2. Wireless Communication: Introduction
Wireless Communication is the fastest growing and most
vibrant technological areas in the communication field
Wireless Communication is a method of transmitting
information from one point to other, without using any
connection like wires, cables or any physical medium.
Wireless Communication doesn’t require any physical
medium but propagates the signal through space. Since,
space only allows for signal transmission without any
guidance.
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3. Types of Wireless Communication
Technologies/systems
Television and Radio Broadcasting
Satellite Communication
Radar
Mobile Telephone System (Cellular Communication)
Global Positioning System (GPS)
Infrared Communication
WLAN (Wi-Fi)
Bluetooth
Paging
Cordless Phones
Radio Frequency Identification (RFID)
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4. Why Wireless Communication?/Advantages of
Wireless Communication
Low Cost
Mobility
Ease of Installation
Reliability
Disaster Recovery: In case of accidents due to fire, floods
or other disasters, the loss of communication infrastructure
in wireless communication system can be minimal.
Expandability
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5. Disadvantages of Wireless
Communication
Even though wireless communication has a number of
advantages over wired communication, there are a few
disadvantages as well.
Interference
Security
Health Concerns
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6. What is Wireless Communication ?
Transmitting voice and data using electromagnetic waves
in open space (atmosphere)
Electromagnetic waves
Travel at speed of light (c = 3x108 m/s)
Has a frequency (f) and wavelength (l)
c = f x l
Higher frequency means higher energy photons
The higher the energy photon the more penetrating is the
radiation
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7. Types of wireless communication
celullar wireless computer network radio service
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8. Electromagnetic radiation spectrum
The electromagnetic spectrum provides an unguided medium
(channel) for point-to-point and/or broadcast radio transmission.
Radio transmission is usually (frequency)-bandlimited by
design.
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9. Wavelength of Some Technologies
GSM Phones:
frequency ~= 900 Mhz
wavelength ~= 33cm
PCS Phones
frequency ~= 1.8 Ghz
wavelength ~= 17.5 cm
Bluetooth:
frequency ~= 2.4Gz
wavelength ~= 12.5cm
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10. Types of electromagnetic carriers
when the distance between the sender and receiver is
short (e.g. TV box and a remote control) infrared waves
are used
for long range distances between sender and receiver
(e.g. TV broadcasting and cellular service) both
microwaves and radio waves are used
radio waves are ideal when large areas need to be coverd and
obstacles exist in the transmission path
microwaves are good when large areas need to be coverd and no
obstacles exist in the transmission path
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12. Advantages and disadvantages of wireless
communication
advantages:
mobility
a wireless communication network is a solution in areas where
cables are impossible to install (e.g. hazardous areas, long
distances etc.)
easier to maintain
disadvantages:
has security vulnerabilities
high costs for setting the infrastructure
unlike wired comm., wireless comm. is influenced by physical
obstructions, climatic conditions, interference from other
wireless devices
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13. Frequency Carries/Channels
The information from sender to receiver is carrier over a
well defined frequency band.
This is called a channel
Each channel has a fixed frequency bandwidth (in KHz)
and Capacity (bit-rate)
Different frequency bands (channels) can be used to
transmit information in parallel and independently.
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14. Example
Channel 1 (b - b+30)
Channel 2 (b+30 - b+60)
Channel 3 (b+60 - b+90)
Station A Station B
Assume a spectrum of 90KHz is allocated over a base frequency b
for communication between stations A and B
Assume each channel occupies 30KHz.
There are 3 channels
Each channel is simplex (Transmission occurs in one way)
For full duplex communication:
Use two different channels (front and reverse channels)
Use time division in a channel
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16. Radio waves generation
when a high-frequency alternating current (AC) passes
through a copper conductor it generates radio waves which
are propagated into the air using an antenna
radio waves have frequencies between:
3 Hz – 300 KHz - low frequency
300 KHz – 30 MHz – high frequency
30 MHz – 300 MHz – very high frequency
300 MHz – 300 GHz – ultra high frequency
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17. Radio propagation
1. Reflection: A propagating wave impinges on an object that is large
compared to the wavelength. (e.g., the surface of the Earth, buildings,
walls).
2. Diffraction: A radio path between the transmitter and receiver is
obstructed by a surface with sharp irregular edges; waves bend around
the obstacle, even when line of sight (LOS) does not exist.
3. Scattering: Objects smaller than the wavelength of the propagating
wave are encountered along the way (e.g., foliage, street signs,
lampposts).
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18. Radio propagation (2)
radio waves are generated by an antenna and they
propagate in all directions as a straight line
radio waves travel at a velocity of 186.000 miles per
second
radio waves become weaker as they travel a long
distance
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19. Radio propagation (3)
There are 3 modes of propagation:
Surface(Ground) Wave /Mode Propagation:– Follows the contour of the
earth
and meant for low frequency waves. Ex: AM radio, submarine Comn
Direct (Line of Sight) Wave/Mode Propagation : Signal reflected from
ionized layer of atmosphere back down to earth and meant for high
frequency waves. Ex:amateur radio, International broadcasts
Ionospheric (Sky) Wave/Mode Propagation is the propagation of radio
waves bent (refracted) back to the Earth's surface by the ionosphere
.frequency waves
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20. ASN_Unit-I_MVSR 20
20
Propagation Impairments
Attenuation and attenuation distortion
Free space loss
Atmospheric absorption
Multipath (diffraction, reflection, refraction…)
Noise
Thermal noise
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21. ASN_Unit-I_MVSR 21
2.21
Signal can take many different paths between sender and
receiver due to reflection, scattering, diffraction
Time dispersion: signal is dispersed over time
interference with “neighbor” symbols, Inter
Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
distorted signal depending on the phases of the
different parts
Multipath propagation
signal at sender
signal at receiver
LOS pulses
multipath
pulses
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24. Nokia N95
s
Operating Frequency: WCDMA2100 (HSDPA),
EGSM900, GSM850/1800/1900 MHz (EGPRS)
Memory: Up to 160 MB internal dynamic memory;
memory card slot - microSD memory cards (up to 2 GB)
Display: 2.6" QVGA (240 x 320 pixels) TFT – ambient
light detector - up to 16 million colors
Data Transfer:
WCDMA 2100 (HSDPA) with simultaneou voice
and packet data (Packet Switching max speed
UL/DL= 384/3.6MB, Circuit Switching max speed
64kbps)
Dual Transfer Mode (DTM) support for
simultaneous voice and packet data connection in
GSM/EDGE networks - max speed DL/UL:
177.6/118.4 kbits/s
EGPRS class B, multi slot class 32, max speed DL/
UL= 296 / 177.6 kbits/s
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25. Services
E-mail file
10 Kbyte
Web Page
9 Kbyte
Text File
40 Kbyte
Large Report
2 Mbyte
Video Clip
4 Mbyte
Film with TV
Quality
2G
8 sec
9 sec
33 sec
28 min
48 min
1100 hr
PSTN
3 sec
3 sec
11 sec
9 min
18 min
350 hr
ISDN
1 sec
1 sec
5 sec
4 min
8 min
104 hr
2G+
0.7 sec
0.8 sec
3 sec
2 min
4 min
52 hr
UMTS/3G
0.04 sec
0.04sec
0.2 sec
7 sec
14 sec
>5hr
Source: UMTS Forum
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26. Computer Networks
A computer network is two or more computers
connected together using a telecommunication system
for the purpose of communicating and sharing resources
Why they are interesting?
Overcome geographic limits
Access remote data
Separate clients and server
Goal: Universal Communication (any to any)
Network
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27. Type 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 Firewire (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.
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29. Reference model
Physical layer: conversion of stream of bits into
signals – carrier generation - frequency selection
– signal detection – encryption
Data link layer: accessing the medium –
multiplexing - error correction – synchronization
Network layer: routing packets – addressing -
handover between networks
Transport layer: establish an end-to-end
connection – quality of service – flow and
congestion control
Application layer: service location – support
multimedia – wireless access to www
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30. Wireless Network
The difference between wired and wireless is the
physical layer and the data link layer
Wired network technology is based on wires or
fibers
Data transmission in wireless networks take place
using electromagnetic waves which propagates
through space (scattered, reflected, attenuated)
Data are modulated onto carrier frequencies
(amplitude, frequency)
The data link layer (accessing the medium,
multiplexing, error correction, synchronization)
requires more complex mechanisms.
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32. IEEE standard 802.11
mobile terminal
fixed
terminal
application
TCP
802.11 PHY
802.11 MAC
IP
application
TCP
802.3 PHY
802.3 MAC
IP
LLC
802.11 MAC 802.3 MAC
802.11 PHY 802.3 PHY
infrastructure
network
access point
LLC LLC
Transport layer
Network layer
Data link layer
Physical link l.
CSMA/CA = Carrier Sense Multiple Access / Collision Avoidance
CSMA/CA = Carrier Sense Multiple Access / Collision Detection
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34. CSMA/CA
Request to Send
(RTS) packet sent
by the sender S,
and a Clear to
Send (CTS) packet
sent by the
intended receiver R.
Alerting all nodes
within range of the
sender, receiver or
both, to not
transmit for the
duration of the
main transmission.
http://en.wikipedia.org/wiki/
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35. Mobile Communication Technologies
WLAN 802.11
802.11a
802.11b
802.11i/e/…/w
802.11g
WiFi
Local wireless networks 802.11h
Personal wireless nw
WPAN 802.15
802.15.2
802.15.1
Bluetooth
802.20 (Mobile Broadband WirelessAccess)
Wireless distribution networks
WMAN 802.16 (Broadband WirelessAccess)
+ Mobility
WiMAX
ZigBee
802.15.4 802.15.4a/b
802.15.5
802.15.3 802.15.3a/b
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36. Bluetooth Characteristics
Operates in the 2.4 GHz band - Packet switched
1 milliwatt - as opposed to 500 mW cellphone
Low cost
10m to 100m range
Uses Frequency Hop (FH) spread spectrum, which divides
the frequency band into a number of hop channels.
During connection, devices hop from one channel to
another 1600 times per second
Data transfer rate 1-2 megabits/second (GPRS is
~50kbits/s)
Supports up to 8 devices in a piconet (= two or more
Bluetooth units sharing a channel).
Built-in security
Non line-of-sight transmission through walls and briefcases
Easy integration of TCP/IP for networking.
http://www.bluetooth.com/English/Technology/Pages/Basics.aspx
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37. Wi-Fi
Wi-Fi is a technology for WLAN based on the IEEE
802.11 (a, b, g) specifications
Originally developed for PC in WLAN
Increasingly used for more services:
Internet and VoIP phone access, gaming, …
and basic connectivity of consumer electronics such
as televisions and DVD players, or digital cameras,
…
In the future Wi-Fi will be 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).
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38. Wi-Fi
An access point (AP) broadcasts its SSID (Service Set
Identifier, "Network name") via packets (beacons)
broadcasted every 100 ms at 1 Mbit/s
Based on the settings (e.g. the SSID), the client may
decide whether to connect to an AP
Wi-Fi transmission, as a non-circuit-switched wired
Ethernet network, can generate collisions
Wi-Fi uses CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance) to avoid collisions
CSMA = the sender before transmitting it senses the
carrier – if there is another device communicating then it
waits a random time an retry
CA = the sender before transmitting contacts the receiver
and ask for an acknowledgement – if not received the
request is repeated after a random time interval.
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39. WiMAX
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 150
km/h.
http://wimax.retelit.it/index.do
http://www.wimax-italia.it/
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41. Advantages of wireless LANs
Very flexible within the reception area
Ad-hoc networks without previous planning
possible
(almost) no wiring difficulties (e.g. historic
buildings, firewalls)
More robust against disasters like, e.g.,
earthquakes, fire - or users pulling a
plug...
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42. Wireless networks disadvantages
Higher loss-rates due to interference
emissions of, e.g., engines, lightning
Restrictive regulations of frequencies
frequencies have to be coordinated, useful frequencies are
almost all occupied
Low data transmission rates
local some Mbit/s, regional currently, e.g., 53kbit/s with GSM/
GPRS
Higher delays, higher jitter
connection setup time with GSM in the second range, several
hundred milliseconds for other wireless systems
Lower security, simpler active attacking
radio interface accessible for everyone, base station can be
simulated, thus attracting calls from mobile phones
Always shared medium
secure access mechanisms important
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43. What is modulation ?
Modulation = modulation = adding information (e.g.
voice) to a carrier electromagnetic (radio) signal
The sine wave on which the characteristics of the
information signal are modulated is called a carrier
signal
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46. Signal characteristics that can
be modified
signal x(t) = A cos(2πft + Φ)
• A – amplitude
• f – frequency
• Φ – phase (initial angle of the sinusoidal function at its
origin
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50. ASK
ASK On-off keying (Amplitude Shift Keying) –
frequency is kept constant, amplitude has 2 levels (for
bit 1 and for bit 0)
The binary sequence 0010110010
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55. ASN_Unit-I_MVSR
Spread Spectrum Modulation
Problem of radio transmission: frequency dependent fading can wipe out
narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a
special code
protection against narrow band interference
protection against narrowband interference
Side effects:
coexistence of several signals without dynamic coordination
tap-proof
Alternatives: Direct Sequence, Frequency Hopping
detection at
receiver
interference spread
signal
signal
spread
interference
f f
power power
56. ASN_Unit-I_MVSR
Effects of spreading and interference
dP/df
f
i)
dP/df
f
ii)
sender
dP/df
f
iii)
dP/df
f
iv)
receiver
f
v)
user signal
broadband interference
narrowband interference
dP/df
57. ASN_Unit-I_MVSR
Spreading and frequency selective fading
frequency
channel
quality
1 2
3
4
5 6
narrow band
signal
guard space
2
2
2
2
2
frequency
channel
quality
1
spread
spectrum
narrowband channels
spread spectrum channels
58. ASN_Unit-I_MVSR
DSSS (Direct Sequence Spread Spectrum)
XOR of the signal with pseudo-random number (chipping sequence)
many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages
reduces frequency selective
fading
in cellular networks
base stations can use the
same frequency range
several base stations can
detect and recover the signal
soft handover
Disadvantages
precise power control necessary
user data
chipping
sequence
resulting
signal
0 1
0 1 1 0 1 0 1 0
1 0 0 1 1
1
XOR
0 1 1 0 0 1 0 1
1 0 1 0 0
1
=
tb
tc
tb: bit period
tc: chip period
59. ASN_Unit-I_MVSR
DSSS (Direct Sequence Spread Spectrum)
X
user data
chipping
sequence
modulator
radio
carrier
spread
spectrum
signal
transmit
signal
transmitter
demodulator
received
signal
radio
carrier
X
chipping
sequence
lowpass
filtered
signal
receiver
integrator
products
decision
data
sampled
sums
correlator
60. ASN_Unit-I_MVSR
FHSS (Frequency Hopping Spread Spectrum)
Discrete changes of carrier frequency
sequence of frequency changes determined via pseudo random number
sequence
Two versions
Fast Hopping:
several frequencies per user bit
Slow Hopping:
several user bits per frequency
Advantages
frequency selective fading and interference limited to short period
simple implementation
uses only small portion of spectrum at any time
Disadvantages
not as robust as DSSS
simpler to detect
61. ASN_Unit-I_MVSR
FHSS (Frequency Hopping Spread Spectrum)
user data
slow
hopping
(3 bits/hop)
fast
hopping
(3 hops/bit)
0 1
tb
0 1 1 t
f
f1
f2
f3
t
td
f
f1
f2
f3
t
td
tb: bit period td: dwell time
62. ASN_Unit-I_MVSR
FHSS (Frequency Hopping Spread Spectrum)
modulator
user data
hopping
sequence
modulator
narrowband
signal
spread
transmit
signal
transmitter
received
signal
receiver
demodulator
data
frequency
synthesizer
hopping
sequence
demodulator
frequency
synthesizer
narrowband
signal
63. 2/17/2023 ASN_Unit-I_MVSR 63
Medium Access Control -Motivation
Can we apply media access methods from fixed networks?
Example CSMA/CD
Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision
occurs (legacy method in IEEE 802.3)
Problems in wireless networks
signal strength decreases proportional to the square of the distance
the sender would apply CS and CD, but the collisions happen at the
receiver
it might be the case that a sender cannot “hear” the collision, i.e., CD
does not work
furthermore, CS might not work if, e.g., a terminal is “hidden”
64. CSMA/CD fails in wireless N/w because CSMA/CD is not
really interested in collisions at the sender , but rather in
those at the receiver.
The signal should reach the receiver without collisions. But
sender is the one detecting collisions
This is not a problem using wire, as more or less the same
signal strength can be assumed all over the wire
The strength of a signal in wireless N/w decreases
proportionally to the square of the distance to the sender
The sender start sending but a collision happens at the
receiver due to a second sender.
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65. 2/17/2023 ASN_Unit-I_MVSR 65
Motivation - hidden and exposed terminals
Hidden terminals
A sends to B, C cannot receive A
C wants to send to B, C senses a “free” medium (CS fails)
collision at B, A cannot receive the collision (CD fails)
A is “hidden” for C ,and vice versa.
Exposed terminals
B sends to A, C wants to send to another terminal (not A or B)
C has to wait, CS signals a medium in use
but A is outside the radio range of C, therefore waiting is not necessary
C is “exposed” to B
B
A C
66. 2/17/2023 ASN_Unit-I_MVSR 66
Motivation - near and far terminals
Terminals A and B send, C receives
signal strength decreases proportional to the square of the distance
the signal of terminal B therefore drowns out A’s signal
C cannot receive A
If C for example was an arbiter for sending rights, terminal B would
drown out terminal A already on the physical layer
Also severe problem for CDMA-networks - precise power control
needed!
A B C
67. 2/17/2023 ASN_Unit-I_MVSR 67
Access methods SDMA/FDMA/TDMA
SDMA (Space Division Multiple Access)
segment space into sectors, use directed antennas
cell structure
FDMA (Frequency Division Multiple Access)
assign a certain frequency to a transmission channel between a
sender and a receiver
permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast
hopping (FHSS, Frequency Hopping Spread Spectrum)
TDMA (Time Division Multiple Access)
assign the fixed sending frequency to a transmission channel
between a sender and a receiver for a certain amount of time
68. SDMA is used for allocating a separated space to users in wireless
networks.
A typical application involves assigning an optimal base station to a
mobile phone user
The mobile phone may receive several base stations with different
quality.
A MAC algorithm could now decide which base station is best, taking
into account with frequencies (FDM), time slots(TDM) or code(CDM)
are still available(depending on technology)
Typically SDMA is never used in isolation but always in combination
with one or more other schemes
The basis for the SDMA algorithm is formed by cell and sectorized
antennas which constitute the infrastructure implementing SDM
Fig ., shows a spatialy filtered base station antenna serving different
users by using spot beams.
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69. FDMA ( Frequency Division Multiple Access)
FDMA assigns individual channels to individual users
Each user is allocated a unique freq., band or channel
These channel are assigned on demand to users who request service
During the period of the call, no other user can share the same channel
The FDMA channel carries only one phone circuit at a time.
If an FDMA channel is not in use, then it sits idle and can not be used by other
users.
After assignment of a voice channel , the base station and the mobile transmit
simultaneously and continuously
The B.W of FDMA is relatively narrow
The complexity of FDMA is lower compared to TDMA
The FDMA mobile unit user duplexer for simultaneous transmission and
reception
FDMA requires tight RF filtering to minimize adjacent channel interference
FDMA have higher cell site system costs compared to TDMA
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72. TDMA (Time Division Multiple Access)
2/17/2023 ASN_Unit-I_MVSR 72
TDMA (Time Division Multiple Access) System divide the
ratio spectrum into time slots.
In each slot only one user is allowed to either transmit or
receive
Each user occupies a cyclically repeating time slot
transmission for any user is non continuous
Listening to different frequencies at the same time is
quite difficult
75. 2/17/2023 ASN_Unit-I_MVSR 75
Aloha/slotted aloha
Mechanism
random, distributed (no central arbiter), time-multiplex
Slotted Aloha additionally uses time-slots, sending must always
start at slot boundaries
Aloha
Slotted Aloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t
76. 2/17/2023 ASN_Unit-I_MVSR 76
DAMA - Demand Assigned Multiple Access
Channel efficiency only 18% for Aloha, 36% for Slotted
Aloha (assuming Poisson distribution for packet arrival
and packet length)
Reservation can increase efficiency to 80%
a sender reserves a future time-slot
sending within this reserved time-slot is possible without collision
reservation also causes higher delays
typical scheme for satellite links
Examples for reservation algorithms:
Explicit Reservation according to Roberts (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA
77. 2/17/2023 ASN_Unit-I_MVSR 77
Access method DAMA: Explicit Reservation
DAMA(demand assigned multiple access)
Explicit Reservation (Reservation Aloha):
two modes:
ALOHA mode for reservation:
competition for small reservation slots, collisions possible
reserved mode for data transmission within successful reserved slots (no
collisions possible)
it is important for all stations to keep the reservation list consistent
at any point in time and, therefore, all stations have to synchronize
from time to time
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t
78. 2/17/2023 ASN_Unit-I_MVSR 78
Access method DAMA: PRMA
Implicit reservation (PRMA - Packet Reservation MA):
a certain number of slots form a frame, frames are repeated
stations compete for empty slots according to the slotted aloha
principle
once a station reserves a slot successfully, this slot is automatically
assigned to this station in all following frames as long as the station
has data to send
competition for this slots starts again as soon as the slot was empty
in the last frame
frame1
frame2
frame3
frame4
frame5
1 2 3 4 5 6 7 8 time-slot
collision at
reservation
attempts
A C D A B A F
A C A B A
A B A F
A B A F D
A C E E B A F D
t
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservation
79. 2/17/2023 ASN_Unit-I_MVSR 79
Access method DAMA: Reservation-TDMA
Reservation Time Division Multiple Access
every frame consists of N mini-slots and x data-slots
every station has its own mini-slot and can reserve up to k
data-slots using this mini-slot (i.e. x = N * k).
other stations can send data in unused data-slots according to
a round-robin sending scheme (best-effort traffic)
N mini-slots N * k data-slots
reservations
for data-slots
other stations can use free data-slots
based on a round-robin scheme
e.g. N=6, k=2
80. 2/17/2023 ASN_Unit-I_MVSR 80
MACA - collision avoidance
MACA (Multiple Access with Collision Avoidance) uses short signaling
packets for collision avoidance
RTS (request to send): a sender request the right to send from a receiver
with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as soon as it is
ready to receive
Signaling packets contain
sender address
receiver address
packet size
Variants of this method can be found in IEEE802.11 as DFWMAC
(Distributed Foundation Wireless MAC)
81. 2/17/2023 ASN_Unit-I_MVSR 81
MACA examples
MACA avoids the problem of hidden terminals
A and C want to
send to B
A sends RTS first
C waits after receiving
CTS from B
MACA avoids the problem of exposed terminals
B wants to send to A, C
to another terminal
now C does not have
to wait for it cannot
receive CTS from A
A B C
RTS
CTS
CTS
A B C
RTS
CTS
RTS
82. 2/17/2023 ASN_Unit-I_MVSR 82
MACA variant: DFWMAC in IEEE802.11
idle
wait for the
right to send
wait for ACK
sender receiver
packet ready to send; RTS
time-out;
RTS
CTS; data
ACK
RxBusy
idle
wait for
data
RTS; RxBusy
RTS;
CTS
data;
ACK
time-out
data;
NAK
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
time-out
NAK;
RTS
83. 2/17/2023 ASN_Unit-I_MVSR 83
Polling mechanisms
If one terminal can be heard by all others, this “central”
terminal (a.k.a. base station) can poll all other terminals
according to a certain scheme
now all schemes known from fixed networks can be used (typical
mainframe - terminal scenario)
Example: Randomly Addressed Polling
base station signals readiness to all mobile terminals
terminals ready to send can now transmit a random number
without collision with the help of CDMA or FDMA (the random
number can be seen as dynamic address)
the base station now chooses one address for polling from the list of
all random numbers (collision if two terminals choose the same
address)
the base station acknowledges correct packets and continues
polling the next terminal
this cycle starts again after polling all terminals of the list
84. 2/17/2023 ASN_Unit-I_MVSR 84
ISMA (Inhibit Sense Multiple Access)
Current state of the medium is signaled via a “busy tone”
the base station signals on the downlink (base station to terminals)
if the medium is free or not
terminals must not send if the medium is busy
terminals can access the medium as soon as the busy tone stops
the base station signals collisions and successful transmissions via
the busy tone and acknowledgements, respectively (media access is
not coordinated within this approach)
mechanism used, e.g.,
for CDPD (cellular digital packet data)
(USA, integrated
into AMPS)
85. 2/17/2023 ASN_Unit-I_MVSR 85
Access method CDMA
CDMA (Code Division Multiple Access)
all terminals send on the same frequency probably at the same time and
can use the whole bandwidth of the transmission channel
each sender has a unique random number, the sender XORs the signal with
this random number
the receiver can “tune” into this signal if it knows the pseudo random
number, tuning is done via a correlation function
Disadvantages:
higher complexity of a receiver (receiver cannot just listen into the medium
and start receiving if there is a signal)
all signals should have the same strength at a receiver
Advantages:
all terminals can use the same frequency, no planning needed
huge code space (e.g. 232) compared to frequency space
interferences (e.g. white noise) is not coded
forward error correction and encryption can be easily integrated
86. 2/17/2023 ASN_Unit-I_MVSR 86
CDMA in theory
Sender A
sends Ad = 1, key Ak = 010011 (assign: “0”= -1, “1”= +1)
sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)
Sender B
sends Bd = 0, key Bk = 110101 (assign: “0”= -1, “1”= +1)
sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)
Both signals superimpose in space
interference neglected (noise etc.)
As + Bs = (-2, 0, 0, -2, +2, 0)
Receiver wants to receive signal from sender A
apply key Ak bitwise (inner product)
Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6
result greater than 0, therefore, original bit was “1”
receiving B
Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0”
87. 2/17/2023 ASN_Unit-I_MVSR 87
CDMA on signal level I
data A
key A
signal A
data key
key
sequence A
Real systems use much longer keys resulting in a larger distance
between single code words in code space.
1 0 1
1
0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1
0
1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Ad
Ak
As
88. 2/17/2023 ASN_Unit-I_MVSR 88
CDMA on signal level II
signal A
data B
key B
key
sequence B
signal B
As + Bs
data key
1 0 0
0
0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1
1
1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1
Bd
Bk
Bs
As
89. 2/17/2023 ASN_Unit-I_MVSR 89
CDMA on signal level III
Ak
(As + Bs)
* Ak
integrator
output
comparator
output
As + Bs
data A
1 0 1
1 0 1 Ad
90. 2/17/2023 ASN_Unit-I_MVSR 90
CDMA on signal level IV
integrator
output
comparator
output
Bk
(As + Bs)
* Bk
As + Bs
data B
1 0 0
1 0 0 Bd
92. 2/17/2023 ASN_Unit-I_MVSR 92
Aloha has only a very low efficiency, CDMA needs complex receivers to
be able to receive different senders with individual codes at the same
time
Idea: use spread spectrum with only one single code (chipping
sequence) for spreading for all senders accessing according to aloha
SAMA - Spread Aloha Multiple Access
1
sender A
0
sender B
0
1
t
narrow
band
send for a
shorter period
with higher power
spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)
Problem: find a chipping sequence with good characteristics
1
1
collision
93. 2/17/2023 ASN_Unit-I_MVSR 93
Comparison SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
Terminals only one terminal can
be active in one
cell/one sector
all terminals are
active for short
periods of time on
the same frequency
every terminal has its
own frequency,
uninterrupted
all terminals can be active
at the same place at the
same moment,
uninterrupted
Signal
separation
cell structure, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
Advantages very simple, increases
capacity per km²
established, fully
digital, flexible
simple, established,
robust
flexible, less frequency
planning needed, soft
handover
Dis-
advantages
inflexible, antennas
typically fixed
guard space
needed (multipath
propagation),
synchronization
difficult
inflexible,
frequencies are a
scarce resource
complex receivers, needs
more complicated power
control for senders
Comment only in combination
with TDMA, FDMA or
CDMA useful
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
