BROADBAND The aim in the cellular system

 

 

 

 

 

 

BROADBAND
COMMUNICATION

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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                                Submitted To: Naveed
Rehman                                                                                       

                                Submitted by:
Jaspreet Singh

 

 

 

 

 

 

 

 

 

 

 

 

 

Comparison

and

Contrast

 of

FDMA, CDMA and TDMA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CONTENTS

 

1. Introduction

  1.1 FDMA

1.1.1
Features

1.1.2
Advantages

1.1.3
Disadvantages

 

  1.2 CDMA

1.2.1
Features

1.2.2
Advantages

1.2.3
Disadvantages

 

  1.3 TDMA

1.3.1
Features

1.3.2
Advantages

1.3.3
Disadvantages

 

2. Comparison of FDMA, CDMA and TDMA

3. Conclusion

4. Reference

 

 

 

 

 

 

 

 

 

 

 

 

1.
INTRODUCTION

 

 

In any mobile technology, it is necessary to have a method that enables several multiple users who can use it simultaneously. As
cellular/mobile technology has
progressed, different multiple access
schemes have been used like TDMA, FDMA, CDMA  are three major access
technique, that is used in wireless communication system. 1st Generation (analog cellular
systems) systems use analog Frequency Modulation (FM) and have  Frequency Division Multiple Access (FDMA) media access control
architecture. 2G cellular systems
use digital modulation and processing techniques. Time Division Multiple Access
(TDMA) based and Code
Division Multiple Access (CDMA) based standards are used in 2nd Generation. 3rd Generation network supports voice calling, mobile internet, video
calls and it uses CDMA technology.

In wireless systems, it is often desirable to allow the subscriber to send
information simultaneously from the mobile station to the base station while
receiving information from the base station to the mobile station.

Cellular
system is one divides any given area into cells where a mobile unit in each cell
communicates with a base station. The aim in the cellular system design is to increase the capacity of the channel,
i.e., to handle as many calls as possible in a given bandwidth with a
sufficient level of quality of service.

There are several different ways to allow access to the channel.
These includes mainly the following ?

n  Frequency
division multiple-access (FDMA)

n  Time
division multiple-access (TDMA)

n  Code
division multiple-access (CDMA)

The
available bandwidth is allocated to the users depending on these
techniques can be classified as narrow-band and wide-band
systems.

 

Narrow-band Systems

In this systems operating with channels substantially narrow than
the coherence bandwidth. The narrow band TDMA allows users to use the same channel but
allocates a unique time slot to each user on the channel, thus separating a
small number of users in time.

Wide-band Systems

In this wide-band systems, the transmission bandwidth of a single channel is much
larger than the coherence bandwidth of the channel. Thus, multipath fading
doesn’t greatly affect the received signal, and frequency selective fades occur
only in a small fraction of the signal bandwidth.

 

1.1 Frequency Division Multiple Access (FDMA):

 

It is a technology by
which the total bandwidth available to the system is divided into different
frequencies.
This division is done between non overlapping frequencies that are then
assigned to each of the communicating pair. FDMA is used mainly for analog signal transmission. Its not that this
technology is not capable of carrying digital information, but just that it is
not considered to be an efficient method,
specially for digital transmission. Because just imagine if the frequencies to
handle the customers gets over? What if more capacity is required? The only
option would be to drill down the existing frequencies to a much narrower
amount which will not be very competent. In FDMA all
users share the satellite simultaneously but each user transmits at single
frequency.

 

To understand this
technology better, see how FM radio works. All
the radios have their own frequency bands
at different levels and they send their signals at the allocated unique
frequencies within the available bands.

 

 

The best example of this system is the cable system used in television. The medium is a single coax cable that is used to broadcast
hundreds of channels of video or audio programming to destination.
The coax cable has a useful bandwidth from about 4 MHz to 1 GHz. This bandwidth
is divided up into 6-MHz wide channels. Initially, one TV station or channel
used a single 6-MHz band. But with such digital techniques, multiple TV channels
may share a single band today thanks to compression and multiplexing techniques
used in each channel.

This technique is also used in fiber optic communications systems. A
single fiber optic cable has enormous bandwidth that can be subdivided to
provide FDMA. Different data or information sources are each assigned a
different light frequency for transmission. Light generally isn’t referred to
by frequency but by its wavelength (?). As a result of this, fiber optic
FDMA is called wavelength division multiple access (WDMA) or just wavelength
division multiplexing (WDM).

FDMA systems is the original analog telephone system  using a hierarchy of frequency multiplex
techniques to put multiple telephone calls on single line. The analog 300-Hz to
3400-Hz voice signals were used to modulate sub carriers in 12
channels from 60 kHz to 108 kHz. Modulator/mixers created single side band (SSB)
signals, both upper and lower side bands. These sub carriers were then
further frequency multiplexed on sub carriers in the 312-kHz to 552-kHz range using
the same modulation methods. At the receiving end of the system, the signals
were sorted out and recovered with filters and demodulators.

Original aerospace telemetry systems used an FDMA system to
accommodate multiple sensor data on a single radio channel. Early satellite
systems shared individual 36-MHz bandwidth transponders in the 4-GHz to 6-GHz
range with multiple voice, video, or data signals via FDMA

1.1.1
Features:

n  In
FDMA, every user shares the frequency channel or satellite transponder
simultaneously; however, every user transmits at single frequency.

n  FDMA
is compatible with both digital and analog signals.

n  FDMA
demands highly efficient filters in the radio hardware, contrary to CDMA and
TDMA.

n  FDMA
is devoid of timing issues that exist in TDMA.

n  As
a result of the frequency filtering, FDMA is not prone to the near-far problem
that exists in CDMA.

n  All
users transmit and receive at different frequencies because every user receives
an individual frequency slot.

1.1.2 Advantages-

n  All
channels in a cell are available to all the mobiles systems.

n  Channel
assignment is carried out on a FIFO
basis. ?

n  The
number of channels depends on the modulation technique and the guard bands
between the channels.

n  These
guard bands can be used to minimize adjacent channel interference in system.

n  If
channel is not in use
then it sits idle ?

n  Channel
bandwidth is relatively narrow
I.e. 30kHz ? ?

n  It is fairly efficient when the
number of stations is small and the traffic is uniformly constant ?

n  Capacity
can be increased by reducing the
information bit rate and using efficient digital code No need for network
timing ?


No restriction regarding
the type of base band signal or type of modulation

 

1.1.3 Disadvantages-

 ?

n  The
presence of guard bands
is a problem

n  Requires
right RF filters
to minimize adjacent channel interference

n  Maximum
bit rate per channel is fixed ?

n  Small
inhibiting flexibility in bit rate capability ?

n  Does
not differ significantly from analog system

 

1.2 Code division multiple
access (CDMA):

 

The technology of code-division multiple access
channels has long been known. In the Soviet Union (USSR), the first work devoted to this subject
was published in 1935 by Dmitry
Ageev. It
was shown that through the use of linear methods, there are three types of
signal separation: frequency, time and compensatory. The technology of CDMA was
used in 1957, when the young military radio engineer Leonid
Kupriyanovich in Moscow made an experimental model of a wearable automatic mobile
phone, called LK-1 by him, with a base station. LK-1 has a weight of 3 kg,
20–30 km operating distance, and 20–30 hours of battery life. The base
station, as described by the author, could serve several customers. In 1958,
Kupriyanovich  made the new experimental “pocket”
model of mobile phone. In 1958, the USSR also started the development of the
“Altai” national civil mobile phone service for cars, based on the
Soviet MRT-1327 standard. The phone system weighed 11 kg (24 lb). It
was placed in the trunk of the vehicles of high-ranking officials and used a
standard handset in the passenger compartment. The main developers of the Altai
system were VNIIS (Voronezh Science Research Institute of Communications) and
GSPI (State Specialized Project Institute). In 1963 this service started in
Moscow, and in 1970 Altai service was used in 30 USSR cities.

 

Unlike FDMA, CDMA
separates calls by code. Every bit of a conversation is been tagged with a
specific and unique code. The system gets a call, it allocates a unique code to
that particular conversation, now the data is split into small parts and is
tagged with the unique code given to the conversation of which they are part
of. Now, this data in small pieces is sent over a number of the discrete
frequencies available for use at any time in the specified range. The system
then at the end reassembles the conversation from the coded bits and deliver it. Does it make sense?

Just think about how you
recollect your luggage at the end of the flight journey. When you check in, a
tag with a code is given to you which is also given to your luggage. And at the destination,
you collects your luggage on the basis of that I know you will say that you
recognize your bag, but then I have a habit of always matching the codes of my
bag and the one on the tag given to me and that is how I become sure of not
picking up the wrong luggage.

CDMA (Code Division Multiple
Access) also called spread-spectrum and
code division multiplexing, one of
the competing transmission technologies for digital MOBILE PHONES. The
transmitter mixes the packets constituting a message into the digital signal
stream so that it could be determined by a PSEUDO-RANDOM NUMBER sequence
that is also known to the intended receiver, which uses. it to extract those
parts of the signal intended for itself. Hence each different random sequence
corresponds to a separate communication channel in the system.

n  Unlike TDMA, in CDMA all
stations can transmit data simultaneously, there is no timesharing.

n  CDMA allows each station to
transmit over the entire frequency spectrum all the time.

n  Multiple simultaneous
transmissions are separated using coding theory.

n  In CDMA each user is given a
unique code sequence.

1.2.1 Features-

n  Many
users of a CDMA system share the same frequency. Either TDD or FDD may be used.

n  Unlike
TDMA or FDMA, CDMA has a soft capacity limit. Increasing the number of users in
a CDMA system raises the noise floor in a linear manner. Thus there is no
absolute limit on the number of users in CDMA but the system performance
gradually degrades for all users as the number of users is increased.

n  CDMA
is a interference limited system.

n  Multipath
fading is substantially reduced because the signal is spread over a large
spectrum. If the spread spectrum bandwidth is greater than the coherence
bandwidth of the channel, the inherent diversity will reduce the effect of
small scale fading.

n  Channel
data rates are very high in
this system. The symbol duration is very short
and usually much less than the channel delays spread. A RAKE receiver can be
used to improve reception by collecting time delayed versions of the required
signal.

n  CDMA
uses co-channel cells thus it can use macroscopic spatial diversity to provide
soft handoff.

n  Self-jamming
is a problem in CDMA system. Self-jamming arises from the fact that the
spreading sequences of different users are not exactly orthogonal.

n  The
near-far problem occurs at a CDMA receiver if an undesired user has a high
detected power as compared to the desired powers

1.2.2 Advantages-

 ?

n  Potentially
larger capacity (more users can communicate simultaneously) If users don’t use
the medium all the time (e.g., they are just reading e- mail), CDMA will allow
much more users to communicate simultaneously.

n  In
other words, CDMA will use the resource (the radio spectrum) more efficiently.
Provides larger spread spectrum, thus more robust against noise bursts and
multipath frequency selective fading ? GSM bandwidth = 200 kHz ? IS-95
bandwidth = 1.25 MHz ? W-CDMA (3G) bandwidth = 10MHz The transition from one BS
to another (handoff) is not abrupt, as in TDMA, and provides better quality

n  No
absolute limit on the number of users ?

n  Easy
addition of more users ?

n  Impossible
for hackers to decipher the code sent ?


Better signal quality

 

1.2.3 Disadvantage-

 

As the number of users increases, the
overall quality of service decreases ? Self-jamming, Near- Far- problem arise

 

1.3 Time division multiple access (TDMA):

 

Unlike FDMA and CDMA, in
TDMA
the division of calls happens on the basis
of time. The system first digitizes the calls, and then combines those
conversations into a unified digital stream on a single radio channel. Now it
divides each cellular channel into three time slots that means three calls get
put on a single frequency and then a time slot is assigned to each call during
the conversation, a regular space in a digital stream. The users transmit in
fast succession, one after the other, each using its own time slot. This allows
multiple stations to share the same transmission medium (e.g. radio frequency
channel) while using only a part of its channel capacity.

This technology enables
three different users to use one frequency at the same time.

Here there is no need for
three separate frequencies like in FDMA. As in FDMA, instead of monopolizing a
single radio channel for a single call, TDMA efficiently carries three calls at
the same time.

 

Examples of TDMA include IS-136, personal
digital cellular (PDC), integrated digital enhanced network (iDEN) and the
second generation (2G) Global System for Mobile Communications (GSM).

TDMA allows a mobile’s radio component to listen and broadcast only in its assigned time
slot. During the remaining time period, the station may apply network
measurements by detecting surrounding transmitters in different frequencies.
This feature allows inter frequency handover, which differs from code division multiple access
(CDMA), where frequency handover is difficult to achieve. However, CDMA allows
handoffs, which enable mobile stations to simultaneously communicate with up to
six base stations.

TDMA is used in most 2G cellular systems, while 3G systems are based on CDMA.
However, TDMA remains relevant to modern systems. For example, combined TDMA,
CDMA and time division duplex (TDD) are universal terrestrial radio access
(UTRA) systems that allow multiple users to share one time slot

 

1.3.1 Features

n  Shares
single carrier frequency with multiple users at a time

n  Non-continuous
transmission makes hand off
easy

n  Slots
can be assigned on demand in dynamic TDMA system

n  Higher
synchronization overhead than CDMA system

n  Advanced
equalization is necessary for high data rates

n  Frequency or slot allocation
complexity

1.3.2
Advantages

 

n  It
carry data rates of 64 kbps to 120 Mbps .

n  It
provides the user with extended battery life and talk time.

n  It
is the most cost effective technology to convert an analogue system to digital.

n  TDMA
technology separates users according to time and it ensures that there
will be no interference
at all

n  TDMA
allows the operator to do services like fax, voice band data and SMS as well as
bandwidth-intensive application such as multimedia etc.

 

1.3.3 Disadvantages

n  Each
user has a predefined time slot.When moving from one cell to other, if all the
time slots in this cell are full the user might be disconnected.

n  It
is subjected to multipath distortion. A signal coming from a tower to a handset
might come from any one of several directions. It might have bounced off
several different buildings before arriving.

1.2 Comparison of FDMA, TDMA and CDMA

Approach

TDMA

FDMA

CDMA

Idea

Segments sending time
into disjoint time slots demand driven or fixed patterns.

Segment the frequency
band into disjoint sub-bands

Spread the spectrum
using orthogonal codes.

Terminals

All terminals are active
for short periods of time on same frequency.

Every terminal has its
own frequency uninterrupted

All terminals can be
active at the same place at the same moment uninterrupted.

Signal separation

Synchronization in time
domain

Filtering in the
frequency domain.

Code plus special
receivers.

Transmission scheme

Discontinuous

Continuous

Continuous

Cell capacity

Limited

Limited

No absolute limit on
channel capacity but it is an interference limited system

Advantages

Established fully
digital, flexible

Simple, established,
robust

Flexible, less frequency
planning needed, soft handover

Disadvantages

Guard space needed
(multipath propagation), synchronization difficult

Inflexible, frequencies
are scarce resource

Complex receivers, needs
more complicated power control for senders

Comment

Standards in fixed
networks, together with FDMA or SDMA used in many mobile networks

Typically combined with
TDMA and SDMA

Still faces some
problems, higher complexity, lowered expectations, will be integrated with
TDMA or FDMA

 

 

 

 

 

3. CONCLUSION

 

In this topic, we have mainly discussed the fixed assignment
type of MA techniques, namely, FDMA, TDMA and CDMA.

 

The main idea to discuss only the basic MA techniques has been to grow up a fair idea
about the resource sharing in a wireless media when there are many users, keeping the QoS view point
in mind.

 

CDMA has three times the capacity of TDMA. CDMA is the
first technology to use soft handoff, which allows a handset to communicate with different base stations simultaneously.

 

In contrast to CDMA, TDMA users experience interruption
when signal is handed off, resulting in higher interference during handoff and
increased call drops. CDMA has higher immunity to interference.  

 

With better advancements and qualities in CDMA, it has several disadvantages such as the system is
little complicated, the overall performance degrades with the increase in
number of users.

 

4. REFERENCES

 

1.       
Frenzel, Louis E., Principles
of Electronic Communication Systems, 3rd Edition, McGraw Hill, 2008.

2.       
Gibson, Jerry D., Editor,
The Communications Handbook, CRC Press, 1997.

3.       
Skylar, Bernard,
Digital Communications, 2nd Edition, Prentice Hall, 2001.

4.       
Tomasi, Wayne, Advanced
Electronic Communications Systems, 4th Edition, Prentice Hall, 1998

5.       
T. S. Rappaport, Wireless
Communications: Principles and Practice, 2nd ed. Singapore: Pearson Education, Inc., 2002.

6.       
K. Feher, Wireless Digital
Communications: Modulation and Spread Spectrum

Applications. Upper Saddle River, NJ:
Prentice Hall, 1995.

7.       
J. G. Proakis, Digital Communications, 4th ed. NY: McGraw Hill,
2000.