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`UMTS
`NETWORKS
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`Architecture. Mnhiliw and Services
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`51.1.11 .'I.H NAHUM-1
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`Copyright 1? 2005 John Wiley 3: Sons Ltd. The Atrium. Southern Gate. Chietester.
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`112
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`Lib-{TS Networks
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`Intra-BSIFinter—cell handover {softer handover).
`Inter‘BS handover. including hard and soft handovers.
`lnterARNC handover, including hard. soft and soft—softer handovers.
`interrMSC handover.
`
`Inter-SEEN [Serving GPRS Support Node) handover.
`inter-system handover.
`
`5.3.1.2 REM—Power Control
`
`Power control is an essential feature of any CDMAFbased cellular system. Without
`utilising an accurate power control mechanism. theSe systems cannot operate. In the
`following subsections we first describe why power control
`is so essential for these
`cellular systems and outline the main factors underlying this fact. We then describe
`the kinds of power control mechanisms utilised in WCDMA-FDD radio access.
`The main reasons for implementing power control are the near-far problem (see p.
`119]. the interference-dependent capacity of WCDMA and the limited power source of
`the LEE. Unlike Frequency Division Multiple Access (FDMA) and Time Division
`Multiple Access [TDMA]. both of which are bandwidth-limited multi-aecesses.
`WCDMA is an interference-limited multiple access. In FDMA and TDMA power
`control is applied to reduce inter—cell interference within the cellular system which
`arises from frequency reuse. while in WCDMA systems the purpose of power control
`is mainly to reduce intra-ccll interference. Meeting these targets requires optimisation of
`radio transmission power (i.e.. that the power of every transmitter is adjusted to the
`level required to meet the requested QoS}. Determining the transmission power level is.
`however. a very sophisticated task due to unpredictable variation of the radio channel.
`Whatever the radio environment. power received should be at an acceptable level
`(cg. at the BS for the uplink to support the requested QoS). The target of power
`control is to adjust the power to the desired level without any unnecessary increase
`in UE transmit power. This ensures that transmit power isjust within the required level
`[neither higher nor less}. taking into account the existing interference in the system.
`The influence of the multipaih propagation characteristics and the technical char-
`acteristics of the WCDMA system te.g.. simultaneous bandwidth sharing and near—far
`phenomena} has the effect of power control being essential for the WCDMA system, to
`overcome the drawbacks caused by the radio environment and the nature ofelectro-
`
`magnetic waves. Without power control such phenomena as fading and interference
`will drive down system stability and ultimately degrade its performance dramatically.
`Maximising system capacity is an invaluable asset for both advanced cellular tech-
`nology suppliers and cellular network operators. System capacity is maximise-:3 if the
`transmitted power of each terminal is controlled such that its signal arrives at the BS
`with the minimum required SIR. if a terminal‘s signal arrives at the BS with a power
`value that is too low. then the required QcS for radio connection cannot be met. If the
`received power value is too high. then. although the performance of this terminal is
`good. interference to all the other terminal transmitters sharing the channel is increased
`and may result in unacceptable performance for other users. unless their number is
`reduced.
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`UMTS Radio Access Network 12.3
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`Power 1
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`Power 2
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`
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`D1. D2 = Distances between UE and as
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`Figure 5.22 Near—tar effect in the CDMA system
`
`Due to the fact that the total handwidth in the WCDMA system is shared simul-
`taneously. other users can be experienced as a noise-like interference for a specific user.
`When the power control mechanism is missing or operates imperfectly. common
`sharing of’ the bandwidth creates a severe problem. called the “near—far" effect. In
`near—far situations the signal of the terminal
`that
`is close to the serving BS maltr
`dominate the sigml of those terminals that are distant from the same BS. Figure
`5.22 illustrates a situation in which the near—far problem could occur. The main
`factors that cause the near—far problem include the path loss variation of simultaneous
`users at different distances from the 35. the fading variation and other signal power
`variation of users caused by radio wave propagation mechanisms [described in
`Chapter 3}.
`In WCDMA the near—far eFfect can be mitigated by applying power control mech-
`anisms. diversity techniques. soft handovers. multi-user receivers and. more generally.
`near—Far resistance receitrers. HecaUse of the crucial drawback of the near—far effect on
`
`the performance of the WCDMA system. its mitigation is one of the pivotal purposes
`of power control mechanisms. These mechanisms have a considerable impact on
`WCDMA system capacity.
`
`5.3.L2J Basic Approaches Used in Power Control
`For the reasons just outlined. it is relativer easy to determine that the optimal situation
`in the uplink case. From the BS receiver point of view, is that the power representing one
`UE‘s signal shotdd always be equal to another UE's signal regardless of their distance
`From the BS.
`11" so. the SIR will be optimal and the BS receiver able to decode the
`maximum number of transmissions. in reality. however, the radio channel is extremely
`unstable and radio services requested vary for difi'ercnt users [even for the same user
`and during the same radio connection]. Therefore.
`the transmission power of LIE
`should be controlled very accurater by utilising efficient mechanisms.
`To achieve this. power control has been thoroughly investigated and. as a conse-
`quence. man)r power control algorithms have been developed since the advent of the
`CDMA scheme. These include distributed. centralised. synchronous. asynchronous.
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`114
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`UMTS Networks
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`iterative and non-iterative algorithms. Most current algorithms either utilise 31R or
`transmit power as a reference point in the power control decision-matting process.
`The primary principie of Centralised Power Control [CFC] schemes is that they keep
`the overall power control mechanism centralised. As a result they require a central
`controller. which should have knowledge of all the radio connections in the RAN.
`In contrast to CFC methods. distributed power control methods do not utilise a
`central controller.
`Instead.
`they distribute the controlling mechanism within the
`RAN and toward its edge. This feature makes them of special interest. CFC approaches
`bring about added complexity. latency and network vulnerability. The main advantage
`of a distributed power control algorithm is that it can respond more adaptively to a
`variable QoS. which is greatly important for cellular systems with packet-based trans-
`mission characteristics. like WCDMA.
`
`5.3.}.2J Power Control Mechanism in UTRAN t" WCDMA-FDD}
`
`ln WCDMA. power control is employed in both uplink and downlinlt directions.
`Downlirtlt power control is basically for minimising interference with other cells and
`compensating for the interference from other eells. as well as achieving an acceptable
`Slit. However. power control for the downlinl-t is not as vital as it is for the uplinlt. it is
`still implemented for the downiink. because it improves system performance by con—
`trolling interference from other cells.
`is to mitigate the near—far problem by
`The main target of uplinlt power control
`making the transmission power level received from all terminals as equal as possible
`at the hotne cell for the same QoS. Therefore. uplinlt power control is used for fine-
`tnning terminal transmission power. resulting in mitigation of intra-oell interference
`
`and the near —far efi'ect. Note that the power control mechanism specified for WCDMA
`is. in principle. a distributed approach.
`The power control mechanisms used in the GSM are clearly inadequate to guarantee
`this situation in WCDMA and.
`thus. WCDMA takes a different approach to the
`matter. In the GSM. power control is applied to the connection once or-twice per
`second. but. due to its critical nature in WCDMA. the power used in the connection
`is adjusted Liflfl times per second {i.e.. the power control cycle is repeated for each
`radio frame in association with the DCH). Therefore. power adjustment steps are
`considerably faster than in the GSM.
`To manage power control properly in WCDMA. the system uses two different power
`control mechanisms. as defined in Figure 5.23. These power control mechanisms are:
`
`a Open Loop Power Control {DLPC}.
`I Closed Loop Power Control {CLPC}. including inner and outer loop power control
`mechanisms.
`
`the UTRAN
`these different power control mechanisms together.
`By applying all
`benefits from the advantages of CPC as well by overlaying the inner CLPC with the
`outer CLFC mechanism in order to keep the target SIR. at an acceptable level.
`
`5.3J.2.3 Open Loop Power Control (ULPC'J
`the UE adjusts its
`in ULPC. which is basically used for uplink power adjusting.
`transmission power based on an estimate of the received signal level from the BS
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`UMTS Radio Access Network
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`115
`
`[E
`
`f
`
`
`
`apenteepfiewsrcuenent
`
`
`.1— filiniflmeinflm
`Innu-Lonpflufli'ewfleoflfl—"
`
`eumtnapmconea
`
`Figure 5.13 The main power control mechanisms employed in WCDMA
`
`CPI-CH when the UE is in idle mode and prior to Physical Random Access Channel
`{PEACH} transmission. in addition. the UE receives information about the allowed
`
`power parameters from the cell ECCH when in idle mode. The UE evaluates the path
`loss. and from this together with figures received from the BCCH and the UE it is able
`to estimate an appropriate power level to initialise the connection.
`Figure 5.24 illustrates the DLPC as applied For the uplink case. in this proCess. the
`LTE estimates the strength of the transmission signal by measuring the received power
`level ofthe pilot signal from the BS in the downlinlt. and adjusts its transmission power
`level in a way that is inverser proportional to the pilot signal power level. Consev
`quentlv. the stronger the received pilot signal the lower the LIE power transmitted.
`In the ease of WCDMA-FDD. ULPC alone is insufficient to adjust UE transmission
`poWer. because the l'ading characteristics of the radio channel var},r rapidly and inde—
`pendently between uplinlt and downlinlt. Therefore.
`to compensate for the rapid
`changes in signal strength a CLPC mechanism is also needed. Nevertheless. ULPC is
`
`useful for determining the initial value of transmitted power and for mitigating draw-
`backs of the log-nonnal-distrihuted path loss and shadowing.
`
`5.3J.2.4 Closed Loop Poser Contra! (CLPC)
`CLPC is utilised to adjust transmission power when the radio connection has alreadyr
`been established. its principal purpose is to compensate for the effect of rapid changes
`in radio signal strength and. hence.
`it needs to he Fast enough to respond to these
`changes.
`
`Pilot Strength Es‘tirnatim
`—h~ UE
`
`
`
`P_tnr:“PilotStrengthEstimation g
`
`Figure 5.14 Open Loop Power Control For uplinit
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`UMTS Networks
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`Figure 5.25 illustrates the basic uplink CLPC mechanism specified for WCDMA. In
`this case the BS commands the LE to either increase or decrease its transmission power
`using a cycle of LfiltHz {LSfll} times per second} by |-. L or 3-dE step sizes. The
`decision to increase or decrease power is based on the received SIR as estimated by
`the BS. 1|When the BS receives the UE signal it compares the signal strength with the
`predefined threshold value at the BE. If the UE transmission power exceeds the thresh—
`old value. the BS sends a Transmission Power Command {TPC} to the LIE to decrease
`its signal power. if the received signal is lower than the threshold target the BS sends a
`command to the UE to increase its transmission power. It should be emphasised that
`various measurement parameters. such as SIR. signal strength, Frame Error Ratio
`{PERI and Bit Error Ratio [HE R]. can he used to compare the quality of the power
`received and to make a decision about whether to control transmission power or not.
`Note also that CLPC is utilised to adjust transmission power in the downlink as well.
`In the case ol'downlink CLFC. the roles of the BS and the UE are interchanged; that is.
`the UE compares the received signal strength from the BS with a predefined threshold
`and sends the TPE to the ES to adjust its transmission pD'WEI aocordingly'.
`ln WCDMA. the CLPC mechanism consists of inner loop and outer loop variants.
`What we have so far described is related to the inner loop variant; this is the fastest loop
`in the WCDMA power control mechanism and. hence. it is occasionally called "fast
`power control“.
`
`Another variant of the CLPC is the OLPC mechanism. The main target of DLPC is
`to keep the target SIR for the uplink inner loop power control mechanism at a satis-
`factory quality level. Thanks to macrodiversitg the ENC is aware of current radio
`connection conditions and qualityr and. therefore. is able to define the allowed power
`levels of the cell and target SIR to he used by the BS when determining TP'Cs. To
`maintain the quality of the radio connection. the RNC uses this power control method
`to adjust the target SIR and keep anv variation in the quality of the connection in
`check. By doing this. the network is able to compensate for changes in radio interface
`propagation conditions and to achieve the masimurn target quality for BER connection
`and FER observation. in fact. ULPC fine-tunes the performance of inner loop power
`control.
`
`P1: Decisist Matting
`
`{fa—2‘53:
`
`TF'G Getter-ands
`
`as
`
`
`
`x as
`if,—
`
`TPC Gonmands
`
`.r'
`LIEP tr:
`~
`I when
`l.
`according to
`
`a TPCoo-wnw-ds
`
`
`
`
`
`Figure 5.25 Basic llC'loscd Loop Power Control mechanism in uplink
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