Cell-system-for-frequency-rex

rabidwestvirginiaNetworking and Communications

Oct 26, 2013 (3 years and 7 months ago)

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CELLULAR SYTEM

:

BASIC CONCEPT

There are a variety of wireless communication systems for transmitting voice, video, and data in
local or wide areas. There are point
-
to
-
point wireless bridges, wireless local area networks,
multidirectional wireless
cellular systems, and satellite communication systems
.

A cell in a cellular system is a roughly circular area with a central transmitter/receiver base

station
(although the base station may be located off
-
center to conform to local topology). The
station is

raised up on a tower or placed on top of a building. Some are located on church
steeples. The station has a 360
-
degree omnidirectional antenna (except when directional
transmissions are required) that is tuned to create a cellular area of a specific size.

Cells are
usually pictured as hexagonal in shape and arranged in a honeycomb pattern. Cell size varies
depending on the area. In a city, there are many small cells, while rural area may have very large
cells
.

When a user turns a phone on, its phone number

and serial number are broadcast within the local
cell. The base station picks up these signals and informs the switching office that the particular
device is located within its area. This information is recorded by the switching office for future
referenc
e. An actual call takes place when the user enters a phone number and hits the Send
button. The cellular system selects a channel for the user to use during the duration of the call.

Overview
:

The first generation of cellular systems used analog radio tech
nology. Analog cellular systems
consist of three basic elements: a mobile telephone (mobile radio), cell sites, and a mobile
switching center (MSC). Figure 2.1 shows a basic cellular system in which a geographic service
area such as a city is divided into
smaller radio coverage area cells. A mobile telephone
communicates by radio signals to the cell site within a radio coverage area. The cell site s base
station (BS) converts these radio signals for transfer to the MSC via wired (landline) or wireless
(micr
owave) communications links. The MSC routes the call to another mobile telephone in the
system or the appropriate landline facility. These three elements are integrated to form a
ubiquitous coverage radio system that can connect to the public switched tele
phone network
(PSTN).






Figure
1
a). Basic Cellular System block diagram 1b)cross sectional view

Cell
systems for frequency re
-
use
:

The method that is employed is to enable the frequencies to be re
-
used. Any radio transmitter
will only have a certain
coverage area. Beyond this the signal level will fall to a limited below
which it cannot be used and will not cause significant interference to users associated with a
different radio transmitter. This means that it is possible to re
-
use a channel once out
side the
range of the radio transmitter. The same is also true in the reverse direction for the receiver,
where it will only be able to receive signals over a given range. In this way it is possible to
arrange split up an area into several smaller regions,

each covered by a different transmitter /
receiver station.

These regions are conveniently known as cells, and give rise to the name of a "cellular"
technology used today. Diagrammatically these cells are often shown as hexagonal shapes that
conveniently
fit together. In reality this is not the case. They have irregular boundaries because
of the terrain over which they travel. Hills, buildings and other objects all cause the signal to be
attenuated and diminish differently in each direction.


Interference
and System Capacity:



Sources of interference



another mobile in the same cell



a call in progress in the neighboring cell



other base stations operating in the same frequency band



noncellular system leaks energy into the cellular frequency band



Two major
cellular interference



co
-
channel interference



adjacent channel interference


Co
-
channel Interference and System Capacity
:



Frequency reuse
-

there are several cells that use the same set of frequencies



co
-
channel cells



co
-
channel interference



To reduce
co
-
channel interference, co
-
channel cell must be separated by a minimum
distance.



When the size of the cell is approximately the same



co
-
channel interference is independent of the transmitted power



co
-
channel interference is a function of



R
: Radius of the cell



D
: distance to the center of the nearest co
-
channel cell



Increasing the ratio
Q=D/R
, the

interference is reduced.



Q

is called the co
-
channel reuse ratio
.
For a hexagonal geometry






From Analog to Digital Systems
:

Analog
cellular

services were introduced by AT&
T in the 1970s and became widespread in the
1980s. The primary analog service in the United States is called AMPS (Advanced Mobile
Phone Service). There are similar systems around the world that go by different names. The
equivalent system in England is ca
lled TACS (Total Access Communications System).

The AMPS system is a circuit
-
oriented communication system that operates in the 824
-
MHz to
894
-
MHz frequency range. This range is divided into a pool of 832 full
-
duplex channel pairs (1
send, 1 receive). Any
one of these channels may be assigned to a user. A channel is like physical
circuit, except that it occupies a specific radiofrequency range and has a bandwidth of 30 kHz.
The circuit remains dedicated to a subscriber call until it is disconnected, even if

voice or data is
not being transmitted.

Cellular systems are described in multiple generations, with third
-

and fourth
-
generation (3G and
4G) systems just emerging:



1G systems


These are the
analog

systems such as AMPS that grew rapidly in the 1980s
and

are still available today. Many metropolitan areas have a mix of 1G and 2G systems,
as well as emerging 3G systems. The systems use frequency division multiplexing to
divide the bandwidth into specific frequencies that are assigned to individual calls.



2G

systems


These second
-
generation systems are
digital
, and use either TDMA (Time
Division Multiple Access) or CDMA (Code Division Multiple Access) access methods.
The European GSM (Global System for Mobile communications) is a 2G digital system
with its
own TDMA access methods. The 2G digital services began appearing in the late
1980s, providing expanded capacity and unique services such as caller ID, call
forwarding, and short messaging. A critical feature was seamless roaming, which lets
subscribers mov
e across provider boundaries.



3G systems


3G has become an umbrella term to describe cellular data communications
with a target data rate of 2 Mbits/sec. The ITU originally attempted to define 3G in its
IMT
-
2000 (International Mobile Communications
-
2000)

specification, which specified
global wireless frequency ranges, data rates, and availability dates. However, a global
standard was difficult to implement due to different frequency allocations around the
world and conflicting input. So, three operating m
odes were specified. According to
Nokia, a 3G device will be a personal,

mobile, multimedia communications device that
supports speech, color pictures, and video, and various kinds of information content.
Nokia's Web site (
http://www.Nokia.com
) provides interesting information about 3G
systems. There is some doubt that 3G systems will ever be able to deliver the
bandwidth to support these features because bandwidth is shared. However, 3G
systems will certain
ly support more phone calls per cell.



4G Systems


On the horizon are 4G systems that may become available even
before 3G matures (3G is a confusing mix of standards). While 3G is important in
boosting the number of wireless calls, 4G will offer true
high
-
speed data services. 4G
data rates will be in the 2
-
Mbi
t/sec to 156
-
Mbit/sec range.


1G
-

First Generation networks
:

1G

-

First Generation

mobile phone

networks were the earliest cellular systems to develop, and
they relied on a network of distributed transceivers to communicate with the

mobile phones
. Fi
rst
Generation phones were also analogue, used for

voice calls

only, and their signals were
transmitted by the method of

frequency modulation
. These systems typically allocated one
25

MHz

frequency band

for the signals to be sent from the cell

base station

to the

handset
, and a
second
different 25 MHz

band

for signals being returned from the handset to the base station.
These bands were then split into a number of communications
channels, each of which would be
used by a particular caller.

In the case of

AMPS
, the first 1G system to start operating in the USA (in July 1978)
, each
channel was separated from the adjacent channels by a spacing of 30

kHz
, which was not
particularly efficient in terms of the available radio
spectrum, and this placed a limitation on the
number of calls that could be made at any one time. However, the system was a multiple access
one, because a second caller could use the same channel, once the first caller had hung up. Such
a system is called
"
frequency division multiple access
" (
FDMA
).

In addition, because the cell transmitter's power output is restricted and designed to cover a
specific area, it is possible to use the same frequencies in other cells that are far enough
away for
there to be no interference
-

this system is called

frequency

re
-
use, and enables the network
capacity to be increased. The cell
ular structure of the network is also responsible for another
feature of

cell phone

communications, i.e. that it is necessary for some
sort of

handover

to take
place when the mobile phone passes from one cell area to another, and this requires that the pair
of frequencies u
sed by the phone are changed at the time of handover.

NMT 450, the Nordic Mobile Telephone System using the 450 MHz band, was the first cell
phone network to start operating in Europe (i.e. Scandinavia) in 1981. Later, in 1985, the United
Kingdom began ope
rations with its TACS (Total Access Communications System). With the
introduction of

2G

networks, the 1G phones were destined to become obsolete, as
they were not
adaptable to the new 2G standards and also had other drawbacks, such as their poor security due
to the lack of

encryption
, and the fact that anyone with a receiver tuned to the right frequency
could overhear the conversation.

Nowadays, everybody has a mobile phone. This article will be looking at the the mobile phone's
history
-

& its future
-

in order to discover more about

the now
-
essential telecommunications
device.


Ever since the launch of 3G mobile telephone technology, people have been discussing 4
-
G. 4
-
G
technology will signify the future of mobile telephones, producing the most advanced handsets &
best services to
date. In actual fact, one of the next services to be developed is

thought to be the
live streaming of radio and television shows to 3G handsets is & businesses including Disney &
Real recently announced that they'll be offering services like these.

This se
cond
-
generation system, widely deployed in the United States, Canada, and South
America, goes by many names, including North American TDMA, IS
-
136, and D
-
AMPS
(Digital AMPS). For the sake of clarity, we will refer to it as North American TDMA, as well as
s
imply
TDMA
, when the context makes it clear. TDMA has been used in North America since
1992 and was the first digital technology to be commercially deployed there. As its name
indicates, it is based on
Time Division Multiple Access
. In TDMA the resources a
re shared in
time, combined with frequency
-
division multiplexing (that is, when multiple frequencies are
used). As a result, TDMA offers multiple digital channels using different time slots on a shared
frequency carrier. Each mobile station is assigned bot
h a specific frequency and a time slot
during which it can communicate with the base station, as shown in Figure 3.4.


Figure 2
:
Time Division Multiple Access.


The TDMA transmitter is active during the assigned time slot and inactive during other time
slots, which allows for power
-
saving terminal designs, among other advantages. North American
TDMA supports three time slots, at 30 kHz each, further divided into three or six channels to
maximize air interface utilization. A sequence of time
-
division mult
iplexed time slots in TDMA
makes up frames, which are 40 ms long. The TDMA traffic channel total bit rate is 48.6 Kbps.
Control overhead and number of users per channel, which is greater than one, decrease the
effective throughput of a channel available fo
r user traffic to 13 Kbps. TDMA is a dual
-
band
technology, which means it can be deployed in 800
-
MHz and 1900
-
MHz frequency bands. In
regions where both AMPS and TDMA are deployed, TDMA phones are often designed to
operate in dual mode, analog and digital,

in order to offer customers the ability to utilize
coverage of the existing analog infrastructure
.

Global System for Mobile Communications (GSM)

There are still some analog
cellular

systems

in operations in Europe, but their number is
declining, and some
regional networks are being completely shut down or converted to Global
System for Mobile Communications. The GSM
cellular

system initiative was initiated in 1982
by the Conference of European Posts and Telecommunications Administrations (CEPT) and is
curr
ently governed by European Telecommunications Standards Institute (ETSI), which in turn
has delegated GSM specifications maintenance and evolution to 3GPP (reviewed in part in
Chapter 1). The intent behind GSM introduction was to have a common approach to
the creation
of digital
systems

across. In hindsight, this was a smart political decision, which contributed to
the worldwide success of European
cellular

infrastructure providers and equipment
manufacturers.

Let's look at some details of the GSM air inter
face technology. The GSM standard, similarly to
North American TDMA, is based on the use of two simultaneous multiplexing technologies,
TDMA and FDMA. Each radio frequency (RF) channel in GSM supports eight time slots
(compared to three for North American
TDMA) grouped into TDMA frames, which are in turn
grouped into
multiframes

consisting of 26 TDMA frames carrying traffic and control channels.
Multiframes are built into
superframes

and
hyperframes
. This yields an 8
-
to
-
1 capacity increase
over NMT or TACS
in the same RF spectrum. The allocation of the time slots is essentially static
on a short
-
term basis; for instance, the eighth time slot of a given RF channel is assigned to the
same user each time it comes around, whether or not the user has voice or dat
a to send.

The GSM system, emphasizing not only physical properties but also service definitions (unlike
some 1G
systems
), supports three major types of services: bearer services, tele
-
services, and
supplementary services. GSM
bearer

services allow for tra
nsparent or acknowledged user data
transfer and define access attributes, information transfer attributes, and general attributes with
specific roles.
Access

attributes define access channel properties and parameters such as bit rate;
transfer

attributes d
efine data transfer mode (bidirectional, unidirectional), information type
(speech or data), and call setup mode;
general

attributes define network
-
specific services such as
QoS and internetworking options.
Tele
-
services

are what GSM subscribers actually u
se. They are
based on the foundation provided by bearer services and govern user
-
to
-
user communications for
voice or data applications. Examples of tele
-
services include Group 3 Fax, telephony, Short
Message Service (SMS), and circuit data IP and X.25 comm
unications. GSM
supplementary

services provide additional value
-
added features such as call waiting, call forwarding, call
barring, and conference calling used by wireless operators to further differentiate their offerings.


GSM or GLOBAL SYSTEM FOR
MOBILE COMMUNICATIONS

is a digital
cellphone technology that is based on TDMA (Time Division Multiple Access) and is
predominantly used in European countries and it is also used in other countries of the world
including India. GSM was developed in the 1980
s and was distributed to be used in around 7
European countries in 1992. Nowadays, GSM is used in Asia, Europe, Australia, North America,
and Chile. GSM operates on 1.9GHz PCS bands in USA and 900MHz and 1.8GHz bands in
Europe. Global System for Mobile Com
munications defines the entire mobile system and is not
just used as a radio interface but also TDMA and CDMA which is Code Division Multiple
Access. In the year 2000, world had more than 250 million GSM users which represents more
than half of world’s pop
ulation of cellphone users. GSM use is increasing day by day due to its
reliability and easy connectivity services. WAV and AIFF files are used for audio coding of the
GSM standard used in IP telephony

Cellular Standards

Keeping track of the analog and dig
ital cellular standards can be difficult. Table W
-
2 lists the
most common standards.

Common

Reference Name

Standard

Category

Frequency
Band(s)

Comments

Analog cellular

TIA/EIA
-
553

FDMA analog
cellular

800 MHz

The AMPS
standard. Does
not support N
-
AMPS.

Analog cellular
(enhanced)

IS
-
91

FDMA analog
cellular

800 MHz

Same as
above, but also
includes N
-
AMPS and
authentication
support.

Narrowband
-
AMPS (N
-
AMPS)

IS
-
88

FDMA analog
cellular

800 MHz

Divides one
FDMA
channel into
three smaller
channels.
Meant for
PDA and
messaging.

Local AMPS

IS
-
94

FDMA analog
cellular

800 MHz

A low
-
power
cellular system
designed for
local (in
-
building) use.

TDMA digital
cellular, also

called D
-
AMPS
(digital
-
AMPS)

IS
-
54

TDMA digital
cellular

800 MHz

Same as
AMPS, except
uses
digital
TDMA to
divide each
channel into
three time
-
slotted
channels. Does
not directly
support data.

TDMA digital
cellular
(enhanced)

IS
-
136

TDMA digital
cellular

800 MHz

An
enhancement
to above
(TIA/EIA/IS
-
54) that
supports
circuit
-

switched data
at 9,
600
bits/sec.

CDMA digital
cellular

IS
-
95a

CDMA digital
cellular

800 MHz

1,900 MHz

Uses spread
spectrum radio
and code
division
multiplexing
to put up to

20
conversations
on a single
band. Data
rate is 16
Kbits/sec.

GSM



TDMA

900 MHz

GSM was
designed

by
the European
community as
a digital
system to
replace analog
system.

Table W
-
2: Wireless mobile standards


Multiple Access Schemes for Cellular Systems

-

a summary or tutorial about the basics of the different multiple access schemes including
FDMA,
TDMA, CDMA and OFDMA, used within cellular technology to allow multiple users to
access the cellular system. These include FDMA, TDMA, CDMA and OFDMA.

In any cellular system or cellular technology, it is necessary to have a scheme that enables
several mult
iple users to gain access to it and use it simultaneously. As cellular technology has
progressed different multiple access schemes have been used. They form the very core of the
way in which the radio technology of the cellular system works.

There are four

main multiple access schemes that are used in cellular systems ranging from the
very first analogue cellular technologies to those cellular technologies that are being developed
for use in the future. The multiple access schemes are known as FDMA, TDMA, C
DMA and
OFDMA
.



Requirements for a multiple access scheme

In any cellular system it is necessary for it to be able have a scheme whereby it can handle
multiple users at any given time. There are many ways of doing this, and as cellular technology
has
advanced, different techniques have been used.

There are a number of requirements that any multiple access scheme must be able to meet:



Ability to handle several users without mutual interference.



Ability to be able to
maximize

the spectrum efficiency



Must

be robust, enabling ease of handover between cells.



FDMA
-

Frequency Division Multiple Access

FDMA is the most straightforward of the multiple access schemes that have been used. As a
subscriber comes onto the system, or swaps from one cell to the next,

the network allocates a
channel or frequency to each one. In this way the different subscribers are allocated a different
slot and access to the network. As different frequencies are used, the system is naturally termed
Frequency Division Multiple Access.

This scheme was used by all analogue systems.

TDMA
-

Time Division Multiple Access

The second system came about with the transition to digital schemes for cellular technology.
Here digital data could be split up in time and sent as bursts when required. As speech was
digitised it could be sent in short data bursts, any small delay caused

by sending the data in
bursts would be short and not noticed. In this way it became possible to organise the system so
that a given number of slots were available on a give transmission. Each subscriber would then
be allocated a different time slot in whi
ch they could transmit or receive data. As different time
slots are used for each subscriber to gain access to the system, it is known as time division
multiple access. Obviously this only allows a certain number of users access to the system.
Beyond this
another channel may be used, so systems that use TDMA may also have elements of
FDMA operation as well.



CDMA
-

Code Division Multiple Access

CDMA uses one of the aspects associated with the use of direct sequence spread spectrum. It can
be seen from the
article in the cellular telecoms area of this site that when extracting the required
data from a DSSS signal it was necessary to have the correct spreading or chip code, and all
other data from sources using different orthogonal chip codes would be rejecte
d. It is therefore
possible to allocate different users different codes, and use this as the means by which different
users are given access to the system.

The scheme has been likened to being in a room filled with people all speaking different
languages.
Even though the noise level is very high, it is still possible to understand someone
speaking in your own language. With CDMA different spreading or chip codes are used. When
generating a direct sequence spread spectrum, the data to be transmitted is multi
plied with
spreading or chip code. This widens the spectrum of the signal, but it can only be decided in the
receiver if it is again multiplied with the same spreading code. All signals that use different
spreading codes are not seen, and are discarded in
the process. Thus in the presence of a variety
of signals it is possible to receive only the required one.

In this way the base station allocates different codes to different users and when it receives the
signal it will use one code to receive the signal
from one mobile, and another spreading code to
receive the signal from a second mobile. In this way the same frequency channel can be used to
serve a number of different mobiles.