Transcript Wireless Networks

Asstt. Professor
Adeel Akram
What is signal encoding?
 In communications systems, the altering of the
characteristics of a signal to make the signal more
suitable for an intended application, such as
optimizing the signal for transmission
 Modifying the signal spectrum, increasing the
information content, providing error detection and/or
correction, and providing data security
 A single coding scheme usually does not provide more
than one or two specific capabilities.
 Different codes have different sets of advantages and
disadvantages.
Reasons for Choosing Encoding
Techniques
 Digital data, digital signal
 Equipment less complex and less expensive than digitalto-analog modulation equipment
 Analog data, digital signal
 Permits use of modern digital transmission and
switching equipment
Reasons for Choosing Encoding
Techniques
 Digital data, analog signal
 Some transmission media will only propagate analog
signals
 E.g., Fax/Modem
 Analog data, analog signal
 Analog data in electrical form can be transmitted easily
and cheaply
 Done with voice transmission over voice-grade lines
Signal Encoding Criteria
 What determines how successful a receiver will be
in interpreting an incoming signal?
 Signal-to-noise ratio
 Data rate
 Bandwidth
 An increase in data rate increases bit error rate
 An increase in SNR decreases bit error rate
 An increase in bandwidth allows an increase in
data rate
Comparing Encoding Schemes
 Signal interference and noise immunity
 Performance in the presence of noise
 Cost and complexity
 The higher the signal rate to achieve a given data rate,
the greater the cost
Digital Data to Analog Signals
 Keying is a form of modulation where the modulating
signal takes one of two or more values at all times. For
example: "on" or "off“
 The name derives from the Morse code key used for
telegraph signaling
 Amplitude-shift keying (ASK)

Amplitude difference of carrier frequency
 Frequency-shift keying (FSK)

Frequency difference near carrier frequency
 Phase-shift keying (PSK)

Phase of carrier signal shifted
Amplitude-Shift Keying
 One binary digit represented by presence of
carrier, at constant amplitude
 Other binary digit represented by absence of
carrier

binary1
 A cos2f ct 
s t   



0
where the carrier signal is Acos(2πfct)
binary 0
Amplitude-Shift Keying
 Susceptible to sudden gain changes
 Inefficient modulation technique
 On voice-grade lines, used up to 1200 bps
 Used to transmit digital data over optical fiber
Binary Frequency-Shift Keying
(BFSK)
 Two binary digits represented by two different
frequencies near the carrier frequency


s t   



A cos2f1t 
A cos2f 2t 
binary1
binary 0
where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Binary Frequency-Shift Keying
(BFSK)
 Less susceptible to error than ASK
 On voice-grade lines, used up to 1200bps
 Used for high-frequency (3 to 30 MHz) radio
transmission
 Can be used at higher frequencies on LANs that use
coaxial cable
Phase-Shift Keying (PSK)
 Two-level PSK (BPSK)
 Uses two phases to represent binary digits

binary1
 A cos2f ct 
s t   
binary
0


A
cos
2

f
t



c


 A cos2f ct 


 A cos2f ct 
binary1
binary 0
Phase-Shift Keying (PSK)
 Differential PSK (DPSK)
 Phase shift with reference to previous bit


Binary 0 – signal burst of same phase as previous signal burst
Binary 1 – signal burst of opposite phase to previous signal
burst
Phase-Shift Keying (PSK)
 Four-level PSK (QPSK)
 Each element represents more than one bit


s t   




A cos 2f ct  
4

3 

A cos 2f ct  
4 

3 

A cos 2f ct  
4 



A cos 2f ct  
4

11
01
00
10
Quadrature Amplitude
Modulation
 QAM is a combination of ASK and PSK
 Two different signals sent simultaneously on the same
carrier frequency
st   d1 t cos2f ct  d2 t sin 2f ct
Quadrature Amplitude
Modulation
Analog Data to Analog Signal
 Modulation of digital signals
 When only analog transmission facilities are available,
digital to analog conversion required
 Modulation of analog signals
 A higher frequency may be needed for effective
transmission
Modulation Techniques
 Amplitude modulation (AM)
 Angle modulation
 Frequency modulation (FM)
 Phase modulation (PM)
Outline
 Cellular Concept
 Cellular Architecture
 Frequency Reuse
 Multiple Access Methods
 FDMA, TDMA, and CDMA

In particular, we focus on CDMA.
Different Generations
 1G
 analog
 2G
 digital
 3G
 higher data rate for multimedia applications
1G Cellular Systems
 Many Different Standards:
 AMPS (US)
 NMT (Northern Europe)
 TACS (Europe)
 NTT (Japan)
 many others...
 Spectrum
 around 800 and 900 MHz
Frequency Division Duplex (FDD)
Forward Link
mobile
Reverse Link
base
station
Two separate frequency bands are used for
forward and reverse links.
Typically, 25 MHz in each direction.
AMPS: 824-849 MHz (forward)
869-894 MHz (reverse)
Frequency Division Multiple Access
(FDMA)
 The spectrum of each link (forward or reverse)
is further divided into frequency bands
frequency bands
 Each station assigned fixed frequency band
idle
idle
idle
Number of User Channels in AMPS
 Bandwidth allocated to each user in each link
(forward or reverse) is 30 KHz.
 No. of user channels
= Total bandwidth / user bandwidth
= 25 MHz / 30 kHz
= 833
 Is it enough?
Frequency Reuse
Radio coverage,
called a cell.
f
f
The same frequency can be
reused in different cells, if they
are far away from each other
Cellular Architecture
MS
MS – Mobile Station
BSC – Base Station Controller
MSC or MTSO– Mobile Switching Center
PSTN – Public Switched Telephone Network
BSC
MSC
PSTN
segmentation
of the area
into cells
Geometric Representation
 Cells are commonly represented by hexagons.
 Why hexagon?
 How about circle?
 How about square, or triangle?
Hexagon vs Circles
 Notice how the circles below would leave gaps in our
layout. Still, why hexagons and not triangles or
rhomboids?
Hexagonal
Cells
Cell site and Cell
 The cell site is a location or a point, the cell is a wide geographical area
 Cells site covers a portion or a sector of each cell, not the whole thing.
Antennas from other cell sites cover the other portions. The covered
area, if you look closely, resembles a sort of rhomboid
In reality, the cell is the red hexagon
Channel Reuse
 The total number of channels are divided into N
groups.
 N is called reuse factor.
 Each cell is assigned one of the groups.
 The same group can be reused by two different cells
provided that they are sufficiently far apart.
Example:
N=7
Reuse Distance
 How far apart can two users share the same channel?
 It depends on whether signal quality is acceptable or
not.
 The larger the distance between the two users, the better
the signal quality.
 How to measure signal quality?
Nyquist Bandwidth
 Given a bandwidth of B, the highest signal
transmission rate for binary signals (two voltage levels)
is:
 C = 2B
 Ex: B=3100 Hz; C=6200 bps
 With multilevel signaling
 C = 2B log2 M

M = number of discrete signal or voltage levels
Signal Quality
 The signal quality depends on the ratio between
signal power and interference (noise) power.
S
S

I  Ii
i
Interference from the i-th
interfering BS.
 This is called signal-to- noise (interference) ratio
(SNR or SIR).
Signal-to-Noise Ratio
 Ratio of the power in a signal to the power
contained in the noise that’s present at a particular
point in the transmission
 Typically measured at a receiver
 Signal-to-noise ratio (SNR, or S/N)
signal power
( SNR) dB  10 log10
noise power
 A high SNR means a high-quality signal, low
number of required intermediate repeaters
 SNR sets upper bound on achievable data rate
Shannon Capacity Formula
 Equation:
C  B log2 1  SNR 
 Represents theoretical maximum that can be
achieved
 In practice, only much lower rates achieved
 Formula assumes white noise (thermal noise)
 Impulse noise is not accounted for
 Attenuation distortion or delay distortion not accounted for
Classifications of Transmission
Media
 Transmission Medium
 Physical path between transmitter and receiver
 Guided Media
 Waves are guided along a solid medium
 E.g., copper twisted pair, copper coaxial cable, optical
fiber
 Unguided Media
 Provides means of transmission but does not guide
electromagnetic signals
 Usually referred to as wireless transmission
 E.g., atmosphere, outer space
Propagation Model
 The received signal power depends on the
distance between the transmitter and the
receiver

d 
Pr  P0  
 d0 
 P0 is the power received at a reference distance
d0
  is called the path loss exponent
 Typically, 2 ≤  ≤ 6 *
Typical values of α
Table : Path Loss Exponents for Different Environments
Propagation Environment
Path Loss Exponent
Free Space
2
Urban Area
2.7 to 3.5
Shadowed Urban Area
3 to 5
In-Building Line-of-Sight
1.6 to 1.8
Obstructed In Building
4 to 6
Obstructed In Factory
2 to 3
As shown in Table typical values for the path loss exponent are between 2 to 6
2G Cellular Systems
 Four Major Standards:
 GSM (European)
 IS-54 (later becomes IS-136, US)
 JDC (Japanese Digital Cellular)
 IS-95 (CDMA, US)
Example: GSM
 Frequency Band
 935-960, 890-915 MHz
 Two pieces of 25 MHz band
(same as AMPS)
 AMPS has 833 user channels
 How about GSM?
Time Division Multiple Access
(TDMA)
 The mobile users access the channel in roundrobin fashion.
 Each station gets one slot in each round.
Slots 2, 5 and 6 are idle
FDMA/TDMA, example GSM
f
960 MHz
124
200 kHz
1
935.2 MHz
20 MHz
915 MHz
124
1
890.2 MHz
t
1 2 3
7 8
Each freq. carrier is divided into 8 time slots.
Number of channels in GSM
 Freq. Carrier: 200 kHz
 TDMA: 8 time slots per freq carrier
 No. of carriers = 25 MHz / 200 kHz
= 125
 No. of user channels = 125 * 8
= 1000
Capacity Comparison
 Reuse factor
 7 for AMPS
 3 for GSM
(why smaller reuse factor?)
 What’s the capacity of GSM relative to AMPS?
A. one half of AMPS
B. the same
C. 3 times larger
D. 10 times larger
Answer
 AMPS
 reuse factor = 7
 no. of users / cell = 833 / 7 = 119
 GSM
 reuse factor = 3
 no. of users / cell = 1000 / 3 = 333
 almost 3 times larger than AMPS!
Multiple Access Methods
Three major types:
 Frequency Division Multiple Access (FDMA)
 Time Division Multiple Access (TDMA)
 Code Division Multiple Access (CDMA)


Frequency hopping (FH-CDMA)
Direct sequence (DS-CDMA)
Frequency-Time
Plane
Frequency
Partition of signal
space into time slots
and frequency bands
Time
FDMA
Frequency
Different users
transmit at different
frequency bands
simultaneously
Time
TDMA
Frequency
Different users
transmit at different
time slots
Each user occupy the
whole freq. spectrum
Time
Frequency
Hopping
CDMA
Frequency
At each successive time
slot, the frequency
band assignments are
reordered
Time
Each user employs a
code that dictates the
frequency hopping
pattern
Assignment
 Write note on 3G Mobile technology
 Write note on 3.5G Mobile technology
 Write note on 3.75G Mobile technology
 Write note on 4G Mobile technology
 Give an Overview of GSM network Architecture
 Difference between CDMAOne and CDMA2000
Questions
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