BROADBAND station to the mobile station. A

 

 

 

 

 

 

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 cellular technology,
it is required to have a method that enables several multiple users and use it simultaneously. As cellular
technology has progressed, different multiple access schemes
have been used. TDMA, FDMA, CDMA  are three major access technique, that is used in
wireless communication system. 1G (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
2G. 3G network supports voice calling, mobile internet,
video calls, and it uses CDMA based standards.

In wireless
communication 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.

A cellular
system divides any given area into cells where a mobile unit in each cell
communicates with a base station. The main aim in the cellular system design is
to be able 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)

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

 

Narrow-band Systems

Systems operating with channels substantially narrower than
the coherence bandwidth are called as Narrow band systems. 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 on a
single channel.

Wide-band Systems

In 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 within a wide-band channel, 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 frequencies. This division is done between non
overlapping frequencies that are then assigned to each communicating pair. FDMA is used mainly for analog 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 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, just imagine how FM
radio works. All the radios have their own frequency bands and they send their
signals at the carefully allocated unique frequencies within the available
bands.

 

 

The best
example of this is the cable television system. The medium is a single coax
cable that is used to broadcast hundreds of channels of video/audio programming
to homes. 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 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, fiber optic FDMA is called wavelength division
multiple access (WDMA) or just wavelength division multiplexing (WDM).

One of the
older FDMA systems is the original analog telephone system, which used 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.

n  Channel
assignment is carried out on a first-come first- served basis. ?

n  The
number of channels, given a frequency spectrum BT , depends on the modulation
technique and the guard bands between the channels.

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

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

n  Channel
bandwidth is relatively narrow (30kHz) ?

n  Simple
algorithmically, and from a hardware standpoint ?

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

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


No restriction regarding
the type of baseband or type of modulation

 

1.1.3 Disadvantages-

 ?

n  The
presence of guard bands ?

n  Requires
right RF filtering 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 in an
order 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. CDMA is most used in the USA.


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


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


Multiple simultaneous transmissions are separated using coding theory.


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 CDMA systems. 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 time basis.  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 rapid 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 station’s radio component to listen and broadcast only in
its assigned time slot. During the remaining time period, the mobile 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

n  Non-continuous
transmission makes hand off
simpler

n  Slots
can be assigned on demand in dynamic TDMA

n  Higher
synchronization overhead than CDMA

n  Advanced
equalization is necessary for high data rates

n  Frequency/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, it ensures that there will be no
interference

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.

 

 

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