EEE 6207
Broadband Wireless Communications
Dr. Md. Forkan Uddin
Professor
Dept. of EEE, BUET
Dhaka 1205, Bangladesh
1
Vision
 Course syllabus covers background materials of wireless
communications
 Knowledge of networking is also very important
 We will focus on background materials for research in wireless
communications and networking
 Knowledge of this course will help to work in telecom industries
and to pursue research is wireless communications
2
Outline
 Overview of broadband (wireless) communications
 Layering architecture and cross-layer concept
 Data link layer techniques: TDMA, FDMA, ALOHA, CSMA
 Spread spectrum communications: DSSS, FHSS, THSS
 Propagation in radio channel: path loss, fading, shadowing
Physical layer techniques:
 Multi-user detection: MMSE, MLSE, SIC
 Multiple antenna system: MISO, SIMO, MIMO
 Multi-carrier communication: OFDM
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Outline (Contd..)
Broad band wireless standards
IEEE 802.11 (WiFi)
IEEE 802.16 (WiMax)
LTE (Long Term Evaluation)
Bluetooth
4
Books
1. Modern Wireless Communications
S. Haykin and M. Moher
2. Wireless Communications
T.S. Rapaport
3. Wireless Communications and Networking
J. W. Mark and W. Zhuang
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Marks Distribution
 Project: 30% (Report:15% and Presentation:15%)
 Final: 70%
Project:
 Simulation/analytical results on a topic of this course
Some project ideas will be given
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Broadband Communications
Communication technology with transmission rates above the
fastest speed available over a telephone line, e.g., 64 kbps
Examples:
 Integrated Services Digital Network (ISDN)
 Digital subscriber line (DSL)/ Asynchronous DSL (ADSL)
Cable (CATV) Data Networks
Asynchronous transfer mode (ATM)
Broadband wireless techniques (WPAN, WLAN, WMAN)
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Broadband Communications
 ISDN
 Implemented over existing copper telephone cables
 Data rate: 128 Kbps to several Mbps
 DSL /ADSL
 High-bandwidth digital services over copper telephone cable
 Rates can reach into the multimegabit/sec rates
 ATM
 High-bandwidth service provided from the carriers
 Provide guarantee of quality of service (QoS)
8
Broadband Communications
 Cable (CATV) Data Networks
Well-established broadband network in US
Provides Internet as well as TV channels
 Coaxial cable is the media of transmission
 Broadband Wireless Networks
WPAN (Bluetooth)
WLAN (WiFi)
WMAN (WiMax, LTE)
 3G, 4G and 5G Cellular
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Layering Architecture
 Concept of layering is used in our daily life
 Communication through air mail
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OSI Model
 International
standard
organization
(ISO)
established
a
committee in 1977 to develop an architecture for computer
communication
 Open Systems Interconnection (OSI) model is the result of this
effort
 In 1984, OSI model is approved as reference model
11
OSI Model
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Application Layer
 Application Layer
 Allows access to network resource
 Make applications into data format
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Presentation and Session Layers
 Presentation Layer
 Translate, encrypt and compress data
 Uses various source coding techniques based on applications
 Creates packet from the compressed information bits
 Session Layer
 The session layer is responsible for dialog control and
synchronization
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Transport Layer
 Transport Layer
 Controls congestion
 Provides end-to-end transmission reliability between source and
destination
 Examples:
 Transport control protocol (TCP) – use for high reliability, use
for data
 User datagram protocol (UDP) – allow some loss, use for voice
or video
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Network Layer
The network layer is responsible for the delivery of individual
packets from the source to the destination
 Determine the routes for the packets
 Assign the address of the hosts
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Data Link Layer
 Who will transmit, when to transmit, whom to transmit to
 Attempts to provide reliable communication over the physical
layer interface in one hop distance
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Physical layer
 Provides physical interface for transmission of information
through a medium (wired/wireless)
Covers all - mechanical, electrical, functional and procedural aspects for physical communication
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Interaction Among the Layers
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Why Layering?
 Layer architecture simplifies the network design
 It is easy to debug network applications in a layered architecture
network
 The network management is easier
 Research/work on a layer can be done independently
20
Cross-layer
 The performance of one layer depends on the activity/status of
the other layers, especially in wireless networks
 Information from one layer need to pass to another layer and
the activity of a layer should be modified with those information
 Wireless system need to be designed by cross-layering of two or
more layers to achieve better performance
 Cross-layer examples:
MAC+PHY
Network+PHY
Network+MAC+PHY
Transport+MAC
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Classification of MAC Protocols
Random access (contention methods): No station is superior to another station and none is
assigned the control over another. No station permits, or does not permit, another station to
send. distributed, low cost, poor performance
Controlled access: The stations consult one another to find which station has the right to
send. A station cannot send unless it has been authorized by other stations. They were
designed for computer networks. Not popular now.
Channelization techniques: The available bandwidth of a link is shared in time, frequency, or
through code, between different stations. Usually, it is controlled by a system administrator.
Conflict free; centralized, high cost, good performance
Frequency Division Multiple Access
(FDMA)
 The frequency spectrum is divided into unique frequency
bands or channels
 These channels are assigned to users on demand
 Multiple users cannot share a channel at the same time
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FDMA
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FDMA
 In FDMA, the available bandwidth of the common
channel is divided into bands that are separated by
guard bands
 FDMA requires tight RF filtering to reduce adjacent
channel interference
25
Time Division Multiple Access
(TDMA)
 Time is slotted
 One user per slot
 Buffer and burst method
 Non-continuous transmission
 Digital data
 Digital modulation
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TDMA
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Code Division Multiple Access
(CDMA)
 Transmitters may transmit at the same time and the same
channel
 Each signal is modified by spreading it over a large bandwidth
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CDMA
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CDMA
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CDMA
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Space Division Multiple Access
 SDMA is not really a multiple access method but rather a
technique to reuse frequency spectrum
 Use TDMA / CDMA at the same location
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FDMA vs. TDMA vs. CDMA
FDMA:
 Many channels - same antenna
 For maximum power efficiency operate near saturation
 Near saturation power amplifiers are nonlinear
 Nonlinearities causes signal spreading
 Inter modulation (IM) frequencies
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FDMA vs. TDMA vs. CDMA
 IM are undesired harmonics
 Interference with other channels in the FDMA system
 Decreases user C/I - decreases performance
 Interference outside the mobile radio band: adjacent-channel
interference
 FDMA requires RF filtering to minimize adjacent channel
interference
 RF filters needed - higher costs
 Even through, it could be a good choice for high speed
communications
34
FDMA vs. TDMA vs. CDMA
TDMA:
 Digital equipments used in time division multiplexing is increasingly
becoming cheaper
 There are advantages in digital transmission techniques. Ex: error
correction
 Lack of inter modulation noise means increased efficiency
 Handoff process much simpler
 Low battery consumption
 Extra signaling bits required for synchronization and framing
 High synchronization overhead and guard slots
 Selected for next generation communications
35
FDMA vs. TDMA vs. TDMA
CDMA:
 Near far problem in CDMA
 Requires power control
 Highly cost for power control circuit
 Good if the distance of the users are very close from the base
station
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Reservation Based Access
 A reservation frame is used and its number of bits is equal to the
number of users
 A user wants to reserve time, it puts the value 1 to the
corresponding bit during the reservation frame using time
37
Polling
 The transmitter sends selection or poll message to the receiver
 If receiver sends ACK then data transmitted
 If receiver sends NAK then data is not transmitted and sends
poll message to another user
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Token Based Access
 A token is used
 The user who has the token, sends data to its receiver and
then pass the token to another user
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ALOHA
 Each nodes access the channel distributed fashion
 When a packet arrive a node, it tries to access the channel
and sends the packet the receiver
 When multiple users access simultaneously collision occurs
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ALOHA: Vulnerable time
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Throughput Model
 Let G is the traffic arrivals per packet time T
 Arrival rate=G/T
 In a frame time T, the probability of k transmission
Pr(N=k)=(GT/T)^k*exp(-GT/T)/k!
 Vulnerable period is 2T, where T is duration of transmission of a
packet
 Probability of 0 transmission in 2T, i.e., probability of successful
transmission=exp(-2G)
 Throughput per packet duration, S=G*exp(-2G)
 Maximum throughput= 0.186, it means efficiency is 18.6%
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Slotted ALOHA
 Time is slotted
 Duration of each slot is equal to T
 Each nodes access the channel in a distributed fashion
 If a packet arrives to a node, it tries to access the channel in
the next slot and sends the packet the receiver
43
Slotted ALOHA: Vulnerable time
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Throughput Model
 Vulnerable period is T
 Probability of 0 transmission in T, i.e., probability of successful
transmission=exp(-G)
 Throughput per packet duration, S=G*exp(-G)
 Maximum throughput, S(max)= 0.368 , it means efficiency is
36.8%
45
ALOHA vs. Slotted ALOHA
Slotted ALOHA: the maximum throughput
Smax = 0.368=1/e when G = 1.
ALOHA: the maximum throughput
Smax = 0.184=1/2e when G = 0.5.
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CSMA
 In ALOHA and slotted ALOHA, a node does not have any
knowledge, whether or not the other node(s) are accessing the
channel as a result collision is huge
 In CSMA, a node sense the channel by energy detection of the
channel before a transmission
 However, collision can happen due to the propagation delay and
detection time
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CSMA
48
Types of CSMA
 1-persistent: Each node
senses the channel
continuously and send
whenever the channel is idle
 Non-persistent: Senses the
channel in a time interval
 P-persistent: Senses
continuously but decision of
how many time slots after it
will transmit is based on
random probability
distribution
49
Types of CSMA
 1-persistent: collision is very high since more than one node can
start transmission at the same time
 Non-persistent: efficiency is better than 1-persistent
 P-persistent: efficiency is very good with appropriate selection of
p
50
ALOHA vs. CSMA
0.01-persistent CSMA
1.0
0.9
Nonpersistent CSMA
0.8
0.1-persistent CSMA
0.7
S
0.6
0.5-persistent CSMA
1-persistent CSMA
0.5
0.4
0.3
Slotted Aloha
0.2
Aloha
0.1
0
0
1
2
3
4
5
6
7
8
9
G
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CSMA with collision detection
CSMA/CD
 In CSMA, if 2 terminals begin sending packet at the same time,
each will transmit its complete packet (although collision is
taking place).
 Wasting medium for an entire packet time.
 CSMA/CD is used to detect collision and stop transmission
 Used in Ethernet, very efficient
52
Steps in CDMA/CD
Step 1: If the medium is idle, transmit
Step 2: If the medium is busy, continue to listen until
the
channel is idle then transmit
Step 3: If a collision is detected during transmission, cease
transmitting
Step 4: Wait a random amount of time and repeats the same
algorithm
53
CSMA/CA (CSMA with collision
Avoidance)
 All terminals listen to the same medium as CSMA/CD.
 Terminal ready to transmit senses the medium.
 If medium is busy it waits until the end of current transmission.
 It again waits for an additional predetermined time period DIFS
(Distributed inter frame Space).
 Then picks up a random number of slots (the initial value of
back-off counter) within a contention window to wait before
transmitting its frame.
 If there are transmissions by other terminals during this time
period (back-off time), the terminal freezes its counter.
 It resumes count down after other terminals finish transmission
+ DIFS. The terminal can start its transmission when the
counter reaches to zero.
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CSMA/CA
Node B’s frame
Node A’s frame
Delay: B
Node C’s frame
Time
Delay: C
Nodes B & C sense
the medium
Nodes B resenses the medium
and transmits its frame.
Nodes C starts
transmitting.
Node C freezes its counter.
Nodes C resenses the
medium and starts
decrementing its counter.
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CSMA/CA
DIFS Contention
window
DIFS
Medium Busy
Defer access
Contention window
Next Frame
Time
Slot
Backoff after defer
DIFS – Distributed Inter Frame
Spacing
56
CSMA/CA with ACK
 Immediate Acknowledgements from receiver upon
reception of data frame without any need for
sensing the medium
 ACK frame transmitted after time interval SIFS
(Short Inter-Frame Space) (SIFS < DIFS)
 Receiver transmits ACK without sensing the medium
 If ACK is lost, retransmission done
57
CSMA/CA/ACK
DIFS
Source
Time
Data
SIFS
Destination
ACK
DIFS
Contention window
Next Frame
Other
Defer access
Backoff after defer
SIFS – Short Inter Frame Spacing
58
CSMA/CA with RTS/CTS
DIFS
SIFS
SIFS
Destination
Time
Data
RTS
Source
SIFS
CTS
ACK
DIFS
Contention
window
Next Frame
Other
Defer access
Backoff after
defer
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Collision/Contention
Resolution
 Collision/ contention resolution technique used in CSMA/CA is the
heart of the protocol
 Adaptive CW management for collision/contention resolution
 For a packet, the initial CW is randomly selected from 0 to
(CW_min-1)
 For the k-th transmission of a packet in case of (k-1) consecutive
failure, the CW is randomly selected from 0 to 2(k-1)CW_min-1
 It’s also called binary exponential back-off (BEB) mechanism
 After a successful transmission, for the next packet, the CW is reset
to randomly selected from 0 to (CW_min-1)
 Thus, the mechanism automatically reduce the collision if number
of stations increases
CSMA/CA
 Basic CSMA/CA is widely using in WiFi
 Very efficient for WLAN with a low number of stations
 Poor performance for multi-hop due to hidden and exposed
terminal problem
A
Collision
B
C
D
C
D
HTP
A
B
Missing opportunity ETP
CSMA/CA
 Basic CSMA/CA is widely using in WiFi
 Very efficient for WLAN with a low number of stations
 Poor performance for multi-hop due to hidden and exposed
terminal problem
 Busy tone multiple access (BTMA) protocol is proposed to solve
the problem but it is not yet practically implemented or
standardized
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Multiple Access Techniques in use
System
Multiple Access
Advanced Mobile Phone System (AMPS)
FDMA/FDD
Global System for Mobile (GSM)
TDMA/FDD
US Digital Cellular (USDC)
TDMA/FDD
Digital European Cordless Telephone (DECT)
FDMA/TDD
US Narrowband Spread Spectrum (IS-95)
CDMA/FDD
Satellite Communication
TDMA and FDMA
WiMax
TDMA
WiFi
CSMA/CA
Ethernet
CSMA/VD
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