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Advanced Wireless and Mobile Computing Networks
(CSC544)
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Lecture 2 – Wireless Systems
Dr. Sarmad Ahmed Shaikh
Email: sarmad.ahmed107@gmail.com
Sindh Madressatul Islam University (SMIU), Karachi
Spring-2022

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 In previous lecture, we discussed
– Introduction to
• Communication systems
• Wireless comm
– History of Wireless Comm
– Current trends
– Challenges
 In this lecture, we will study
– Basic Concept of Multiple Access
– Wireless systems
– The Wireless Revolution
• 1G, 2G, 3G, 4G
– Modern Wireless Comm Systems
• Cellular Telephone Systems

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Basic Concept: Multiple Access
 Allow many mobile users to share the finite radio spectrum
 Achieve high capacity by simultaneously allocating the bandwidth
 Constraint:
– Should not have severe performance degradation
– Quality of Service (QoS)

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FDMA – Frequency Division Multiple Access
 Each user transmits and receives at different frequencies as each
user gets a unique frequency slot
 Requires high-performing filters in the radio hardware
 Not vulnerable to the timing problems that TDMA has

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TDMA – Time Division Multiple Access
 A fixed sequence of time slots is transmitted repeatedly over a
single transmission channel
 TDMA works by dividing a radio frequency into these time slots
and then allocating the slots to multiple calls
 In this way, a single frequency can support multiple, simultaneous
data channels
– GSM uses TDMA
TDMA requires
synchronization –
important issue.

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CDMA - Code Division Multiple Access
 Users in a CDMA cellular environment simultaneously share the
same radio frequency band
 They can be separated at the receiver end with the knowledge of
their unique code
– people could take turns speaking (time division)
– speak at different pitches (frequency division)
– or speak in different languages (code division)

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 Each user in a CDMA system uses a different code to modulate
their signal.
 Technology known as spread spectrum – because frequency is
increased in modulated signal

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Wireless Systems
 Range Comparison
Satellite
Links
SW
Radio
MW
Radio
FM
Radio
Mobile
Telephony
WLANs
Blueooth
1,000 Km
100 Km
10 Km
1 Km
100 m
10 m
1 m

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 Wireless LANs
 Satellite Systems
 Paging Systems
 Bluetooth
 Cellular systems
 Ultra Wide Band Systems

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Wireless Local Area Networks (WLANs)
 WLANs connect “local” computers (100m range)
 Breaks data into packets
 Channel access is shared (random access)
 Backbone Internet provides best-effort service
 Poor performance in some apps (e.g. video)
01011011
Access
Point
0101 1011

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 802.11b (Current Generation)
– Standard for 2.4GHz ISM band
– 1.6-10 Mbps, 500 ft range
 802.11a (Emerging Generation)
– Standard for 5GHz band
– OFDM with time division
– 20-70 Mbps, variable range
 802.11g (Newer Standard)
– Standard in 2.4 GHz and 5 GHz bands
– Speeds up to 54 Mbps
Wireless LAN Standards
Since 2008,
all WLAN
Cards have
all 3
standards

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Satellite Systems
 Cover very large areas
 Different orbit heights
– GEOs (35786 Km)
– LEOs (2000 Km)
 Optimized for one-way
transmission
– Radio and movie (SatTV)
 Most two-way systems struggling
– Expensive alternative to terrestrial system
– A few ambitious systems on the horizon

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Paging Systems
 Send brief messages to a subscribers
– Messages are received but not acknowledged
 Paging systems are typically used to notify a subscriber of the need
to call a particular telephone
– Broad coverage for short messaging
 Message broadcast from all base stations
 Simple terminals
 Optimized for 1-way transmission
– Simplex
 Answer-back hard
 Overtaken by cellular
 In modern time
– Fax machines

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Bluetooth
 Cable replacement RF technology (low cost)
 Short range (10m, extendable to 100m)
 2.4 GHz band (crowded)
 1 Data (700 Kbps) and 3 voice channels
 Widely supported by telecommunications, PC, and consumer
electronics companies
 Few applications beyond cable replacement

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Cellular Systems
 Geographic region divided into hexagonal shaped cells
 Frequencies/timeslots/codes reused at spatially-separated
locations.
 Co-channel interference between same color cells.
 Base stations/MTSOs (Mobile Telephone Switching Office)
coordinate handoff and control functions
 Shrinking cell size increases capacity, as well as networking
burden
BASE
STATION
MTSO

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Types of Cells
Satellite
Macrocell
Microcell
Urban
In-Building
Picocell
Global
Suburban
Basic Terminal
PDA Terminal
Audio/Visual Terminal

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 Cell radii can vary from 10’s of meters in buildings to 100’s of
meters in the cities, up to several km’s in the countryside.
 Macrocells, provide overall area coverage
 Microcells, focus on slow moving subscribers moving between
buildings.
 Picocells, focus on the halls of a theater, or exhibition centre.

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BS
BS
MTSO
PSTN
MTSO
BS
Karachi
Lahore
Internet

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Evolution of Cellular Networks
 First Generation
– Launched in mid 1980s
– Analog System
– Analog Modulation
– Voice traffic only
– FDMA scheme used
– AMPS (Advanced Mobile Phone Service)
 Second Generation
– Voice focus only
– Digital systems, Digital Modulation
– TDMA and CDMA multiple access scheme
– Could provide data rate of around 9.6 kbps

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 Examples of 2G
– Global System for Mobile (GSM)
▪ TDMA/FDMA , 900 MHz and 1800 MHz band
– Personal Digital Communication (PDC)
• Popular in Japan
• Couldn’t sell anywhere else
– IS-95
• CDMA based
• Popular in US/South Korea
 Limitations of 2G
– Developed for Voice Communication
– Unsuitable for data traffic
– Average rate of the order of tens of kbps
– Not suitable for internet (packet switching)
– Multiple standards (no true global coverage)

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 2.5G
– Effort to remove the impediment of 2G
– Digital System
– Voice + Low data rate
– Short message service (SMS)
• Not real time. SMS is a store and forward message mechanism
• Uses signaling channels (i.e. very low cost to operators)
• Immense growth
– About 20% of service providers revenue come from SMS
– SMS text messaging is the most widely used data application in the world,
with 2.4 billion active users
– Internet access through GPRS (General Packet Radio Service)
– Enhanced data rates for Global Evolution (EDGE)
• Uses better modulation techniques

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3G
 A mobile standard that links personal communication users into
network broad-band multimedia, information technology and
telecommunication services and applications
– Digital Modulation
– Simultaneous Voice + High Speed Data
– Multi mega-bit Internet Access
– Multimedia Transmission
 You can do it all while moving
 Would speed at which you are travelling matter?
– QoS is different for stationary, walking and vehicular
 WCDMA & CDMA2000
 Similar but different

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3G standardization process
 International Mobile Telecommunications-2000 (IMT-2000)
– Global standard for third generation (3G) wireless communications, defined
by a set of interdependent ITU Recommendations
– includes GSM EDGE, UMTS, and CDMA2000 as well as DECT and WiMAX
 International Telecommunication Union (ITU) activities on IMT-
2000 comprise international standardization, including
– frequency spectrum and technical specifications for radio and network
components
– tariffs and billing
– technical assistance
– studies on regulatory and policy aspects

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WCDMA (UMTS)
 WCDMA (Wideband Code Division Multiple Access) belongs to the
UMTS family of 3G networks and sometimes used as a synonym
for UMTS
 Universal Mobile Telecommunications System (UMTS) is one of
the third-generation (3G) cell phone technologies, which is also
being developed into a 4G technology.
– Currently, the most common form of UMTS uses W-CDMA as the underlying
air interface. It is standardized by the 3GPP, and is the European answer to
the ITU IMT-2000 requirements for 3G cellular radio systems.
 W-CDMA transmits on a pair of 5 MHz-wide radio channels, while
CDMA2000 transmits on one or several pairs of 1.25 MHz radio
channels.

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4G
 Extension of 3G with Evolved UMTS Terrestrial Radio Access
Network (E-UTRAN), OFDMA
– Long term evolution (LTE)
– Data rates 20 to 100 Mbps

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Roadmap of Wireless Communication

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Bits
per
second

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Open System Interconnection (OSI) Model
 In the OSI model, a communication network comprises nodes that
implement the procedures described by the seven layers
 This class is about L1: the physical layer
L7: Application
L6: Presentation
L5: Session
L4: Transport
L3: Network
L2: Data link
L1: Physical

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Modern Wireless Communication Systems

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Cellular Telephone Systems
 High capacity due to cell
 High quality service often comparable to landline telephone
systems
 Handoff enables a user to move uninterrupted
 Mobile switching center (MSC) responsible to connect all users to
public switching telephone network (PSTN)
 Base station (BS) consists of several transceivers and Antennas

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 Base station serves as a bridge
 Base station connected to MSC via microwave link or telephone
lines
 MSC can handle 5000 conversations at a time
 MSC also handles billing and system maintenance functions

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 Communication between base station (BS) and mobile station
(MS) using
 Common air interface (CAI)
– Forward Voice Channels: BS to MS
– Reverse Voice Channels: MS to BS
– Forward Control Channels (FCC)
– Reverse control channels

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 When a cellular phone is turned on, it first scans the group of FCC
 5% channels are used for control purpose
 95% used for voice and data
 When a telephone call is placed to a mobile user
– MSC dispatches the request to all BSs
 Mobile Identification Number (MIN) is broadcast as a paging
message over all of the FCC channels through out the cellular
system

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 The mobile receives the paging message and responds over
reverse control channel
 The BS informs the MSC of hand shake and MSC instruct the BS to
move the call to unused voice channel
 All these events occur in few seconds
 Once the call is in progress, the MSC adjusts the power of the
mobile to maintain call quality. Handoff !!!

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Figure 1.6 Timing diagram illustrating how a call to a mobile user initiated by a landline
subscriber is established.

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Figure 1.7 Timing diagram illustrating how a call initiated by a mobile is established.

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 When a mobile originates a call, a call request is sent on RCC
 With this request, mobile also send its telephone number (MIN),
Electronic serial number (ESN) and the telephone number of the
called party
 Mobile also transmits station class mark (SCM), which means,
what is the transmitter power level