Wireless Broadband Networks

MOBILE COMMUNICATION SYSTEMS AND
STANDARDS
Wireless Broadband Networks
Prepared by Dr.T.Deepa 129-04-2020

Outline
• History of Communication Network
• Wired and wireless Communication Technologies
• Orthogonal frequency division Multiplexing (OFDM)
• WiMAX- Features
• Challenges for WiMAX system Design
29-04-2020 Prepared by Dr.T.Deepa 2

Wireless Comes of Age
• Guglielmo Marconi invented the wireless telegraph in 1896
• Communication by encoding alphanumeric characters in analog signal
• Sent telegraphic signals across the Atlantic Ocean
• Communications satellites launched in 1960s
• Advances in wireless technology
• Radio, television, mobile telephone, communication satellites
• More recently
• Satellite communications, wireless networking, cellular technology

Network
• A computer network is two or more computers connected together
using a telecommunication system for the purpose of communicating
and sharing resources
• Why Networks?
Overcome geographic limits
Access remote data
Separate clients and server
• Goal: Universal Communication (any to any)

Client /server network
client/server network. A computer network in which one
centralized, powerful computer (called the server) is a
hub to which many less powerful personal computers or
workstations (called clients) are connected.
The clients run programs and access data that are stored
on the server. Compare peer-to-peer network.
Need for Computer Networking. Computer networks help
users on the network to share the resources and in
communication. ...
File sharing: Networking of computers helps the network users
to share data files. Hardware sharing: Users can share devices
such as printers, scanners, CD-ROM drives, hard drives etc.

Types of Networks
• PAN: a personal area network is a computer network (CN) used for communication
among computer devices (including telephones and personal digital assistants) close to
one person
• Technologies: USB and Fire wire (wired), IrDA and Bluetooth (wireless)
• LAN: a local area network is a CN covering a small geographic area, like a home,
office, or group of buildings
• Technologies: Ethernet (wired) or Wi-Fi (wireless)
• MAN: Metropolitan Area Networks are large CNs usually spanning a city
• Technologies: Ethernet (wired) or WiMAX (wireless)
• WAN: Wide Area Network is a CN that covers a broad area,
• e.g., cross metropolitan, regional, or national boundaries
• Examples: Internet
• Wireless Technologies: HSDPA, EDGE, GPRS, GSM.

Mobile Communication Technologies

The IEEE Family
IEEE 802 is a family of IEEE standards dealing with local
area networks and metropolitan area networks. More
specifically, the IEEE 802 standards are restricted to
networks carrying variable-size packets. By contrast, in
cell relay networks data is transmitted in short, uniformly
sized units called cells.

Salient Features of IEEE 802.16 Family

Optical Wireless Networks
• Optical wireless communication enables communication using infrared ray.
• Operates outdoor up to 5 Km and indoor a few meters.
• Advantages:
• Abundance of unregulated bandwidth: 200 THz in the 700 – 1500 nm range
• No multipath fading: Intensity modulation and direct detection
• Higher capacity per unit volume
• Cost effective at rates near 100 Mbps
• Small cell size
• At 800 – 890 nm and 1550 nm absorption effects are minimal.
• Disadvantages:
• Multipath dispersion
• Limited range
• Difficult to operate outdoor
• High power requirement
• SNR can vary significantly with the distance
• costly
10Prepared by Dr.T.Deepa29-04-2020

Broadband vs Baseband
• baseband transmission the whole bandwidth of the cable is utilized
by a single signal, Eg., LAN.
• Conversely, in the broadband transmission, multiple signals are sent
on multiple frequencies simultaneously using a single channel, Eg.,
WAN.

Broadband Wireless Technology
• Higher data rates obtainable with broadband wireless technology
• Graphics, video, audio
• Shares same advantages of all wireless services: convenience and
reduced cost
• Service can be deployed faster than fixed service
• No cost of cable plant
• Service is mobile, deployed almost anywhere

Wireless Broadband
• Wireless broadband is high-speed Internet and data service delivered
through a wireless local area network (WLAN) or wide area
network (WWAN).
• FIXED
• MOBILE

Fixed line broadband
• DSL (Digital Subscriber Line) is a technology for bringing high-
bandwidth information to homes and small businesses over ordinary
copper telephone lines. xDSL refers to different variations of DSL,
such as ADSL, HDSL, and RADSL.
• Delivers broadband over twisted pair telephone wires , and cable
modem technology
• Delivers over coaxial cable TV Plant

Fixed Wireless Broadband
• Wireless broadband may be either fixed or mobile.
• Fixed Wireless Broadband
• Alternative to DSL technology
• The service is similar to that provided through digital subscriber line (DSL) or cable
modem but the method of transmission is wireless.
• A fixed wireless service provides wireless Internet for devices in relatively permanent
locations, such as homes and offices.
• Fixed wireless broadband technologies include LMDS (Local Multipoint Distribution
System) and MMDS (Multichannel Multipoint Distribution Service) systems for
broadband microwave wireless transmission direct from a local antenna to homes and
businesses within a line-of-sight radius.

Applications
• Provides Web surfing and quicker file downloads
• Enables several multimedia applications
• Real time audio and video streaming
• Multimedia conferencing and interactive gaming.
• Voice telephony using voice over Internet Protocol
(VoIP)
• Advanced broadband systems
• FTTH
• VDSL
• Enable such applications as entertainment quality
video including HDTV and video on demand (VoD)

Benefits of Fixed Broadband
• IEEE 802.16d is optimized for fixed access and 802.16e for mobile access.
• Benefits of fixed networks are as follows:
• Less complex modulation. As compared to SOFDMA, OFDM is a simpler modulation technique.
For markets where mobility is not to be supported, OFDM is a less complex option, making the
fixed network quickly deployable at a lower cost.
• License-exempt bands. Fixed deployments successfully use license exempt bands in areas where
interference levels are acceptable, in comparison to mobile services, which require a licensed
spectrum to provide coverage in wide areas. Thus, 802.16-2004 includes most of the profiles
targeting license-exempt bands.
• Higher throughput. Higher-spectrum bands selected for 802.16-2004 profiles have an advantage of
higher throughput.
• Better time to market. Earlier commercial availability of 802.16-2004 products has enabled
operators to meet the pent-up demand for broadband connectivity in underserved areas and to start
gaining market share ahead of competitors.

Mobile Broadband
• A mobile broadband service provides connectivity to users who may
be in temporary locations, such as coffee shops.
• Mobile broadband works through a variety of devices, including
portable modems and mobile phones, and a variety of technologies
including WiMAX, GPRS, and LTE.
• Mobile broadband does not rely on a clear line of sight because
connectivity is through the mobile phone infrastructure.
• Mobile devices can connect from any location within the area of
coverage.
• WiMAX supports both fixed and mobile wireless and is often predicted to
become the standard for wireless broadband.

Mobile WiMAX
• Mobile WiMAX has its own added advantages.
• Most of the companies are bringing to market dual-band products that
enable deployment of both mobile and fixed WiMAX environments.
• The choice between 802.16-2004 and 802.16e products largely
depends on the type of services provided and the business model of the
operator.
• A mobile operator building an overlay network to complement a 3G
network will head straight for 802.16e.
• A wireless Internet service provider (WISP) supplying wireless access
to a rural community will typically choose the less complex, OFDM-
based, 802.16-2004 WiMAX products.

Advantages of Mobile Broadband (IEEE 802.16e)
• Advantages that allow 802.16e to stand in competition with 3G cellular networks and that lead 802.16e
to migrate toward 4G [9] are as follows:
 Technology. 802.16e is based on OFDMA technology paired with MIMO smart antenna technology,
which is best suited for 4G.
 Coverage. WiMAX has a coverage range of 4 to 6 miles (30 miles maximum).
 Spectrum. 802.16 can work in both the licensed as well as the unlicensed frequency bands.
 Interference. OFDM, which is supported by 802.16, utilizes multiple channels to send and receive
data, which results in less interference.
 IP connectivity. 802.16e supports asynchronous transfer mode (ATM), IP versions 4 (IPv4) and 6
(IPv6), Ethernet, and virtual
 local area network (VLAN) services, which provide a rich choice of service possibilities to voice
and data network service providers.
 Interoperability. Interfaces are IP based, which permits reuse of mobile client software across
operator domains.
 Backhaul. 802.16 provides backhaul connections to cellular services.
 Standardization and economies of scale. The WiMAX Forum works on standardization, which will
provide the ability for mass production of WiMAX-enabled products, lowering the service,
development, and deployment costs.

Limitations and Difficulties of Wireless
Technologies
• Wireless is convenient and less expensive
• Limitations and political and technical difficulties inhibit wireless
technologies
• Lack of an industry-wide standard
• Device limitations
• E.g., small LCD on a mobile telephone can only displaying a few lines of text
• E.g., browsers of most mobile wireless devices use wireless markup language
(WML) instead of HTML

23
History of Mobile Technologies
Technology 1G 2G 2.5G 3G 4G
Design Begin 1970 1980 1985 1990 2000
Implementation 1984 1991 1999 2002 2010 ?
Service Analog voice Digital voice,
SMS
Higher
capacity,
Packet data,
MMS
Higher
capacity,
Broadband
data
Higher capacity,
Complete IP,
multimedia
Standards AMPS,
TACS,NMT
TDMA,CDMA,
GSM,PDC
GPRS,
EDGE
WCDMA,
CDMA2000
Single standard
Bandwidth 1.9kbps 14.4kbps 384kbps 2Mbps 100+Mbps
Multiplexing FDMA TDMA,
CDMA
TDMA,
CDMA
CDMA CDMA ?
Core Network PSTN PSTN PSTN,
Packet
network
Packet
network
IP network
(Internet)
Prepared by Dr.T.Deepa29-04-2020

Wireless Fidelity Systems (WiFi)
• Wireless Fidelity (WiFi) is the standard
for the high-speed wireless LAN.
• A Wi-Fi network can be used to connect
computers to each other, to the Internet,
and to wired networks (which use IEEE
802.3 or Ethernet).
• Wi-Fi networks operate in the unlicensed
2.4 and 5 GHz radio bands, with an
11/54 Mbps (802.11b/g) or 54 Mbps
(802.11a) data rate

Wi-Fi
• Wi-Fi is a technology for WLAN based on the IEEE 802.11 (a, b, g)
specifications
• Developed for PC in WLAN
• Used for more services:
• Internet and VoIP phone access, gaming, …etc.,
• and basic connectivity of consumer electronics such as televisions and
DVD players,or digital cameras,
• Wi-Fi -used by cars in highways in support of an Intelligent Transportation
System to increase safety, gather statistics, and enable mobile commerce
(IEEE 802.11p).
• Wi-Fi supports structured (access point) and ad-hoc networks (a PC and a
digital camera).

WiMAX
• WiMAX is an acronym that stands for
Worldwide Interoperability for Microwave
Access.
• The WiMAX Forum is an industry-led, non-
profit corporation formed to promote and certify
compatibility and interoperability of broadband
wireless products.
• The WiMax forum supports the industry-wide
acceptance of the IEEE 802.16 and ETSI
HiperMAN wireless MAN standards.

WiMAX(Contd)
• IEEE 802.16: Broadband Wireless Access / WirelessMAN / WiMax (Worldwide
Interoperability for Microwave Access) Connecting Wi-Fi hotspots with each
other and to other parts of the Internet
• Providing a wireless alternative to cable and DSL for last mile broadband access
• Providing high-speed mobile data and telecommunications services
• Providing Nomadic connectivity
• 75 Mbit/s up to 50 km LOS, up to 10 km NLOS; 2-5 GHz band
• Initial standards without roaming or mobility support 802.16e adds mobility
support, allows for roaming at 150km/h.

Introduction- Wireless Broadband Communication
28
 Increasing demand on high data rate communication services
 Orthogonal frequency division multiplexing(OFDM) technology can satisfy the
demand on high data rate services
 OFDM- Multicarrier modulation (MCM) system
 Can be deployed in many wireless standards
 WLAN –IEEE 802.11, WMAN-IEEE 802.16
 Digital audio/video broadcasting (DAB/DVB-T), Multi Band - OFDM Ultra Wide Band (MB-
OFDM UWB)
 Asymmetric Digital Subscriber Line(ADSL), Very High Speed Digital Subscriber Line
(VDSL) and Power Line Communications (PLC).

 Total transmission bandwidth is split into many subchannels(subcarriers) narrow
subcarriers and are transmitted in parallel
 An OFDM signal is the sum of orthogonal subcarriers, with data on each
subcarrier being independently modulated
 By making all subcarriers narrowband they experience almost flat fading
 subcarriers are orthogonally overlapped
29
Introduction- OFDM

OFDM
30
 Orthogonal Frequency Division Multiplexing (OFDM)
 Harmonically related narrowband sub-carriers
 The sub-carriers spaced by 1/Ts
 The peak of each sub-carrier coincides with trough of other
sub-carriers
 Splitting a high-speed data stream into a number
of low-speed streams
 Different sub-carrier transmitted simultaneously

Conventional OFDM system model
31
S/P- Serial to Parallel converter
P/S-Parallel to Serial converter
IFFT- Inverse Fast Fourier Transform
FFT –Fast Fourier Transform
CP-Cyclic Prefix
IDHT
Input
Data
Symbol
Mapping
S/P P/S
DHTP/SDemapping
Output
Data
Channel
Add
CP
Remove
CP
S/P

Representation of OFDM Symbols
• The complex baseband representation of a multicarrier OFDM signal
x(n) consisting of N subcarriers is given by
32
(1)
•PAPR for the discrete time OFDM signal x(n) is defined as,
    1Nn0ekX
N
1
nx nk/Nj2
1N
0k
 



 2
2
1Nn0
x(n)E
x(n)max
PAPR 

2
1Nn0
x(n)max

 2
x(n)E
is the maximum power of OFDM symbols
is the average power of OFDM symbols
Where,
(2)

33
 What is OOFDM
 By introducing OFDM in the optical domain, Optical
OFDM (OOFDM) was generated
 Advantages of OOFDM
 Cost-effective
 High-speed
 Excellent flexibility and robustness
Optical OFDM

OFDM- Advantages
 High data rate services
 Equalization is very simple compared to Single Carrier systems
 Robustness in multi-path environments.
• CP preserves orthogonality between sub carriers.
 Spectrally efficient
• IFFT/FFT operation ensures that subcarriers do not interfere with each
other.

OFDM- Drawbacks
 Sensitivity to Carrier frequency offset (CFO)
• caused by Doppler shift due to channel time variation result in
Inter Carrier Interference (ICI)
 The FFT/IFFT becomes one of the most critical modules in OFDM
transceivers.
• The rapidly increasing demand of OFDM based applications for
wireless broadband communications makes processing speed an
important major consideration in FFT architecture design.
 Peak to Average Power Ratio (PAPR)
• measure the ratio of the peak power level to the average power
level
• Large variations between the average and the peak signal power
in OFDM systems. 35Prepared by Dr.T.Deepa29-04-2020

High PAPR Problem
• Non constant envelope
signals
• Requires expensive and
high linear power
amplifier with high
dynamic range, if not
• Non linearity or clipping
introduces out of band
distortion and spectral
regrowth
Time domain Representation of OFDM
signal
36
0 10 20 30 40 50 60 70
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Amplitude
Sample Index

37
Existing PAPR Reduction Techniques
 Amplitude Clipping
 Clipping and Filtering
 Partial Transmit Sequence (PTS)
 Selective Mapping (SLM) technique
 Non-linear companding transforms (NCT)
 Tone Reservation (TR) and Tone Injection (TI) techniques
 Active Constellation Extension (ACE)
 Interleaving
 Coding Schemes

Existing PAPR Reduction Techniques(Cont.)
38
Schemes Power
increase
Data rate
loss
Distortion
less
BER
degradation
Computational
complexity
Clipping No No No Yes Low
Coding No Yes Yes No High
PTS/SLM No Yes Yes No High
NCT No No Yes Yes Low
TR/TI Yes Yes/No Yes No High
ACE Yes No Yes Yes Low
Reference: Rashmatallah, Y., Mohan,S, “Peak-To-Average Power Ratio
Reduction in OFDM Systems: A Survey And Taxonomy” IEEE Communications
Surveys & Tutorials, Vol.14.No.4,pp. 1567 – 1592, 2013.Prepared by Dr.T.Deepa29-04-2020

Factors for Effective PAPR reduction
techniques
 High capability PAPR reduction
 Low average power in transmit sequence
 No BER performance degradation at the receiver
 No loss in data rate
 Low computational complexity
 No spectral spillage
 High Power Amplifier (HPA) efficiency
 Effect of non-linear devices
o DACs, mixers and HPAs.

Technical Challenges –Wireless Broadband
Systems
• Developing reliable transmission through a wireless radio channel
• Achieving high spectral efficiency and coverage using limited available
spectrum
• Supporting and efficiently multiplexing services with a variety of QoS
requirements
• Supporting Mobility through seamless handover & roaming
• Achieving low power consumption to support handheld battery operated
devices.
• Providing robust security
• Adapting IP based protocols & Architecture for the wireless environment
to achieve lower cost and convergence with wired networks

Wireless Radio Channel
• Distance dependent decay of signal Power- Path Loss
• Blockage due to large obstructions - Shadowing
• Large Variation in Received Signal Envelope
• ISI due to time dispersion
• Frequency dispersion due to motion- Doppler Spread

Distance dependent decay of signal Power
• The wireless radio channel puts fundamental limitations to the
performance of wireless broadband systems
• Radio channels are extremely random, and are not easily analysed.
• Suppose s(t) of power Pt is transmitted through a given channel The
received signal r(t) of power Pr is averaged over any random
variations due to shadowing.
• We define the linear path loss of the channel as the ratio of transmit
power to receiver power

Path Loss
• Path loss, or path attenuation, is the reduction in
power density (attenuation) of an electromagnetic
wave as it propagates through space.
• It may be due to many effects, such as free-space
loss, refraction, diffraction, reflection, aperture-
medium coupling loss, and absorption.
• It is also influenced by terrain contours, environment
(urban or rural, vegetation and foliage), propagation
medium (dry or moist air), the distance between the
transmitter and the receiver, and the height and
location of antennas.

Blockage due to large obstructions shadowing
• Large obstructions such as buildings cause localized blockage of
signals.
• Shadowing is the effect that the received signal power fluctuates due
to objects obstructing the propagation path between transmitter and
receiver.
• These fluctuations are experienced on local-mean powers, that is,
short-term averages to remove fluctuations due to multipath fading.

Multipath Fading
• MULTIPATH FADING occurs
when a transmitted signal
divides and takes more than
one path to a receiver and
some of the signals arrive out
of phase, resulting in a weak
or fading signal.

Large Variation in Received Signal Envelope
• Multipath fading
• The amplitude of the received signal over very small durations
Signal
Strength
(dB)
Distance
Path Loss
Slow Fading
(Long-term
fading)
Fast Fading
(Short-term
fading)

Slow Fading vs Fast Fading
• Fading refers to variation in signal strength with respect to time as it is
received at the antenna from the transmitter at distant end. The variation
can be result of communication channel between the transmitter and
receiver.
• Slow fading can be caused by events such as shadowing, where a large
obstruction such as a hill or large building obscures the main signal path
between the transmitter and the receiver.
• Fast fading occurs when the coherence time of the channel is small relative
to the delay requirement of the application.

Solution -
• BWSs need to be designed to cope with these
large and rapid variations in received signal
strength. This is done through the use of one /
more diversity techniques.
• In this technique multiple antennas are
strategically spaced and connected to
common receiving system.
• While one antenna sees a signal null, one of
the other antennas may see a signal peak,
and the receiver is able to select the antenna
with the best signal at any time.

ISI due to time dispersion
• Caused by time delayed multipath signals
• ISI has been recognized as the major obstacle to high
speed data transmission over mobile radio channels.
• In a multipath environment, when the time delay
between significant fraction of transmitted signal’s
symbol period, a transmitted symbol may arrive at the
receiver during the next symbol period and cause ISI.
• Second multipath is delayed and is received during
next symbol
• At higher data rates, the symbol time is shorter , hence
it takes only a smaller delay to cause ISI. This makes
ISI a bigger concern for broadband wireless and
mitigating it more challenging

50
Intersymbol Interference (ISI)
Time
Time
Time
Transmission
signal
Received signal
(short delay)
Received signal
(long delay)
1
0
1
Propagation time
Delayed signals

Solution - Equalization & OFDM
Equalization is a technique used to combat inter symbol interference
(ISI).
 OFDM/WIMAX – the solution of choice for mitigating ISI in
broadband systems
51

Frequency dispersion due to motion
• Frequency dispersion results from different frequencies
propagating at different speeds.
• The relative motion between the transmitter and the receiver
causes carrier frequency dispersion called Doppler spread.
• Doppler spread is measure of spectral broadening caused by
motion.
• One of the challenging issues is the large Doppler spread, which is
caused by the high mobility of a wireless terminal and may lead to
severe communication performance loss. Doppler spread is a
measure of spectral broadening of the rate of change in a mobile
fading channel which is proportional to the mobile speed.
• Leads to loss of SNR & can make carrier recovery and
synchronization more difficult
• It is particular concern for OFDM systems . It may corrupt the
orthogonality of the OFDM subcarriers.

Spectrum Scarcity
• Scarcity of radio-spectrum resources.
• limited amount of spectrum for commercial use.
• The need to accommodate an ever-increasing number of users and
offering bandwidth-rich applications using a limited spectrum
challenges the system designer to continuously search for solutions
that use the spectrum more efficiently.
• Spectral-efficiency considerations impact many aspects of broadband
wireless system design.

Spectrum management is the process of regulating the use of radio frequencies to promote
efficient use and gain a net social benefit. The term radio spectrum typically refers to the
full frequency range from 3 kHz to 300 GHz that may be used for wireless communication.

Spectral Efficiency
• Spectral efficiency, spectrum
efficiency or bandwidth efficiency refers
to the information rate that can be
transmitted over a given bandwidth in a
specific communication system.

Concept of Cellular Architecture
• The most fundamental tool used to achieve higher
system-wide
• spectral efficiency is the concept of a cellular
architecture, whereby instead of using a single high-
powered transmitter to cover a large geographic area,
several lower-power transmitters that each cover a
smaller area, called a cell, are used.
• The cells themselves are often subdivided into a few
sectors through the use of directional antennas.
• Typically, a small group of cells or sectors form a
cluster, and the available frequency spectrum is divided
among the cells or sectors in a cluster and allocated
intelligently to minimize interference to one another.

Frequency reuse
• Principles of cellular frequency reuse. In the cellular
concept, frequencies allocated to the service are re-used in a regular
pattern of areas, called 'cells', each covered by one base station. In
mobile-telephone nets these cells are usually hexagonal.
• The pattern of frequency allocation within a cluster is then repeated
throughout the desired service area and is termed frequency reuse.

Concept of Cellular Architecture (Cont’d)
• For higher capacity and spectral efficiency, frequency reuse must be
maximized.
• Increasing reuse, however, leads to a larger potential for interference.
Therefore, to facilitate tighter reuse, the challenge is to design
transmission and reception schemes that can operate under lower
signal-to interference-plus-noise ratio (SINR) conditions or implement
effective methods to deal with interference.
• One effective way to deal with interference is to use multiple-antenna
processing.

Several signal processing techniques
• Several signal processing techniques can be used to maximize the
spectral efficiency and hence capacity of the system. Many of these
techniques exploit channel information to maximize capacity.
• Examples of these are included below
• Adaptive modulation and coding
• Spatial multiplexing
• Efficient multiaccess techniques

Adaptive modulation and coding:
• The idea is to vary the modulation and coding rate on a per user and/or per packet basis
based on the prevailing SINR conditions.
• By using the highest level modulation and coding rate that can be supported by the SINR,
the user data rates—and hence capacity—can be maximized.
• Definition: matching of the modulation, coding and other signal and protocol parameters
to the conditions on the radio link (e.g. the path loss, the interference due to signals
coming from other transmitters, the sensitivity of the receiver, the available transmitter
power margin, etc.).
• For example, WiMAX uses a rate adaptation algorithm that adapts the modulation and
coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate
and robustness of data transmission.
• The process of link adaptation is a dynamic one and the signal and protocol parameters
change as the radio link conditions change—for example in High-Speed Downlink Packet
Access (HSDPA) in Universal Mobile Telecommunications System (UMTS).Prepared by Dr.T.Deepa 6029-04-2020

ACM in WiMAX
• Adaptive modulation allows WiMAX system to adjust channel
modulation scheme, according to SNR ratio in radio link.
• If good SNR is achieved, system can switch to the highest throughput
modulation (64QAM).
• If fading occurs system can shift to other low-throughput modulation,
but still not dropping connection.

Spatial multiplexing:
• Multiple independent streams can be transmitted in parallel
over multiple antennas and can be separated at the receiver
using multiple receive chains through appropriate signal
processing.
• Spatial multiplexing is a transmission technique in MIMO
wireless communication to transmit independent and
separately encoded data signals, so-called streams, from
each of the multiple transmit antennas. Therefore, the space
dimension is reused, or multiplexed, more than one time.
• This can be done as long as the multipath
• Spatial multiplexing provides data rate and capacity gains
proportional to the number of antennas used.

Efficient multi access techniques
• Besides ensuring that each user uses the spectrum as efficiently as possible, effective
methods must be devised to share the resources among the multiple users efficiently.
• This is the challenge addressed at the MAC layer of the system.
• Greater efficiencies in spectrum use can be achieved by coupling channel-quality
information in the resource-allocation process.
• If one were concerned purely with high spectral efficiency or capacity, an obvious way to
achieve that would be to decrease the cell radius or to pack more base stations per unit
area. Obviously, this is an expensive way to improve capacity. Therefore, it is important
to look at spectral efficiency more broadly to include the notion of coverage area.
• The big challenge for broadband wireless system design is to come up with the right
balance between capacity and coverage that offers good quality and reliability at a
reasonable cost.

Efficient multi access techniques (Contd)
• Frequency-division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)/Spread spectrum multiple
access (SSMA)
• Space division multiple access (SDMA)
• Power division multiple access (PDMA)

Quality of Service(QoS)
• QoS is the “collective effect of service,” as perceived by the user.
• QoS refers to the capability of a network to provide better service to
selected network traffic over various technologies, including Frame
Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1
networks, SONET, and IP-routed networks that may use any or all of
these underlying technologies.

QoS (Cont’d)
• QoS is the overall performance of a telephony or computer network,
particularly the performance seen by the users of the network.
• To quantitatively measure quality of service, several related aspects of
the network service are often considered, such as error rates, bit
rate, throughput, transmission delay, availability, jitter, etc.
• Achieving the required QoS by managing the delay, delay variation
(jitter), bandwidth, and packet loss parameters on a network becomes
the secret to a successful end-to-end business solution.
• Thus, QoS is the set of techniques to manage network resources.

• Broadband wireless
networks must support a
variety of applications,
such as voice, data, video,
and multimedia, and each
of these has different
traffic patterns and QoS
requirements, as shown in
Table 1.4.

QoS Challenges
• Delivering QoS is more challenging for mobile broadband than for fixed.
• The time variability and unpredictability of the channel become more acute,
and complication arises from the need to hand over sessions from one cell
to another as the user moves across their coverage boundaries.
• Handovers cause packets to be lost and introduce additional latency.
• Reducing handover latency and packet loss is also an important aspect of
delivering QoS.
• Handover also necessitates coordination of radio resources across multiple
cells.

QoS Challenges (Cont’d)
• QoS has been limited to delivering it across the wireless link. From a
user perspective, however, the perceived quality is based on the end-
to-end performance of the network.
• To be effective, therefore, QoS has to be delivered end-to-end across
the network, which may include, besides the wireless link, a variety of
aggregation, switching, and routing elements between the
communication end points.
• IP-based networks are expected to form the bulk of the core network;
hence, IP-layer QoS is critical to providing end-to-end service quality.

Mobility
• For the end user, mobility is one of the truly distinctive values that
wireless offers. The fact that the subscriber station moves over a
large area brings several networking challenges.
• the ability to move or be moved freely and easily.
• Two of the main challenges are
• (1) providing a means to reach inactive users for session initiation and
packet delivery, regardless of their location within the network, and
• (2) maintaining an ongoing session without interruption while on the
move, even at vehicular speeds. The first challenge is referred to as
roaming; the second, handoff.
• Together, the two are referred to as mobility management, and
performing them well is critical to providing a good user
experience.

Mobility Management
• Mobility management is one of the major functions of a GSM or
a UMTS network that allows mobile phones to work.
• The aim of mobility management is to track where the subscribers are,
allowing calls, SMS and other mobile phone services to be delivered
to them.

Roaming
• One of the fundamental mobility management procedures of
all cellular networks.
• Definition: the ability for a cellular customer to automatically
make and receive voice calls, send and receive data, or access
other services, including home data services, when travelling
outside the geographical coverage area of the home network,
by means of using a visited network.
• For example; should a subscriber travel beyond their cell
phone company's transmitter range, their cell phone would
automatically hop onto another phone company's service, if
available.
• This can be done by using a communication terminal or else
just by using the subscriber identity in the visited network.

Roaming (Cont’d)
• Roaming is technically supported by a mobility
management, authentication, authorization and billing procedures.
• Roaming ensures that a traveling wireless device (typically a cell
phone) is kept connected to a network without breaking the
connection.

Handoff:
• To meet the second challenge of mobility, the system should provide a method
for seamlessly handing over an ongoing session from one base station to another
as the user moves across them.
• A handoff process typically involves detecting and deciding when to do a
handoff, allocating radio resources for it, and executing it.
• It is required that all handoffs be performed successfully and that they happen as
infrequently and imperceptibly as possible.
• The challenge for handoff-decision algorithms is the need to carefully balance
the dropping probability and handoff rate.
• Excessive handoff can lead to an unnecessary signaling load.
• The other challenge is to ensure that sufficient radio resources are set aside so
that ongoing sessons are not dropped midsession during handoff. Some system
designs reserve bandwidth resources for accepting handoff or at least prioritize
handoff requests over session-initiation requests.

IP based Mobility
• Another aspect of mobility management that will become increasingly important in the
future is layer 3, IP mobility.
• Traditionally, in mobile networks, mobility is handled by the layer 2 protocol, and the
fact that the terminal is moving is hidden from the IP network.
• The terminal continues to have a fixed IP address, regardless of its changing its point of
attachment to the network. Although this is not an issue for most IP applications, IP-
based mobility-management solutions can solve this problem, but it is trick it poses a
challenge for certain IP applications, such as Web-caching and multicasting.
• A web cache (or HTTP cache) is an information technology for the temporary storage
(caching) of web documents, such as HTML pages and images, to reduce bandwidth
usage, server load, and perceived lag.
• Multicasting send (data) across a computer network to several users at the same time.
• IP-based mobility management is also required to support roaming and handover across
heterogeneous networks, such as between a WiMAX network and a Wi-Fi network.

Portability
• Like mobility, portability is another unique value
provided by wireless.
• the ability of a computer program to be ported from
one system to another in computer science
• Portability is desired for not only full-mobility
applications but also nomadic applications.
• living the life of a nomad
Limitation: Power consumption: When a power outlet or portable generator is
not available, mobile computers must rely entirely on battery power. Combined
with the compact size of many mobile devices, this often means unusually
expensive batteries must be used to obtain the necessary battery life.

Portability
• Devices/nodes connected within the mobile broadband system should
facilitate mobility.
• These devices may have limited device capabilities and limited power
supply, but should have a sufficient processing capability and physical
portability to operate in a movable environment.

Challenges-Portability
• Dictates that the subscriber device be battery powered and lightweight and therefore
consume as little power as possible.
• Unfortunately, advances in battery technology have been fairly limited, especially when
compared to processor technology.
• The problem is compounded by the fact that mobile terminals are required to pack greater
processing power and functionality within a decreasing real estate.
• Given the limitations in battery power, it is important that it be used most efficiently.
• Solution: The need for reducing power consumption challenges designers to look for
• power-efficient transmission schemes,
• power-saving protocols,
• computationally less intensive signal-processing algorithms,
• low-power circuit-design and fabrication, and battery technologies with longer life.

Challenges/Solution-Portability (Cont’d)
• The requirement of low-power consumption drives physical-
layer design toward the direction of using power-efficient
modulation schemes: signal sets that can be detected and
decoded at lower signal levels.
• Unfortunately, power-efficient modulation and coding schemes
tend to be less spectrally efficient.
• Since spectral efficiency is also a very important requirement for
broadband wireless, it is a challenge to make the appropriate
trade-off between them.
• This often results in portable wireless systems offering
asymmetric data rates on the downlink and the uplink. The
power-constrained uplink often supports lower bits per second
per Hertz than the downlink.

Challenges-Portability (Cont’d)
• It is not only the transmitter power that drains the battery. Digital signal processors
used in terminal devices are also notorious for their power consumption.
• This motivates the designer to come up with computationally more efficient
signal-processing algorithms for implementation in the portable device.
• Protocol design efforts at power conservation focus on incorporating low power
sleep and idle modes with methods to wake up the device as and when required.
• Fast switching technologies to ensure that the transmitter circuitry is turned on
only when required and on an instantaneous demand basis can also be used to
reduce overall power consumption.

Security
• Security is an important consideration in any communications system
design.
• The fact that connections can be established in a untethered fashion
makes it easier to intrude in an inconspicuous and undetectable
manner than is the case for wired access.
• Further, the shared wireless medium is often perceived by the general
public to be somewhat less secure than its wired counterpart.
• Therefore, a robust level of security must be built into the design of
broadband wireless systems.

Security (Cont’d)
• Safety is always important, especially when it comes to the safety of
your documents stored on your computer.
• Using a wireless connection can be very unsafe, even when you are at
home, anyone close by can scan your connection and try to hack you,
get into your internet connection and your pc.

Security (Cont’d)
• The technology used for mobile broadband has evolved from mobile phone networks.
• Safety of our calls has always been important to us, and mobile phone companies have
done a lot to ensure full safety of your calls.
• Using this technology, internet access through the mobile broadband is much safer than
through any other wireless network available on the market.
• Your connection is encrypted as the backbone of the mobile broadband is GSM network.
Encryption used is 128 bit meaning that it is unbreakable.
• However, keep in mind that once you use wireless router your safety will go down as
connection from router to your pc is not encrypted anymore. This does lower your safety
but it does not mean that you cannot enhance it for router usage.

How Secure is mobile broadband ?
• From the perspective of an end user, the primary security concerns are privacy and data integrity.
• Users need assurance that no one can eavesdrop on their sessions and that the data sent across the
communication link is not tampered with. This is usually achieved through the use of encryption.
• From the service provider’s perspective, an important security consideration is preventing
unauthorized use of the network services. This is usually done using strong authentication and
access control methods.
• Authentication and access control can be implemented at various levels of the network:
• the physical layer, the network layer, and the service layer.
• The service provider’s need to prevent fraud should be balanced against the inconvenience that it
may impose on the user.
• Besides privacy and fraud, other security concerns include denial-of-service attacks in which
malignant users attempt to degrade network performance, session hijacking, and virus insertion.

Supporting IP in Wireless
• Need of IP in Wireless
• Networking protocol of choice for modern communication systems.
• Used to support not only data but also voice, video, and multimedia.
• Video over IP and IPTV are also emerging as potential rivals to traditional cable TV. Because more and more
applications will migrate to IP, IP-based protocols and architecture must be considered for broadband wireless
systems.
• Features: of IP-based protocols and architecture for broadband wireless.
IP-based systems tend to be cheaper because of the economies of scale they enjoy from widespread
adoption in wired communication systems.
Adopting an IP architecture can make it easier to develop new services and applications rapidly.
The large IP application development community can be leveraged.
An IP-based architecture for broadband wireless
Enable easier support for such applications as IP multicast and anycast.
Makes it easy to integrate broadband wireless systems with other access technologies and
thereby enable converged services. Prepared by Dr.T.Deepa 8629-04-2020

Challenges IP based Protocols
• IP-based protocols are simple and flexible but not very efficient or robust.
• These deficiencies were not such a huge concern as IP evolved largely in
the wired communications space, where transmission media, such as fiber-
optic channels, offered abundant bandwidth and very high reliability.
• In wireless systems, however, introducing IP poses several challenges:
1. Making IP-based protocols more bandwidth efficient,
2. Adapting them to deliver the required QoS (delay, jitter, throughput, etc.)
when operating in bandwidth-limited and unreliable media,
3. Adapting them to handle terminals that move and change their point of
attachment to the network.

Conclusion
Broadband wireless could be a significant growth market for the telecom industry.
Broadband wireless systems can be used to deliver a variety of applications and
services to
both fixed and mobile users.
WiMAX could potentially be deployed in a variety of spectrum bands: 2.3GHz,
2.5GHz,3.5GHz, and 5.8GHz.
WiMAX faces a number of competitive challenges from both fixed-line and third
generation mobile broadband alternatives.
The service requirements and special constraints of wireless broadband make the
technical design of broadband wireless quite challenging.

References
 Jeffrey G. Andrews, ArunabhaGhosh and RiasMuhamed, “Fundamentals of
WiMAX: Understanding Broadband Wireless Networking”, Pearson
Education, 2007.
 Yan Zhang and Hsiao-Hwa Chen, “Mobile WiMAX : toward broadband
wireless metropolitan area networks”,Auerbach Publications, 2007
 Moray Rumney, “LTE and Evolution to 4G Wireless: Design and
Measurement Challenges”, Agilent Technologies, 2008.
 StefaniaSesia, IssamToufik, Matthew Baker, “LTE – The UMTS Long Term
Evolution: From Theory to Practice”, John Wiley & Sons, 2e, 2011.
 Luis M. Correia, “Mobile Broadband Multimedia Networks: Techniques,
Models and Tools for 4G”, Elseiver, 2006.
29-04-2020 Prepared by Dr.T.Deepa 90

Wireless Broadband Networks

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