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Outline
Frequency Concepts
Frequency Specifications
Wavelength
Frequency Allocation – US
Bandwidth
Channels
Duplex Distance
Carrier Separation
Capacity
Frequency Reuse
Transmission Rate
Modulation Method
Access Method
GSM Overview
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Outline
Analog and Digital Transmission
Analog and Digital Signals
Advantages of Using Digital Signals
Transmission Problems
Path Loss
Shadowing
Carrier to Interference C/I Ratio
Carrier to Adjacent C/A Ratio
Multipath Fading
Rayleigh Fading
Time Dispersion
Time Alignment
Combined Signal Loss
GSM Overview
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Outline
Solutions to Transmission Problems
Cell Planning
Channel Coding
Interleaving
Antenna Space Diversity
Antenna Polarization Diversity
Adaptive Equalization
Frequency Hopping
Time Advance
GSM Overview
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Outline
GSM Transmission Process
Analog to Digital Conversion
Sampling
Quantization
Coding
Speech Coding
Channel Coding
Adaptive Multi Rate AMR
Interleaving
Ciphering / Encryption
Burst Formatting
Modulation and Transmission
GSM Overview
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First Generation
The first generation of mobile cellular telecommunications systems appeared in the
1980s. The first generation was not the beginning of mobile communications, as
there were several mobile radio networks in existence before then, but they were
not cellular systems either.
The capacity of these early networks was much lower than that of cellular networks, and
the support for mobility was weaker.
In mobile cellular networks the coverage area is divided into small cells, and thus the
same frequencies can be used several times in the network without disruptive
interference. This increases the system capacity.
The first generation used analog transmission techniques for traffic, which was almost
entirely voice.
Second Generation
The second-generation (2G) mobile cellular systems use digital radio transmission for
traffic. Thus, the boundary line between first- and second generation systems is
obvious: It is the analog/digital split.
The 2G networks have much higher capacity than the first-generation systems. One
frequency channel is simultaneously divided among several users (either by code
or time division). Hierarchical cell structures—in which the service area is covered
by macrocells, microcells, and picocells—enhance the system capacity even
further.
There are four main standards for 2G systems: Global System for Mobile (GSM)
communications and its derivatives; digital AMPS (Advanced Mobile Phone
Service) (D-AMPS); code division multiple access (CDMA) IS-95; and personal
digital cellular (PDC). GSM is by far the most successful and widely used 2G
system.
Second Generation
• The basic GSM uses the 900-MHz band, but there are also several
derivatives, of which the two most important are Digital Cellular System
1800 (DCS-1800; also known as GSM-1800) and PCS-1900 (or GSM-1900).
• The latter is used only in North America and Chile, and DCS-1800 is seen in
other areas of the world. The prime reason for the new frequency band
was the lack of capacity in the 900-MHz band.
• The 1,800-MHz band can accommodate a far greater user population, and
thus it has become quite popular, especially in densely populated areas.
• The coverage area is, however, often smaller than in 900-MHz networks,
and thus dual band mobiles are used, where the phone uses a 1,800-MHz
network when such is available and otherwise roams onto a 900-MHz
network.
Generation 2.5
“Generation 2.5” is a designation that broadly includes all advanced upgrades
for the 2G networks. These upgrades may in fact sometimes provide almost
the same capabilities as the planned 3G systems.
The boundary line between 2G and 2.5G is a hazy one. It is difficult to say
when a 2G becomes a 2.5G system in a technical sense. Generally, a 2.5G
GSM system includes at least one of the following technologies:
a) High-speed Circuit-Switched Data (HSCSD),
b) General Packet Radio Services (GPRS),
c) Enhanced Data rates for Global Evolution (EDGE).
An IS-136 system becomes 2.5G with the introduction of GPRS and EDGE,
and an IS-95 system is called 2.5G when it implements IS-95B, or
CDMA2000 1xRTT upgrades.
Generation 2.5
The biggest problem with plain GSM is its low air interface data rates. The
basic GSM could originally provide only a 9.6-Kbps user data rate. Later,
14.4-Kbps data rate was specified, although it is not commonly used.
Anyone who has tried to Web surf with these rates knows that it can be a
rather desperate task.
HSCSD is the easiest way to speed things up. This means that instead of one
time slot, a mobile station can use several time slots for a data connection.
In current commercial implementations, the maximum is usually four time
slots. One time slot can use either 9.6-Kbps or14.4-Kbps speeds. The total
rate is simply the number of time slots times the data rate of one slot.
This is a relatively inexpensive way to upgrade the data capabilities, as it
requires only software upgrades to the network (plus, of course, new
HSCSD-capable phones), but it has drawbacks. The biggest problem is the
usage of scarce radio resources.
3rd Generation
3G is the third generation of wireless technologies. It comes with enhancements over
previous wireless technologies, like high-speed transmission, advanced multimedia
access and global roaming. 3G is mostly used with mobile phones and handsets as a
means to connect the phone to the Internet or other IP networks in order to make
voice and video calls, to download and upload data and to surf the net.
How is 3G Better?:
3G has the following enhancements over 2.5G and previous networks:
a)
b)
c)
d)
e)
Several times higher data speed;
Enhanced audio and video streaming;
Video-conferencing support;
Web and WAP browsing at higher speeds;
IPTV (TV through the Internet) support.
GSM Overview
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4th Generation
• 4G wireless is the term used to describe the fourth-generation of wireless service.
4G is a step up from 3G, which is currently the most widespread, high-speed
wireless service. 4G is only available in limited areas.
• While all 4G service is called "4G," the underlying technology is not the same.
Sprint, for example, uses WiMax technology for its 4G network, while Verizon
Wireless uses a technology called Long Term Evolution, or LTE.
• No matter what technology is behind it, 4G wireless is designed to deliver speed.
On average, 4G wireless is supposed to be anywhere from four to ten times faster
than today's 3G networks. Sprint says its 4G WiMax network can offer download
speeds that are ten times faster than a 3G connection, with speeds that top out at
10 megabits per second. Verizon's LTE network, meanwhile, can deliver speeds
between 5 mbps and 12 mbps.
WiMax is a wireless broadband technology that delivers WiFi-like speeds to wide areas
GSM Overview
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Frequency Concepts
Frequency Specifications
Frequency specifications of various GSM systems
GSM Overview
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Frequency Specifications
The above diagram displays the frequencies used by major
mobile standards
The frequency of a wave is the number of times the wave
oscillates per second
GSM Overview
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Wavelength
Wavelength is the length of one complete signal oscillation (λ)
in meters
Frequency and wavelength are related via the speed of
propagation (c = λ * f)
The higher the frequency, the shorter the wavelength
Lower frequencies (longer wavelengths) are suitable for
transmission over long distances
Higher frequencies are more suitable for transmission over
short distances
GSM Overview
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Frequency Allocation - US
FCC auctions licenses to mobile network operators (Federal
Communications Commission)
FCC specified six blocks within the frequency band
Three 30 MHz duplex blocks: A, B and C
Three 10 MHz duplex blocks: D, E and F
Major Trading Areas (MTAs) and Basic Trading Areas (BTAs)
GSM Overview
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Bandwidth
Amount of frequency range allocated to one application
Depends on available frequency spectrum
Important factor in determining capacity of mobile systems
Capacity is the number of simultaneous calls that can be
handled
GSM Overview
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Channels
A channel is a set of frequencies allocated for transmission
and reception of information
Types of communication channels are listed in the above table
GSM Overview
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Channels
The direction from the MS
to the network is the uplink
The direction from the
network to the MS is the
downlink
GSM Overview
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GSM cellular technology
• The GSM cellular technology had a number of design aims
when the development started:
a)
b)
c)
d)
e)
f)
It should offer good subjective speech quality
It should have a low phone or terminal cost
Terminals should be able to be handheld
The system should support international roaming
It should offer good spectral efficiency
The system should offer ISDN compatibility
GSM Overview
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Duplex Distance
Uplink and downlink channels must be separated by duplex
distance
This prevents both channels from interfering with each other
GSM Overview
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Carrier Separation
Carrier separation is distance on frequency band between channels being
transmitted in same direction
Prevents overlapping of information in one channel into an adjacent
channel
Dependent on amount of information to be transmitted within the
channel
GSM Overview
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Capacity
Number of frequencies in a cell determines the capacity
Each company is allocated a limited number of frequencies
Frequencies are distributed throughout cells in the network
GSM Overview
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Frequency Reuse
Frequencies must be reused
several times to provide
sufficient capacity
Same frequencies cannot
be reused in neighboring
cells
GSM Overview
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Transmission Rate
Amount of information transmitted over a radio channel per
period of time
Expressed in bits per second
In GSM, transmission rate over air interface is 270 kbps
GSM Overview
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Modulation Method
Modulation technique used in GSM is GMSK (Gaussian
Minimum Shift Keying : MSK + Gaussian filter)
GMSK is a form of phase shift keying
Carrier bandwidth is 200 KHz
GMSK provides :
–
–
–
–
high interference resistance level
Quite insensitive to non-linearities of power amplifier
Robust to fading effects
But moderate spectral efficiency
GSM Overview
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Multiple Access Method
GSM uses TDMA to transmit and
receive speech signals over air
interface
With TDMA, one carrier is used to
carry a number of calls at
designated periods of time
TDM frame consists of 8 time
slots
GSM Overview
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Analog and Digital Transmission
Analog & Digital Signals
Analog signal is continuous
waveform which changes
smoothly over time
Analog Signal
Digital signal consists of a
set of discrete values
Digital Signal
GSM Overview
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Advantages of Digital Signals
Digital signals provide better quality for transmission than
analog signals:
Error detection and correction
Data Compression
Ciphering / Encryption
GSM Overview
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