unit-1

SUGUMAR.D,

Assistant Professor,

ECE Department,

Karunya University.

12EC244-Mobile Communications

UNIT-1 (Wireless Transmission)

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Unit-1 Wireless Transmission 12EC244 Mobile Communications

Over View 1. Frequencies

2. Signals

3. Antennas

4. Signal propagation

5. Multiplexing

6. Modulation

7. Spread spectrum

8. Cellular systems

9. Medium Access Control

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1.Frequencies for communication

VLF = Very Low Frequency UHF = Ultra High Frequency

LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency

HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

Frequency and wave length: = c/f wave length , speed of light c @ 3x108 m/s, frequency f

1 Mm

300 Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100 m

3 THz

1 m

300 THz

visible light VLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmission coax cable twisted

pair

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Frequencies for mobile communication VLF, LF, MF HF not used for wireless VHF-/UHF-ranges for mobile radio

simple, small antenna for cars deterministic propagation characteristics, reliable connections

SHF and higher for directed radio links, satellite communication small antenna, beam forming large bandwidth available

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

(resonance frequencies)

weather dependent fading. E.g. signal loss caused by heavy rain

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Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands

worldwide (WRC, World Radio Conferences)

Examples Europe USA Japan

Cellular phones

GSM 880-915, 925-960, 1710-1785, 1805-1880

UMTS 1920-1980, 2110-2170

AMPS, TDMA, CDMA, GSM 824-849, 869-894

TDMA, CDMA, GSM, UMTS 1850-1910, 1930-1990

PDC, FOMA 810-888, 893-958

PDC 1429-1453, 1477-1501

FOMA 1920-1980, 2110-2170

Cordless phones

CT1+ 885-887, 930-932

CT2 864-868

DECT 1880-1900

PACS 1850-1910, 1930-1990

PACS-UB 1910-1930

PHS 1895-1918

JCT 245-380

Wireless

LANs

802.11b/g 2412-2472 802.11b/g 2412-2462

802.11b 2412-2484

802.11g 2412-2472

Other RF systems

27, 128, 418, 433, 868 315, 915 426, 868

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Frequencies and regulations

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2.Signals physical representation of data

function of time and location

signal parameters: parameters representing the value of data

classification

continuous time/discrete time

continuous values/discrete values

analog signal = continuous time and continuous values

digital signal = discrete time and discrete values

signal parameters of periodic signals:

period T, frequency f=1/T, amplitude A, phase shift

sine wave as special periodic signal for a carrier:

s(t) = At sin(2 p ft t + t)

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Fourier representation of periodic

signals

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1

0

1

0

t t

ideal periodic signal real composition

(based on harmonics)

)2cos()2sin(2

1)(

11

nftbnftactgn

n

n

n


Fourier representation of periodic signals

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Representations of Signals

Different representations of signals

amplitude (amplitude domain)

frequency spectrum (frequency domain)

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

Composed signals transferred into frequency domain using Fourier

transformation

Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!)

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3.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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z y

x

z

y x ideal

isotropic

radiator


Antennas: simple dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths

/4 on car roofs or /2 as Hertzian dipole

shape of antenna proportional to wavelength

Example: Radiation pattern of a simple Hertzian dipole

Gain: maximum power in the direction of the main lobe compared to

the power of an isotropic radiator (with the same average power)

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side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simple

dipole

/4 /2


Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

y y z

side view (xy-plane)

x

side view (yz-plane)

z

top view (xz-plane)

x

top view, 3 sector

x

z

top view, 6 sector

x

z

directed

antenna

sectorized

antenna

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Antennas: diversity

Grouping of 2 or more antennas

multi-element antenna arrays

Antenna diversity

switched diversity, selection diversity receiver chooses antenna with largest output

diversity combining

combine output power to produce gain co phasing needed to avoid cancellation

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4.Signal propagation ranges

Transmission range

communication possible

low error rate

Detection range

detection of the signal possible

no communication possible

Interference range

signal may not be detected

signal adds to the background noise

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Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d in vacuum much more in real

environments

(d = distance between sender and receiver) Receiving power additionally influenced by

fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges

reflection scattering diffraction shadowing refraction

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Multipath propagation 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

signal at sender

signal at receiver

LOS pulses multipath

pulses

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short term fading

long term

fading

t

power

Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts

quick changes in the power received (short term fading) Additional changes in

distance to sender obstacles further away

slow changes in the average power received (long term fading)

Effects of mobility

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5.Multiplexing

Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c)

Goal: multiple use of a shared medium

Important: guard spaces needed! s2

s3

s1 f

t

c

k2 k3 k4 k5 k6 k1

f

t

c

f

t

c

channels ki

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Frequency Multiplex Separation of the whole spectrum

into smaller frequency bands

A channel gets a certain band of the spectrum for the whole time

Advantages: no dynamic coordination

necessary

works also for analog signals Disadvantages:

waste of bandwidth if the traffic is distributed unevenly

inflexible guard spaces

k2 k3 k4 k5 k6 k1

f

t

c

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Time Multiplex A channel gets the whole spectrum for a certain amount of time

Advantages:

only one carrier in the medium at any time

throughput high even for many users

Disadvantages:

Precise synchronization necessary

f

t

c

k2 k3 k4 k5 k6 k1

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Time and Frequency Multiplex

Combination of both methods

A channel gets a certain frequency band for a certain amount of time

Example: GSM

Advantages: better protection

against tapping

protection against frequency selective interference

higher data rates compared to code multiplex

but: precise coordination required

f

t

c

k2 k3 k4 k5 k6 k1

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Code Multiplex

Each channel has a unique code All channels use the same spectrum

at the same time

Advantages: bandwidth efficient no coordination and

synchronization necessary

good protection against interference and tapping

Disadvantages: lower user data rates more complex signal

regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6 k1

t

c

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6.Modulation Digital modulation

digital data is translated into an analog signal (baseband)

ASK, FSK, PSK - main focus in this chapter

differences in spectral efficiency, power efficiency, robustness

Analog modulation

shifts center frequency of baseband signal up to the radio carrier

Motivation

smaller antennas (e.g., l/4)

Frequency Division Multiplexing

medium characteristics

Basic schemes

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation (PM)

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Modulation and Demodulation

synchronization

decision

digital

data analog

demodulation

radio

carrier

analog

baseband

signal

101101001 radio receiver

digital

modulation

digital

data analog

modulation

radio

carrier

analog

baseband

signal

101101001 radio transmitter

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Digital Modulation

Modulation of digital signals known as

Shift Keying

Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference

Frequency Shift Keying (FSK): needs larger bandwidth

Phase Shift Keying (PSK): more complex robust against interference

1 0 1

t

1 0 1

t

1 0 1

t

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Advanced Frequency Shift Keying

Bandwidth needed for FSK depends on the distance between the carrier frequencies

special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying)

bit separated into even and odd bits, the duration of each bit is doubled

depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen

the frequency of one carrier is twice the frequency of the other

Equivalent to offset QPSK

even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM

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Example of MSK

data

even bits

odd bits

1 1 1 1 0 0 0

t

low

frequency

high

frequency

MSK

signal

bit

even 0 1 0 1

odd 0 0 1 1

signal h n n h

value - - + +

h: high frequency

n: low frequency

+: original signal

-: inverted signal

No phase shifts!

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Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying):

bit value 0: sine wave

bit value 1: inverted sine wave

very simple PSK

low spectral efficiency

robust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying):

2 bits coded as one symbol

symbol determines shift of sine wave

needs less bandwidth compared to BPSK

more complex

Often also transmission of relative, not absolute phase shift: DQPSK -Differential

QPSK (IS-136, PHS) 11 10 00 01

Q

I 0 1

Q

I

11

01

10

00

A

t

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Quadrature Amplitude Modulation Quadrature Amplitude Modulation

(QAM): combines amplitude and phase

modulation

it is possible to code n bits using one symbol

2n discrete levels, n=2 identical to QPSK

bit error rate increases with n, but less errors compared to comparable PSK

schemes

Example: 16-QAM (4 bits = 1 symbol)

Symbols 0011 and 0001 have the same phase f , but different amplitude a. 0000

and 1000 have different phase, but same

amplitude. used in standard 9600 bit/s

modems

0000

0001

0011

1000

Q

I

0010

a

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Hierarchical Modulation DVB-T modulates two separate data streams onto a

single DVB-T stream

High Priority (HP) embedded within a Low Priority (LP) stream

Multi carrier system, about 2000 or 8000 carriers

QPSK, 16 QAM, 64QAM

Example: 64QAM

good reception: resolve the entire 64QAM constellation

poor reception, mobile reception: resolve only QPSK portion

6 bit per QAM symbol, 2 most significant determine QPSK

HP service coded in QPSK (2 bit), LP uses remaining 4 bit

Q

00

10

000010 010101

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7.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

detection at

receiver

interference spread

signal

signal

spread

interference

f f

power power

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detection at receiver

signal

spread interference

f

interference spread signal

f

power power


Effects of spreading and interference

user signal

broadband interference

narrowband interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiver f

v)

dP/df

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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

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DSSS (Direct Sequence Spread

Spectrum) I 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

chipping

sequence

user data

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

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DSSS (Direct Sequence Spread

Spectrum) II

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

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FHSS (Frequency Hopping Spread

Spectrum) I 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

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Spectrum) II

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

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Spectrum) III

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

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8.Cell structure

Implements space division multiplex: base station covers a certain transmission area (cell)

Mobile stations communicate only via the base station Advantages of cell structures:

higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

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Frequency planning I Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies:

Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells

Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already

used in neighbor cells

more capacity in cells with more traffic assignment can also be based on interference measurements

f4 f5

f1 f3

f2

f6

f7

f3 f2

f4 f5

f1

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Frequency planning II

f1 f2

f3 f2

f1

f1

f2

f3 f2

f3 f1

f2 f1

f3 f3

f3 f3

f3

f4 f5

f1 f3

f2

f6

f7

f3 f2

f4 f5

f1 f3

f5 f6

f7 f2

f2

f1 f1 f1 f2

f3

f2

f3

f2

f3 h1

h2

h3 g1

g2

g3

h1 h2

h3 g1

g2

g3 g1

g2

g3

3 cell cluster

7 cell cluster

3 cell cluster

with 3 sector antennas

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Cell breathing

CDM systems: cell size depends on current load Additional traffic appears as noise to other users If the noise level is too high users drop out of cells

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Can we apply media access methods from fixed networks?

Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection

send when medium is free, listen to medium if collision occurs (IEEE 802.3)

Problems in wireless networks

signal strength decreases with distance

sender applies CS and CD, but collisions happen at receiver

sender may not hear collision, i.e., CD does not work

Hidden terminal: CS might not work

9.Medium Access

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Motivation - 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 (CS fails) Collision at B, A cannot receive the collision (CD fails) C is hidden from A 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 radio range of C, waiting is not necessary C is exposed to B

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Motivation - near and far terminals

Terminals A and B send, C receives signal strength decreases proportional to the square of the distance

Bs signal drowns out As signal

C cannot receive A

If C was an arbiter, B would drown out A

Also severe problem for CDMA-networks - precise power control needed!

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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 frequency to a transmission channel 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

The multiplexing schemes are now used to control medium access!

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FDD/FDMA - general scheme

example GSM f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

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TDD/TDMA - general scheme

example DECT

1 2 3 11 12 1 2 3 11 12

t downlink uplink

417 s

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Aloha/Slotted Aloha Mechanism

random, distributed (no central arbiter), time-multiplex Slotted Aloha uses time-slots, sending must start at slot boundaries

ALOHA

SLOTTED ALOHA

sender A

sender B

sender C

collision

sender A

sender B

sender C

collision

t

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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

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Access method 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 in 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

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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 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

frame1

frame2

frame3

frame4

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

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

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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

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MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) 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)

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MACA examples 1. 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

2. 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

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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

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Polling mechanisms

If base station can poll other terminals according to a certain scheme

schemes known from fixed networks can be used

Example: Randomly Addressed Polling

base station signals readiness to all mobile terminals

terminals ready to send transmit random number without collision using CDMA or FDMA

the base station chooses one address for polling from 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

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D/ISMA (Digital/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 (USA, integrated into AMPS)

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Access method CDMA CDMA (Code Division Multiple Access)

all terminals send on same frequency at the same time using ALL the 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

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 interference (e.g. white noise) is not coded forward error correction and encryption can be easily integrated

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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

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CDMA on signal level I

Real systems use much longer keys resulting in a larger distance

between single code words in code space.

data A

key A

signal A

data key

key

sequence A

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

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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

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CDMA on signal level III

Ak

(As + Bs)

* Ak

integrator

output

comparator

output

As + Bs

data A

1 0 1

1 0 1 Ad

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CDMA on signal level IV

integrator

output

comparator

output

Bk

(As + Bs)

* Bk

As + Bs

data B

1 0 0

1 0 0 Bd

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comparator

output

CDMA on signal level V

wrong

key K

integrator

output

(As + Bs)

* K

As + Bs

(0) (0) ?

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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

send for a

shorter period

with higher power

1 sender A 0 sender B

0

1

t

narrow

band

spread the signal e.g. using the chipping sequence 110101 (CDMA without CD)

Problem: find a chipping sequence with good characteristics

1

1

collision

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Comparison

SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA

Idea segment space intocells/sectors

segment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

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References

Chapter # 2 and Chapter # 3 from Mobile Communications, Second Edition, By Prof. Dr. Jochen H. Schiller.

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unit-1

Documents

karunya university unit

high frequency frequency

wireless vhfuhf

high frequency hf

high frequency lf

high frequency uv

mobile communication

low frequency shf