The strengths of the waves decrease as the distance between the transmitter
and receiver increases.
• Fading is deviation of the attenuation, affecting a signal over certain
propagation media.

Fading is of two types:

• Large scale fading: Average signal power attenuation or path loss due to
motion over large areas.
• Small scale fading: Large variation in signal power due to small changes in
the distance between the transmitter and receiver.
 Fading
• Fading is caused by destructive interference between two or more versions
of the transmitted signal being slightly out of phase due to the different
propagation time. This is also called multipath propagation.
• The different components are due to reflection and scattering form trees
buildings and hills etc.
• The transmitted signal experience attenuation, delay, and phase shift while
traveling from to source to the receiver resulting in reduced signal strength.
This phenomenon is called channel fading.
Reasons for Fading:
• Propagation Loss
• Multipath Effect
– Time delay
– Phase shift
– Attenuation (addition of noise)

• If the mobile unit moves fast, the rate of fluctuation is fast, which leads to
Severe Fading.
• Large scale fading • Small scale fading
• Tx , Rx distance is large • Tx , Rx distance is small
• For average distance strength is • For every instance strength is
calculated calculated
 Fading - Propagation Models
• Propagation models have traditionally focused on predicting the average
received signal strength at a given distance from the transmitter.
• Propagation models are of two types depending on the power measured in
certain distance/time.
Large-scale propagation model
➢ Propagation models that predict the mean signal strength for an
arbitrary transmitter-receiver (T-R) separation distance are useful in
estimating the radio coverage area of a transmitter.
➢ They are called large-scale propagation models, Since they characterize
signal strength over large T-R separation distances (several hundreds or
thousands of meters).

Small-scale propagation or fading model

➢ Propagation models that characterize the rapid fluctuations of the
received signal strength over very short travel distances (a few
wavelengths) or short time durations (on the order of seconds) are called
small-scale fading models.
 Long-term Fading
• Long-term fading occurs when the propagation environment is changing
significantly. But this fading is typically much slower than short-term
fading.
• Long-term fading means slower variation in mean signal strength and is
produced by movement over much longer distances.
• Long-term is caused by terrain configuration (hill, flat area etc.), which
results in local mean attenuation and fluctuation.
• Long term fading is also called as slow fading or shadowing.
 Large Scale Fading
• The free space propagation model is used to predict received signal
strength when the Transmitter and Receiver have a clear, unobstructed line-
of-sight path between them.
• This may applicable for,
– Satellite Communication systems and
– Microwave line-of-sight radio link.
 Free Space Propagation Model
• The free space power received by a receiver antenna which is separated
from a radiating transmitter antenna by a distance (d), is given by the Friis
free space equation,
• The free space power received by a receiver is
𝑃𝑡 𝐺𝑡 𝐺𝑟 𝜆2
𝑃𝑟 𝑑 =
4𝜋 2 𝑑 2 𝐿

▪ Pt is the transmitted power

▪ Pr(d) is the received power
▪ Gt is the transmitter antenna gain,
▪ Gr is the receiver antenna gain
▪ d is the Tx-Rx separation distance in meters,
▪ L is the system loss factor/spatial attenuation.
 Free Space Propagation Model
• The gain of an antenna is related to its effective aperture( Ae).
4𝜋𝐴𝑒
𝐺=
𝜆2
• The effective aperture is related to the physical size of the antenna.

• The frequency is related as

𝑐 2𝜋𝑐
𝜆= =
𝑓 𝜔𝑐

• The path loss for the free space model when antenna gains are included is
given by
𝑃 𝐺𝑡 𝐺𝑟 𝜆2
𝑃𝐿 𝑑𝐵 = 10𝑙𝑜𝑔 𝑡 = −10𝑙𝑜𝑔
𝑃𝑟 4𝜋 2 𝑑 2
• When antenna gains are excluded, the antennas are assumed to have unity
gain, and path loss is given by
𝑃 𝜆2
• 𝑃𝐿 𝑑𝐵 = 10𝑙𝑜𝑔 𝑡 = −10𝑙𝑜𝑔
𝑃𝑟 4𝜋 2 𝑑 2
 Small Scale Fading
• Small-scale fading, or simply fading, is used to describe the rapid
fluctuation of the amplitude of a radio signal over a short period of time or
travel distance.
• Multipath in the radio channel creates small-scale fading effects.
▪ Rapid changes in signal strength
- small travel distance or time interval
▪ Random frequency modulation - varying Doppler shifts.
▪ Time dispersion (echoes) caused by multipath propagation delays.
Relationship between Wavelength & Frequency
❑ Doppler Shift
• The Doppler effect (or Doppler shift) is the change in frequency of a wave
for an observer moving relative to the source of the wave.
• In classical physics (waves in a medium), the relationship between the
observed frequency f and the emitted frequency fo is given by:

• where
▪ v is the velocity of waves in the medium,
▪ vs is the velocity of the source relative to the medium and
▪ vr is the velocity of the receiver relative to the medium.

• If the car (mobile) is moving towards the direction of the arriving wave, the
Doppler shift is positive.
• Multipath: Different Doppler shifts at different angles.
• Many Doppler shifts → Doppler spread.
❑ What is a positive Doppler shift?
• As a convention, the velocity is positive if the source is moving away from
us and negative if the source is moving towards the observer.
• Thus: if the source is moving away(positive velocity) the observed
frequency is lower and the observed wavelength is greater
❑ Doppler shift
• The Doppler shift is the change in frequency of a wave in relation to an
observer who is moving relative to the wave source.
• Hence, the time between the arrivals of successive wave crests (peaks) at
the observer is reduced, causing an increase in the frequency.
❑ Does Doppler effect depend on distance?
• The Doppler effect causes the received frequency of a source to differ from
the sent frequency if there is motion that is increasing or decreasing the
distance between the source and the receiver.
❑ What affects the Doppler effect?
• A similar change in observed frequency occurs if the source is still and the
observer is moving towards or away from it. In fact, any relative motion
between the two will cause a Doppler shift/effect in the frequency
observed.
• The pitch we hear depends on the frequency of the sound wave.
❑ How is the Doppler effect used in everyday life?
• Radar. The Doppler effect is used to measure the velocity detected objects
where a radar beam is fired at a moving target.
• For example, the police use radar to detect a speeding vehicle. Radio waves
are fired using a radar gun at the moving vehicle.

❑ Why is the Doppler effect important?

• The Doppler effect is important in astronomy because it enables the
velocity of light emitting objects in space, such as stars or galaxies, to be
worked out.
• As a consequence an observer (mobile user) in front of the approaching
sound actually hears a sound of higher frequency, since more than f
vibrations reach her in a second
 Factors Influencing Short Term Fading
❑ Many physical factors in the radio propagation channel influence small
scale fading.
➢ Multipath propagation.
➢ Speed of the mobile.
➢ Speed of surrounding objects.
➢ The transmission bandwidth of the signal.

❑ Large scale fading is used to describe the signal level at the receiver after
traveling over a large area (hundreds of wavelengths)
❑ Small scale fading is used to describe the signal level at the receiver after
encountering obstacles near the receiver (several wavelengths to fractions
of wavelengths)
 Factors Influencing Short Term Fading
• Multipath Propagation
• The presence of reflecting objects and scatterers in the channel creates a
constantly changing environment that dissipates the signal energy in
amplitude, phase, and time.
• These effects result in multipath propagation.
• The multipath propagation results to fluctuations in signal strength, thereby
inducing small-scale fading, signal distortion, or both.
• Multipath propagation often lengthens the time required for the baseband
portion of the signal to reach the receiver which can cause signal smearing
due to intersymbol interference.
or
• Multipath is the propagation phenomenon that results in radio signals
reaching the receiving antenna by two or more paths.
• The effects of multipath include constructive and destructive interference,
and phase shifting of the signal.
 Factors Influencing Short Term Fading
• Speed of the Mobile
• The relative motion between the base station and the mobile results in
random frequency modulation due to different Doppler shifts on each of the
multipath components.
▪ Doppler shift will be positive - moving toward BS.
▪ Doppler shift will be negative - away from the BS.

• The phase change in the received signal due to the difference in path,
results a change in frequency.
▪ Doppler shift positive - increase in frequency.
▪ Doppler shift negative - decrease in frequency.

• When the objects are closing range, the Doppler frequency shift will be
positive, meaning the return signal will have a higher frequency than that
transmitted.
 Factors Influencing Short Term Fading
• Speed of Surrounding Objects
• If objects in the radio channel are in motion, they induce a time varying
Doppler shift on multipath components.
• If the surrounding objects move at a greater rate than the mobile, then this
effect dominates the small-scale fading.
• Otherwise, motion of surrounding objects may be ignored, and only the
speed of the mobile need be considered.
 Factors Influencing Short Term Fading
• The Transmission Bandwidth of the Signal
• If the transmitted radio signal bandwidth is greater than the "bandwidth" of
the multipath channel, the received signal will be distorted, but the received
signal strength will not fade much over a local area.
• The bandwidth of the channel can be quantified by the coherence
bandwidth.
• The coherence bandwidth is a measure of the maximum frequency
difference for which signals are still strongly correlated in amplitude.

• If the transmitted signal has a narrow bandwidth as compared to the

channel, the amplitude of the signal will change rapidly, but the signal will
not be distorted in time.
 Parameters of Mobile Multipath Fading
• Wired communication - channel is Static
• Wireless system - channel is not static (continuously varying)
– Have to Increase Strength & Decrease Noise

• To analyze the multipath channel assume either transmitter or receiver is

moving, hence multiple version of transmitted signal is created in channel.
• All the multipath components which reaches the receiver at various time
are summed up.
• Averaging instantaneous power delay measurements is done over a local
area.
• Local area not greater than 2m are considered as Indoor. Local area not
greater than 6m are considered as Outdoor.
• The multipath channel parameters are derived from the Power Delay
Profile (PDP).
 Power delay profile
• Power Delay Profiles are generally represented as plots of relative
Received Power as a function of excess delay with respect to a fixed time
delay reference.
• Power delay profiles are found by averaging instantaneous power delay
profile measurements over a local area in order to determine an average
small-scale power delay profile.
 Parameters of Mobile Multipath Fading
• Multipath propagation causes severe dispersion of the transmitted signal
and the expected degree of dispersion is determined through the
measurement of the power-delay profile of the channel.
• The power-delay profile provides an indication of the dispersion or
distribution of transmitted power over various paths in a multipath model
for propagation.

• Parameters of Mobile Multipath Fading are:

1. Time Dispersion Parameters
▪ Determined from Power Delay Profile
▪ Parameters include
✓ Mean Access Delay
✓ RMS Delay Spread
✓ Excess Delay Spread (X dB)
2. Coherence Bandwidth
3. Coherence Time & Doppler Spread
• Dispersion means breaking up
 Time Dispersion Parameters
1. Mean Access Delay ( τҧ )
• Mean excess delay is the first moment of the power delay profile and is
defined as

• where,
– ak is the amplitude,
– τk is the excess delay and
– P(τk) is the power of the individual multipath signals
• The power delay profile (PDP) gives the intensity of a signal received
through a multipath channel as a function of time delay. The time delay is
the difference in travel time between multipath arrivals.
 Time Dispersion Parameters
2. RMS Delay Spread ( 𝜎𝜏 )
• RMS delay spread is the square root of the second central moment of the
power delay profile, it can be written as

• where,

• Depends only on the relative amplitude of the multipath components

• Typical RMS delay spreads
▪ In the order of microseconds : Outdoor
▪ In the order of nanoseconds : Indoor
 Time Dispersion Parameters
3. Maximum Excess delay ( 𝜏𝑚 )
• Maximum excess delay (X dB) is defined to be the time delay during which
multipath energy falls to X dB below the maximum.

𝐸𝑥𝑐𝑒𝑠𝑠 𝑑𝑒𝑙𝑎𝑦, 𝜏𝑚 = 𝜏𝑋 − 𝜏0
• where,
– 𝜏𝑋 is the maximum delay at which a multipath component is within X dB
– 𝜏0 is the first arriving signal

• It is also called as Excess Delay Spread

• This is measured with respect to a specific power level, which is

characterized as the threshold of the signal.
• When the signal level is lower than the threshold, it is processed as Noise.
Time Dispersion Parameters
 Frequency Dispersion Parameters
• Coherence Bandwidth
• Coherent Bandwidth, Bc , is the range of frequencies over which the
channel can be considered to be “Flat” (i.e. No Slope).
• A channel which passes all spectral components with approximately equal
gain and linear phase.
• Practically, coherence bandwidth (Bc) is the minimum separation over
which the two frequency components are affected differently.

• If the coherent bandwidth is defined as the bandwidth over which the

frequency correlation function is above 0.9, then the coherent bandwidth is
approximately.
𝜎𝜏 is rms delay spread
• If the frequency correlation function is above 0.5
 Frequency Dispersion Parameters
• Coherence Time
• Coherent Time, Tc, is a statistical measure of the time duration over which
the channel impulse response is almost invariant.
• When channel behaves like this, it is said to be slow faded.
• It is the minimum time duration over which two received signals are
affected differently.

• Two signals arriving with a time separation greater than Tc are affected
differently by the channel.
• A statistic measure of the time duration over which the channel impulse
response is essentially invariant.
• If the coherent time is defined as the time over which the time correlation
function is above 0.5, then
 Frequency Dispersion Parameters
• Doppler Spread
• Doppler spread and coherent time are parameters which describe the time
varying nature of the channel in a small-scale region.
• When a pure sinusoidal tone of fc is transmitted, then the received signal
spectrum, called the Doppler spectrum, will have components in the range
fc-fd and fc+fd, where fd is the Doppler shift.
 Types of Small Scale Fading
• The type of fading experienced by a signal propagating through a mobile
radio channel depends on the nature of the transmitted signal with respect
to the characteristics of the channel.

• Signal parameters as Bandwidth, Symbol period.

• Channel parameters as RMS delay and Doppler spread.

• Multipath delay spread leads to time dispersion and frequency selective

fading
• Doppler spread leads to frequency dispersion and time selective fading
Types of Fading
 Types of Small Scale Fading
Small Scale Fading
(Based on Multipath Time Delay Spread)

Flat Fading Frequency Selective Fading

1. BW of Signal < BW of Channel 1. BW of Signal > BW of Channel
2. Delay spread < Symbol period 2. Delay spread > Symbol period

Small Scale Fading

(Based on Doppler Spread)

Fast Fading Slow Fading

1. High Doppler Spread 1. Low Doppler Spread
2. Coherence Time < Symbol period 2. Coherence Time > Symbol period
3. Channel Variation Faster than 3. Channel Variation Slower than
baseband Signal Variations baseband Signal Variations
 Flat Fading
• Small scale fading based on Multipath time delay spread.
▪ Flat Fading
▪ Frequency Selective Fading

• Flat Fading:
• Time dispersion due to multipath causes the transmitted signal to undergo
either flat or frequency selective fading.
• If the mobile radio channel has a constant gain and linear phase response
over a bandwidth which is greater than the bandwidth of the transmitted
signal, then the received signal will undergo flat fading.
• The received signal strength changes with time due to fluctuations in the
gain for the channel caused by multipath.
 Flat Fading
• The characteristics of a flat fading channel are illustrated in Figure

• It can be seen from that if the channel gain changes over time, a change of
amplitude occurs in the received signal.
• Over time, the received signal r(t) varies in gain, but the spectrum of the
transmission is preserved.
• Flat fading channel is also called amplitude varying channel.
• Also called narrow band channel: bandwidth of the applied signal is narrow
as compared to the channel bandwidth.
• Increase the transmit power to combat this situation.
 Flat Fading
• Time varying statistics: Rayleigh flat fading.
• A signal undergoes flat fading if,
• Bandwidth of the applied signal is smaller than the coherence bandwidth of
the channel

𝐵𝑆 ≪ 𝐵𝐶
• and
• The multipath time delay spread of the channel is smaller than the signal
duration of the transmitted signal
𝑇𝑆 ≫ 𝜎𝜏

▪ 𝑇𝑆 : Symbol period
▪ 𝜎𝜏 : rms Delay Spread
▪ 𝐵𝑆 : Signal bandwidth
▪ 𝐵𝐶 : Coherence bandwidth
 Frequency Selective Fading
• If the channel possesses a constant-gain and linear phase response over a
bandwidth that is smaller than the bandwidth of transmitted signal, then the
channel creates Frequency Selective Fading.

• Frequency selective fading is due to time dispersion of the transmitted

symbols within the channel.
– Induces inter symbol interference.
• Frequency selective fading channels are much more difficult to model than
flat fading channels.
 Frequency Selective Fading
• Statistic impulse response model
➢ 2-ray Rayleigh fading model
• For frequency selective fading
𝐵𝑆 > 𝐵𝐶 & 𝑇𝑆 < 𝜎𝜏
▪ 𝐵𝑆 : Signal bandwidth & 𝐵𝐶 : Coherence bandwidth
▪ 𝑇𝑆 : Symbol period & 𝜎𝜏 : rms Delay Spread

• Frequency selective fading channel characteristics

 Fast Fading
• Small scale Fading Effects due to Doppler Spread are of two types.
• Fast Fading.
• Slow Fading.

• Fast Fading:
• Fast fading is the rapid variation of the signal levels when the user device
moves a short distances.
• Fast fading is due to reflections of local objects and the motion of the user
device relative to those objects.
• The received signal is the sum of number of signals reflected from the local
surfaces.
• These signals sum in a constructive or destructive manner, depending on
their relative phase relationships.
 Fast Fading
• Fast Fading:
• The channel impulse response changes rapidly within the symbol duration.
• That is, the coherence time of the channel is smaller than the symbol period
of the transmitted signal.
• This causes frequency dispersion due to Doppler spreading, which leads to
signal distortion.
• A signal undergoes fast fading if
𝑇𝑆 > 𝑇𝐶
• and
𝐵𝑆 < 𝐵𝐷

• 𝑇𝑆 : Symbol Period & 𝑇𝐶 : Coherence Bandwidth

• 𝐵𝑆 : Bandwidth of the signal & 𝐵𝐷 : Doppler Spread
 Slow Fading
• Slow Fading:
• Most of the large reflectors and diffracting objects along the transmission
path are distant from the user device.
• The motion of the user devices relative to these distant objects is small. So
the propagation changes are slow.
• These factors contribute to the mean path losses between a fixed transmitter
and a fixed receiver.
• The variation of these mean losses was modeled as lognormal distribution.
• The slow-fading process is also referred to as shadowing or lognormal
fading.
 Slow Fading
• Slow Fading:
• The channel impulse response changes at a rate much slower than the
transmitted baseband signal s(t).
• The Doppler spread of the channel is much less then the bandwidth of the
baseband signal.
• i.e., A signal undergoes slow fading if
𝑇𝑆 ≪ 𝑇𝐶 & 𝐵𝑆 ≫ 𝐵𝐷
Types of Fading