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1. 3G refers to the third generation of developments in wireless technology,

especially mobile communications.

Standard 3G

International Telecommunications Unit (ITU): IMT-2000 consists of five radio interfaces

W-CDMA

CDMA2000

TD-CDMA / TD-SCDMA

UWC-136

DECT+

UMTS WCDMA specification summary

the UMTS WCDMA system offered a significant improvement in capability over the previous 2Gservices.

Parameter Specification

Data rate2048 kbps low range384 kbps urban and outdoor

RF channel bandwidth 5 MHz

Multiple access scheme CDMA

Duplex schemes FDD and also TDD

UMTS / WCDMA Network Architecture

- the UMTS / WCDMA network architecture including the User Equipment - UE -, the Radio Network

Subsystem, and the Core Network.

UMTS WCDMA tutorial includes:

UMTS WCDMA Tutorial

UMTS 3G history

UMTS WCDMA network architecture

UMTS / WCDMA radio or air interface

CDMA multiple access technology

UMTS / WCDMA modulation schemes

UMTS WCDMA channels UMTS TDD

TD-SCDMA

UMTS WCDMA handover / handoff

The UMTS network architecture is required to provide a greater level of performance to that of the originalGSM network. However as many networks had migrated through the use of GPRS and EDGE, theyalready had the ability to carry data. Accordingly many of the elements required for the WCDMA / UMTS

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network architecture were seen as a migration. This considerably reduced the cost of implementing theUMTS network as many elements were in place or needed upgrading.

With one of the major aims of UMTS being to be able to carry data, the UMTS network architecture wasdesigned to enable a considerable improvement in data performance over that provided for GSM.

UMTS network constituents

The UMTS network architecture can be divided into three main elements:

1. User Equipment (UE): The User Equipment or UE is the name given to what was previous

termed the mobile, or cellphone. The new name was chosne because the considerably greaterfunctionality that the UE could have. It could also be anything between a mobile phone used fortalking to a data terminal attached to a computer with no voice capability.

2. Radio Network Subsystem (RNS): The RNS is the equivalent of the previous Base Station

Subsystem or BSS in GSM. It provides and manages the air interface fort he overall network.

3. Core Network: The core network provides all the central processing and management for thesystem. It is the equivalent of the GSM Network Switching Subsystem or NSS.

The core network is then the overall entity that interfaces to external networks including the public phonenetwork and other cellular telecommunications networks.

UMTS Network Architecture Overview

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User Equipment, UE

The USER Equipment or UE is a major element of the overall UMTS network architecture. It forms thefinal interface with the user. In view of the far greater number of applications and facilities that it canperform, the decision was made to call it a user equipment rather than a mobile. However it is essentiallythe handset (in the broadest terminology), although having access to much higher speed data

communications, it can be much more versatile, containing many more applications. It consists of avariety of different elements including RF circuitry, processing, antenna, battery, etc.

There are a number of elements within the UE that can be described separately:

UE RF circuitry: The RF areas handle all elements of the signal, both for the receiver and forthe transmitter. One of the major challenges for the RF power amplifier was to reduce the powerconsumption. The form of modulation used for W-CDMA requires the use of a linear amplifier.These inherently take more current than non linear amplifiers which can be used for the form ofmodulation used on GSM. Accordingly to maintain battery life, measures were introduced intomany of the designs to ensure the optimum efficiency.

Baseband processing: The base-band signal processing consists mainly of digital circuitry.This is considerably more complicated than that used in phones for previous generations. Again

this has been optimised to reduce the current consumption as far as possible. Battery: While current consumption has been minimised as far as possible within the circuitry of

the phone, there has been an increase in current drain on the battery. With users expecting thesame lifetime between charging batteries as experienced on the previous generation phones, thishas necessitated the use of new and improved battery technology. Now Lithium Ion (Li-ion)batteries are used. These phones to remain small and relatively light while still retaining or evenimproving the overall life between charges.

Universal Subscriber Identity Module, USIM: The UE also contains a SIM card, although inthe case of UMTS it is termed a USIM (Universal Subscriber Identity Module). This is a moreadvanced version of the SIM card used in GSM and other systems, but embodies the same typesof information. It contains the International Mobile Subscriber Identity number (IMSI) as well asthe Mobile Station International ISDN Number (MSISDN). Other information that the USIM holdsincludes the preferred language to enable the correct language information to be displayed,

especially when roaming, and a list of preferred and prohibited Public Land Mobile Networks(PLMN).

The USIM also contains a short message storage area that allows messages to stay with the usereven when the phone is changed. Similarly "phone book" numbers and call information of thenumbers of incoming and outgoing calls are stored.

The UE can take a variety of forms, although the most common format is still a version of a "mobilephone" although having many data capabilities. Other broadband dongles are also being widely used.

UMTS Radio Network Subsystem

This is the section of the UMTS / WCDMA network that interfaces to both the UE and the core network.The overall radio access network, i.e. collectively all the Radio Network Subsystem is known as theUTRAN UMTS Radio Access Network.

The Radio Network Subsystem comprises two main components:

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Radio Network Controller, RNC: This element of the radio network subsystem controls theNode Bs that are connected to it. The RNC undertakes the radio resource management andsome of the mobility management functions, although not all. It is also the point at which the dataencryption / decryption is performed to protect the user data from eavesdropping.

Node B: Node B is the term used within UMTS to denote the base station transceiver. Itcontains the transmitter and receiver to communicate with the UEs within the cell.

In order to facilitate effective handover between Node Bs under the control of different RNCs, the RNCnot only communicates with the Core Network, but also with neighbouring RNCs.

UMTS Radio Network Subsystem Architecture

UMTS Core Network

The UMTS core network architecture is a migration of that used for GSM with further elements overlaid to

enable the additional functionality demanded by UMTS.

In view of the different ways in which data may be carried, the UMTS core network may be split into twodifferent areas:

Circuit switched elements: These elements are primarily based on the GSM network entitiesand carry data in a circuit switched manner, i.e. a permanent channel for the duration of the call.

Packet switched elements: These network entities are designed to carry packet data. Thisenables much higher network usage as the capacity can be shared and data is carried as packetswhich are routed according to their destination.

Some network elements, particularly those that are associated with registration are shared by bothdomains and operate in the same way that they did with GSM.

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UMTS Core Network

Circuit switched elementsThe circuit switched elements of the UMTS core network architecture include the following networkentities:

Mobile switching centre (MSC): This is essentially the same as that within GSM, and itmanages the circuit switched calls under way.

Gateway MSC (GMSC): This is effectively the interface to the external networks.

Packet switched elementsThe packet switched elements of the UMTS core network architecture include the following networkentities:

Serving GPRS Support Node (SGSN): As the name implies, this entity was first developedwhen GPRS was introduced, and its use has been carried over into the UMTS networkarchitecture. The SGSN provides a number of functions within the UMTS network architecture.

o Mobility management When a UE attaches to the Packet Switched domain of the UMTS

Core Network, the SGSN generates MM information based on the mobile's currentlocation.

o Session management: The SGSN manages the data sessions providing the required

quality of service and also managing what are termed the PDP (Packet data Protocol)

contexts, i.e. the pipes over which the data is sent.o Interaction with other areas of the network: The SGSN is able to manage its elements

within the network only by communicating with other areas of the network, e.g. MSC andother circuit switched areas.

o Billing: The SGSN is also responsible billing. It achieves this by monitoring the flow of

user data across the GPRS network. CDRs (Call Detail Records) are generated by theSGSN before being transferred to the charging entities (Charging Gateway Function,CGF).

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Gateway GPRS Support Node (GGSN): Like the SGSN, this entity was also first introducedinto the GPRS network. The Gateway GPRS Support Node (GGSN) is the central element withinthe UMTS packet switched network. It handles inter-working between the UMTS packet switchednetwork and external packet switched networks, and can be considered as a very sophisticatedrouter. In operation, when the GGSN receives data addressed to a specific user, it checks if theuser is active and then forwards the data to the SGSN serving the particular UE.

Shared elementsThe shared elements of the UMTS core network architecture include the following network entities:

Home location register (HLR): This database contains all the administrative information abouteach subscriber along with their last known location. In this way, the UMTS network is able toroute calls to the relevant RNC / Node B. When a user switches on their UE, it registers with thenetwork and from this it is possible to determine which Node B it communicates with so thatincoming calls can be routed appropriately. Even when the UE is not active (but switched on) itre-registers periodically to ensure that the network (HLR) is aware of its latest position with theircurrent or last known location on the network.

Equipment identity register (EIR): The EIR is the entity that decides whether a given UEequipment may be allowed onto the network. Each UE equipment has a number known as the

International Mobile Equipment Identity. This number, as mentioned above, is installed in theequipment and is checked by the network during registration.

Authentication centre (AuC) : The AuC is a protected database that contains the secret keyalso contained in the user's USIM card.

UMTS / WCDMA radio air interface

- the air interface, frequencies and power control used within UMTS or Wideband CDMA, WCDMA,

cellular telecommunications system

Physical layer within UMTS / WCDMA is totally different to that employed by GSM. It employs a spreadspectrum transmission in the form of CDMA rather than the TDMA transmissions used for GSM.Additionally it currently uses different frequencies to those allocated for GSM.

UMTS Uplink and Downlink

When looking at the radio air interface and its associated properties, it is necessary to define thedirections in which the transmissions are occurring. Being a full duplex system, i.e. transmittingsimultaneously in both directions, it is necessary to be able to define which direction is which.

Uplink; This may also sometimes be known as the reverse link, and it is the link from the UserEquipment (UE) to the Node B or base station.

Downlink; This may also sometimes be known as the forward link, and it is the link from theNode B or base station to the User Equipment (UE).

The terms Uplink and Downlink are the terms that are used with UMTS, and especially within Europe. Theterms forward link and reverse link are more commonly used with the CDMA2000 technologies and alsowithin North America.

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Uplink and downlink transmission directions

UMTS frequencies

There are currently six bands that are specified for use for UMTS / WCDMA although operation on otherfrequencies is not precluded. However much of the focus for UMTS is currently on frequency allocationsaround 2 GHz. At the World Administrative radio Conference in 1992, the bands 1885 - 2025 and 2110 -2200 MHz were set aside for use on a world wide basis by administrations wishing to implementInternational Mobile Telecommunications-2000 (IMT-2000). The aim was that allocating spectrum on aworld wide basis would facilitate easy roaming for UMTS / WCDMA users.

Within these bands the portions have been reserved for different uses:

1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paired uplink anddownlink, channel spacing is 5 MHz and raster is 200 kHz. An Operator needs 3 - 4 channels(2x15 MHz or 2x20 MHz) to be able to build a high-speed, high-capacity network.

1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA) Unpaired, channelspacing is 5 MHz and raster is 200 kHz. Transmit and receive transmissions are not separated in

frequency. 1980-2010 and 2170-2200 MHz Satellite uplink and downlink.

UMTS carrier frequencies are designated by a UTRA Absolute Radio Frequency Channel Number(UARFCN). This can be calculated from:

UARFCN = 5 x (frequency in MHz)

UMTS uses wideband CDMA as the radio transport mechanism. The UMTS channels are spaced by 5MHz.

Synchronisation

The level of synchronisation required for the WCDMA system to operate is provided from the PrimarySynchronisation Channel (P-SCH) and the Secondary Synchronisation Channel (S-SCH). Thesechannels are treated in a different manner to the normal channels and as a result they are not spreadusing the OVSFs and PN codes. Instead they are spread using synchronisation codes. There are twotypes that are used. The first is called the primary code and is used on the P-SCH, and the second isnamed a secondary code and is used on the S-SCH.

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The primary code is the same for all cells and is a 256 chip sequence that is transmitted during the first256 chips of each time slot. This allows the UE to synchronise with the base station for the time slot.

Once the UE has gained time slot synchronisation it only knows the start and stop of the time slot, but itdoes not know information about the particular time slot, or the frame. This is gained using the secondarysynchronisation codes.

There is a total of sixteen different secondary synchronisation codes. One code is sent at the beginning ofthe time slot, i.e. the first 256 chips. It consists of 15 synchronisation codes and there are 64 differentscrambling code groups. When received, the UE is able to determine before which synchronisation codethe overall frame begins. In this way the UE is able to gain complete synchronisation.

The scrambling codes in the S-SCH also enable the UE to identify which scrambling code is being usedand hence it can identify the base station. The scrambling codes are divided into 64 code groups, eachhaving eight codes. This means that after achieving frame synchronisation, the UE only has a choice ofone in eight codes and it can therefore try to decode the CPICH channel. Once it has achieved this it isable to read the BCH information and achieve better timing and it is able to monitor the P-CCPCH.

UMTS power control

As with any CDMA system it is essential that the base station receives all the UEs at approximately thesame power level. If not, the UEs that are further away will be lower in strength than those closer to thenode B and they will not be heard. This effect is often referred to as the near-far effect. To overcome thisthe node B instructs those stations closer in, to reduce their transmitted power, and those further away toincrease theirs. In this way all stations will be received at approximately the same strength.

It is also important for node Bs to control their power levels effectively. As the signals transmitted by thedifferent node Bs are not orthogonal to one another it is possible that signals from different ones willinterfere. Accordingly their power is also kept to the minimum required by the UEs being served.

To achieve the power control there are two techniques that are employed: open loop; and closed loop.

Open loop techniques are used during the initial access before communication between the UE and nodeB has been fully established. It simply operates by making a measurement of the received signal strengthand thereby estimating the transmitter power required. As the transmit and receive frequencies aredifferent, the path losses in either direction will be different and therefore this method cannot be any morethan a good estimate.

Once the UE has accessed the system and is in communication with the node B, closed loop techniquesare used. A measurement of the signal strength is taken in each time slot. As a result of this a powercontrol bit is sent requesting the power to be stepped up or down. This process is undertaken on both theup and downlinks. The fact that only one bit is assigned to power control means that the power will be

continually changing. Once it has reached approximately the right level then it would step up and thendown by one level. In practice the position of the mobile would change, or the path would change as aresult of other movements and this would cause the signal level to move, so the continual change is not aproblem.

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Note on CDMA:

CDMA, Code Division Multiple Access, is a multiple access scheme used by many 3G cellulartechnologies, and other forms of wireless technology. It uses a process called Direct Sequence SpreadSpectrum where spreading codes are used to spread a signal out over a given bandwidth and thenreconstituting the data in the receiver by using the same spreading code. By supplying different spreading

codes to different users, several users are able to utilises the same frequency without mutualinterference.

Click on the link for aCDMA tutorial

UMTS CDMA format

The data to be transmitted is encoded using a spreading code particular to a given user. In this way onlythe desired recipient is able to correlate and decode the signal, all other signals appearing as noise. Thisallows the physical RF channel to be used by several users simultaneously.

The data of a CDMA signal is multiplied with a chip or spreading code to increase the bandwidth of thesignal. For WCDMA, each physical channel is spread with a unique and variable spreading sequence.The overall degree of spreading varies to enable the final signal to fill the required channel bandwidth. Asthe input data rate may vary from one application to the next, so the degree of spreading needs to bevaried accordingly.

For the downlink the transmitted symbol rate is 3.84 M symbols per second. As the form of modulationused is QPSK this enables two bits of information to be transmitted for every symbol, thereby enabling amaximum data rate of twice the symbol rate or 7.68 Mbps. Therefore if the actual rate of the data to betransmitted is 15 kbps then a spreading factor of 512 is required to bring the signal up to the required chiprate for transmission in the required bandwidth. If the data to be carried has a higher data rate then alower spreading rate is required to balance this out. It is worth remembering that altering the chip rate

does alter the processing gain of the overall system and this needs to be accommodated in the signalprocessing as well. Higher spreading factors are more easily correlated by the receiver and therefore alower transmit power can be used for the same symbol error rate.

The codes required to spread the signal must be orthogonal if they are to enable multiple users andchannels to operate without mutual interference. The codes used in W-CDMA are Orthogonal VariableSpreading Factor (OVSF) codes, and they must remain synchronous to operate. As it is not possible toretain exact synchronisation for this, a second set of scrambling codes is used to ensure that interferencedoes not result. This scrambling code is a pseudo random number (PN) code. Thus there are two stagesof spreading. The first using the OSVF code and the second using a scrambling PN code. These codesare used to provide different levels of separation. The OVSF spreading codes are used to identify theuser services in the uplink and user channels in the downlink whereas the PN code is used to identify theindividual node B or UE.

On the uplink there is a choice of millions of different PN codes. These are processed to include amasked individual code to identify the UE. As a result there are more than sufficient codes toaccommodate the number of different UEs likely to access a network. For the downlink a short code isused. There are a total of 512 different codes that can be used, one of which will be assigned to eachnode B.

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UMTS / WCDMA modulation

- tutorial, or overview of the basics of the UMTS, WCDMA modulation formats or schemes used

The modulation schemes used within the CDMA signal format have been chosen to optimise theperformance of the overall UMTS, WCDMA system. One major driver that influenced the choice of theUMTS modulation formats was experience gained from 2G systems. By using a careful choice ofmodulation scheme it has been possible to overcome the problems experienced in 2G. However theremany other requirements that need to be met in addition to this.

UMTS modulation schemes

There are several considerations that were taken into account when making the choice for the overallformat for the UMTS WCDMA modulation formats. Some of the considerations were:

It is necessary to ensure that the data is carried efficiently over the available spectrum, andtherefore maximum use is made of the available spectrum, and hence the capacity of the systemis maximised.

The modulation scheme should be chosen to ensure that the efficiency of the RF power amplifierin the handset or UE is made as high as possible. By enabling the power amplifier to bemaximised, less battery power is consumed for the same transmitted power. As battery power isof particular importance to users, this is a key requirement.

The modulation format should be chosen to avoid the audio interference caused to many nearbyelectronics equipment resulting from the pulsed transmission format used on many 2G systemssuch as GSM

As the uplink and downlink have different requirements, the exact format for the modulation format usedon either direction is slightly different.

UMTS modulation schemes for both uplink and downlink, although somewhat different are both basedaround phase shift keying formats. This provides many advantages over other schemes that could beused in terms of spectral efficiency and other requirements.

Note on PSK:

Phase shift Keying, PSK is a form of modulation used particularly for data transmissions. If offers aneffective way of transmitting data. By altering the number of different phase states which can be adopted,the data speeds that can be achieved within a given channel can be increased, but at the cost of lowerresilience to noise an interference.

Click on the link for aPSK tutorial

Downlink modulation

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The UMTS modulation format for the downlink is more straightforward than that used in the uplink. Thedownlink uses quadrature phase shift keying, QPSK.

The QPSK modulation used in the downlink is used with time-multiplexed control and data streams. Whiletime multiplexing would be a problem in the uplink, where the transmission in this format would give riseto interference in local audio systems, this is not relevant for the downlink where the NodeB is sufficiently

remote from any local audio related equipment to ensure that interference is not a problem.

Uplink modulation

However the uplink uses two separate channels so that the cycling of the transmitter on and off does notcause interference on the audio lines, a problem that was experienced on GSM. The dual channels (dualchannel phase shift keying) are achieved by applying the coded user data to the I or In-phase input to theDQPSK modulator, and control data which has been encoded using a different code to the Q orquadrature input to the modulator.

UMTS / WCDMA channels

- tutorial, or overview of the basics of UMTS / WCDMA logical, physical and transport channels.

There are many UMTS channells that are used within the UMTS system. The data carried by theUMTS / WCDMA transmissions is organised into frames, slots and channels. In this way all the payloaddata as well as the control and status data can be carried in an efficient manner.

UMTS uses CDMA techniques (as WCDMA) as its multiple access technology, but it additionally usestime division techniques with a slot and frame structure to provide the full channel structure.

A channel is divided into 10 ms frames, each of which has fifteen time slots each of 666 microsecondslength. On the downlink the time is further subdivided so that the time slots contain fields that containeither user data or control messages.

On the uplink dual channel modulation is used so that both data and control are transmittedsimultaneously. Here the control elements contain a pilot signal, Transport Format Combination Identifier(TFCI), FeedBack Information (FBI) and Transmission Power Control (TPC).

The channels carried are categorised into three: logical, transport and physical channels. The logicalchannels define the way in which the data will be transferred, the transport channel along with the logicalchannel again defines the way in which the data is transferred, the physical channel carries the payloaddata and govern the physical characteristics of the signal.

The channels are organised such that the logical channels are related to what is transported, whereas the

physical layer transport channels deal with how, and with what characteristics. The MAC layer providesdata transfer services on logical channels. A set of logical channel types is defined for different kinds ofdata transfer services.

Logical Channels:

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Broadcast Control Channel (BCCH), (downlink). This channel broadcasts information to UEs relevant tothe cell, such as radio channels of neighbouring cells, etc.

Paging Control Channel (PCCH), (downlink). This channel is associated with the PICH and is used forpaging messages and notification information.

Dedicated Control Channel (DCCH), (up and downlinks) This channel is used to carry dedicated controlinformation in both directions.

Common Control Channel (CCCH), (up and downlinks). This bi-directional channel is used to transfercontrol information.

Shared Channel Control Channel (SHCCH), (bi-directional). This channel is bi-directional and only foundin the TDD form of WCDMA / UMTS, where it is used to transport shared channel control information.

Dedicated Traffic Channel (DTCH), (up and downlinks). This is a bidirectional channel used to carry userdata or traffic.

Common Traffic Channel (CTCH), (downlink) A unidirectional channel used to transfer dedicated userinformation to a group of UEs.

Transport Channels:

Dedicated Transport Channel (DCH), (up and downlink). This is used to transfer data to a particular UE.Each UE has its own DCH in each direction.

Broadcast Channel (BCH), (downlink). This channel broadcasts information to the UEs in the cell toenable them to identify the network and the cell.

Forward Access Channel (FACH),(down link). This is channel carries data or information to the UEs thatare registered on the system. There may be more than one FACH per cell as they may carry packet data.

Paging Channel (PCH) (downlink). This channel carries messages that alert the UE to incoming calls,SMS messages, data sessions or required maintenance such as re-registration.

Random Access Channel (RACH), (uplink). This channel carries requests for service from UEs trying toaccess the system

Uplink Common Packet Channel (CPCH), (uplink). This channel provides additional capability beyondthat of the RACH and for fast power control.

Downlink Shared Channel (DSCH) (downlink).This channel can be shared by several users and is usedfor data that is "bursty" in nature such as that obtained from web browsing etc.

Physical Channels:

Primary Common Control Physical Channel (PCCPCH) (downlink). This channel continuously broadcastssystem identification and access control information.

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UMTS TDD (Universal mobile telecommunications system - time division duplex) is a growing cellulartechnology. Although UMTS TDD or TD WCDMA is not as widely deployed as the more popular UMTSFDD which is being deployed for the 3G mobile phone systems, UMTS TDD is nevertheless being widelyused and providing a viable service for many applications. In particular it is being used to provide mobilebroadband data services, and other applications may include its use in providing mobile TV applications.In this way, UMTS is a growing cellular technology which will be far more widely used in the years to

come

TDD - time division duplex

A communications system requires that communication is possible in both directions: to and from thebase station to the remote station. There are a number of ways in which this can be achieved. The mostobvious is to transmit on one frequency and receive on another. The frequency difference between thetwo transmissions being such that the two signals do not interfere. This is known as frequency divisionduplex (FDD) and it is one of the most commonly used schemes, and it is used by most cellular schemes.

It is also possible to use a single frequency and rather than using different frequency allocations, usedifferent time allocations. If the transmission times are split into slots, then transmissions in one directiontake place in one time slot, and those in the other direction take place in another. It is this scheme that isknown as time division duplex, TDD, and it is used for UMTS-TDD.

Note on TDD and FDD duplex schemes:

In order for radio communications systems to be able to communicate in both directions it is necessary tohave what is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter andreceiver so that they can transmit and receive. There are several methods that can be adopted. For

applications including wireless and cellular telecommunications, where it is required that the transmitterand receiver are able to operate simultaneously, two schemes are in use. One known as FDD orfrequency division duplex uses two channels, one for transmit and the other for receiver. Another schemeknown as TDD, time division duplex uses one frequency, but allocates different time slots for transmissionand reception.

Click on the link for more information on TDD FDD duplex schemes

When using a TDD system, there are a number of characteristics that are pertinent for TDD systems.These characteristics need to be accommodated when developing or using TDD systems.

Utilisation of unpaired bands: Typically there is more traffic in the downlink (network to themobile) than in the uplink (mobile to network). Accordingly the operator is able to allocate moretime to the downlink transmission than the uplink. This is not possible with the paired spectrumrequired for FDD systems where it is not possible to re-allocate the use of the different bands. Asa result of this, it is possible to make very efficient use of the available spectrum.

Discontinuous transmission: In any TDD system it is necessary to switch between transit andreceive. This takes a certain amount of time. Not only does it take time for the mobile and thebase station to change between transmit and receive in terms of ramping up or down the power,along with the settling of any transients. In addition to this the time is required between transmit

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UMTS TDD within 3GPP

All the standards for UMTS 3G systems have been defined under the auspices of 3GPP - the thirdgeneration partnership project. The standards not only define the FDD systems, but also the TDD system.

In these specifications, it was the original intent of UMTS that the TDD spectrum would be used to provide

high data rates in selected areas forming what could be termed 3G hot zones.

UMTS TDD details

UMTS TDD uses many of the same basic parameters as UMTS FDD. The same 5 MHz channelbandwidths are used. UMTS TDD also uses direct sequence spread spectrum and different users andwhat can be termed "logical channels" are separated using different spreading codes. Only when thereceiver uses the same code in the correlation process, is the data recovered. In W-CDMA all otherlogical channels using different spreading codes appear as noise on the channel and ultimately limit thecapacity of the system. In UMTS TDD, a scheme known as multi user detection (MUD) is employed in thereceiver and improves the removal of the interfering codes, allowing higher data rates and capacity.

In addition to the separation of users by using different logical channels as a result of the differentspreading codes, further separation between users may be provided by allocating different time slots.There are 15 time slots in UMTS TDD. Of these, three are used for overhead such as signalling, etc andthis leaves twelve time slots for user traffic. In each timeslot there can be 16 codes. Capacity is allocatedto users on demand, using a two dimensional matrix of timeslots and codes.

In order for UMTS TDD to achieve the best overall performance, the transport format, i.e. the modulationand forward error correction can be altered for each user. The schemes are chosen by the network, andwill depend on the signal characteristics in both directions. Higher order forms of modulation enablehigher data speeds to be accommodated, but they are less resilient to noise and interference, and thismeans that the higher data rate modulation schemes are only used when signal strengths are high.

Additionally the levels of forward error correction can be changed. When errors are likely, i.e. when signalstrengths are low or interference levels are high, Similarly higher levels of forward error correction areneeded under low require additional data to be sent and this slows the payload transfer rate. Thus it ispossible to achieve much higher data transfer rates when signals are strong and interference levels arelow.

Spectrum allocations for UMTS TDD

Standard allocations of radio spectrum have been made for 3G telecommunications systems in mostcountries around the globe. In Europe and many other areas spectrum has been allocated for UMTS FDDbetween 1920MHz to 1980MHz and 2110MHz to 2170MHz. For UMTS TDD spectrum is primarily located

between 1900MHz and 1920MHz and between 2010MHz and 2025MHz. In addition to this there aresome other allocations around 3 GHz.

UMTS TDD performance

UMTS TDD is able to support high peak data rates. Release 5 of the UMTS standard provides HSDPA(high-speed downlink packet access). The scheme allows the use of a higher order modulation scheme

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called 16-QAM (16 point quadrature amplitude modulation), which enables peak rates of 10 Mbps persector in commercial deployments. The next release increases the modulation to 64-QAM, and introducesintercell interference cancellation (called Generalized MUD) and MIMO (multiple in, multiple out). Incombination, these increase the peak rate to 31 Mbps per sector.

3G TD-SCDMA Tutorial

- overview or tutorial of the basics of TD-SCDMA, the 3G UMTS TDD system that has been widely

deployed in China.

TD-SCDMA is an additional TDD version of UMTS. Devised in China, the system provides a number ofadvantages in several applications. TD-SCDMA has been adopted as a 3G standard by the InternationalTelecommunications Union (ITU), and it is part of the 3GPP UMTS system being defined in the 3GPPstandards.

Much of the initial work for the system was undertaken by the China Academy of Telecommunications

Technology (CATT). Apart from the advantages of the basoc TDD approach, TD-SCDMA is able tosupport IP services, and it has been designed to incorporate new technologies such as joint detection,adaptive antennas, and dynamic channel allocation

While similar in many was to UMTS TDD, TD-SCDMA is has a number of differences and handsets forthe two systems would not be compatible unless the capability for both systems was specifically built in tothem.

TD-SCDMA basics

One of the key elements of TD-SCDMA is the fact that it uses a TDD, Time Division Duplex approach. As

seen with UMTS TDD this has advantages in a number of areas, enabling the balance to be changedbetween uplink and downlink to accommodate the different levels of data transfer. It also has advantagesin terms of using unpaired spectrum, spectrum efficiency for certain loads and it does not requireexpensive diplexers in the handsets to enable simultaneous transmission on the uplink and downlink,although transmit / receive switching times must be accommodated and can reduce the efficiency of thesystem.

Note on TDD and FDD duplex schemes:

In order for radio communications systems to be able to communicate in both directions it is necessary tohave what is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter andreceiver so that they can transmit and receive. There are several methods that can be adopted. Forapplications including wireless and cellular telecommunications, where it is required that the transmitterand receiver are able to operate simultaneously, two schemes are in use. One known as FDD orfrequency division duplex uses two channels, one for transmit and the other for receiver. Another schemeknown as TDD, time division duplex uses one frequency, but allocates different time slots for transmissionand reception.

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2. The link between the existing NodeB and the UE is broken.3. A new link is established between the new NodeB and the UE.

Although this is a simplification of the process, it is basically what happens. The major problem is that anydifficulties in re-establishing the link will cause the handover to fail and the call or connection to bedropped.

UMTS hard handovers may be used in a number of instances:

When moving from one cell to an adjacent cell that may be on a different frequency.

When implementing a mode change, e.g. from FDD to TDD mode, for example.

When moving from one cell to another where there is no capacity on the existing channel, and achange to a new frequency is required.

One of the issues facing UMTS hard handovers was also experienced in GSM. When usage levels arehigh, the capacity of a particular cell that a UE is trying to enter may be insufficient to support a new user.To overcome this, it may be necessary to reserve some capacity for new users. This may be achieved byspreading the loading wherever possible - for example UEs that can receive a sufficiently strong signalfrom a neighbouring cell may be transferred out as the original cell nears its capacity level.

UMTS soft handover

Soft handover is a form of handover that was enabled by the introduction of CDMA. Soft handover occurswhen a UE is in the overlapping coverage area of two cells. Links to the two base stations can beestablished simultaneously and in this way the UE can communicate with two base stations. By havingmore than one link active during the handover process, this provides a more reliable and seamless way inwhich to perform handover.

In view of the fact that soft handover uses several simultaneous links, it means that the adjacent cells

must be operating on the same frequency or channel as UEs do not have multiple transmitters andreceivers that would be necessary if they were on different frequencies.

When the UE and NodeB undertake a soft handover, the UE receives signals from the two NodeBs andcombines them using the RAKE receiver capability available in the signal processing of the UE.

In the uplink the situation is more complicated as the signal combining cannot be accomplished in theNodeB as more than one NodeB is involved. Instead, combining is accomplished on a frame by framebasis. The best frames are selected after each interleaving period. The selection is accomplished byusing the outer loop power control algorithm which measures the signal to noise ratio (SNR) of thereceived uplink signals. This information is then used to select the best quality frame.

Once the soft handover has been completed, the links to the old NodeB are dropped and the UEcontinues to communicate with the new NodeB.

As can be imagined, soft handover uses a higher degree of the network resources than a normal link, oreven a hard handover. However this is compensated by the improved reliability and performance of thehandover process. However with around 5 to 10% of handovers falling into this category, networkoperators need to account for it.

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Note on the RAKE receiver

A RAKE receiver is a form of radio receiver that has been made feasible in many areas by the use ofdigital signal processing, DSP. It is often used to overcome the effects of multipath propagation. Itachieves this by using several sub-receivers known as "fingers" which are given a particular multipathcomponent. Each finger then processes its component and decodes it. The resultant outputs from the

fingers are then combined to provide the maximum contribution from each path. In this way rake receiversand multipath propagation can be used to improve the signal to noise performance.

UMTS softer handover

A form of handover referred to as softer handover is really a special form of soft handover. It is a form ofsoft handover that occurs when the new radio links that are added are from the same NodeB. This occurswhen several sectors may be served from the same NodeB, thereby simplifying the combining as it canbe achieved within the NodeB and not require linking further back into the network.

UMTS softer handover is only possible when a UE can hear the signals from two sectors served by thesame NodeB. This may occur as a result of the sectors overlapping, or more commonly as a result ofmultipath propagation resulting from reflections from buildings, etc.

In the uplink, the signals received by the NodeB, the signals from the two sectors can be routed to thesame RAKE receiver and then combined to provide an enhanced signal.

In the downlink, it is a little more complicated because the different sectors of the NodeB use differentscrambling codes. To overcome this, different fingers of the RAKE receiver apply the appropriate de-spreading or de-scrambling codes to the received signals. Once this has been done, they can becombined as before.

In view of the fact that a single transmitter is used within the UE, only one power control loop is active.

This may not be optimal for all instances but it simplifies the hardware and general operation.

Inter-RAT / Intersystem UMTS / GSM handover

In many instances it is necessary for the UMTS radio access network to handover to the 2G GSMnetwork. These handovers are given a variety of names including Inter-RAT handover as they arehanding over between different forms of Radio Access Technology, Intersystem Handover, and UMTS /GSM Handover. These handovers may be required for one of a variety of reasons including:

Limited UMTS coverage

UMTS network busy whereas spare capacity is available on GSM network

The most common form of intersystem or inter-RAT handover is between UMTS and GSM. There are twodifferent types of inter-RAT handover:

UMTS to GSM handover: There are two further divisions of this category of handover:

o Compressed mode handover: Using compressed mode handover the UE uses the gaps

in transmission that occur to analyse the reception of local GSM base stations. The UE

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uses the neighbour list provided by the UMTS network to monitor and select a suitablecandidate base station. Having selected a suitable base station the handover takesplace, but without any time synchronisation having occurred.

o Blind handover: This form of handover occurs when the base station hands off the UE

by passing it the details of the new cell to the UE without linking to it and setting thetiming, etc of the mobile for the new cell. In this mode, the network selects what it

believes to be the optimum GSM based station. The UE first locates the broadcastchannel of the new cell, gains timing synchronisation and then carries out non-synchronised intercell handover.

Handover from GSM to UMTS : This form of handover is supported within GSM and a"neighbour list" was established to enable this occur easily. As the GSM / 2G network is normallymore extensive than the 3G network, this type of handover does not normally occur when the UEleaves a coverage area and must quickly find a new base station to maintain contact. Thehandover from GSM to UMTS occurs to provide an improvement in performance and cannormally take place only when the conditions are right. The neighbour list will inform the UE whenthis may happen.

UMTS handover methodology

The decisions about handover are generally handled by the RNC. It continually monitors informationregarding the signals being received by both the UE and NodeB and when a particular link has fallenbelow a given level and another better radio channel is available, it initiates a handover. As part of thismonitoring process, the UE measures the Received Signal Code Power (RSCP) and Received SignalStrength Indicator (RSSI) and the information is then returned to the node B and hence to the RNC on theuplink control channel.

Cellular Telecommunications and Cell Phone Technology

- articles and information on the basics of cellular telecommunications and cell phone technology

For wireless technologies (Wi-Fi, WiMAX, etc), please refer to ourWireless technologiespages.

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Mobile phone or cell phone history

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Pacific or Personal Digital Cellular (PDC)

UMA - unlicensed mobile access

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Cellular repeater

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LTE Femtocells: Opportunities and Challenges Sanket Nesargi, Aricent [2010-03]

Femtocell trends and developments Rupert Baines, picoChip [2010-03]

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Enabling Any-to-Any Communication for Next-Generation Mobile Networks Tareq Bustami andCharlie Kawwas, LSI [2010-03]

Mobile Network Optimisation with SON Chris Larmour, CMO, Actix [2010-02]

Femtocells: turning cellular communications inside out Simon Saunders, Chairman of FemtoForum

Designing the perfect femtocell Steve Shaw, Kineto Wireless

3G LTE: testing times Evan Gray, Aeroflex