Mobility Parameter Planning for 3GPP LTE: Basic …

1 Mobility Parameter Planning for 3 GPP LTE: Basic Concepts andIntra-Layer MobilityJari SaloThis white paper discusses the design of intra-layer Mobility parameters for 3 GPP LTE radio networks. Firstly, Basic UE measurements defined in 3 GPP Release 9 are reviewed. A measurement example illustrates their basicdifferences. Both idle mode and connected mode measurements and cell change rules from 3 GPP are summarizedin a vendor-independent way. Secondly, design criteria for setting handover margin, time to trigger and measure-ment filtering coefficient are discussed in detail. A simple Parameter design example is given. The scope of thispaper is limited to a review Basic of concepts and design of intra-layer (intra-frequency) Mobility INTRODUCTIONThis technical white paper introduces idle andconnected mode Mobility Parameter design for3 GPP LTE.

2 The target audience are radio plan-ning and optimization engineers with some ex-perience in LTE. Principles of OFDMA and SC-FDMA, as described in 3 GPP LTE specifications,will not be repeated in this paper. Instead thereader is referred to well-known literature ref-erences [1 3]. Both FDD and TDD variants ofLTE radio interface are addressed and the dif-ference between the two is highlighted, Section 2 Reference Signal Received Power(RSRP) and Reference Signal Received Quality(RSRQ) are defined and their properties dis-cussed. The relation of RSRQ to signal-to-noise-plus-interference ratio (SINR) is shown, andcomplemented by a measurement example il-lustrating the properties and interdependenciesof the different RF measurement quantities. Tokeep the paper self-contained, Section 3 sum-marizes the idle mode Mobility rules for 3 GPPRel9, while Section 4 does the same for con-nected mode handovers; the treatment is keptas vendor-independent as possible.

3 In the keysection of this paper we outline criteria on howto design handover parameters for practical net-work deployment. The selection of handovermargin, time to trigger and L3 filtering coef-ficient is a trade-off between network interfer-ence, probability of unnecessary ("ping-pong")handovers and too late handover triggering. Fi-nally, a design example summarizes the keypoints of the primary goal of the paper is to introducesome ideas for Planning intra-layer Mobility pa-rameters. Post-launch Mobility Parameter tun-ing based on network measurements are verycase-specific topics and beyond the scope : 16 June 201312J. SaloAbbreviationsBCCH Broadcast ChannelCPICH Common Pilot ChannelFDDF requency Division MultiplexingLTELong Term EvolutionNASNon-Access StratumOAMO peration and MaintenancePCHP aging ChannelPCIP hysical Cell IdentityPDSCH Physical Downlink Shared ChannelPRBP hysical Resource BlockPSSP rimary Synchronization SignalRATR adio Access TechnologyREResource ElementRRCR adio Resource ControlRSReference SignalRSCPR eceived Signal Code PowerRSRQ Reference Signal Received QualityRSRPR eference Signal Received SignalRSSIR eceived Signal Strength IndicatorSIBS ystem Information BlockSINRS ignal-to-Interference-Noise-RatioSNRS ignal-to-Noise RatioSSSS econdary Synchronization SignalTDDTime Division MultiplexingTTIT ransmission Time IntervalUEUser EquipmentUSIMUMTS Subscriber

4 Identity Module2. DEFINITION OF RSRP AND RSRQIn this section, RSRP and RSRQ are defined,including their relation to SINR. A measure-ment example is shown to illustrate the Reference Signal Received Power (RSRP)RSRP is defined asRSRP=1KK k=1 Prs,k,(1)wherePrs,kis the estimated received power (inWatts) of thekth Reference Signal Resource El-ement transmitted from the first BTS antennaport. In Figure 1 these REs are denoted withR0. To improve the accuracy of the RSRP esti-mate, the UE may optionally also measure theRS transmitted from the second antenna port(R1), if present. In case of four BTS transmit an-tennas, Reference Signals of the third and fourthBTS antenna ports are not used in the RSRP measurement. Since all LTE UEs have at leasttwo receive antennas, it is also mandated by [6]that the RSRP must be equal or higher than thestronger of the two receive antennas individualmeasured maximum number of PRBs over whichRSRP should be measured1is sent to the UEover RRC signalling, and is denoted in thispaper withNprb.

5 For example, in case of10 MHz measurement bandwidth (Nprb=50),the OFDM symbol carryingR0(Figure 1) con-tains a total of 100 REs forR0, hence in this caseK=100 in Eq. (1) for this OFDM symbol2as-suming UE implementation does not useR1inRSRP measures only the RS power and ex-cludes all noise and interference power. ThusRSRP is a purely coverage-based handover trig-ger which is ideally independent of the networkload, similar to CPICH RSCP in 3G. It shouldbe noted that, since it is defined as the averagepower of RS resource elements, RSRP does notdepend on the number of BTS transmit reporting range of RSRP is divided in 1dBbins over 140 .. 44 dBm, the lowest RSRP value having reported value of 0 .The 3 GPP specification does not specify howthe RSRP measurement should be implementedin a UE.

6 However, the RSRP measurement accu-racy requirements are stated in some detail in[5], see Table 1 for a summary. Under normaloperating conditions, absolute measurement ac-1 Measurement bandwidth can be less than the actual down-link system bandwidth. This is useful for keeping measure-ment Parameter configuration simple in multi-bandwidthdeployment specification does not state any requirements on howmany OFDM symbols UE should average. Only measure-ment accuracy requirement is Parameter Planning for 3 GPP LTE30R0R0R0R0R0R0R0R1R1R1R1R1R1R1R1 Ran OFDM symbol containing R0subframeFreqTimeFigure 1. Reference Signals as seen by UE,shown for transmit antenna port 1 (R0) andtransmit antenna port 2 (R1) for one 1ms sub-frame and 12 is allowed to have up to 6dB error forintra-frequency RSRP measurement.

7 MeasuredRSRP difference between serving and a neigh-bour cell on the same carrier frequency shouldhave at most 3dB error. For inter-frequencyRSRP measurement, the absolute and relativeerror under normal conditions should both beless than 6dB. It can be seen that inter-frequencyRSRP measurement is considerably less accu-rate for "power budget" type handover trigger-ing than intra-frequency RSRP above cited measurement accuracy re-quirements hold for SNR= 6dB and withoutany higher layer measurement filtering. HigherSNR and application of L3 filtering results inimproved measurement accuracy. Therefore, intypical network conditions, measurement errorcan be expected to be smaller than shown in Ta-ble 1UE RF measurement L1 accuracy RelativeAccuracy RSRP intra-freq 6dB 3dBRSRP inter-freq 6dB 6dBRSRQ intra-freq inter-freq 4dB When SNR 6dB, RSRP 127.

8 124dBmdepending on frequency band [5]. Reference Signal Received Quality(RSRQ)Reference Signal Received Quality is the ra-tio of RSRP and total received signal and noisepower normalized to 1 PRB bandwidth. As a for-mula we haveRSRQ=NprbRSRPRSSI(2)=Nprb1K Kk=1 Prs,k Nren=1Pn,(3)whereNre=12 Nprbis the total number of REs(including RS REs) in the measurement band-widthNprb, andPnis the total received poweron thenth RE. The RSSI term in the denomima-tor of Eq. (3) is the sum power received over theOFDM symbol that containsR0, including owncell power, thermal noise and interference fromother cells. For example, for 10 MHz (50 PRBs)measurement bandwidthNre=600 and, assum-ing that onlyR0is measured,K= with RSRP, implementation of the RSRQ measurement is not defined in detail by accuracy requirements are givenin Table 1.

9 It can be seen that no rela-tive accuracy requirement has been defined forintra-frequency RSRQ. This is because in intra-frequency case the ratio of RSRQs depends onlyon the ratio of RSRPs of serving and neighbourcells. This can be seen by writing down theRSRQ ratio between two cells on the same car-4J. Salorier frequency, that is,RSRQ servRSRQ neigh=NprbRSRP servRSSINprbRSRP neighRSSI=RSRP servRSRP neigh, (4)since RSSI does not depend on the measured cellas it is simply the total received signal , the ratio of RSRQs only depends onthe ratio of the RSRPs. This essentially makesrelative RSRQ redundant as a trigger for intra-frequency handover. In contrast, ratio of RSRQ smay be a useful trigger for inter-frequency han-dover where it can be used to direct UEs to aless loaded reporting range of RSRQ is divided bins over.

10 3 dB, the lowestRSRQ value having reporting value 0 . For un-loaded cell having one transmit antenna port,we have RSRQ= 3dB, assuming no other-cellinterference and negligible thermal noise. Simi-larly, a cell with two transmit antenna ports hasRSRQ= 6dB. On the other hand a fully loaded1Tx cell has RSRQ 11dB, again neglect-ing any neighbour cell interference and ther-mal noise. Therefore, from Mobility parame-ter Planning viewpoint, setting any RSRQ-basedhandover trigger to a value higher than 11dBcould result in "ping-ponging" if UE happens tomeasure RSRQ at a time instant when there isdata download ongoing in the serving cell. Be-cause of this dependency on the instantaneoustraffic load, RSRQ measurements should be av-eraged over a longer period than RSRP. This isvery similar to the fluctuation of CPICHEc/N0seen in HSDPA Relation of RSRQ and SINRIt is a common practice to use Signal-to-Interference Ratio (SINR) as an indicator fornetwork quality.