UMTS Tutorial 2

8/3/2019 UMTS Tutorial 2

1/19

UMTS Tutorial

La terza generazione di sistemi di telecomunicazioni radiomobili comunemente identicata

con la sigla breve 3G, acronimo di 3rd Generation, o pi comunemente con la sigla breve

UMTS, acronimo di Universal Mobile Telecommunications System. Lo standard 3G fu

originariamente pensato per essere uno standard univoco ed unicato a livello mondiale

mentre, in realt, stato implementato in modi differenti a seconda del tipo di accesso radio

impiegato.

Architettura

The UMTS network architecture is required to provide a greater level of performance to that of

the original GSM network. With one of the major aims of UMTS being to be able to carry data,

the UMTS network architecture was designed to enable a considerable improvement in data

performance over that provided for GSM. The UMTS network architecture can be divided into three

main elements:

User Equipment (UE)

The User Equipment or UE is the name given to what was previous termed the mobile, orcellphone. The new name was chosne because the considerably greater functionality that theUE could have. Ovviamente lo UE costituito sia dal dispositivo mobile di terza generazione,sia dalla USIM (UMTS SIM), la scheda rimovibile che si trova nel dispositivo cellulare. LUSIM contiene

lidenticativo di un dato utente e i servizi a cui permesso che egli acceda in base al rapporto contrattuale

con loperatore mobile. LUSIM specica per ogni utenza e consente laccesso in maniera sicura ai servizi.

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. The overall radio access network, i.e. collectively all

the Radio Network Subsystem is known as the UTRAN (UMTS Radio Access Network). The Radio

Network Subsystem comprises two main components:

y Radio Network Controller, RNC: This element of the radio network subsystemcontrols the Node Bs that are connected to it. The RNC undertakes the radio resourcemanagement and some of the mobility management functions, although not all. It isalso the point at which the data encryption / decryption is performed to protect the user

data from eavesdropping. E anche possibile per un RNC collaborare con le BaseStation Subsystems (BSS) che formano linterfaccia aerea di collegamento GERAN(GSM/EDGE Radio Access Network). Questa cooperazione permette lesecuzione dialgoritmi Common Radio Resource Management (CRRM) tra sistemi UMTS andGSM/GPRS. While Node B has a rather limited view of the world, and limited controlover its own resources, the RNC has an overview of all radio resources attached to it. Itis responsible for these resources and controls the set-up, maintenance and release ofradio connections ( Radio Bearers ). Radio bearer control also involves the planning ofresources and the calculation of interference and utilization levels, as well as thecontrol of CDMA codes. Furthermore, the RNC is involved in power control. Powercontrol actually has two stages: the inner loop performed by Node B as described in


2/19

the previous section, and the outer loop performed by the RNC. As one might expect,the RNC outer loop controls the Node B inner loop. This means that the RNC determinesthe target power level the UE should achieve based on the overall radio resourcepicture, and Node B is responsible for enforcing this power level. The RNC, the ServingRNC to be precise, controls the small-scale mobility of the UE, i.e. mobility across asmall number of cells, whereas the SGSN/MSCcontrols large scale mobility, includingroaming. The Serving RNC decides, based on measurement reports received from both

UE and UTRAN, whether a handover is necessary and then initiates this handover. TheServing RNC is also responsible for the control of macrodiversity (cf. Chapter 5, Section5.2.4.2), i.e. for deciding whether the UEshould attach to more than one cell, and ifyes, which cells. The RNCtransports IP-based trafc in the same way as an ordinaryrouter. Additionally, the RNC must protect the trafc against a variety of securitythreats on the radio interface by means of encryption and integrity protection Inaddition, the RNC is responsible for broadcasting system information on the radiointerface.

LRNC pu avere pi ruoli logici:

i) CRNC (Controlling RNC). Specica il ruolo dellRNC rispetto al dato Node B. Siriferisce al controllo che lRNC ha su un set di Node B.

ii) SRNC (Serving RNC). Specica il ruolo dellRNC rispetto al dato UE. LSRNC lRNC che mantiene la connessione di un dato UE con la CN attraverso linterfaccia

Iu. Cos pu essere considerato come lRNC che controlla lRNS al quale il mobile

collegato in un dato momento. Quando lUE si muove nella rete e esegue gli

Handover tra celle differenti pu richiedere una procedura di rilocazione da parte


3/19

dell SRNS (Serving RNS) quando la cella di destinazione appartiene ad un RNC

differente. Questo tipo di procedura richiede la comunicazione tra SRNC e la nuova

RNC attraverso linterfaccia Iur per permettere alla nuova RNC di stabilire una nuova

connessione con la CN attraverso la sua interfaccia Iu.

iii) DRNC (Drift RNC) Anche questo ruolo descritto rispetto allUE ed unaconseguenza di uno specico tipo di Handover che esiste con i sistemi di tipo CDMA() denotato come soft Handover. In questo caso un UE pu essere simultaneamentecollegato a pi celle. Cos quando un UE si muove sul bordo tra piu RNS possibileche instauri nuovi collegamenti radio con celle appartenenti ad un nuovo RNCmentre mantiene il collegamento con alcune celle dellSRNC. In questo caso lanuova RNC prende il ruolo di DRNC e la connessione con la CN non ottenutaattraverso lIu del DRNC ma ancora attraverso lIu dell SRNC sebbene sia necessariostabilire risorse per lUE nellinterfaccia Iur fra DRNC e SRNC.Solo quando tutti icollegamenti radio della vecchia RNC sono rilasciati e la Ue connessa solo allanuova RNC la procedura di rilocazione SRNS sar eseguita. Tutte le RNC sono CRNCe una fata RNC pu essere SRNC per certi UE e simultaneamente DRCN per altri.

y 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.

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 two different

areas:

y Circuitswitched 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.y Packetswitched elements: These network entities are designed to carry packet data. This

enables much higher network usage as the capacity can be shared and data is carried as packets

which are routed according to their destination.

Circuitswitched elements

The circuit switched elements of the UMTS core network architecture include the following network

entities:

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

y GatewayMSC(GMSC): This is effectively the interface to the external networkPacket switched elements

The packet switched elements of the UMTS core network architecture include the following network

entities:


4/19

y Serving GPRSSupport Node (SGSN): As the name implies, this entity was first developedwhen GPRS was introduced, and its use has been carried over into the UMTS network

architecture. The SGSN provides a number of functions within the UMTS network architecture.

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 current location.

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.

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 and other circuit

switched areas.

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 the SGSN before being

transferred to the charging entities (Charging Gateway Function, CGF).

y GatewayGPRSSupportNode (GGSN): Like the SGSN, this entity was also first introducedinto the GPRS network. The Gateway GPRS Support Node (GGSN) is the central element within

the UMTS packet switched network. It handles inter-working between the UMTS packet switched

network and external packet switched networks, and can be considered as a very sophisticated

router. In operation, when the GGSN receives data addressed to a specific user, it checks if the

user is active and then forwards the data to the SGSN serving the particular UE.

y Shared elements : The shared elements of the UMTS core network architecture include thefollowing network entities:

i) Home location register (HLR): This database contains all the administrative informationabout each subscriber along with their last known location. In this way, the UMTS network is

able to route calls to the relevant RNC / Node B. When a user switches on their UE, itregisters with the network and from this it is possible to determine which Node B it

communicates with so that incoming calls can be routed appropriately. Even when the UE is

not active (but switched on) it re-registers periodically to ensure that the network (HLR) is

aware of its latest position with their current or last known location on the network.

ii) Equipmentidentityregister(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 the

equipment and is checked by the network during registration.

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

WCDMA

When looking at the radio air interface and its associated properties, it is necessary to define the

directions in which the transmissions are occurring being a full duplex system.


5/19

y 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.

y 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).

Much of the focus for UMTS is currently on frequency allocations around 2 GHz. At the World

Administrative radio Conference in 1992, the bands 1885 - 2025 and 2110 - 2200 MHz were set . Within

these bands the portions have been reserved for different uses:

y 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.

y 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.

y 1980-2010 and 2170-2200 MHz Satellite uplink and downlink.UMTS uses wideband CDMA as the radio transport mechanism. The UMTS channels are spaced by 5 MHz.

La tecnica di accesso al canale utilizzata in UMTS la tecnica CDMA (Code Division Multiple Access).

CDMA is a form of spread spectrum transmission technology. It has a number of distinguishing features

that are key to spread spectrum transmission technologies:

y U se of wide bandwidth: CDMA, like other spread spectrum technologies uses a widerbandwidth than would otherwise be needed fort he transmission of the data. This results in a

number of advantages including an increased immunity to interference or jamming, and multiple

user access.

y Spreading codes used: In order to achieve the increased bandwidth, the data is spread by useof a code which is independent of the data. Codes are sequences of one and minus one, so-called chips. The sender multiplies the bit sequence by the code before sending. The receiver, inturn, multiplies the received sequence of chips again with the code, thereby obtaining back theoriginal sequence of bits. Of course, because of the non-zero travelling time between sender andreceiver, the receiver must apply the code with the right time-shift, i.e. we need synchronizationbetween sender and receiver.

y Multiple access: The use of the spreading codes which are independent for each user alongwith synchronous reception allow multiple users to access the same channel simultaneously.

y Enhanced security: The use of spread spectrum and the multiple spreading codes for CDMAsignificantly reduces the possibility of eavesdropping.

y Improvement in handover / handoff: Using CDMA it is possible for a terminal tocommunicate with two base stations at once. As a result, the old link only needs to be brokenwhen the new one is firmly established. This provides significant improvements in terms of thereliability of handover / handoff from one base station to another. Within CDMA it is possible todo what is termed a "soft handover" where the UE communicates with two base stations at thesame time. This significantly improves handover reliability.


6/19

Direct sequence spread spectrum (DS-SS). The data is directly coded by a high chip rate (spreading)code by multiplying the information-bearing signal with a pseudorandom binary waveform. Each bit inthe spreading sequence is called a chip, and this is much shorter than each information bit. Supponiamoche :

: durata del bit dinformazione da trasmettere. Con W =

che rappresenta la banda del segnale

: indica il bit rate del codice di spreading.

=

rappresenta la durata del chip.

As the bandwidth is the inverse of the chip duration, the bandwidth of the total signal is nowalso W = 1/ i.e., larger than the bandwidth of a narrowband-modulated signal by a factorN, che rappresenta il rapporto tra la banda trasmessa e quella del segnale originario ed chiamato fattore di guadag . As we assume that the spreading operation does not change thetotal transmit power, it also implies that the power-spectral density decreases by a factor .Thus, without changing the signal power, the power spectral density (PSD) of the signal wouldbe N times lower than it would be in non-spread transmission and the signal is less likely to bedetected.


7/19

The basic spreading process in a direct sequence spread-spectrum system is illustrated in theconceptual block diagram of a DSSS transmitter in Figure 2.1. The information-bearing signal,d(t), is multiplied by the spreading code, c(t), and modulated onto a RF carrier frequency toobtain a nal spread output signal, s(t)

where fRF is the RF carrier frequency.

In addition to the desired signal, the received signal also contains noise, other widebandinterferers, and possibly narrowband interferers. Note that the effective bandwidth of noiseand wideband interferers is not signicantly affected by the despreading operation, whilenarrowband interferers are actually spread over a bandwidth NW. The incoming signal isreceived by the RF front-end, is down-convert


8/19

the RF signal to IF. This IF DSSS signal is despread and bandpass ltered, whereafter thedespread signal is demodulated by means of a BPSK demodulator to recover the originalinformation-bearing signal, d(t).

In the case of a high-power narrow-band interference or jamming signal, the interference orjamming signal is added to the spread data signal in the radio channel.

WCDMA is resistive to interference from a narrowband signal whose bandwidth is much smallerin comparison and is uncorrelated to the wanted signal. In the detection process, thecomposite received signal is multiplied by the spreading code of a wanted user. This causes ade-spreading of the narrowband interference power over the band of the WCDMA signaldetermined by the code chip rate. At this point the power spectral density of the narrowbandinterfering signal has been reduced by the ratio of the chip rate (3.84 Mcps in WCDMA) andthe bandwidth of the narrowband signal. Subsequent filtering to pull out the wanted userssignal results in capturing only the portion of the reduced interfering power that lies within theband of the wanted signal. This amount will be insignificant depending on the ratio of thebandwidths,and the power of the interfering signal compared to the power of the wantedsignal. To quantify the resistivity to the narrowband interferer, assume that the power of the

received wanted signal and the narrowband interferer are Psig, and Pint , respectively. Forsimplifying the analysis, assume the only noise or interference present is due to thenarrowband signal. Then, the signal-to-interference power ratios before and after the de-spreading operations are

where BW is the bandwidth of the wanted signal. Substituting Equation (3) into Equation (4)gives

Thus the improvement achieved against the interferer is seen to be W/BW, which is just theprocessing gain. If this gain is enough to result in the required value for the service (C/I)Aft ,


9/19

the resistivity to the narrowband signal is achieved so the larger the processing gain, the morethe resistance to interference.

In termini di rapporto SNR le cose restano invariate visto che For a DS-SS system, the noisepower at the receiver input is N0/TC = N0 N/Tb , which is reduced by narrowband ltering by afactor of N ; thus, at the detector input, it is N0/Tb. A similar effect occurs for widebandinterference.

Let us next discuss the spreading signals for DS-SS systems. In order to perfectly reverse thespreading operation in the receiver by means of a correlation operation, we want theAutoCorrelation Function (ACF) of the spreading sequence to be a Dirac delta function. In sucha case, the convolution of the original information sequence with the concatenation of spreaderand despreader is the original sequence.

These ideal properties can only be approximated in practice. One group of suitable codesequences is a type of Pseudo Noise (PN) sequences called maximum length sequence (m-sequence). PN sequences have the following ACF:


10/19

At the receiver, the desired signal is obtained by correlating the received signal with thespreading signal of the desired user. Other users thus become wideband interferers; afterpassing through the despreader, the amount of interference power seen by the detector isequal to the Cross CorrelationFunction (CCF) between the spreading sequence of theinterfering user and the spreading sequence of the desired user. Thus, we ideally wish for

for all users j and k. In other words, we require code sequences to be orthogonal. Perfectorthogonality can be achieved for at most N spreading sequences; this can be immediatelyseen by the fact that N orthogonal sequences span an N-dimensional space, and any othersequence of that duration can be represented as a linear combination.

WCDMA codes

WCDMA relies on CDMA for multiple access. However, transmission timing is still based on ahierarchical timeslot structure similar to GSMs: frames of duration Tf = 10 ms are divided into15 timeslots, each of which has a 12-bit-long System Frame Number (SFN). Each timeslot hasa duration of 0.667 ms which equals 2,560 chips. The conguration of frames and timeslots isdifferent for uplink and downlink.

WCDMA uses two types of code for spreading and multiple access: channelization codes and

scrambling codes.

channelization codes : They spread the signal by increasing the occupied bandwidth inaccordance with the basic principle of CDMA. Channelization codes in WCDMA are OrthogonalVariable Spreading Factor (OVSF) codes.

scrambling codes : They do not lead to bandwidth expansion but help to distinguish betweencells and/or users. Spreading with the orthogonal channelization codes alone is insufcientbecause orthogonal codes are rather sensitive to synchronization: Two orthogonal codes thatare time-shifted relative to each other can have a substantial cross-correlation. This becomes aproblem because, e.g. UEs are synchronized with their respective cell, however, cells are not


11/19

synchronized among themselves. Therefore, with orthogonal codes the receiving cell cannotproperly despread a signal which contains contributions from several UEs, possibly attached todifferent cells. Scrambling codes, in contrast, are quasiorthogonal. This means that their auto-correlation is high, and their cross-correlation is almost, but not quite zero. But then, theyremain quasi-orthogonal even when time-shifted relative to each other. Therefore, each senderrst applies a channelization code, and then a scrambling code on top.

Scrambling codes are much longer than channelization codes, they have 384 000 chips. Withchips code length, the number of possible scrambling codes is very large. UMTS only utilizes8192 different scrambling codes which is still large enough to allow the exibility describedabove to serve user and load shifts between cells without too much bookkeeping.

A UE close to a cell border can be attached to two or more cells simultaneously, as illustratedin Figure 5.8. The downlink signal is sent via all cells to which the UE is attached. The uplinksignal from the UE is received and processed by all cells it is attached to. So The assignment ofchannelization code and scrambling code is different in uplink and downlink direction:

y Uplink : The uplink uses OVSF codes for spreading. However, they are not used forchannelization (distinguishing between users in the uplink). Therefore, different userscan use the same OVSF codes. As mentioned above, signals from different users aredistinguished by different scrambling codes. So more, each individual UE is in command

of the entire set of channelization codes, allowing the UE to manage the bandwidths ofits sessions independently

y Downlink : each cell uses its own scrambling code, because cells among themselvesare not synchronized, either. The scrambling code is thus a cells nger print. Eachcell has the full set of channelization codes at its disposal, assigning a different one toeach UE it is serving.


12/19

WCDMA Modulation

As the uplink and downlink have different requirements, the exact format for the modulationformat used on either direction is slightly different. UMTS modulation schemes for both uplinkand downlink, although somewhat different are both based around phase shift keying formats.

y Downlink : The UMTS modulation format for the downlink is more straightforward thanthat used in the uplink. The downlink uses quadrature phase shift keying, QPSK. The

QPSK modulation used in the downlink is used with time-multiplexed control and data

streams. While time multiplexing would be a problem in the uplink, where the

transmission in this format would give rise to 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.

y Uplink : However the uplink uses two separate channels so that the cycling of thetransmitter on and off does not cause interference on the audio lines, a problem that

was experienced on GSM. The dual channels (dual channel phase shift keying) are

achieved by applying the coded user data to the I or In-phase input to the DQPSK

modulator, and control data which has been encoded using a different code to the Q or

quadrature input to the modulator.


13/19

WCDMA Handover

Within UMTS it is possible to define a number of different types of UMTS handover or handoff. With the

advent of generic CDMA technology, new possibilities for effecting more reliable forms of handover

became possible, and as a result one of a variety of different forms of handover are available depending

upon the different circumstances.

For purely inter W-CDMA technology, there are three basic types of handover:

y Hardhandover: This form of handover is essentially the same as that used for 2G networkswhere one link is broken and another established.

y Softhandover: This form of handover is a more gradual and the UE communicatessimultaneously with more than one Node B or base station during the handover process.

y Softerhandover: Not a full form of UMTS handover, but the UE communicates with more thanone sector managed by the same NodeB.

y UMTSGSMinterRAThandover: This form of handover occurs when mobiles have to changebetween Radio Access Technologies.

UMTS hard handover

The name hard handover indicates that there is a "hard" change during the handover process. For hard

handover the radio links are broken and then re-established. Although hard handover should appear

seamless to the user, there is always the possibility that a short break in the connection may be noticed

by the user.

The basic methodology behind a hard handover is relatively straightforward. There are a number of basic

stages of a hard handover:

1. The network decides a handover is required dependent upon the signal strengths of the existinglink, and the strengths of broadcast channels of adjacent cells.

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

any difficulties in re-establishing the link will cause the handover to fail and the call or connection to be

dropped.

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

y When moving from one cell to an adjacent cell that may be on a different frequency.y When implementing a mode change, e.g. from FDD to TDD mode, for example.y When moving from one cell to another where there is no capacity on the existing channel, and a

change to a new frequency is required.

One of the issues facing UMTS hard handovers was also experienced in GSM. When usage levels are high,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 by

spreading the loading wherever possible - for example UEs that can receive a sufficiently strong signal

from a neighbouring cell may be transferred out as the original cell nears its capacity level.


14/19

UMTS soft and softwer handover

A soft or softer handover occurs when the mobile station is in the overlapping coverage area of two

adjacent cells. The user has two simultaneous connections to the UTRAN part of the network using

different air interface channels concurrently. In the case of soft handover the mobile station is in the

overlapping cell coverage area of two sectors belonging to different base stations; softer handover is the

situation where one base station receives two user signals from two adjacent sectors it serves. Although

there is a high degree of similarity between the two handover types there are some significant

differences.

In the case of softer handover the base station receives 2 separated signals through multi-path

propagation. Due to reflections on buildings or natural barriers the signal sent from the mobile stations

reaches the base station from two different sectors. The signals received during softer handover are

treated similarly as multi-path signals. In the uplink direction the signals received at the base station are

routed to the same rake receiver and then combined following the maximum ratio combining technique.

In the downlink direction the situation is slightly different as the base station uses different scrambling

codes to separate the different sectors it serves. So it is necessary for the different fingers of the rake

receiver in the mobile terminal to apply the appropriate de-spreading code on the signals received from

the different sectors before combining them together. According to [3] soft handover occurs in 5-10% of

the connections. Due to the nature of the softer handover there is only one power control loop active

For soft handover the situation is very similar in the downlink direction. In the mobile station the signalsreceived from the two different base stations are combined using MRC Rake processing. In the uplink

direction on the other hand there are significant differences. The received signals can no longer be

combined in the base station but are routed to the RNC. The combining follows a different principle; in

the RNC the two signals are compared on a frame-by-frame basis and the best candidate is selected after

each interleaving period; i.e. every 10, 20, 40 or 80ms. As the outer loop power control algorithm

measures the SNR of received uplink signals at a rate between 10 and 100Hz, this information is used to

select the frame with the best quality during the soft handover

Basically the soft handover is composed of two main functions:- Acquiring and processing measurements

- Executing the handover algorithm

Before starting the in-depth analysis of these functions some terms used for describingthe handover process have to be defined:- Set: list of cells or Node Bs- Active set: list of cells having a connection with the mobile station- Monitored set: list of (neighbouring) cells whose pilot channel Ec/I0 is continuously measured but notstrong enough to be added to the active set.


15/19

Measurements

Accurate measurements of the Ec/I0 of the pilot channel (CPICH) form the main input for obtaining theRRC measurement report, necessary for making handover decisions. Mainly three parameters can bemeasured. Besides the Ec/I0 of the CPICH also the received signal code power (RSCP) and the receivedsignal strength indicator (RSSI) are measured. RSCP is the power carried by the decoded pilot channeland RSSI is the total wideband received power within the channel bandwidth. Ec/I0 is defined as:

EC/I0 = RSCP/RSSI

It is important to apply filtering on the handover measurements to average out the effect of fast fading.Measurement errors can lead to unnecessary handovers. Appropriate filtering can increase theperformance significantly. As long filtering periods can cause delays in the handovers13, the length of thefiltering period has to bechosen as a trade-off between measurement accuracy and handover delay. Alsothe speed of the user matters, the slower the user equipment is moving the harder it is to average outthe effects of fast fading. Often a filtering time of 200ms is chosen. Other essential information neededduring the so-called intra-mode handovers soft and softer handover is timing information. As theWCDMA network is of asynchronous nature there exist relative timing differences between the cells.

To allow easy combining in the Rake receiver and avoid delays in the power control loops, thetransmissions have to be adjusted in time. After the UE has measured the timing difference between theCPICH channels of the serving cell and the target cell, the RNC sends DCH timing adjustment info to thetarget cell.

The soft handover algorithm

Based on the Ec/I0 measurements of the set of cells monitored, the mobile station decides which of threebasic actions to perform; it is possible to add, remove or replace a node B in the active cell. These tasksare respectively called Radio Link Addition and Radio Link Removal, while the latter is Combined RadioLink Addition and Removal. The example below is directly taken from the original 3GPP specifications.Discussing this scenario gives a good insight into the algorithm itself and forms an introduction to theillustrating simulations included in the next paragraph. This scenario can be based on a user following atrajectory as shown below.

At the start of the scenario the user is connected to cell number 1 which has the strongest pilot signal.Due to the user moving or to slow fading the perception of the signal strengths to the mobile user canchange and following actions are taken:


16/19

The set of NodeBs that have a connection to a mobile are called the Active Set (AS) of the mobile. Thedetermination of the AS is highly dynamic. In UMTS it is controlled by a series of parameters: theReporting Range, the Addition Hysteresis, the Removal Hysteresis, and the Time To Trigger. We explainthe meaning of the parameters by means of Fig. 2.14. A mobile moves from NodeB X to NodeB Y. Theupper part shows the events that take place and the lower part shows the Ec/I0 of the pilot signals of Xand Y. The pilot signal is broadcast with constant power by all NodeBs. The Ec/I0 is the measured andaveraged chip energy per interference ratio of the pilot signal. At the beginning the pilot of X is clearlystronger than the pilot of Y. When the mobile moves from X to Y the pilot signal strength of X gets

continuously weaker and that of Y grows continuously. At some time the pilot Ec/I0 of Y is stronger thanthe pilot Ec/I0 of X minus the Reporting Range plus the Addition Hysteresis:

Ec/I0(Y ) > Ec/I0(X) ReportingRange + AdditionHysteresis

At that moment a timer is started that expires after the Time To Trigger. If during that time Ec/I0(Y )does not drop below

Ec/I0(X) ReportingRange + AdditionHysteresis,

NodeB Y enters the AS. Otherwise, the timer is stopped and started anew when the pilot of Y becomeslarger than the threshold again. In general, the timer is started, whenever the pilot Ec/I0 of a candidateuser becomes stronger than the Ec/I0 of the strongest NodeB in the AS minus the Reporting Range plusthe Addition Hysteresis. In the example the AS contains only X so its pilot signal is of course thestrongest one. When the mobile moves further towards NodeB Y, the pilot of Y becomes stronger thanthe pilot of X and finally the pilot of X is weaker than

Ec/I0(Y ) ReportingRange RemovalHysteresis.

The timer of length Time To Trigger is started and when it expires the RNC removes X from the AS. Sothe Reporting Range is the parameter that essentially determines the AS size. The Addition and RemovalHysteresis control how aggressively a new NodeB is added to the AS or an old NodeB is removed fromthe AS. The hysteresis and also the Time To Trigger further avoid an oscillation of add and drop events.The explanation in this section describes only the main principle of handover control.

Inter-RAT / Intersystem UMTS / GSM handover

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

different types of inter-RAT handover:

y UMTSto GSMhandover: 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 uses

the neighbour list provided by the UMTS network to monitor and select a suitable

candidate base station. Having selected a suitable base station the handover takes place,

but without any time synchronisation having occurred.

o Blind handover: This form of handover occurs when the base station hands off the UE bypassing it the details of the new cell to the UE without linking to it and setting the timing,

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 broadcast channel of the new

cell, gains timing synchronisation and then carries out non-synchronised intercell

handover.

y 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 normally

more extensive than the 3G network, this type of handover does not normally occur when the UE

leaves a coverage area and must quickly find a new base station to maintain contact. The

handover from GSM to UMTS occurs to provide an improvement in performance and can normally

take place only when the conditions are right. The neighbour list will inform the UE when this may

happen.


17/19

Power Control

In WCDMA system there is a mechanism of transmitted power control: without it, a single overpowered

mobile user could block a whole cell. Power control is needed both in the uplink and in the downlink,

although for different reasons.

In the uplink direction, all signals should arrive at the base stations receiver with the same signal power.The mobile stations cannot transmit using fixed power levels, as then cells would be dominated by users

closest to the base station and distant users couldnt get their signals heard in the station. Thisphenomenon is called the near-far effect.

The situation is different in the downlink direction: there is no near-far effect. The signals transmitted byone base station are orthogonal and so they dont interfere with each other. However, it is impossible toachieve full orthogonality in typical usage environments: signal reflections cause non-orthogonalinterference even if a single base station is considered. Moreover, signals sent from other base stationsare non-orthogonal and thus they increase the interference level. This happens because the orthogonalspreading codes lose their orthogonality on the uplink due to asynchronous transmission from mobilestations in different locations in the cell (their signals are therefore received at the base station withdifferent delays).

We must also remember that the neighboring cells can use the same downlink frequency carrier. Notethat a mobile station close to the base station would not suffer if the signals it receives have been sentusing too much power. But other users, especially those in other cells, could receive this signal as non-

orthogonal noise. Therefore, power control is also needed in the downlink. The signal should betransmitted with the lowest possible power level, which maintains the required signal quality.

Open loop power control

OL power control is the ability of the user equipment (UE) to set its power to a specified value suitable forthe receiver. This method is used for setting up initial uplink transmission powers. The desired powerlevel is calculated from measurement information about the pathloss, the target SIR and the interferenceat the cells receiver, broadcasted on the BCH (Broadcasting Channel).

Figure : open loop power control algorithm

Closed (inner) loop power control

Open loop power control methods, based on characteristics of a downlink pilot channel, are far tooinaccurate to accomplish this; mainly because fast fading patterns for UL and DL channels in FDD arepractically uncorrelated due to the large frequency separation between those two bands . Hence open

loop power control methods are only used in the initial phase of setting up a connection, as explainedabove.

The fast power control thus tries to equalize the channel effects for different users and ensure theirsignals are received at the base station with just the necessary power to meet the required Eb/N0 foreach. This is done in a closed loop fashion by having the mobile station adjust its power using thefeedback received from the base station.

Every 667ms (1/1500Hz) the base station compares the estimated SIR of each mobile stations signal,with a SIR target value. If the measured SIR is higher than the target SIR, the base station will commandthe MS to power down; in the other case the base station sends a power up command. The basic stepsize to which the user adjusts its transmit power following received TPC commands is 1dB or 2dB


18/19

with an accuracy of 0,5dB. The SIR target value used in the CL power control method is provided by theouter loop power control algorithm, as will be explained below.

Figure : closed loop power control algorithm

Outer loop power control

There is also an outer loop power control, which is used to set and adjust the Eb/N0 required The Eb/N0required for a given service quality is generally dependent on the channel multipath profile, as well as themobile speed. As these change, the outer loop power control prepares updated target Eb/N0 based onreal time quality measurements and sends the target value to the inner loop power control. The innerloop power control then uses the information to increase or reduce the transmit power in order to meetthe indicated target Eb/N0 at the receiving end. The outer loop power control helps to prevent excesstransmit powers and hence interference in the network by setting the Eb/N0 to just what is required foreach channel condition rather than setting it to a fixed value for the worse case conditions. Since theEb/N0 required for the service should be determined after a possible soft handover, the outer loop powercontrol is implemented in the RNC.

Figure : outer loop power control algorithm

Capacity vs Coverage

CDMA networks are not hard capacity limited. This means that additional users cannot be hard blockeddue to a lack of timeslots or shortage in the number of copper wires available, as is the case in for

example GSM and POTS networks respectively. Instead of being hardware limited, CDMA networks are

interference limited. This means that every additional user will gradually degrade the noise figure in the

system until the network is fully loaded. The interference-determined behaviour of soft handover

networks makes it possible for loaded cells to borrow capacity from surrounding cells with lower traffic

density.

Intuitively it can be seen that capacity and coverage are not independent parameters in a UMTS system.

Imagine the situation where a lot of users are concentrated in the central cell area. As the users are

sending and receiving more and more application traffic over the WCDMA air interface, the total amount


19/19

of noise present in the system will increase. Hence a user near the cell edge will be ordered by the power

control algorithm to power up in order to overcome the increase in noise and still reach the Node B with a

power level similar to the users in cell centre. The remote mobile station will be transmitting at increasing

power levels and will soon reach its maximum transmission power as the traffic load generated in the

network keeps increasing. The other users overshout the user near the cell edge area, as the high

power transmitted is not sufficient to reach the Node B. The user will no longer be able to establish

communication with this Node B. Otherwise stated, the area covered by the Node B becomes smaller.

This phenomenon is characteristic for CDMA networks; the coverage decreases with increasing trafficload. The above-described scenario also causes the effect of breathing cells, which means that the

coverage area of a cell is not strictly defined but can move depending on the load present in the system.

Documents

UMTS Tutorial 2