Mobile Computing Unit 1 -...
Transcript of Mobile Computing Unit 1 -...
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
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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
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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
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1 2 311
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tdownlink
uplink
417 µs
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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
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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
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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
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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
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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
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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
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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
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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.
a)CDMA b)CSMA c)FDMA d)TDMA
5. ------------ comprises all technologies that allocate certain time slots for communication.
a)CDMA b)CSMA c)FDMA d)TDMA
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
(a)SDM (b)FDM (c)TDM (d)CDM
9. Each Channel has unique code and all the channels use the same spectrum at
the same time is
(a)SDM (b)FDM (c)TDM (d)CDM
10. Which are the following multiplexing are used for secured wireless
transmission?
(a)SDM (b)FDM (c)TDM (d)CDM
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Part-A (2 Marks)
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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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