Introduction to UMTS[Compatibility Mode]

7/29/2019 Introduction to UMTS[Compatibility Mode]

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Universal Mobile Telecommunication System



2.5 Generation Mobile Networks

• High Speed Circuit Switched Data (HSCSD)

– 9.6 Kbps or 14.4 Kbps (× upto 4 Slots) => 57.6 Kbps

ax.

– Advantage: minimum change in n/w architecture

– Disadvanta e: inefficient in terms of resource usa e

• General Packet Radio Services (GPRS) – Uses 1 to 8 radio channels in the 200 KHz frequency

– Offer speed upto 115 Kbps

• Enhanced Data Rate for Global Evolution (EDGE)

– Enhanced GPRS

– Better modulation techniques

– Speed up to 384Kbps



• User Equipment (UE)

• Access Network (AN) – UMTS Terrestrial Radio Access Network (UTRAN)

• Core Network (CN)



•

– Universal Subscriber Identity Module (USIM)

•

• Permanent identity (IMSI) of user, shared secret

key, phone book.

– Mobile Equipment (ME)

• Mobile Terminal

– Radio Transmission Termination, authentication, MM

• Terminal Equipment (TE)

– . . , ,

camera and user display



• Access Network

– resides between the UE and the Core Network – performs functions specific to the WCDMA air

interface

• Access Network in UMTS allows different types of

network-

– BSS ………….legacy of GSM era

– RNS………….newly standardized access network



• The core network in WCDMA may be used with any

access ec n que.

• This functionality split between the core network and theaccess network provides flexibility to keep the corene wor xe , w e a e same me a ow ng or different access techniques.

• Functions performed by core network includes- – Mobility Management

– Call Control

– Switchin

– Session Management – Routing

–

– Equipment Identification – Etc.



CHANNELS



SPREADING PROCEDURE IN THE

• The spreading procedure in the UTRAN consists of two separate

– Channelization• uses orthogonal codes

– Scrambling.• uses PN codes

• Channelization occurs before scrambling in the transmitter both inthe u link and the downlink.



CHANNELIZATION

• Channelization transforms each data s mbol into

multiple chips. This ratio (number of chips/symbol) iscalled the spreading factor (SF).

• us, s s proce ure a ac ua y expan s e s gna

bandwidth.

• Channelization codes are ortho onal codes more

precisely, orthogonal variable spreading factor [OVSF]

codes), meaning that in an ideal environment they don’t

.• However, orthogonally requires that the codes be time

synchronized.



SCRAMBLING

• The orthogonal codes alone cannot handle thesprea ng unc on

• seudorandom codes are used in the second art of thespreading procedure,

• which is called the scramblin sta e.

• already spread is further combined (XORed) with a.

• There are millions of scrambling codes available in the



CHANNELIZATION CODES



USE OF CHANNELIZATION CODES



MULTIUSER SPREADING



CODE TYPES IN THE AIR INTERFACE



STANDARD WCDMA PARAMETERS



VARIABLE DATA RATES IN UMTS



UMTS - 2 MBPS DATA RATE

• 3G systems must be able to support 2 Mbps

over-the-air interface.

BUT How ?????



UMTS - 2 Mbps DATA RATE

• As we have seen that with a fixed chip rate of 3.84 Mcps,

, , ,

4, we can only get to 480 kbps raw data rate.• Once you account for overhead information, like power

control commands, it is less than that.

• There are two other factors that allow us to meet our

.

– The first is the modulation technique used in W-CDMA:

Quadrature Phase Shift Keying (QPSK)

• QPSK allows the transmitter to send two bits of information (in thiscase, symbols) for every modulated bit transmitted over-the-air. So,

with QPSK, the data rate of the physical channel is doubled.

’• , .



UMTS - 2 MBPS DATA RATE

• With QPSK, and the minimum amount of protection and

spreading, our raw user data rate is up to 960 kbps.

• In order to achieve the goal of 2 Mbps for a single user,

.

• With two channelization codes, a raw data rate of 1920

kb s is achievable.

• The maximum single user configuration in the downlink

allows three channelization codes to be assigned to one

u .• Note that this is the raw channel data rate. With control

or overhead information taken into account the actual

user data rate is a little over 2 Mbps.



MODES OF OPERATION

•

the W-CDMA air interface. – Frequency Division Duplex (FDD)

– Time Division Duplex (TDD)



MODES OF OPERATION

• Most of the initial deployments in UMTS are expected toe - n mo e, o owe a er y ep oymen

of the TDD mode.

• FDD mode provides the capability to provide a widerange of services including voice, multimedia and all

types of packet data applications.

• TDD may be used to deal with asymmetric traffic as wellas relieving hot spots in traffic.

• TDD is also useful in cases where spectrum is notavailable for a channel in both the uplink and downlink.



WCDMA POWER CONTROL

• In a W-CDMA s stem the rocess of the UE and the

UTRAN telling each other to speak up and quiet down is

called power control.

• Why do we need power control in the W-CDMA system?

– ’ ’ .

– In order to combat this interference ower control is

introduced.



WCDMA POWER CONTROL

• ,

results to support a higher number of users inthe RF spectrum.



WCDMA POWER CONTROL- CLOSED LOOP

MODE

• There are two functions within closed loo ower control:

– Inner Loop – Outer Loop



POWER CONTROL MODE - INNER LOOP

• In inner loop power control mode the receiver continuouslymeasures the strength of received signal and instructs the

.

• Inner loop power control can run as fast as 1500 cycles per secondin UMTS.

• Inner loop provides the ability for the transmitter to track thecons an y c ang ng con ons.



INNER & OUTER LOOP POWER



DOWNLINK POWER CONTROL

• Downlink power control is concerned with controlling the transmitower of the Node B.

• Both the inner loop and outer loop functions are implemented in the

UE. Therefore, the power allocated by the Node B to each individualUE is controlled by the UE.

, ,individual closed loop power control functions in operationcontrolling the aggregate user transmit power at the Node B.



UPLINK POWER CONTROL

• Uplink power control is concerned with controlling theransm power o e .

• In the UTRAN, the inner loop and outer loop functionsare split between the Node B and the RNC.

• The Node B is closest to the air interface, so it handlesthe inner loop received signal measurements and fast

feedback.• The RNC handles the outer loop function.



POWER & CAPACITY

• There is a direct relationship between power allocated to a singleuser and the capacity of the system.

• For instance, in the downlink, a Node B has a finite amount of power v u u .

• If each user requires more power, the capacity, or number of userssupported, is reduced!

• Conversel if ower can be mana ed effectivel the ca acit of the system can be increased. This is a critical function in theoptimization of a UMTS system, and is difficult to perfect since RFconditions vary greatly.



SOFT HANDOVER IN WCDMA

• WCDMA uses soft Handover

• Soft handover is based on make-before-break approach

break-before-make approach & used in TDMA systems.

• MAKE BEFORE BREAK approach

– UE establishes connection with a new Node B beforebreakin communication with the current servin

Node B.




• When soft handover is active the UE is connected to

multiple Node Bs – up to six at once.

• The RNC multicasts the data to all of the active Node Bs,

eac o w c rans ers e a a over- e-a r o e .

The UE combines the signals from all of the Node Bs

resulting in a combined signal that has greater qualityrelative to that from any individual Node B.

• When a UE transmits over-the-air, each of the active

packet to the RNC. The RNC selects the best packet

from all received and discards the rest.




• Soft handover presents a unique requirement for the design of the- rece ver w t respect to t e an o e .

• The key aspects of W-CDMA that come into play in the design of thereceiver structure include:

– ransm ers s are a common z ra o c anne

– each transmission is uniquely identified by a scrambling codeand channelization code combination

– simultaneously

• The Rake receiver is the receiver used in WCDMA to support theabove re uirements.

– It is named so because it contains “fingers,” each of which areused in communication with a Node B.



RAKE RECEIVER

• A RAKE receiver is made of correlators, also known as RAKEfingers, each receiving a multipath signal.

• er esprea ng y corre a ors w a oca copy o eappropriately delayed version of the transmitter’s spreading code,the signals are combined.

• Since the received multipath signals are fading independently, thismethod improves the overall combined signal quality and

performance.



RAKE RECEIVER

• The benefits of Rake receiver operations include:

– higher reliability since while one Node Bs signal maydegrade, the connection to another Node B may be

– smoother handoff versus hard, or break-before-make,handover. There is never a time when data is not

,muting due to the UE retuning to another frequency

– increased spectral efficiency since the UE can

com ne e s gna rom a ac ve o e s, so esspower is required from each

Introduction to UMTS[Compatibility Mode]

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