introduction to umts[compatibility mode]
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7/29/2019 Introduction to UMTS[Compatibility Mode]
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Universal Mobile Telecommunication System
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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
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• User Equipment (UE)
• Access Network (AN) – UMTS Terrestrial Radio Access Network (UTRAN)
• Core Network (CN)
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•
– 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
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• 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
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• 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.
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CHANNELS
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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.
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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.
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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
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CHANNELIZATION CODES
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USE OF CHANNELIZATION CODES
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MULTIUSER SPREADING
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CODE TYPES IN THE AIR INTERFACE
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STANDARD WCDMA PARAMETERS
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VARIABLE DATA RATES IN UMTS
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UMTS - 2 MBPS DATA RATE
• 3G systems must be able to support 2 Mbps
over-the-air interface.
BUT How ?????
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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.
’• , .
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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.
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MODES OF OPERATION
•
the W-CDMA air interface. – Frequency Division Duplex (FDD)
– Time Division Duplex (TDD)
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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.
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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.
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WCDMA POWER CONTROL
• ,
results to support a higher number of users inthe RF spectrum.
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WCDMA POWER CONTROL- CLOSED LOOP
MODE
• There are two functions within closed loo ower control:
– Inner Loop – Outer Loop
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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.
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INNER & OUTER LOOP POWER
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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.
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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.
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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.
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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.
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SOFT HANDOVER IN WCDMA
• 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.
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SOFT HANDOVER IN WCDMA
• 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.
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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.
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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
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