3G Cellular Systems

7/31/2019 3G Cellular Systems

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3G Cellular Systems:

Design Issues on the Air Interface

Presented to:

Dr. Oussama BaziDone by:

Nabil Maher


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Outline

1. Introduction

2. Evolution of generations relative to design techniques

3. Towards the 3G

4. operational constraints facing 3G systems

5. Problems that impose constraints on the physical layer

6. Air Interface Solutions

7. Conclusion


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Introduction

3G cellular systems are starting to be introduced worldwide,

offering much higher transmission rates for users and much

better flexibility and integration in service provisioning.

Despite the increase in the performances of successivegeneration of cellular systems, same problems in radio

communications applied to new services requiring higher

performances continued to exist.

The basic concepts of cellular systems are essentially two:

Frequency Reuse

Handover


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Evolution of generations relative to design

techniques

In 1G Systems:

FDMA technique was used

Each user channel used analog

FM modulation

The service provided was forvoice only

The architecture was based on

circuit switching only

The service quality was low to

medium

There was no worldwide standard


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Evolution of generations (Continued)

In 2G Systems: there was a major improvement in

communication quality due to the use of

digital techniques for communications.

In the first phase of 2G systems:

GSM networks and IS-95 networks

where deployed. voice and limited data using circuit

switching techniques were offered.

GSM Networks used:

a combination of FDMA and TDMA

techniques.

a channel BWof 200 KHz and eighttimeslots in each frame.

In IS-95:

CDMA technique was used

a bandwidthof 1.25MHz at a chip

rateof 1.25Mcps.


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Towards the 3G

In following phases of GSM development, packet switching was introducedfor theGPRS andEDGE with much higher rates for users.

In 3G systems:

requirements for high data rate transmission are specified, and rates ashigh as 2Mbps over a bandwidth of 5MHz must be possible.

Integrated services must be offered to single users, permitting multiplesessions at the same time for any user.

The CDMA technique with a wide spreading bandwidth have beennormalized for these systems.

the design must be adapted to the broadband ISDN and the internet,offering similar services and equivalent quality.

The types of services considered for these systems are very wide, andinclude voice, video, multimedia and high data rate services.


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Factors that impose constraints on the physical layer:

Area coverage:

The parameter of interest in this case is the mean value of the transmitted signal

in any point of a cell.

It is dependent on many factors (geographical distance and path loss exponent,major obstacles, presence of water area, trees, antennas height, streetorientation, in-building, propagation inside tunnels, etc)

Doppler and multipath effects: they are related to the small scale variations of the signal.

The Doppler Effect is a function of the speed of a mobile; it shows that thevariations are much faster at high speed than at low speed.

The multipath effect results from the reception of a very big number of waves forthe same signal, coming from different directions and with different amplitudesand phases.

combination of these waves may result in a received signal envelope thatexperiences a wide range of variations, of the order of 40dB.


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Factors imposing the constraints(Continued)

Selective fading:

it is one particular effect when the

bandwidth of the transmitted signal is

much greater than the coherence

bandwidth of the channel.

It is due to the delay following different

paths of propagation, which will cause a

spreading in time of the received signal.

For an example in the UMTS system, a

chip duration is ~ 0.25s, while the

channel impulse response may be spread

over a few s.


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The near-far problem:

problem of fixing the transmission of all users to the minimum level that willensure that the QoS requirements are met.

One criterion to minimize interference in the system is to equalize the receivedpower of all users transmitting with the same rate and using the same service.

it is then essential that a user having a higher radio distance transmit at ahigher power than one that has a smaller one.


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Interference limitations in CDMA:

Depending on:

the number of interfering users (both in-cell and out-of-cell)

the effect of selective fading

the fast variations due to the channel

the compensating techniques that are used (power control and

diversity)

Time-varying channel characteristics :

The mobile radio channel is known for its variation in space and time.

This results into a situation that all constraints can be characterized onlystatistically and follow stochastic processes.

An estimation of the channel for each user at regular interval in time isessential.


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Different QoS requirements:

It is possible to have at the same time users having different

QoS requirements, in terms of the required SNR, although the

interference is ~ similar for all users in a cell in CDMA.

The target received power for users with different QoS will be

different from each other.

It is necessary that the system be able to handle all users askingfor different services.


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Air Interface Solutions Interleaving and channel coding:

both work impair in the transmission and reception chain to improve theperformance of digital modulation.

Interleaving randomizes the burst of errors that are a characteristic of multipatheffect, and channel coding corrects or detects the errors.

Their role is to combat the multipath effect by correcting the errors due to the fast

fading and the limitation of interference in CDMA systems by lowering therequired SNR.

Equalization or time diversity:

it is usually done by means of a RAKE receiver which is equivalent to a structurethat exploits the intrinsic characteristic of the channel response to the wideband

spread signal.

It achieves time diversity for the fading signals, and reduces the variationsaround the mean of the received signals.

Its role is to combat the effects of selective fading and the multipath and Dopplereffects.


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Air Interface Solutions

Channel estimation:

It is done by transmitting a known sequence to estimate the channelparameters, which are then fed to the RAKE receiver for correctdetection of symbols.

In UMTS, a dedicated channel called DPCCH (fordedicated physicalcontrol channel) is used by all mobiles to estimate the channel.

The role of this function is to compensate the time and space variationsof the channel arriving to a mobile, and is included in the transmissionand reception chain.

Power control:

This is an absolute process in all CDMA systems, and its role is tocontrol the transmit power in order to receive the minimum requiredreceived power at the receiver.


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Power Control (Continued)

The logic of the power control is tocompare the received power orthe SIR to a certain level:

if it is above, the transmit

power is decreased by a fixedstep.

if it is below, it is increased bythe same fixed step.

the performance of the powercontrol module also depend onthe speed of the mobiles, and thehigher the speed, the faster thevariations and the poorer theperformance.


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CONCLUSION The design issues of equipments depend on the air interface,

because the major constraints in the system come from there.

The nature of the services in 3G systems is very demanding, andput additional constraints on the expected performance on the airinterface.

The specifications for 3G cellular systems are now attaining maturity,in terms of air interface, service definitions, and logical channelsperforming specific functions for cellular systems.

The specifications have covered all basic compensating techniques,however they leave room for future improvements by more complex

designs such as smart antennas, interference cancellation, etc

Other work and improvements not covered by standardization areabout resource allocation algorithms and call admission handlingdifferent types of services with differing QoS requirements.


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Thanks for your attention !

3G Cellular Systems

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