IT8602 Mobile Communication - Unit I Introduction

IT8602 Mobile Communication
Unit I - Introduction
Kaviya.P
Kamaraj College of Engineering & Technology

Unit I - Introduction
Introduction to Mobile Computing - Applications of
Mobile Computing - Generations of Mobile
Communication Technologies - MAC Protocols -
SDMA - TDMA - FDMA - CDMA

Introduction to Mobile Computing
• What is computing?
– The capability to automatically carry out certain processing related to service
invocations on a remote computer.
• What is the mobility?
– The capability to change location while communicating to invoke computing
service at some remote computers.
• Mobile Computing is an umbrella term used to describe technologies that enable
people to access network services anyplace, anytime, and anywhere.
• Advantage: Flexibility to the users.

User Mobility Vs. Device Mobility
• User Mobility: Refers to a user who has access to the same or similar
telecommunication services at different places. (i.e.) the user can be mobile, and the
services will follow him or her.
• Device Mobility: The communication device moves. Many mechanisms in the
network and inside the device have to make sure that communication is still
possible while the device is moving.

Mobile Computing Vs. Wireless Networking
• Mobile computing is based on wireless networking and helps to invoke computing
services on remote servers while on the move.
• So Wireless networking is an important and necessary ingredient of mobile
computing.
• Mobile computing also requires the applications themselves – their design and
development, the hardware at the client and server sides.

Mobile Computing Vs. Wireless Networking
Mobile Computing Wireless Networking
The ability to compute while on
the move.
Basic communication
infrastructure which is necessary for
mobile computing.
It comes under the category of
computing.
It comes under the category of
networking.
Mobile computing is a part of
wireless networking.
Wireless networking is not a part of
mobile computing.

Characteristics of Mobile Computing
• Ubiquity: The ability of a user to perform computation from anywhere and at
anytime. (Eg: Business Notifications)
• Location awareness: Many applications requires or value additions by location
based services. (Eg: Traffic Control Applications – GPS)
• Adaptation: Ability to adjust to bandwidth fluctuations without inconveniencing the
user. Fluctuations can arise due to a number of factors such as handoff, obstacles,
environmental noise, etc.
• Broadcast: Efficient delivery of data can be made simultaneously to hundreds of
mobile users. (Eg: Advertising information by a taxi service provider)
• Personalization: Services in a mobile environment can be easily personalized
according to a user‟s profile.

Characteristics of Communication Devices
• Fixed and Wired: Desktop Computers
• Mobile and Wired: Laptops
• Fixed and Wireless: Cameras with network connectivity
• Mobile and Wireless: Mobile Phones

Applications of Mobile Computing
• Vehicles
• Emergencies
• Business
• Replacement of wired networks
• Infotainment and more
• Location dependent services
– Follow-on services
– Location aware services
– Privacy
– Information services
– Support services
• Mobile and Wireless devices
– Sensors
– Embedded controllers
– Pager
– Mobile phones
– Personal Digital Assistant (PDA)
– Pocket computer
– Notebook / laptop

• Vehicles
– Transmission of news, road condition, weather, music via DAB
– Personal communication using GSM
– Position via GPS
– Local ad-hoc network with vehicles close-by to prevent accidents, guidance
system, redundancy
– Vehicle data (e.g., from busses, high-speed trains) can be transmitted in advance
for maintenance
• Emergencies
– Early transmission of patient data to the hospital, current status, first diagnosis
– Replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.

UMTS, WLAN,
DAB, DVB, GSM,
cdma2000, TETRA, ...
Personal Travel Assistant,
PDA, Laptop,
GSM, UMTS, WLAN,
Bluetooth, ...

UMTS
2 Mbit/s
UMTS, GSM
384 kbit/s
LAN
100 Mbit/s,
WLAN
54 Mbit/s
UMTS, GSM
115 kbit/s
GSM 115 kbit/s,
WLAN 11 Mbit/s
GSM/GPRS 53 kbit/s
Bluetooth 500 kbit/s
GSM/EDGE 384 kbit/s,
DSL/WLAN 3 Mbit/s
DSL/ WLAN
3 Mbit/s

• Business
– Direct access to customer files stored in a central location
– Consistent databases for all agents
– Mobile office
• Replacement of wired networks
– Remote sensors, e.g., weather, earth activities (Due to economic reasons)
– Flexibility for trade shows
– LANs in historic buildings
• Infotainment and more
– Outdoor Internet access
– Intelligent travel guide with up-to-date location dependent information
– Ad-hoc networks for multi user games

• Location dependent services
– Follow-on services
• Automatic call-forwarding, transmission of the actual workspace to the current
location
– Location aware services
• What services, e.g., printer, fax, phone, server etc. exist in the local environment
– Privacy
• Who should gain knowledge about the location
– Information services
• “push”: e.g., current special offers in the supermarket
• “pull”: e.g., where is the Black Forrest Cherry Cake?
– Support services
• Caches, intermediate results, state information etc. „follow“ the mobile device
through the fixed network

• Mobile and Wireless Devices
– Sensor
– Embedded controllers
– Pager
– Mobile phones
performance
Pager
• Receive only
• Tiny displays
• Simple text messages
Mobile Phones
• Voice, data
• Simple graphical displays
PDA
• Graphical displays
• Character recognition
• Simplified WWW
Packet PC
• Tiny keyboard
• Simple versions of standard
applications
Laptop/Notebook
• Fully functional
• Standard applications
Sensors,
embedded
controllers
• Mobile and Wireless Devices
– Personal Digital Assistant
– Pocket computer
– Notebook/Laptop

Mobile Computing - Advantages & Disadvantages
• Advantages
– Location Flexibility
– Device Portability
– User Mobility
– Saves Time
– Enhanced Productivity
– Entertainment
• Disadvantages
– Less range and bandwidth
– Less secure
– Less power storage
– More Interference
– Limited user interfaces
– Loss of data
– High risk to health

Generations of Mobile Communication Technologies
• 1979 NMT (Nordic Mobile Telephone) at 450MHz (Scandinavian countries)
• 1982 Start of GSM-specification
– Goal: pan-European digital mobile phone system with roaming
• 1983 Start of the American AMPS (Advanced Mobile Phone System, analog)
• 1984 CT-1 standard (Europe) for cordless telephones
• 1986 C-Netz in Germany
– Analog voice transmission, 450MHz, hand-over possible, digital signaling,
automatic location of mobile device
– It was in use until 2000, services: FAX, modem, X.25, e-mail, 98% coverage

• 1991 Specification of DECT
– Digital European Cordless Telephone (today: Digital Enhanced Cordless
Telecommunications)
– Spectrum 1880-1900MHz, ~100-500m range, 120 duplex channels, 1.2Mbit/s
data transmission, voice encryption, authentication, support up to several 10000
user/km2, used in more than 110 countries
• 1992 Start of GSM
– 900MHz, 124 full duplex channels
– Full international roaming, automatic location services, authentication, encryption
on wireless link, efficient interoperation with ISDN systems, high audio quality.
– Services: data with 9.6kbit/s, SMS, FAX, Voice.

• 1994 E-Netz in Germany
– GSM with 1800MHz, smaller cells
• 1996 HiperLAN (High Performance Radio Local Area Network)
– Type 1: 5.2GHz, 23.5Mbit/s
– Type 2 and 3 (both 5GHz) and Type 4 (17GHz)
• 1997 Wireless LAN - IEEE802.11
– IEEE standard, 2.4 GHz and infrared, 2Mbit/s
• 1998 Specification of GSM successors
– For UMTS (Universal Mobile Telecommunication System) as European
proposals for IMT-2000
• 1998 Iridium
– 66 satellites (+6 spare), 1.6GHz to the mobile phone

• 1999 Standardization of additional wireless LANs
– IEEE standard 802.11b, 2.4GHz, 11Mbit/s
– Bluetooth for piconets, 2.4GHz, <1Mbit/s
• Decision about IMT-2000
– Several “members” of a “family”: UMTS, CDMA2000, DECT.
– Start of WAP (Wireless Application Protocol) and i-mode
– First step towards a unified Internet/mobile communication system
– Access to many services via the mobile phone
• 2000 GSM with higher data rates
– HSCSD, GPRS, UMTS
• 2001 Start of 3G systems
– CDMA2000 in Korea, UMTS tests in Europe, FOMA (almost UMTS) in Japan

cellular phones satellites
wireless LANcordless
phones
1992:
GSM
1994:
DCS 1800
2001:
IMT-2000
1987:
CT1+
1982:
Inmarsat-A
1992:
Inmarsat-B
Inmarsat-M
1998:
Iridium
1989:
CT 2
1991:
DECT 199x:
proprietary
1997:
IEEE 802.11
1999:
802.11b, Bluetooth
1988:
Inmarsat-C
analogue
digital
1991:
D-AMPS
1991:
CDMA
1981:
NMT 450
1986:
NMT 900
1980:
CT0
1984:
CT1
1983:
AMPS
1993:
PDC
4G – fourth generation:
when and how?
2000:
GPRS
2000:
IEEE 802.11a
200?:
Fourth Generation
(Internet based)

MAC Protocols
• The MAC protocol is a sub-layer of the data link layer protocol and it directly invokes
the physical layer protocol.
• A channel-access scheme is also based on a multiple access protocol and control
mechanism, also known as media access control (MAC).
• This protocol deals with issues such as addressing, assigning multiplex channels to
different users, and avoiding collisions.

Responsibility & Objective of MAC Protocol
• Responsibility: Enforce discipline in the access of a shared channel when multiple
nodes contend to access that channel.
• Objective: Maximization of the utilization of the channel and minimization of average
latency of transmission.

Properties Required of MAC Protocols
• The design of a MAC protocol depends upon the specific environment in which it is
intended to operate and the specific characteristics of the application for which it is
being designed.
• A good MAC protocol needs to possess the following features:
– It should implement some rules that help to enforce discipline when multiple
nodes contend for a shared channel.
– It should help maximize the utilization of the channel.
– Channel allocation needs to be fair. No node should be discriminated against at
any time and made to wait for an unduly long time for transmission.
– It should be capable of supporting several types of traffic having different
maximum and average bit rates.
– It should be robust in the face of equipment failures and changing network
conditions.

Wireless MAC Protocols: Some Issues
• It is difficult to implement a collision detection scheme in a wireless environment,
since collisions are hard to be detected.
• In infrastructure-less networks, the issue of hidden and exposed terminals & near and
far terminals make a MAC protocol extremely inefficient.

Hidden and Exposed Terminals
Hidden terminals
• A sends to B, C cannot hear A
• C wants to send to B, C senses a “free” medium (Carrier Sense fails)
• Collision at B, A cannot receive the collision (Collision Detection fails)
• C is “hidden” from A
Exposed terminals
• B sends to A, C wants to send to another terminal (not A or B)
• C senses the carrier and detects that the carrier is busy
• C postponed its transmission until it detects the medium become idle
• But A is outside radio range of C, waiting is not necessary (unnecessary delay)
• C is “exposed” to B

Near and Far Terminals
Terminals A and B send, C receives
• Signal strength decreases proportional to the square of the distance
• B‟s signal drowns out A‟s signal
• C cannot receive A
• If C was an arbiter, B would drown out A
• Near/far effect is a severe problem for CDMA networks
• Precise power control needed to receive all senders with the same strength at a receiver.

Space Division Multiple Access (SDMA)
• Used for allocating a separate space to users in wireless
networks.
• 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 should decide which base station is best,
taking into account which frequencies (FDM), time slots
(TDM), or code (CDM) are still available.
• SDMA is never used in isolation.
• Always works in combination with one or more other
schemes.
• SDMA algorithm is formed by cells and sectors.
• Single users are separated in space by individual beams.

Time Division Multiple Access (TDMA)
• Flexible scheme, which comprises all technologies that allocate certain time slots for
communication.
• Now tuning into a certain frequency is not necessary (i.e.) the receiver can stay at the
same frequency the whole time.
• Synchronization between sender and receiver has to be achieved in the time domain.
• This can be done by using a fixed pattern similar to FDMA technique, (i.e.) allocating
a certain time slot for a channel, or by using a dynamic allocation scheme

1. Fixed TDM
• Simplest algorithm for using TDM is allocating time slots for channels in fixed
pattern.
• Assign the fixed sending frequency to a transmission channel between a sender and
a receiver for a certain amount of time.
• If synchronization is assured, each mobile station knows its turn and no interference
will happen.
• Fixed access patterns fit perfectly well for connections with a fixed bandwidth.
• This patterns guarantee a fixed delay - one can transmit.
• TDMA schemes with fixed access patterns are used in IS-54, IS-136, GSM, DECT.

1. Fixed TDM
• Used to implement multiple access and a duplex
channel between a base station and mobile station.
• Assigning different slots for uplink and downlink
using the same frequency is called Time Division
Duplex (TDD).
• The base station uses one out of 12 slots for the
downlink, whereas the mobile station uses one out
of 12 different slots for the uplink.
• A lot of bandwidth is wasted, if the station has no
data to transmit.

2. Classical Aloha
• Neither coordinates medium access nor does it resolve contention on the MAC layer.
• Instead, each station can access the medium at any time.
• Random access scheme, without a central arbiter controlling access and without
coordination among the stations.
• If two or more stations access the medium at the same time, a collision occurs and the
transmitted data is destroyed.
• Resolving this problem is left to higher layers (e.g., retransmission of data).
• It works fine for a light load and does not require any complicated access
mechanisms.
• Data packet arrival follows a Poisson distribution, maximum throughput is achieved
for an 18 percent load.

• Aloha & Slotted Aloha

3. Slotted Aloha
• All senders have to be synchronized, transmission can only start at the beginning
of a time slot.
• Still, access is not coordinated.
• Introduction of slots raises the throughput from 18 percent to 36 percent,
(i.e.) slotting doubles the throughput.
• Aloha systems work perfectly well under a light load.
• But they cannot give any hard transmission guarantees, such as maximum delay
before accessing the medium, or minimum throughput.
• New mobile communication systems like UMTS have to rely on slotted Aloha for
medium access in certain situations.

4. Carrier Sense Multiple Access (CSMA)
• CSMA - One improvement to the basic Aloha is sensing the carrier before
accessing the medium.
• Sensing the carrier and accessing the medium only if the carrier is idle decreases
the probability of a collision.
• Here hidden terminals cannot be detected, so, if a hidden terminal transmits at the
same time as another sender, a collision might occur at the receiver.
• This basic scheme is still used in most wireless LANs.
• Non-persistent CSMA - Stations sense the carrier and start sending immediately
if the medium is idle. If the medium is busy, the station pauses a random amount of
time before sensing the medium again and repeating this pattern.

• In p-persistent CSMA systems nodes also sense the medium, but only transmit
with a probability of p, with the station deferring to the next slot with the
probability 1-p, i.e., access is slotted in addition.
• In 1-persistent CSMA systems, all stations wishing to transmit access the medium
at the same time, as soon as it becomes idle. This will cause many collisions if
many stations wish to send and block each other. To create some fairness for
stations waiting for a longer time, back-off algorithms can be introduced , which
are sensitive to waiting time as this is done for standard Ethernet.

• CSMA with Collision Avoidance (CSMA/CA) is used in wireless LANs
following the standard IEEE 802.11. Here sensing the carrier is combined with a
back-off scheme in case of a busy medium to achieve some fairness among
competing stations.
• Elimination yield - non-preemptive multiple access (EY-NMPA) used in the
HIPERLAN 1 specification. Here several phases of sensing the medium and
accessing the medium for contention resolution are interleaved before one
“winner” can finally access the medium for data transmission. Here, priority
schemes can be included to assure preference of certain stations with more
important data.

5. Demand Assigned Multiple Access (DAMA)
• Reservation mechanisms and combinations with some (fixed) TDM patterns.
• It typically have a reservation period followed by a transmission period.
• During the reservation period, stations can reserve future slots in the transmission
period.
• Collisions may occur during the reservation period, the transmission period can
then be accessed without collision.
• These schemes cause a higher delay under a light load (first the reservation has to
take place), but allow higher throughput due to less collisions.
• It is also called reservation Aloha, a scheme typical for satellite systems.

• DAMA has two modes. During a contention phase following the slotted Aloha
scheme, all stations can try to reserve future slots.
• Collisions during the reservation phase do not destroy data transmission, but only
the short requests for data transmission.
• If successful, a time slot in the future is reserved, and no other station is allowed to
transmit during this slot.
• To maintain the fixed TDM pattern of reservation and transmission, the stations
have to be synchronized from time to time.
• DAMA is an explicit reservation scheme. Each transmission slot has to be
reserved explicitly.

6. Packet Reservation Multiple Access (PRMA)
• An Implicit reservation scheme.
• A certain number of slots form a frame, frames are repeated in time.
• A base station, which could be a satellite, now broadcasts the status of each slot to all
mobile stations.
• All stations receiving this vector will then know which slot is occupied and which slot
is currently free.

6. Packet Reservation Multiple Access (PRMA)
• The base station broadcasts the reservation status „ACDABA-F‟ to all stations, here
A to F.
• This means that slots one to six and eight are occupied, but slot seven is free in the
following transmission.
• Stations compete for empty slots using slotted aloha.
• Once station reserves a slot successfully, slot is assigned to this station in all
following frames as long as the station has data to send.
• Competition for a slot starts again once slot was empty in last frame.

7. Reservation TDMA
• In a fixed TDM scheme N mini-slots followed by N*k data-slots form a frame that is
repeated.
• Every station has its own mini-slot and can use it to reserve up to k data-slots.
• This guarantees each station a certain bandwidth and a fixed delay.
• Other stations can send data in unused data-slots according to a round-robin sending
scheme (best-effort traffic)

8. Multiple Access Collision Avoidance (MACA)
• MACA uses short signaling packets for collision avoidance
– RTS (request to send): A sender uses RTS packet to request right to send 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)

8. Multiple Access Collision Avoidance
• 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

• The sender is idle until a user requests the transmission of a data packet.
• The sender then issues an RTS and waits for the right to send.
• If the receiver gets an RTS and is in an idle state, it sends back a CTS and waits for
data.
• The sender receives the CTS and sends the data.
• Otherwise, the sender would send an RTS again after a time-out (e.g., the RTS could
be lost or collided).
• After transmission of the data, the sender waits for a positive acknowledgement to
return into an idle state.
• The receiver sends back a positive acknowledgement if the received data was
correct.

• If not, or if the waiting time for data is too long, the receiver returns into idle state.
• If the sender does not receive any acknowledgement or a negative
acknowledgement, it sends an RTS and again waits for the right to send.
• Alternatively, a receiver could indicate that it is currently busy via a separate
RxBusy.

9. Polling
• Where one station is to be heard by all others, polling scheme can be applied.
• Strictly centralized scheme - one master station and several slave station.
• The master can poll slaves: using round robin, randomly, according to reservation, etc.
• The master could also establish a list of stations wishing to transmit during contention
phase.
• After this phase, the station polls each station polls each station on the list.
• Schemes are used in Bluetooth wireless LAN.

10. Inhabit Sense Multiple Access (ISMA)
• 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.
• Mechanism used, e.g., for CDPD (USA, integrated into AMPS) – Digital Sense
Multiple Access

Frequency Division Multiple Access (FDMA)
• The available bandwidth is divided into many narrower frequency bands called channels.
• Frequency Division Duplex (FDD) - For full duplex communication to take place, each
user is allocated with
– Forward link – Communicating from MS to BS.
– Reverse link – Communication from BS to MS.
• Thus, each user making a call is allocated two unique frequency bands (channels) for
transmitting and receiving signals during the call.
• When the user is underway, no other user would be allocated the same frequency band to
make a call.
• Unused transmission time in a frequency band that occurs when the allocated caller pauses
between transmissions, or when no user is allocated a band, goes idle and is wasted.
• FDMA does not achieve high channel utilization.

Frequency Division Multiple Access (FDMA)
• All uplinks use the band between 890.2 and 915 MHz, all downlinks use 935.2 to 960
MHz.
• According to FDMA, the base station allocates a certain frequency for up- and
downlink to establish a duplex channel with a mobile phone.
• Up- and downlink have a fixed relation. If the uplink frequency is fu = 890 MHz +
n*0.2 MHz, the downlink frequency is fd = fu + 45 MHz, (i.e.) fd = 935 MHz + n*0.2
MHz for a certain channel n.
• Permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS,
Frequency Hopping Spread Spectrum)

Code Division Multiple Access (CDMA)
• Multiple users are allotted different codes that consists of sequences of 0 and 1 to access
the same channel.
• In CDMA, multiple users use the same frequency at the same time and no time
scheduling is applied.
• The bandwidth of this medium is much larger than the space that would be allocated to
each packet transmission during FDMA and the signals can be distinguished from each
other by means of a special coding schemes that is used.
• This is done with the help of a frequency spreading code known as the m-bit pseudo-
noise (PN) code sequence.
• Using m bits, 2^m-1 different codes can be obtained.
• From these codes, each user will use only one code.
• A code for a user should be orthogonal to the codes assigned to other nodes.

Justify that a Barker code has good autocorrelation.
• A code for a certain user should have a good autocorrelation and should be orthogonal
to other codes.
• The Barker code (+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1), for example, has a good
autocorrelation, (i.e.) the inner product with itself is large, the result is 11.

• All terminals send on same frequency at the same time using entire bandwidth of
transmission channel.
• Each sender has a unique random number, sender XORs the signal with this random
number.
• The receiver can “tune” into this signal if it knows the pseudo random number.
Advantages:
• All terminals can use the same frequency, no planning needed.
• Huge code space compared to frequency space.
• Interference is not coded.
• Forward error correction and encryption can be easily integrated.
Disadvantages:
• Higher complexity of a receiver.
• All signals should have the same strength at a receiver.

• 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”

IT8602 Mobile Communication - Unit I Introduction

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IT8602 Mobile Communication - Unit I Introduction