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UMTS Tutorial
La terza generazione di sistemi di telecomunicazioni radiomobili è comunemente identi¿c con la sigla breve 3G, acronimo di 3rd Generation, o più comunemente con la sigla br UMTS, acronimo acronimo di Universal Mobile Telecommunications Telecommunications System. Lo standard standard 3G originariamente pensato per essere uno standard univoco ed uni¿cato a livello mond mentre, in realtà, è stato implementato in modi differenti a seconda del tipo di accesso r impiegato.
Architettura
The UMTS network architecture is required to provide a greater level of performance to tha the original GSM network. With one of the major aims of UMTS being to be able to carry d the UMTS network architecture was designed to enable a considerable improvement in d performance over that provided for GSM. The UMTS network architecture can be divided into t main elements:
User Equipment (UE)
The User Equipment or UE is the name given to what was previous termed the mobile cellphone. The new name was chosne because the considerably greater functionality that UE could have. Ovviamente lo UE è costitui costituito to sia dal dispositivo mobile di terza terza generazi sia dalla USIM (UMTS SIM), la scheda rimovibile che si trova nel dispositivo cellulare. LUSIM cont
lidenticativo di un dato lidenticativo d ato utente e i servizi a cui è permesso che egli acceda in b ase al rapporto contratt con loperatore mobile. mobile. LUSIM LU SIM è specica per ogni utenza e consente laccesso in maniera sicura ai serv
Radio
Network Subsystem (RNS) Sign up to vote on this title
The RNS is the equivalent of the previous Base Station Subsystem BSS inuseful GSM. It provides Not Useful or manages the air interface fort he overall network. The overall radio access network, network, i.e. collectivel the Radio Network Subsystem is known as the UTRAN (UMTS Radio Access Network) . The R Network Subsystem comprises two main components:
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the previous section, and the ³outer loop´ performed by the RNC. As one might exp the RNC outer loop controls the Node B inner loop. This means that the RNC determ the target power level the UE should achieve based on the overall radio resou picture, and Node B is responsible for enforcing this power level. The RNC, the Ser RNC to be precise, controls the small-scale mobility of the UE, i.e. mobility acros small number of cells, whereas the SGSN/MSCcontrols large scale mobility, includ roaming. The Serving RNC decides, based on measurement reports received from b UE and UTRAN, whether a handover is necessary and then initiates this handover. Serving RNC is also responsible for the control of macrodiversity (cf. Chapter 5, Sec 5.2.4.2), i.e. for deciding whether the UEshould attach to more than one cell, an yes, which cells. The RNCtransports IP-based traf¿c in the same way as an ordin router. Additionally, the RNC must protect the traf¿c against a variety of secu threats on the radio interface by means of encryption and integrity protection addition, the RNC is responsible for broadcasting system information on the ra interface.
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dell¶ SRNS (Serving RNS) quando la cella di destinazione appartiene ad un R differente. Questo tipo di procedura richiede la comunicazione tra SRNC e la nu RNC attraverso l¶interfaccia Iur per permettere alla nuova RNC di stabilire una nu connessione con la CN attraverso la sua interfaccia Iu.
iii)
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(Drift RNC) Anche questo ruolo è descritto rispetto all¶UE ed è conseguenza di uno speci¿co tipo di Handover che esiste con i sistemi di tipo CD () denotato come soft Handover. In questo caso un UE può essere simultaneame collegato a più celle. Così quando un UE si muove sul bordo tra piu RNS è possi che instauri nuovi collegamenti radio con celle appartenenti ad un nuovo R mentre mantiene il collegamento con alcune celle dell¶SRNC. In questo caso nuova RNC prende il ruolo di DRNC e la connessione con la CN non è otten attraverso l¶Iu del DRNC ma ancora attraverso l¶Iu dell SRNC sebbene sia necess stabilire risorse per l¶UE nell¶interfaccia Iur fra DRNC e SRNC.Solo quando tu collegamenti radio della vecchia RNC sono rilasciati e la Ue è connessa solo nuova RNC la procedura di rilocazione SRNS sarà eseguita. Tutte le RNC sono CR e una fata RNC può essere SRNC per certi UE e simultaneamente DRCN per altri. DRNC
Node B : Node B is the term used within UMTS to denote the base station transceive contains the transmitter and receiver t o communicate with the UEs within the cell.
UMTS Core Network
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The UMTS core network architecture is a migration of that used for GSM with further eleme full access demanded with a free trial. overlaid to enable the additional Unlock functionality by UMTS. In view of the diffe ways in which data may be carried, the UMTS core network may be split into two differ areas: Download With Free Trial
These elements are primarily based on the GSM network entiti and carry data in a circuit switched manner, i.e. a permanent channel for the duration of the c
y
s: Circuit sw itched element
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s: Pack et sw itched element
These network entities are designed to carry packet data. This Sign up to vote on this title enables much higher network usage as the capacity can be shared and data is carried as pack which are routed according to their destination. Useful Not useful
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As the name implies, this entity was first develo when GPRS was introduced, and its use has been carried over into the UMTS netw architecture. The SGSN provides a number of functions within the UMTS network architecture. S ervi ng G PRS S upport
S N ): Node ( SG
Mobility management When a UE attaches to the Packet Switched domain of the UMTS C Network, the SGSN generates MM information based on the mobile's current location.
Session management: The SGSN manages the data sessions providing the required qualit service and also managing what are termed the PDP (Packet data Protocol) contexts, i.e. pipes over which the data is sent.
Interaction with other areas of the network: The SGSN is able to manage its elements within network only by communicating with other areas of the network, e.g. MSC and other cir switched areas.
Billing: The SGSN is also responsible billing. It achieves this by monitoring the flow of user d across the GPRS network. CDRs (Call Detail Records) are generated by the SGSN before be transferred to the charging entities (Charging Gateway Function, CGF).
( GG S N ): Like the SGSN, this entity was also first introdu into the GPRS network. The Gateway GPRS Support Node (GGSN) is the central element wi the UMTS packet switched network. It handles inter-working between the UMTS packet switc network and external packet switched networks, and can be considered as a very sophistica router. In operation, when the GGSN receives data addressed to a specific user, it checks if You're Reading a Preview user is active and then forwards the data to the SGSN serving the particular UE.
y
G at ew ay G PRS S upport Node
y
Unlock full access with a free trial. Shared elements : The shared elements of the UMTS core network architecture include
following network entities: i)
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This database contains all the administrative informa about each subscriber along with their last known location. In this way, the UMTS networ able to route calls to the relevant RNC / Node B. When a user switches on their UE registers with the network and from this it is possible to determine which Node B communicates with so that incoming calls can be routed appropriately. Even when the U not active (but switched on) it re-registers periodically totoensure Sign up vote onthat this the titlenetwork (HLR aware of its latest position with their current or last known location on the network. H ome
loc ati on r egi st er ( HLR ):
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ii) Equipment i dentit y r egi st er ( EI R ): The EIR is the entity that decides whether a given equipment may be allowed onto the network. Each UE equipment has a number known as
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This may also sometimes be known as the reverse link, and it is the link from the U Equipment (UE) to the Node B or base station.
y
pl in k : U
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k : Downl in
This may also sometimes be known as the forward link, and it is the link from Node 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 W Administrative radio Conference in 1992, the bands 1885 - 2025 and 2110 - 2200 MHz were set . these bands the portions have been reserved for different uses: y
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1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paired uplink downlink, channel spacing is 5 MHz and raster is 200 kHz. An Operator needs 3 - 4 chann (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, chan spacing is 5 MHz and raster is 200 kHz. Transmit and receive transmissions are not separate frequency. You're Reading a Preview
1980-2010 and 2170-2200 MHz Satellite uplink and downlink. Unlock full access with a free trial. UMTS uses wideband CDMA as the radio transport mechanism. The UMTS channels are spaced by 5 M y
Download FreeCTrial La tecnica di accesso al canale utilizzata in UMTS è With la tecnica DMA (Code Division Multiple Access
CDMA is a form of spread spectrum transmission technology. It has a number of distinguishing featu that are key to spread spectrum transmission technologies: y
of w i de bandw i dt h: CDMA, like other spread spectrum technologies uses a w bandwidth than would otherwise be needed fort he transmission of the data. This results Sign up to vote onor this title number of advantages including an increased immunity to interference jamming, and mult user access. Useful Not useful se U
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c odes u sed: In order to achieve the increased bandwidth, the data is spread by of a code which is independent of the data. Codes are sequences of ³one´ and ³minus one´, g S pr ead in
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Direct sequence spread spectrum (DS-SS). The data is directly coded by a high chip rate (spreadi code by multiplying the information-bearing signal with a pseudorandom ± binary waveform. Each b the spreading sequence is called a chip, and this is much shorter than each information bit. Supponia che :
: durata del bit d¶informazione da trasmettere. Con W = che rappresenta la banda del segnale : indica il bit rate del codice di spreading. = rappresenta la durata del chip.
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the RF signal to IF. This IF DSSS signal is despread and bandpass ¿ltered, whereafter despread signal is demodulated by means of a BPSK demodulator to recover the orig information-bearing signal, d(t).
In the case of a high-power narrow-band interference or jamming signal, the interference jamming 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 sma in comparison and is uncorrelated to the wanted signal. In the detection process, composite received signal is multiplied by the spreading code of a wanted user. This cause Reading a Preview de-spreading of the narrowband You're interference power over the band of the WCDMA sig determined by the code chip rate. At this point the power spectral density of the narrowb Unlock free chip trial. rate (3.84 Mcps in WCDMA) interfering signal has been reduced by full theaccess ratiowith of athe the bandwidth of the narrowband signal. Subsequent filtering to pull out the wanted us signal results in capturing only theDownload portion of With the reduced interfering power that lies within Free Trial band of the wanted signal. This amount will be insignificant depending on the ratio of bandwidths,and the power of the interfering signal compared to the power of the wan signal. To quantify the resistivity to the narrowband interferer, assume that the power of received wanted signal and the narrowband interferer are Psig, and Pint , respectively. simplifying the analysis, assume the only noise or interference present is due to narrowband signal. Then, the signal-to-interference power ratios before and after the spreading operations are Sign up to vote on this title
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the resistivity to the narrowband signal is achieved so the larger the processing gain, the m the resistance to interference.
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In termini di rapporto SNR le cose restano invariate visto che For a DS-SS system, the n power at the receiver input is N 0 /TC = N 0 N/Tb , which is reduced by narrowband ¿ltering b factor of N ; thus, at the detector input, it is N0 /Tb.Sign A similar effect occurs up to vote on this title for wideb interference. Useful Not useful
Let us next discuss the spreading signals for DS-SS systems. In order to perfectly reverse spreading operation in the receiver by means of a correlation operation, we want
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At the receiver, the desired signal is obtained by correlating the received signal with spreading signal of the desired user. Other users thus become wideband interferers; a passing through the despreader, the amount of interference power seen by the detecto equal to the Cross CorrelationFunction (CCF) between the spreading sequence of interfering user and the spreading sequence of the desired user. Thus, we ideally wish for
You're Reading a Preview Unlock full access with a free trial.
for all users j and k. In other words, we require code sequences to be orthogonal. Per orthogonality can be achieved forDownload at most NWith spreading sequences; this can be immedia Free Trial seen by the fact that N orthogonal sequences span an N-dimensional space, and any o sequence of that duration can be represented as a linear combination.
WCDMA
codes
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WCDMA relies on CDMA for multiple access. However, transmission timing is still based o hierarchical timeslot structure similar to GSM¶s: frames of duration Tf = 10 ms are divided
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synchronized among themselves. Therefore, with orthogonal codes the receiving cell can properly despread a signal which contains contributions from several UEs, possibly attached different cells. Scrambling codes, in contrast, are quasiorthogonal. This means that their au correlation is high, and their cross-correlation is almost, but not quite zero. But then, t remain quasi-orthogonal even when time-shifted relative to each other. Therefore, each sen ¿rst applies a channelization code, and then a scrambling code on top.
Scrambling codes are much longer than channelization codes, they have 384 000 chips. W chips code length, the number of possible scrambling codes is very large. UMTS only util 8192 different scrambling codes which is still large enough to allow the Àexibility descri above 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 illustra in Figure 5.8. The downlink signal is sent via all cells to which the UE is attached. The up signal from the UE is received and processed by all cells it is attached to. So The assignmen channelization code and scrambling code is different in uplink and downlink direction:
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WCDMA
Modulation
Sign up to vote on this title As the uplink and downlink have different requirements, the exact format for the modula useful for both up Useful Not format used on either direction is slightly different. UMTS modulation schemes and downlink, although somewhat different are both based around phase shift keying forma
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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 dependin upon the different circumstances. For purely inter W-CDMA technology, there are three basic types of handover:
y
y
y
y
This form of handover is essentially the same as that used for 2G networks where one link is broken and another established. S of t handov er : This form of handover is a more gradual and the UE communicates simultaneously with more than one Node B or base station during the handover pr ocess. r handov er : Not a full form of UMTS handover, but the UE communicates with more th S of te one sector managed by the same NodeB. U MTS G SM i nt er R AT handov er : This form of handover occurs when mobiles have to chan between Radio Access Technologies. H ar d handov er :
UMTS hard handover
The name hard handover indicates that there is a "hard" change during the handover process. For h handover the radio links are broken and then re-established. Although hard handover should app seamless to the user, there is always the possibility that a short break in the connection may be noti by the user. You're Reading a Preview The basic methodology behind a hard handover is relatively straightforward. There are a number of b Unlock full access with a free trial. stages of a hard handover: 1. The network decides a handover is requiredWith dependent upon the signal strengths of the existi Download Free Trial link, 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 t any difficulties in re-establishing the link will cause the handover to fail and the call or connection to Sign up to vote on this title dropped. UMTS hard handovers may be used in a number of instances: Useful Not useful y
When moving from one cell to an adjacent cell that may be on a different frequency.
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UMTS soft and softwer handover
A soft or softer handover occurs when the mobile station is in the overlapping coverage area of adjacent cells. The user has two simultaneous connections to the UTRAN part of the network us different air interface channels concurrently. In the case of soft handover the mobile station is in overlapping cell coverage area of two sectors belonging to different base stations; softer handover is situation where one base station receives two user signals from two adjacent sectors it serves. Altho there is a high degree of similarity between the two handover types there are some signific differences.
In the case of softer handover the base station receives 2 separated signals through multi-p propagation. Due to reflections on buildings or natural barriers the signal sent from the mobile stati reaches the base station from two different sectors. The signals received during softer handover treated similarly as multi-path signals. In the uplink direction the signals received at the base station routed to the same rake receiver and then combined following the maximum ratio combining techniq In the downlink direction the situation is slightly different as the base station uses different scramb codes to separate the different sectors it serves. So it is necessary for the different fingers of the r receiver in the mobile terminal to apply the appropriate de-spreading code on the signals received f the different sectors before combining them together. According to [3] soft handover occurs in 5-10% 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 sign received from the two different base stations are combined using MRC Rake processing. In the up direction on the other hand there are significant differences. The received signals can no longer combined in the base station but are routed to the RNC. The combining follows a different principle the RNC the two signals are compared on a frame-by-frame basis and the best candidate is selected a each interleaving period; i.e. every 10, 20, Reading 40 or 80ms. As the outer loop power control algori You're a Preview measures the SNR of received uplink signals at a rate between 10 and 100Hz, this information is use Unlock full with a free trial. select the frame with the best quality during theaccess soft handover
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Measurements
Accurate measurements of the Ec/I0 of the pilot channel (CPICH) form the main input for obtaining RRC measurement report, necessary for making handover decisions. Mainly three parameters can measured. Besides the Ec/I0 of the CPICH also the received signal code power (RSCP) and the rece signal strength indicator (RSSI) are measured. RSCP is the power carried by the decoded pilot chan and 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 fad Measurement errors can lead to unnecessary handovers. Appropriate filtering can increase performance significantly. As long filtering peri ods can cause delays in the handovers13, the length of filtering period has to bechosen as a trade-off between measurement accuracy and handover delay. the speed of the user matters, the slower the user equipment is moving the harder it is to average the effects of fast fading. Often a filtering time of 200ms is chosen. Other essential information nee during the so-called intra-mode handovers soft and softer handover is timing information. As WCDMA 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, transmissions have to be adjusted in time. After the UE has measured the timing difference between CPICH channels of the serving cell and the target cell, the RNC sends DCH timing adjustment info to target cell.
The soft handover algorithm
Based on the Ec/I0 measurements of the set of cells monitored, the mobile station decides which of th basic actions to perform; it is possible to add, remove or replace a node B in the active cell. These ta You're Reading a Preview are respectively called Radio Link Addition and Radio Link Removal, while the latter is Combined Ra Link Addition and Removal. The example below is directly taken from the original 3GPP specificatio fullinto access a free trial. Discussing this scenario gives a goodUnlock insight thewith algorithm itself and forms an introduction to illustrating simulations included in the next paragraph. This scenario can be based on a user followin trajectory as shown below.
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The set of NodeBs that have a connection to a mobile are called the Active Set (AS) of the mobile. determination of the AS is highly dynamic. In UMTS it is controlled by a series of parameters: Reporting Range, the Addition Hysteresis, the Removal Hysteresis, and the Time To Trigger. We exp the meaning of the parameters by means of Fig. 2.14. A mobile moves from NodeB X to NodeB Y. upper part shows the events that take place and the lower part shows the Ec/I0 of the pilot signals and Y. The pilot signal is broadcast with constant power by all NodeBs. The Ec/I0 is the measured a averaged chip energy per interference ratio of the pilot signal. At the beginning the pilot of X is cle stronger than the pilot of Y. When the mobile moves from X to Y the pilot signal strength of X g continuously weaker and that of Y grows continuously. At some time the pilot Ec/I0 of Y is stronger t the pilot Ec/I0 of X minus the Reporting Range plus the Addition Hysteresis:
Ec/I0(Y ) > Ec/I0(X) í ReportingRange + AdditionHysteresis
At that moment a timer is started that expires after the Time To Trigger. If during that time Ec/I0 does not drop below Ec/I0(X) í ReportingRange + AdditionHysteresis,
NodeB Y enters the AS. Otherwise, the timer is stopped and started anew when the pilot of Y becom larger than the threshold again. In general, the timer is started, whenever the pilot Ec/I0 of a candida user becomes stronger than the Ec/I0 of the strongest NodeB in the AS minus the Reporting Range p the Addition Hysteresis. In the example the AS contains only X so its pilot signal is of course strongest one. When the mobile moves further towards NodeB Y, the pilot of Y becomes stronger t the pilot of X and finally the pilot of X is weaker than Ec/I0(Y ) í ReportingRange í RemovalHysteresis.
The timer of length Time To Trigger isYou're startedReading and when it expires the RNC removes X from the AS. a Preview the Reporting Range is the parameter that essentially determines the AS size. The Addition and Remo Hysteresis control how ³aggressively´ a new NodeB is added to the AS or an old NodeB is removed fr Unlock accessfurther with a free trial.an oscillation of add and drop eve the AS. The hysteresis and also the Time Tofull Trigger avoid The explanation in this section describes only the main principle of handover control. Inter-R AT
Download With Free Trial / Intersystem UMTS / GSM handover
The most common form of intersystem or inter-RAT handover is between UMTS and GSM. There are different types of inter-RAT handover: y
U MTS t o G SM handov er :
There are two further divisions of this category of handover:
o
Signmode up to handover vote on this title Compressed mode handover: Using compressed the UE uses the Useful transmission that occur to analyse the reception local GSM baseuseful stations. The UE u of Not the neighbour list provided by the UMTS network to monitor and select a suit candidate base station. Having selected a suitable base station the handover takes pl
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Control
In WCDMA system there is a mechanism of transmitted power control: without it, a single overpowe mobile user could block a whole cell. Power control is needed both in the uplink and in the downl although for different reasons.
In the uplink direction, all signals should arrive at the base station¶s receiver with the same signal pow The mobile stations cannot transmit using fixed power levels, as then cells would be dominated by us closest to the base station and distant users couldn¶t get their signals heard in the station. phenomenon is called the near-far effect.
The situation is different in the downlink direction: there is no near-far effect. The signals transmitted one base station are orthogonal and so they don¶t interfere with each other. However, it is impossib achieve full orthogonality in typical usage environments: signal reflections cause non-orthogo interference even if a single base station is considered. Moreover, signals sent from other base stat are non-orthogonal and thus they increase the interference level. This happens because the orthog spreading codes lose their orthogonality on the uplink due to asynchronous transmission from mo stations in different locations in the cell (their signals are therefore received at the base station w different delays).
We must also remember that the neighboring cells can use the same downlink frequency carrier. N that a mobile station close to the base station would not suffer if the signals it receives have been s using too much power. But other users, especially those in other cells, could receive this signal as n orthogonal noise. Therefore, power control is also needed in the downlink. The signal should transmitted with the lowest possible power level, which maintains the required signal quality.
Open
loop power control
You're Reading a Preview OL power control is the ability of the user equipment (UE) to set its power to a specified value suitable the receiver. This method is used for setting up initial uplink transmission powers. The desired po full access with a free trial. the target SIR and the interfere level is calculated from measurement Unlock information about the pathloss, at the cell¶s receiver, broadcasted on the BCH (Broadcasting Channel). Download With Free Trial
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Figure : open loop power control algorithm
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with an accuracy of ±0,5dB. The SIR target value used in the CL power control method is provided by outer 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 required for a given ser vice quality is generally dependent on the channel multipath profile, as well as mobile speed. As these change, the outer loop power control prepares updated target Eb/N0 based on real time quality measurements and sends the target value to the inner loop power control. The in loop power control then uses the information to increase or reduce the transmit power in order to m the indicated target Eb/N0 at the receiving end. The outer loop power control helps to prevent exc transmit powers and hence interference in the network by setting the Eb/N0 to just what is required each channel condition rather than setting it to a fixed value for the worse case conditions. Since You're Reading a Preview Eb/N0 required for the service should be determined after a possible soft handover, the outer loop po control is implemented in the RNC. Unlock full access with a free trial.
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of noise present in the system will increase. Hence a user near the cell edge will be ordered by the po control algorithm to power up in order to overcome the increase in noise and still re ach the Node B wi power level similar to the users in cell centre. The remote mobile station will be transmitting at increa power levels and will soon reach its maximum transmission power as the traffic load generated in network keeps increasing. The other users ³overshout´ the user near the cell edge area, as the h power transmitted is not sufficient to reach the Node B. The user will no longer be able to estab communication with this Node B. Otherwise stated, the area covered by the Node B becomes sma This phenomenon is characteristic for CDMA networks; the coverage decreases with increasing tra load. The above-described scenario also causes the effect of breathing cells, which means that coverage area of a cell is not strictly defined but can move depending on the load present in the syste
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