Mobile Computing Unit 1 - chettinadtech.ac.inchettinadtech.ac.in/storage/11-12-29/11-12-29-17-12-49-1365-Gopal.pdf · Mobile Computing Unit 1 Applications II •Traveling salesmen

Mobile Computing Unit 1

WIRELESS COMMUNICATION FUNDAMENTALS

Objective

Unit I present some basics about wireless transmission technology. The topics covered include: frequencies used for communication, signal characteristics, antennas, signal propagation, and several fundamental multiplexing and modulation schemes. This unit does not require profound knowledge of electrical engineering nor does it explore all details about the underlying physics of wireless communication systems. Its aim is rather to help the reader understand the many design decisions in the higher layers of mobile communication systems. Also, it presents a broad range of media access technologies. It explains why media access technologies from fixed networks often cannot be applied to wireless networks, and shows the special problems for wireless terminals accessing ‘space’ as the common medium. Different multiplexing schemes are also discussed.

Introduction

Computers for the next decades?• Computers are integrated

o small, cheap, portable, replaceable -no more separate devices• Technology is in the background

o computer are aware of their environment and adapt (“location awareness”)

o computer recognize the location of the user and react appropriately (e.g., call forwarding, fax forwarding, “context awareness”))

• Advances in technologyo more computing power in smaller deviceso flat, lightweight displays with low power consumptiono new user interfaces due to small dimensionso more bandwidth per cubic metero multiple wireless interfaces: wireless LANs, wireless WANs,

regional wireless telecommunication networks etc. („overlay networks“)

Mobile communication

• Two aspects of mobility:o user mobility: users communicate (wireless) “anytime, anywhere, with

anyone”o device portability: devices can be connected anytime, anywhere to the

network

• Wireless vs. mobile Examples�� x x stationary computer� x √ notebook in a hotel�� √ x wireless LANs in historic buildings��

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√ √ Personal Digital Assistant (PDA)

• The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks:

o local area networks: standardization of IEEE 802.11o Internet: Mobile IP extension of the internet protocol IPo wide area networks: e.g., internetworking of GSM and ISDN, VoIP over

WLAN and POTSApplications

• Vehicleso transmission of news, road condition, weather, music via DAB/DVB-To personal communication using GSM/UMTSo position via GPSo local ad-hoc network with vehicles close-by to prevent accidents,

guidance system, redundancy o vehicle data (e.g., from busses, high-speed trains) can be transmitted in

advance for maintenance • Emergencies

o early transmission of patient data to the hospital, current status, first diagnosis

o replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.

o crisis, war, ...

Typical Application

Mobile and wireless services –Always Best Connected

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Applications II• Traveling salesmen

o direct access to customer files stored in a central locationo consistent databases for all agentso mobile office

• Replacement of fixed networkso remote sensors, e.g., weather, earth activitieso flexibility for trade showso LANs in historic buildings

• Entertainment, education, ...o outdoor Internet access o intelligent travel guide with up-to-date location dependent informationo ad-hoc networks for multi user games

Location dependent services

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• Location aware services o what services, e.g., printer, fax, phone, server etc. exist in the local

environment• Follow-on services

o automatic call-forwarding, transmission of the actual workspace to the current location

• Information serviceso “push”: e.g., current special offers in the supermarketo “pull”: e.g., where is the Black Forrest Cheese Cake?

• Support serviceso caches, intermediate results, state information etc. “follow” the mobile

device through the fixed network• Privacy who should gain knowledge about the location

Mobile devices

Effects of device portability• Power consumption

o limited computing power, low quality displays, small disks due to limited battery capacity

o CPU: power consumption ~ CV2f C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit f: clock frequency, can be reduced temporally

• Loss of datao higher probability, has to be included in advance into the design (e.g.,

defects, theft)

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• Limited user interfaceso compromise between size of fingers and portabilityo integration of character/voice recognition, abstract symbols

• Limited memoryo limited usage of mass memories with moving partso flash-memory or ? as alternative

Wireless networks in comparison to fixed networks

• Higher loss-rates due to interferenceo emissions of, e.g., engines, lightning

• Restrictive regulations of frequencieso frequencies have to be coordinated, useful frequencies are almost all

occupied• Low transmission rates

o local some Mbit/s, regional currently, e.g., 53kbit/s with GSM/GPRS or about 150 kbit/s using EDGE

• Higher delays, higher jittero connection setup time with GSM in the second range, several hundred

milliseconds for other wireless systems• Lower security, simpler active attacking

o radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones

• Always shared mediumo secure access mechanisms important

Wireless Transmissiono Frequencieso Signals, antennas, signal propagationo Multiplexingo Spread spectrum, modulation• Cellular systems

Frequencies for communication

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Frequencies for mobile communication

• VHF-/UHF-ranges for mobile radioo simple, small antenna for carso deterministic propagation characteristics, reliable connections

• SHF and higher for directed radio links, satellite communicationo small antenna, beam formingo large bandwidth available

• Wireless LANs use frequencies in UHF to SHF rangeo some systems planned up to EHFo limitations due to absorption by water and oxygen molecules

(resonance frequencies) weather dependent fading, signal loss caused by heavy

rainfall etc. Frequencies and regulations

• ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

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Signals

• physical representation of data• function of time and location• signal parameters: parameters representing the value of data • classification

o continuous time/discrete timeo continuous values/discrete valueso analog signal = continuous time and continuous valueso digital signal = discrete time and discrete values

• signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift ϕΦo sine wave as special periodic signal for a carrier:

s(t) = Atsin(2 πft t + ϕt)

Fourier representation of periodic signals

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• Different representations of signals o amplitude (amplitude domain)o frequency spectrum (frequency domain)

o phase state diagram (amplitude M and phase ϕ in polar coordinates)

o Composed signals transferred into frequency domain using Fourier transformation

o Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog

signal!

Antennas: isotropic radiator

Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission

Isotropic radiator: equal radiation in all directions (three dimensional) -only a theoretical reference antenna

Real antennas always have directive effects (vertically and/or horizontally)

Radiation pattern: measurement of radiation around an antenna

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Signal propagation ranges

Transmission rangeo communication possibleo low error rate

Detection rangeo detection of the signal possibleo no communication possible

Interference rangeo signal may not be detected o signal adds to the background noise

Multiplexing

• Multiplexing in 4 dimensionso space (si)o time (t)o frequency (f)o code (c)

• Goal: multiple use of a shared medium• Important: guard spaces needed!

Modulation

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• Digital modulationo digital data is translated into an analog signal (baseband)o ASK, FSK, PSK -main focus in this chaptero differences in spectral efficiency, power efficiency, robustness

• Analog modulationo shifts center frequency of baseband signal up to the radio carrier

• Motivationo smaller antennas (e.g., λ/4)o Frequency Division Multiplexingo medium characteristics

• Basic schemeso Amplitude Modulation (AM)o Frequency Modulation (FM)o Phase Modulation (PM)

Spread spectrum technology

• 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

• Side effects: coexistence of several signals without dynamic coordination tap-proof

• Alternatives: Direct Sequence, Frequency Hopping

0

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MEDIUM ACCESS CONTROL

Can we apply media access methods from fixed networks?

Example of 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

(original method in IEEE 802.3)

Problems in wireless networks

a radio can usually not transmit and receive at the same time

signal strength decreases proportionally to the square of the distance or even

more

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”

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

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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

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!

Access methods SDMA/TDMA/FDMA/CDMA

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BA C

A B C


SDMA (Space Division Multiple Access)

segment space into sectors, use directed antennas

cell structure

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

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)

CDMA (Code Division Multiple Access)

assign an appropriate code to each transmission channel (DSSS, Direct Sequency

Spread Spectrum)

frequency hopping over separate channels (FHSS, Frequency Hopping Spread

Spectrum)

Some medium access control mechanisms for wireless

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TDMA/TDD – example: DECT

DECT: Digital Enhanced Cordless Telecommunications

TDD: Time Division Duplex

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TDMA CDMAFDMASDMA

Fixed Aloha ReservationsDAMA

MultipleAccess withCollisionAvoidance

Polling

Pure

CSMA

Slotted

Non-persistent p-persistent CSMA/CA

Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional)

MACAW MACA-BI FAMA

CARMA

Used in 802.11 (mandatory)

FHSS DSSS

Used in GSM

Fixed

Used in Bluetooth Used in UMTS

1 2 311

12

1 2 311

12

tdownlink

uplink

417 µs


FDMA/FDD – example: GSM

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

15

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

downlink

uplink

sender A

sender B

sender C

collision


Carrier Sense Multiple Access (CSMA)

Goal: reduce the wastage of bandwidth due to packet collisions

Principle: sensing the channel before transmitting (never transmit when the

channel is busy)

Many variants:

Collision detection (CSMA/CD) or collision avoidance(CSMA/CA)

Persistency (in sensing and transmitting)

1-Persistent CSMA

Stations having a packet to send sense the channel continuously, waiting until

the channel becomes idle.

As soon as the channel is sensed idle, they transmit their packet.

If more than one station is waiting, a collision occurs.

Stations involved in a collision perform a the backoff algorithm to schedule a

future time for resensing the channel

Optional backoff algorithm may be used in addition for fairness

Non-Persistent CSMA

Attempts to reduce the incidence of collisions

Stations with a packet to transmit sense the channel

If the channel is busy, the station immediately runs the back-off algorithm and

reschedules a future sensing time

If the channel is idle, then the station transmits

Demand Assigned Multiple Accesses (DAMA):

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sender A

sender B

sender C

collision

t


Channel efficiency only 18% for Aloha, 36% for Slotted Aloha

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 (Reservation-ALOHA)

Implicit Reservation (PRMA)

Reservation-TDMA

DAMA / Explicit Reservation

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

DAMA / Packet reservation (PRMA)

Implicit reservation

based on slotted Aloha

a certain number of slots form a frame, frames are repeated

stations compete for empty slots according to the slotted aloha principle

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Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t


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 a slot starts again as soon as the slot was empty in the last frame

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)

Polling mechanisms

If one terminal can be heard by all others, this “central” terminal (e.g., base station) can

poll all other terminals according to a certain scheme

all schemes known from fixed networks can be used (typical mainframe -

terminal scenario)

Example: Randomly Addressed Polling

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frame1

frame2

frame3

frame4

frame5

1 2 3 4 5 6 7 8 time-lot

A C D A B A F

A C A B A

A B A F

A B A F

DA C E E B A F D

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

N mini-slots

N * k data-slots

reservationsfor data-

slots

other stations can use free data-slotsbased on a round-robin scheme

e.g. N=6, k=2


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 a 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

Inhibit 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 (media access is not coordinated within

this approach)

mechanism used, e.g., for CDPD (Cellular Digital Packet Data)

Similar approach was proposed

for Packet Radio Networks

(Kleinrock + Tobagi, 1975)

Code Division Multiple Access

Principles

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all terminals send on the same frequency and can use the whole bandwidth of

the transmission channel

each sender has a unique code

The sender XORs the signal with this code

the receiver can “tune” into this signal if it knows the code of the sender

tuning is done via a correlation function

Disadvantages:

higher complexity of the receiver (receiver cannot just listen into the medium

and start receiving if there is a signal)

all signals should have approximately the same strength at the receiver

Advantages:

all terminals can use the same frequency, no planning needed

huge code space (e.g., 232) compared to frequency space

more robust to eavesdropping and jamming (military applications…)

forward error correction and encryption can be easily integrated

Principle (very simplified)

Example:

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Ak

X As

Ad

Bk

X Bs

Bd

As + B

s

Ak

X

Bk

X

C+D

C+D

Ad

Bd

Spreading Despreading


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“

SAMA (Spread Aloha Multiple Access)

Aloha has only a very low efficiency, CDMA needs complex receivers to be able to

eceive 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

Comparison SDMA/TDMA/FDMA/CDMA

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1sender A 0sender B

0

1

t

narrowbandsend for a shorter periodwith 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


Summary

This unit introduced the basics of wireless communication. As we have only one

‘medium’ for wireless transmission, several multiplexing schemes can be applied to

raise the overall capacity. The standard schemes are SDM, FDM, TDM and CDM. To

achieve FDM, data has to be ‘translated’ into a signal with a certain carrier frequency.

Therefore, tow modulation steps can be applied. Digital modulation encodes data into a

base band signal, whereas analog modulation encodes data into a base band signal,

whereas analog modulation then shifts the centre frequency of the signal up to the

radio carrier. Some advanced schemes have been presented that can code many bits

into a single phase shift, raising the efficiency.

Keywords

SAMA (Spread Aloha Multiple Access)

CDMA(Code Division Multiple Access )

CSMA(Carrier Sense Multiple Access )

FDMA(Frequency Division Multiple Access)

TDMA(Time Division Multiple Access)

SDM – Space Division Multiplexing

FDM- Frequency division multiplexing

TDM- Time Division Multiplexing

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CDM- Code Division Multiplexing

Multiple choice questions

1. CDMA with only a single code, is called

a)SAMA b) CDMA c)FDMA d)TDMA

2. ------------- systems use exactly these codes to separate different users in

code space and to enable access to a shared medium without interference.

a)SAMA b) CDMA c)FDMA d)TDMA

3. In -------------a sender senses the medium (a wire or coaxial cable) to see if it

is free. If the medium is busy, the sender waits until it is free. If the medium is

free, the sender starts transmitting data and continues to listen into the medium.

a)CDMA b)CSMA c)FDMA d)TDMA

4. ------------ comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM) scheme.


5. ------------ comprises all technologies that allocate certain time slots for communication.


6. --------------was to provide a mobile phone system that allows users to roam throughout Europe and provides voice services compatible to ISDN and other PSTN systems (a)GPS (b)GSM (c)CDMA (d)TETRA

7. Separation of whole spectrum into smaller frequency bands is

(a)SDM (b)FDM (c)TDM (d)CDM

8.Precise Synchornization is necessary in


9. Each Channel has unique code and all the channels use the same spectrum at

the same time is


10. Which are the following multiplexing are used for secured wireless

transmission?


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Part-A (2 Marks)

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25


26


Part –B

1. Explain about Mobile services (16)

2. Explain System architecture (16)

3. Explain briefly about TETRA (16)

4. Explain about UTRAN (16)

Review Questions and Exercises

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References

http://cst.mi.fu-berlin.de/resources/mobkom/material/English/PDF-Handout/C01-

Introduction.pdf

http://cst.mi.fu-berlin.de/resources/mobkom/material/English/PDF-Handout/C02-

Wireless_Transmission.pdf

http://cst.mi.fu-berlin.de/resources/mobkom/material/English/PDF-Handout/C03-Media_Access.pdf

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Documents

Mobile Computing Unit 1 - chettinadtech.ac.inchettinadtech.ac.in/storage/11-12-29/11-12-29-17-12-49-1365-Gopal.pdf · Mobile Computing Unit 1 Applications II •Traveling salesmen