02 wireless transmission
TRANSCRIPT
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Wireless Transmission 1-1
zgr Koray AHNGZYrd.Do.Dr.
stanbul Kltr niversitesi
DataData CommunicationCommunicationAnalogAnalog vsvs DigitalDigital
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Electromagnetic Signal
Electromagnetic signals(ES) are used as a means totransmit information
ES is a function of time
Can also be expressed as a function of frequency Signal consists of components of different frequencies
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Analog and Digital Waveforms
Speech
Binary 1s and 0s
Analog signal - signalintensity varies in asmooth fashion overtime No breaks or
discontinuities in thesignal
Digital signal - signalintensity maintains aconstant level for some
period of time andthen changes toanother constant level
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Periodic Signals
Periodic Signal; Samesignal patters repeatsover time
s(t+T ) = s(t ) -< t < + where Tis the period of the
signal
T is the smallest value thatsatisfies the equation.
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Time-Domain Concepts
Aperiodic signal - analog or digital signal patternthat doesn't repeat over time
Peak amplitude (A) - maximum value or strength
of the signal over time; typically measured in volts
Frequency (f ) Rate, in cycles per second, or Hertz (Hz) at which the
signal repeats
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Time-Domain Concepts
Period (T ) - amount of time it takes for one repetition ofthe signal
T= 1 /f
Phase () - measure of the relative position in time withina single period of a signal Wavelength () - distance occupied by a single cycle of
the signal Or, the distance between two points of corresponding phase of two
consecutive cycles
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Sine Wave Parameters
General sine waves(t ) = A sin(2ft + )
Figure (next slide)shows the effect ofvarying each of the three parameters (a) A = 1, f= 1 Hz, = 0; thus T= 1s (b) Reduced peak amplitude; A=0.5 (c) Increased frequency; f= 2, thus T= (d) Phase shift; = /4 radians (45 degrees)
note: 2 radians = 360 = 1 period
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Sine Wave Parameters
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Frequency-Domain Concepts
Fundamental frequency - when all frequencycomponents of a signal are integer multiples of onefrequency, its referred to as the fundamentalfrequency
Spectrum - range of frequencies that a signalcontains
Absolute bandwidth - width of the spectrum of asignal
Effective bandwidth (or just bandwidth) - narrow
band of frequencies that most of the signalsenergy is contained in
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Frequency
Components Any electromagnetic
signal can be shown toconsist of a collectionof periodic analog
signals (sine waves) atdifferent amplitudes,frequencies, and phases
The period of the total
signal is equal to theperiod of thefundamental frequency
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Relationship between Data Rate and Bandwidth
The greater the bandwidth, the higher theinformation-carrying capacityAny digital waveform will have infinite bandwidth BUT the transmission system will limit the
bandwidth that can be transmittedAND, for any given medium, the greater thebandwidth transmitted, the greater the cost
HOWEVER, limiting the bandwidth createsdistortions
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Data Communication Terms
Data - entities that convey meaning, or information
Signals - electric or electromagnetic
representations of data
Transmission - communication of data by thepropagation and processing of signals
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Examples of Analog and Digital Data
Analog Video
Audio
Digital Text
Integers
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Analog Signals
A continuously varying electromagnetic wave thatmay be propagated over a variety of media,depending on frequency
Examples of media: Copper wire media (twisted pair and coaxial cable) Fiber optic cable Atmosphere or space propagation
Analog signals can propagate analog and digitaldata
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Conversion of Voice Input into Analog Signal
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Conversion of PC Input to Digital Signal
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Digital Signals
A sequence of voltage pulses that may betransmitted over a copper wire mediumGenerally cheaper than analog signaling Less susceptible to noise interference
Suffer more from attenuation(g azalmas)Digital signals can propagate analog and digital
data
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Attenuation of Digital Signals
A digital signal is a sequence of voltage pulses thatmay be transmitted over a copper wire medium; forexample a constant positive voltage level mayrepresent binary 0 and a constant negative voltage
level may represent binary 1.
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Analog Signals Carrying Analog and Digital Data
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Digital Signals Carrying Analog and Digital Data
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Analog Transmission
Transmit analog signals without regard to content
Attenuation limits length of transmission link
Cascaded amplifiers boost signals energy forlonger distances but cause distortion Analog data can tolerate distortion
Introduces errors in digital data
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Digital Transmission
Concerned with the content of the signal
Attenuation endangers integrity of data
Digital Signal Repeaters achieve greater distance Repeaters recover the signal and retransmit
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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., /4)
Frequency Division Multiplexing
medium characteristics
Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM)
Phase Modulation (PM)
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Modulation and demodulation
synchronizationdecision
digitaldataanalog
demodulation
radiocarrier
analogbasebandsignal
101101001 radio receiver
digitalmodulation
digitaldata analog
modulation
radiocarrier
analogbasebandsignal
101101001 radio transmitter
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Wireless Transmission1-25
Digital modulation
Modulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):
very simple
low bandwidth requirements
very susceptible to interference
1 0 1
t
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Amplitude Shift Keying (ASK)
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Frequency Shift Keying
Frequency-shift keying (FSK) is a frequencymodulation scheme in which digital information istransmitted through discrete frequency changesof a carrier wave.
needs larger bandwidth
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Frequency Shift Keying (FSK)
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Phase Shift Keying (PSK)
Phase-shift keying (PSK) is a digital modulationscheme that conveys data by changing, ormodulating, the phase of a reference signal (the
carrier wave).
more complex
robust against interference
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PSK Methods
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4 PSK Characteristics
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Quadrature amplitude modulation(QAM)
Quadrature amplitude modulation is a combinationof ASK and PSK so that a maximum contrastbetween each signal unit (bit, dibit, tribit, and soon) is achieved.
Quadrature amplitude modulation (QAM) is amodulation scheme which conveys data by changing(modulating) the amplitude of two carrier waves.These two waves, usually sinusoids, are out of
phase with each other by 90 and are thus calledquadrature carriers
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4 QAM and 8 QAM
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THe domain for 8 QAM
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1 6- QAM constellations
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Bit and Baud
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Assume we need to download text documents at the rate of
100 pages per minute. What is the required bit rate of the
channel?
SolutionA page is an average of 24 lines with 80 characters in each
line. If we assume that one character requires 8 bits, the bit
rate is
Example
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What is the bit rate for high-definition TV (HDTV)?
Solution
HDTV uses digital signals to broadcast high quality video
signals. The HDTV screen is normally a ratio of 16 : 9. There
are 1920 by 1080 pixels per screen, and the screen is renewed
30 times per second. Twenty-four bits represents one color
pixel.
The TV stations reduce this rate to 20 to 40 Mbps through
compression.
Example
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Impairments, such asnoise, limit data rate that canbe achieved
For digital data, to what extent do impairmentslimit data rate?
Channel Capacity the maximum rate at whichdata can be transmitted over a givencommunication path, or channel, under givenconditions
About Channel Capacity
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Concepts Related to Channel Capacity Data rate - rate at which data can be communicated (bps)
Bandwidth - the bandwidth of the transmitted signal asconstrained by the transmitter and the nature of thetransmission medium (Hertz)
Noise - average level of noise over the communicationspath
Error rate - rate at which errors occur Error = transmit 1 and receive 0; transmit 0 and receive 1
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Nyquist Bandwidth
For binary signals (two voltage levels)C= 2B
With multilevel signaling
C= 2B log2MM= number of discrete signal or voltage levels
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Ratio of the power in a signal to the powercontained in the noise thats present at a particularpoint in the transmission
Typically measured at a receiver
Signal-to-noise ratio (SNR, or S/N)
A high SNR means a high-quality signal, lownumber of required intermediate repeaters
SNR sets upper bound on achievable data rate
powernoise
powersignallog10)( 10dB SNR
Signal-to-Noise Ratio
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Equation:
Represents theoretical maximum that can be
achievedIn practice, only much lower rates achieved
Formula assumes white noise (thermal noise)
Impulse noise is not accounted forAttenuation distortion or delay distortion not
accounted for
SNR1log 2 BC
Shannon Capacity Formula
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Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB= 24 dB
Using Shannons formula
251SNR
SNRlog10dB24SNR
MHz1MHz3MHz4
10dB
B
Mbps88102511log10 626 C
Example of Nyquist and Shannon Formulations
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How many signaling levels are required?
16
log4
log102108
log2
2
2
66
2
M
M
MBC
Example of Nyquist and Shannon Formulations
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We need to send 265 kbps over a noiseless channel with abandwidth of 20 kHz. How many signal levels do we need?
Solution
We can use the Nyquist formula as shown:
Since this result is not a power of 2, we need to either
increase the number of levels or reduce the bit rate. If wehave 128 levels, the bit rate is 280 kbps. If we have 64 levels,
the bit rate is 240 kbps.
Example
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Consider a noiseless channel with a bandwidth of 3000 Hz
transmitting a signal with two signal levels. The maximum bit
rate can be calculated as
Example
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Consider the same noiseless channel transmitting a signal
with four signal levels (for each level, we send 2 bits). The
maximum bit rate can be calculated as
Example
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We can calculate the theoretical highest bit rate of a regulartelephone line. A telephone line normally has a bandwidth of
3000Hz. The signal-to-noise ratio is usually 3162. For this
channel the capacity is calculated as
This means that the highest bit rate for a telephone line is
34.860 kbps. If we want to send data faster than this, we can
either increase the bandwidth of the line or improve the
signal-to-noise ratio.
Example
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The signal-to-noise ratio is often given in decibels. Assumethat SNRdB = 36 and the channel bandwidth is 2 MHz. The
theoretical channel capacity can be calculated as
Example
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For practical purposes, when the SNR is very high, we can assume that SNR + 1 is almost the same as SNR. In these
cases, the theoretical channel capacity can be simplified to
For example, we can calculate the theoretical capacity of the
previous example as
Example
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We have a channel with a 1-MHz bandwidth. The SNR for
this channel is 63. What are the appropriate bit rate and
signal level?
Solution
First, we use the Shannon formula to find the upper limit.
Example
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The Shannon formula gives us 6 Mbps, the upper limit. For
better performance we choose something lower, 4 Mbps, for
example. Then we use the Nyquist formula to find the
number of signal levels.
Example
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The Shannon capacity gives us the upper
limit; the Nyquist formula tells us how many
signal levels we need.
Note
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In networking, we use the term bandwidth intwo contexts.
The first, bandwidth in hertz, refers tothe range of frequencies in a
composite signal or the range offrequencies that a channel can pass.
The second, bandwidth in bits per
second, refers to the speed of bittransmission in a channel or link.
Note
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zgr Koray AHNGZYrd.Do.Dr.
stanbul Kltr niversitesi
WirelessWireless TransmissionTransmission
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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 3x108m/s, frequency f
1 Mm300 Hz
10 km30 kHz
100 m3 MHz
1 m300 MHz
10 mm30 GHz
100 m3 THz
1 m300 THz
visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV
optical transmissioncoax cabletwistedpair
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Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellitecommunication
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, signal loss caused by heavy
rainfall etc.
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Signals
Radio waves are used for multicastcommunications, such as radio and television, andpaging systems.
Microwaves are used for unicast communication
such as cellular telephones, satellite networks, andwireless LANs
Infrared signals can be used for short- rangecommunication in a closed area using line-of-sight
propagation.
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Signals physical representation of data function of time and location signal parameters: parameters representing the value of
data (what are they?) 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 ft t + t)
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Antennas
An antenna is an electrical conductor or system ofconductors Transmission - radiates electromagnetic energy into
space Reception - collects electromagnetic energy from
space In two-way communication, the same antenna can
be used for transmission and reception
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Types of Antennas
Whip Antenna
Isotropic antenna (idealized) Radiates power equally in all directions
Dipole antennas
Designed to receive particular wavelength Parabolic Reflective Antenna
Phased Array
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Whip Antennas
Radio antennas, cell phone antennas, walkie talkies, etc. Simple, cheap, easy to make
Generally low gain antennas
Higher gain off axis from the antenna, low gain on axis(above and below)
Low Sensitivity
High Sensitivity High Sensitivity
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Dipole antennas
These antennas are the simplest practical antennasfrom a theoretical point of view; the currentamplitude on such an antenna decreases uniformlyfrom maximum at the center to zero at the ends.
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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
zy
x
z
y x ideal
isotropicradiator
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Parabolic Dish Antenna
Satellite receivers, older Radar systems, radiotelescopes, etc.
Strong, and reasonably easy to produce, fairlycheap compared to other antennas with similar gainproperties
Low gain behind antenna and off to the sides, highgain in front of antenna Very Sensitive
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Phased Array Antenna
High gain in a small area, low gain everywhere else Consists of multiple antenna elements arranged in a plane
where the individual elements are wired together tocontrol directional gain
Allows the antenna to scan a volume quickly withoutphysically moving array
Very Sensitive
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Antennas: directed and sectorized
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
Often used for microwave connections or base stations for
mobile phones (e.g., radio coverage of a valley)
directedantenna
sectorized
antenna
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Example
What is the appropriate antenna size for a GSM1800MHz mobile phone? Hint: Generally, theantenna size is wavelength/4.
the relation between wavelength (L) and
frequency (f) is l = c / f l = c / f = (3x108)/(1800x106) = 1/6 mUsually, the antenna size is wavelength/4
so the appropriate size for a GSM 1800 mobilephone is 1/24 m,which is about 4. 1 67 cm.
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Example
What is the wavelength of 300 MHz frequencyis?
What is the frequency of 30 cm wavelength is ?
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Propagation Modes
Ground-wave propagation
Sky-wave propagation
Line-of-sight propagation
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Ground Wave Propagation
Follows contour of the earth
Can propagate considerable distances
Frequencies up to 2 MHz Example: AM radio (during the day)
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Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down to earth
Signal can travel a number of hops, back and forth betweenionosphere and earths surface Examples: amateur radio, CB radio, AM (at night)
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Line-of-Sight Propagation
Transmitting and receiving antennas must be within line ofsight Satellite communication signal above 30 MHz not reflected by
ionosphere Ground communication antennas within effectiveline of site due torefraction
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Signal propagation ranges
Transmission range communication possible low error rate
Detection range detection of the signal
possible
no communicationpossible
Interference range signal may not be
detected
signal adds to thebackground noise
distance
sender
transmission
detection
interference
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Real world example
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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 pulsesmultipathpulses
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Multiplexing
In telecommunications and computer networks, multiplexing
is a term used to refer to a process where multiple analogmessage signals or digital data streams are combined intoone signal over a shared medium. The aim is to share anexpensive resource. For example, in telecommunications,several phone calls may be transferred using one wire.
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Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c)
Goal: multiple useof a shared medium
Important: guard spaces needed!
s2
s3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
channels ki
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Space Division Multiple Access
Space division multiplexing: In wirelesscommunication, a SDM implies a separate senderfor each communication channel with wide enoughdistance between senders.
For example, This type of multiplexing is used atFM radio stations where the transmission range islimited to a certain region. Many radio stationsaround the world can use the same frequencywithout interference.
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Space Division Multiple Access channels are mapped onto dedicated (sufficiently distant)
space so that different users can access the mediumwithout interfering with each other SDMA can be achieved through:
a) use of spot beam antennasb) by controlling transmission power, in case of omnidirectional
antennas SDMA is typically used in combination with FDMA, TDMA
or CDMA example: cellular networks (e. g. SDMA + FDMA)
network coverage area is divided into a number of small areas called
cells frequencies are reused in different cells users using same
frequency are so far away that they do not interfere with eachother
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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 isdistributed unevenly
inflexible
k2 k3 k4 k5 k6k1
f
t
c
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Frequency multiplex
FDM is an analog multiplexingtechnique that combines signals.
F l l
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FDM Multiplexing
FDM D l l
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FDM Demultiplexing
E l
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Assume that a voice channel occupies a bandwidth of 4 KHz.We need to combine three voice channels into a link with a
bandwidth of 12 KHz, from 20 to 32 KHz. Show the
configuration using the frequency domain without the use of
guard bands.
SolutionSolution
Shift (modulate) each of the three voice channels to a different
bandwidth.
Example
S l i
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Solution
E l
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Five channels, each with a 100-KHz bandwidth, are to bemultiplexed together. What is the minimum bandwidth of the
link if there is a need for a guard band of 10 KHz between the
channels to prevent interference?
SolutionSolution
For five channels, we need at least four guard bands. This
means that the required bandwidth is at least
5 x 100 + 4 x 10 = 540 KHz,
Example
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E l
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Four data channels (digital), each transmitting at 1 Mbps, use asatellite channel of 1 MHz. Design an appropriate
configuration using FDM
SolutionSolution
The satellite channel is analog. We divide it into four
channels, each channel having a 250-KHz bandwidth. Each
digital channel of 1 Mbps is modulated such that each 4 bits
are modulated to 1 Hz. One solution is 16-QAM modulation.
Example
S l ti
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Solution
A l Hi h
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Anolog Hierarchy
E l
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The Advanced Mobile Phone System (AMPS) uses two bands.
The first band, 824 to 849 MHz, is used for sending; and 869
to 894 MHz is used for receiving. Each user has a bandwidth
of 30 KHz in each direction. The 3-KHz voice is modulated
using FM, creating 30 KHz of modulated signal. How many
people can use their cellular phones simultaneously?
SolutionSolution
Each band is 25 MHz. If we divide 25 MHz into 30 KHz, we
get 833.33. In reality, the band is divided into 832 channels.
Example
Tim m ltipl x
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f
t
c
k2 k3 k4 k5 k6k1
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 evenfor many users
Disadvantages
precisesynchronizationnecessary
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TDM Frames
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TDM Frames
Solution
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Wireless Transmission 1-100
Solution
Interleaving
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Wireless Transmission 1-101
Interleaving
Example
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Wireless Transmission 1-102
Four 1-Kbps connections are multiplexed together. A unitis 1 bit. Find
(1) the duration of 1 bit before multiplexing,
(2) the transmission rate of the link,
(3) the duration of a time slot, and
(4) the duration of a frame?
Example
Solution
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Wireless Transmission 1-103
Solution
We can answer the questions as follows:
1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).
2. The rate of the link is 4 Kbps.
3. The duration of each time slot 1/4 ms or 0.250 ms.4. The duration of a frame 1 ms.
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Wireless Transmission 1-104
In a TDM, the data rate of the link is nIn a TDM, the data rate of the link is n
times faster, and the unit duration is ntimes faster, and the unit duration is ntimes shorter.times shorter.
Example
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Wireless Transmission 1-105
Four channels are multiplexed using TDM. If each channelsends 100 bytes/s and we multiplex 1 byte per channel, show
the frame traveling on the link,
the size of the frame,
the duration of a frame,
the frame rate,
and the bit rate for the link.
Example
Solution
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Wireless Transmission 1-106
Solution
Example
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Wireless Transmission 1-107
A multiplexer combines four 100-Kbps channels using a timeslot of 2 bits.
Show the output with four arbitrary inputs.
What is the frame rate?
What is the frame duration?
What is the bit rate?
What is the bit duration?
Example
Solution
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Wireless Transmission 1-108
Solution
Example
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Wireless Transmission 1-109
We have four sources, each creating 250 characters per
second. If the interleaved unit is a character and 1
synchronizing bit is added to each frame, find
(1) the data rate of each source,
(2) the duration of each character in each source,
(3)the frame rate,
(4) the duration of each frame,
(5) the number of bits in each frame, and
(6) the data rate of the link.
Example
Solution
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Wireless Transmission 1-110
We can answer the questions as follows:
1. The data rate of each source is 2000 bps = 2 Kbps.
2. The duration of a character is 1/250 s, or 4 ms.
3. The link needs to send 250 frames per second.4. The duration of each frame is 1/250 s, or 4 ms.
5. Each frame is 4 x 8 + 1 = 33 bits.
6. The data rate of the link is 250 x 33, or 8250 bps.
Solution
Example
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Wireless Transmission 1-111
Two channels, one with a bit rate of 100 Kbps and another
with a bit rate of 200 Kbps, are to be multiplexed. How this
can be achieved? What is the frame rate? What is the frame
duration? What is the bit rate of the link?
Example
Solution
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Wireless Transmission 1-112
Solution
We can allocate one slot to the first channel and
two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second
because it carries 1 bit from the first channel.
The frame duration is 1/100,000 s, or 10 ms. The bit rate is 100,000 frames/s x 3 bits/frame,
or 300 Kbps.
DS Hierarchy
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Wireless Transmission 1-113
DS Hierarchy
Time and frequency multiplex
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Wireless Transmission 1-114
f
Time and frequency multiplex Combination of both methods
A channel gets a certain frequency band for a certain amount oftime
Example: GSM
Advantages
better protection againsttapping
protection against frequencyselective interference
but: precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Code multiplex
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Wireless Transmission 1-115
Code multiplex Each channel has a unique code
All channels use the same spectrumat the same time
k2 k3 k4 k5 k6k1
f
t
c
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CDMA Advantages
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Wireless Transmission 1-117
CDMA Advantages
in CDMA, each user is far more liberated from other users,
than in case of TDMA flexibility in timing and quality oftransmission
CDMA has soft capacity limit there is no absolute limiton the number of users; rather, system performancegradually degrades for all users as # of users an additional user can be added by sacrificing somewhat the link
quality of other users
degradation of performance with an increasing # of users isgraceful, as opposed to hard limits of FDMA / TDMA
soft- handoff can be performed by the MSC, which cansimultaneously monitor a particular user from 2 or morebase stations
CDMA Disadvantages
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Wireless Transmission 1-118
CDMA Disadvantages
self-jamming spreading sequence of different
users may not be exactly orthogonal; causingproblems in despreading of particular code
near-far problem occurs at a CDMA receiver if anundesired user has a high detected power
compared to the desired user
Multiplexing analogy
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Wireless Transmission 1-119
Mult plex ng analogy
CDMA
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Wireless Transmission 1-120Courtesy of Suresh Goyal & Rich Howard
DM
CDMA
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Wireless Transmission 1-121Courtesy of Suresh Goyal & Rich Howard
CDMA
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Wireless Transmission 1-122Courtesy of Suresh Goyal & Rich Howard
CDMA
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Wireless Transmission 1-123Courtesy of Suresh Goyal & Rich Howard
Access method CDMA
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Wireless Transmission 1-124
What is a good code for CDMA?A good autocorrelation (the absolute value of the inner
product of a vector multiplied by itself should be large)
Orthogonal to other codes (Two vectors are calledorthogonal if their inner product is 0.
Examples of a good CDMA code:The Baker code (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1)
a good autocorrelation: the inner product is large, 11.(+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1) (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1) = 11
Orthogonal to other codes
(+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1) (+1, +1, -1, +1, -1, -1, +1, +1, +1, -1, + 1) = 1 used for ISDN and IEEE 802.11.
CDMA in Theory
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Wireless Transmission 1-125
y
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
CDMA Code Division Multiple Access
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Wireless Transmission 1-126
p
(a) Binary chip sequences for fourstations
(b) Bipolar chip sequences(c) Six examples of transmissions(d) Recovery of station Cs signal
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CDMA Code Division Multiple Access
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Wireless Transmission 1-128
Advantages
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Wireless Transmission 1-129
g
Flexible network planning (planning is no longer needed)This is huge code space compared to frequency
spaceGreater coverage (larger area for a given amount of power )
High capacity(greater coverage capacity)
Cost(larger profit for providers due to increased capacity, less infrastructure) Clarity Customer satisfaction (privacy, better call quality longer battery life due to less power
consumption,prevent cross talks)
Compatibility (dual mode analog and digital)
Disadvantages
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Wireless Transmission 1-130
SynchronizationDifficulty to satisfy synchronization requirements.
Self jammingSelf jamming is a steep deterioration of performance as a result of poorsynchronization. Poor synchronization causes partial-correlation with the codesof other users and the result will be a vast increase of the interference.
Near- far problempower control is necessary for mitigating the Near-far problem. There aresome factors for imperfect power control such as: feedback delays, imperfectpower estimates, traffic conditions, errors in the feedback channel.
Network complexityComplex network support is needed for implementing soft handoff, and also forcountering multipath and fading effects.
ThroughputLow throughput efficiency for large number of users.
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Wireless Transmission 1-131
CDMA ha s many a dvantages, but it is still sec ond tec hnology
CDMA System eng ineer should adop t to use global network
system broadly to become a ma jor telec omm unication
in 3G ma rket
Adopting hando ver requirement of GSM and will be
beneficial lead CDMA to g loba l telecommunication system
Conclusion
Comparison SDMA/TDMA/FDMA/CDMA
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Approach SDMA TDMA FDMA CDMA
Ideasegment 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, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
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),synchronization
difficult
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 many
typically combinedwith TDMA(frequency hoppingpatterns) and SDMA
still faces some problems,higher complexity,lowered expectations; willbe integrated with