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Evaluation of Inter-Frequency Quality Handover

Criteria in E-UTRAN

Muhammad Kazmi1, Olof Sjbergh

2, Walter Mller

2

Wireless Access Networks, Ericsson Research1

Research and Development2

SE-164 80 Stockholm, Sweden

{Muhammad.Kazmi, Olof.Sjobergh,

Walter.Muller}.ericsson.com

Jonas Wiorek2

and Bengt Lindoff3

Ericsson Mobile Platforms, Ericsson Research3

SE-223 70 Lund, Sweden

{Jonas.Wiorek, Bengt.Lindoff}.ericsson.com

Abstract In E-UTRAN multiple co-located carriers will

typically be deployed requiring efficient inter-frequency (IF)

handover procedures and algorithms for retaining service

quality, better coverage and load balancing between the carriers.

Five IF handover criteria using RSRP, RSRQ or combination

thereof are investigated in synchronous and asynchronous E-

UTRAN deployment scenarios. The results depict that an IFhandover solely based on RSRP significantly increases number of

handovers. Conversely the handover criterion based only on

RSRQ reduces handovers but it slightly increases the packet loss

rate. The overall best performance is achieved with the combined

handover criterion, which uses both RSRP and RSRQ and thus

guarantees that the received pilot strength as well as the signal

quality stays within the desired limit after the handover.

Keywords; OFDMA, LTE, Radio Resource Management, Co-

located carriers, RSRP, RSRQ, Inter-frequency HO.

I. INTRODUCTIONHandover is one of the most fundamental radio resource

management features in a mobile network. The E-UTRAN orthe so-called 3GPP long term evolution (LTE) [1] supportsmobility in several deployment scenarios: E-UTRA intra-frequency, E-UTRA inter-frequency and inter radio accesstechnology (inter-RAT). The inter-RAT handover enables themobility between E-UTRAN and other access technologies,which may comprise of WCDMA, GSM, High Rate PacketData (HRPD) or cdma2000 1xRTT.

In E-UTRAN the handover decision, which is taken by theserving cell, relies on the downlink measurements and/or thenetwork configured events reported by the user equipment(UE). The evaluation of the handover related events specifiedin [2] are in turn based on one or more downlink measurements

performed by the UE. To support inter-frequency and inter-RAT mobility scenarios the corresponding measurements andevent evaluation are carried out by the UE during the networkconfigured measurement gaps.

Our goal is to devise and analyze suitable criteria forperforming E-UTRA inter-frequency handover in E-UTRANsystem. The E-UTRAN supports both time division duplex(TDD) and frequency division duplex (FDD) modes. Ouranalysis is though based on the E-UTRAN FDD but theconclusions related to the synchronous FDD scenario are alsoapplicable to the E-UTRAN TDD mode.

II. E-UTRAINTER-FREQUENCY HANDOVERSCENARIOSTo support various mobility scenarios in E-UTRAN two E-

UTRAN specific UE downlink measurement quantities arespecified [3]: reference symbol received power (RSRP) andreference symbol received quality (RSRQ).

The RSRP and RSRQ measurements are indeed analogousto WCDMA CPICH Ec/No and CPICH RSCP measurementsrespectively [4]. Thus RSRP is equivalent to the signal strengthmeasurement and is defined as the linear average of thereceived power of the resource elements carrying cell-specificreference signals within the considered measurement frequency

bandwidth. RSRQ is used to depict the cell quality and isdefined as the ratio of RSRP to E-UTRA carrier received signalstrength indicator (RSSI). The E-UTRA carrier RSSI is thelinear average of the total received power in OFDM symbolscontaining the reference symbols. It should be noted thatexcept the first OFDM symbol in a sub-frame, the remainingones can also contain data resource elements [1] [5]. Thismeans the E-UTRA carrier RSSI also incorporates thecontributions from resource elements carrying the user data.This property of the E-UTRA carrier RSSI component in thedenominator of RSRQ enables the depiction of the cell quality.

An operator would typically deploy more than one E-UTRA carrier frequency in the same coverage area. In order toensure efficient use of multiple carrier frequencies deployed inthe same coverage area, the 3GPP E-UTRAN standard

provides necessary procedures, mechanisms and radio resourcemanagement requirements pertaining to the E-UTRA inter-frequency handovers (IF) [2] [6]. In both E-UTRAN FDD andTDD the UE is required to perform RSRP and RSRQmeasurements of at least 4 inter-frequency identified cells perE-UTRA carrier [6]. There is also a requirement on the UE tomonitor up to at least 3 E-UTRA carriers [6]. This means intotal an E-UTRA UE shall be capable of measuring at least 12inter-frequency cells.

An inter-frequency handover allows an operator to achieveone or more of the following objectives: retaining servicequality, load balancing, retaining cell coverage etc. Our focusin this paper is to evaluate the first objective, that is, the

performance of the quality based inter-frequency handover. Inour analysis we make use of both signal strength (i.e. RSRP)and signal quality (i.e. RSRQ) to guarantee the servicecontinuity while retaining the desired quality of service. This

978-1-4244-2517-4/09/$20.00 2009 IEEE

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type of handover is needed when UE experiences poordownlink received quality due to the dominant co-channelinterference from intra-frequency neighbour cells, whichoperate on the same carrier as that of the serving cell. This

phenomena is commonly observed in high rise buildingenvironment, where despite strong received signal strength, thereceived signal quality can be poor due to the strong inter-cellinterference originating from the neighbour cells. In such

scenario an intra-frequency handover is less likely to improvethe received quality since in other intra-frequency neighbourcells the UE is expected to experience similar level ofinterference.

As shown in this paper, in such scenarios if two or morecarriers are available an improvement in the performance can

be achieved by employing a suitable inter-frequency handoverscheme. It is also shown that under these circumstances theinter-frequency handover exhibits its benefits if both RSRP andRSRQ are used. This is because the use of these measurementsensures that the target inter-frequency cell has both bettercoverage and signal quality.

A. Inter-frequency Handover PhasesThe inter-frequency (IF) handover procedure can be

divided into the following four distinct phases:

1) Measurement gap triggeringIn this phase the measurement gaps are triggered according

to an absolute threshold corresponding to one or more intra-frequency measurement quantities; for instance if the intra-frequency RSRQ falls bellow the network configured absolutethreshold. It is therefore based on measurement(s) from theserving cell only.

2) Inter-frequency measurements and event evaluationIn this phase the UE starts measuring inter-frequency cells

on the target carrier frequency during the activated gaps. Inaddition the UE also performs intra-frequency measurementquantities in between the gaps in a usual manner. Based onthese measurements the UE evaluates one or more networkconfigured event.

3) Inter-frequency measurements and/or event reportingIn this phase the UE reports to the serving cell both intra-

frequency and IF measured quantities and/or the correspondingevents evaluated during the previous phase.

4) Handover decisionBased on the UE reported measurements and/or events the

serving cell decides whether to perform the handover or nor bysending or not sending the handover command. The servingcell also chooses the most suitable cell in case multiple eventsare reported by the UE.

B. Inter-frequency Handover CriteriaAs explained in the preceding section that inter-frequency

handover evaluation should preferably be based on both RSRPand RSRQ. We analyze and compare various cases of inter-frequency handovers based on different combinations of RSRP

and RSRQ as expressed in table 1. The set of symbols (;)and (1;2) represent absolute thresholds and handover marginsrespectively.

TABLE I. IF HANDOVER CRITERIA

NoInter-frequency (IF) HO Evaluation Criteria

IF measurement gaptriggering criteria

IF handover decision criteria

1RSRPserving_cell < (RSRPtarget_cell - RSRPserving_cell) > 1

2RSRPserving_cell < (RSRQtarget_cell - RSRQserving_cell) > 2

3RSRQserving_cell < (RSRQtarget_cell - RSRQserving_cell) > 2

4RSRQserving_cell < (RSRPtarget_cell - RSRPserving_cell) > 1

5(RSRPserving_cell < )

OR

(RSRQserving_cell < )

(RSRPtarget_cell - RSRPserving_cell) > 1

AND

(RSRQtarget_cell - RSRQserving_cell) > 2

III. SIMULATION MODELThe performance of the algorithms is evaluated using

dynamic system simulation. The simulated area consists of 7hexagonal cells per E-UTRA FDD carrier; 1 cell per site withsite-to-site distance of 1.5 km. There are two E-UTRA FDDcarrier frequencies (f1 andf2) belonging to E-UTRA band 1 (2.0GHz range).

A. Traffic GenerationA mixed traffic scenario is considered in the simulation.

The mixed traffic comprises of two types of users on both

carriers in the system:

Web surfing traffic Voice over IP (VoIP) traffic

Uneven traffic load is created on the two carriers; where f1is deliberately more loaded than f2 by introducing more Webtraffic on the former. The web traffic on the loaded carriercorresponds to around 50% resource utilization. The objectiveof introducing higher load on one of the carriers (i.e. f1) is toenable the activation of gap-assisted measurements andsubsequent handover to a cell on f2 according to the evaluationcriterion.

Furthermore, there is one VoIP traffic user in the entiresystem whose performance is observed. The VoIP packet sizecomprises of 304 bits including all headers due to Real-timeTransport Protocol (RTP), User Datagram Protocol (UDP) andInternet Protocol (IP). The generated mean VoIP bit rate isapproximately 15.2 kbps. The voice packets which cannot besuccessfully delivered to the UE within 80 ms are dropped.

B. Measurement ModelThe measurement bandwidth of RSRP and RSRQ

comprises of 6 resource blocks (1.4 MHz). One transmitantenna at the eNode B and 2 receive antennas at the UE areused. Both receive antennas are assumed to be uncorrelatedwith equal gain. The physical layer (L1) measurement periodfor both intra-frequency RSRP/RSRQ and inter-frequencyRSRP/RSRQ measurements is 200 ms. Though in E-UTRAN

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standard [6] the inter-frequency L1 measurement period overwhich the minimum accuracy requirements of RSRP andRSRQ are to be met is 480 ms, but this is not likely toinfluence the conclusions of this paper. For each measurementquantity from each cell the UE obtains one snapshot of 4-5 msin time domain once every 60 ms during the physical layermeasurement period. This implies that 3 snapshots areaveraged over 200 ms period to determine the L1 filtered

RSRP and RSRQ. The eventual RSRP and RSRQ comprise ofthe linear average of the measured values from the two receiverdiversity branches.

Higher layer time-domain filtering, which is commonlyknown as layer 3 (L3) filtering [2], is used in dB scale. The L3filter coefficient (k) is set to 4, which corresponds to the timeconstant of approximately 664 ms. Time to trigger, which isused for event evaluation is not configured. Handover margin(for both RSRP and RSRQ) is 1 dB. This means handover tothe target cell target cell is considered provided themeasurement quantity used for the event evaluation from thetarget cell is 1 dB stronger than that from the serving cell.

C. Scheduling StrategyThe resource blocks are assigned by the eNode B scheduler

to the users in round-robin fashion both in time and frequencydomains. This means in the frequency domain, the resource

block allocation starts from the central resource block. The webtraffic and VoIP users are scheduled with equal priority.

D. Radio EnvironmentThe fast fading is used and the channel model consists of

3GPP-TU macro-cells [7]. The mean user speed is 90 km/hr.One transmit and two receive antennas are used at eNode Band UE respectively. The system bandwidth is 5 MHz. TheLog-Normal shadow fading with standard deviation of 8 dB is

used. The shadow fading correlation between cells is 0.5.

IV. SIMULATION RESULTSIn the simulations the voice user is not dropped due to the

bad link quality instead a voice packet not delivered within 80ms is discarded. Thus the performance of the voice users will

be severely degraded if the correct type of handover is nottimely executed i.e. the voice packet loss will increase. In orderto ensure adequate voice reception quality the mean voice

packet loss rate should not exceed 1%.

Thus in order to judge the performance of the 5 differentinter-frequency handover criteria mentioned in table 1, the

mean voice packet loss rate is one of criteria used in ouranalysis. Furthermore, the most suitable handover criterionshould on the average lead to lower number of handovers.Hence another criterion for the performance evaluation is thenormalized mean number of handovers with respect to thereference case which is based on criteria # 1 (i.e. where onlyRSRP is used).

In all scenarios the users perform the normal intra-frequency handover as well as the inter-frequency handoverwhenever necessary.

The simulations are done in both synchronous andasynchronous networks. In the synchronous network scenariothe frame timings of all the simulated cells are assumed to be

perfectly aligned. In asynchronous network the frame timing ofeach cell is set independently. In accordance with the RSRQdefinitions, in both synchronous and asynchronous scenarios,the E-UTRA RSSI part is measured in all the resource elementsin OFDM symbols containing the reference signals.

A. Synchronous Deployment1) RSRP and RSRQ Variation

Figure 1 and figure 2 show the variation of RSRP andRSRQ versus time respectively from the serving and one of thetarget cells in the simulated synchronous deployment scenario.Results are shown for two cases: low system load and highsystem load.

0 5 10 15 20 25 30-135

-130

-125

-120

-115

-110

-105

-100

-95

-90

-85RSRP

t [s]

dB

Starting BS, high load

Starting BS, low load

Target BS, high load

Target BS, low load

Figure 1. RSRP variation in synchronous network

0 5 10 15 20 25 30

-12

-10

-8

-6

-4

-2

0RSRQ

t [s]

dB

Starting BS, high load

Starting BS, low load

Target BS, high load

Target BS, low load

Figure 2. RSRQ variation in synchronous network

Figure 1 shows that the RSRP variation is insensitive to thesystem load. This is expected since RSRP is only measured onthe resource elements containing only the reference signals,which are transmitted in the same manner regardless of thesystem load or user data.

On the other hand figure 2 shows that the variation ofRSRQ is significantly affected by the system load. This is dueto the fact that the denominator in RSRQ (i.e. RSSI)

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incorporates the contributions from the resource elementscontaining the data, which in turn reflects the cell qualityexperienced by the UE. Too large and fast variations in RSRQare not desirable. Interestingly as observed in figure 2 thevariations are not very severe. One reason is the use of higherlayer time domain filtering leading to certain degree of stabilityin RSRQ measurement. Another possible reason is the use ofround-robin scheduling in the simulations.

2) Mean packet loss rateFigure 3 shows the mean voice packet loss rate for all the

five inter-frequency handover criteria in a synchronousnetwork deployment scenario.

The results illustrate that IF handover solely based onRSRP (criterion #1) does not even fulfill the target 1% voice

packet loss rate. Similarly, criterion # 4 according to which thetarget cell is evaluated based on RSRP also leads tosignificantly higher voice packet loss rate. But criterion # 5 inwhich both RSRP and RSRQ are used, guarantees that thevoice packet loss rate remains below the 1% target level.

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5Mean Voice Packet Loss Rate

%

Criteria #1

Criteria #2

Criteria #3

Criteria #4

Criteria #5

Figure 3. Mean packet loss rate in synchronous network

3) Mean number of handoversFigure 4 shows the performance of mean number of

handovers for all the five inter-frequency handover criteria in asynchronous network deployment scenario. The mean value isnormalized with the reference case, which corresponds tocriterion # 1.

The results in figure 4 illustrate that although criteria # 4ensures the lowest number of handovers compared to thereference case (criterion #1) but it also leads to the largest

voice packet loss rate of 1% (as shown figure 3). The mostdesired handover criterion should ensure lower number ofhandovers as well as lower packet loss rate. Thus, from the

perspective of the overall performance, we observe thatcriterion handover criterion # 5 (bases on both RSRP andRSRQ) exhibit the best performance.

0 1 2 3 4 5 60

10

20

30

40

50

60

70

80

90

100Relative Mean Number of IF HO

%

Criteria #1

Criteria #2

Criteria #3

Criteria #4

Criteria #5

Figure 4. Mean number of handovers in synchronous network

B. Asynchronous Deployment1) RSRP and RSRQ Variation

Figure 5 and figure 6 show the variation of RSRP and

RSRQ versus time respectively from the serving and one of thetarget cells in the asynchronous scenario. Results for thisscenario are also shown for the two cases: low and high systemloads.

Figure 5 shows that the RSRP variation is insensitive to thesystem load. This behaviour is very similar to the one observedin synchronous case. Thus as stated before, this is due to thefact that RSRP is only based on the reference signals, whichare transmitted in the same manner regardless of the systemload or user data.

0 5 10 15 20 25 30-135

-130

-125

-120

-115

-110

-105

-100

-95

-90

-85RSRP

t [s]

dB

Starting BS, high load

Starting BS, low load

Target BS, high load

Target BS, low load

Figure 5. RSRP variation in asynchronous network

Figure 6 shows that the variation of RSRQ is significantlyaffected by the system load also in asynchronous scenario. Thisis again due to the E-UTRA carrier RSSI component in RSRQ,which reflects the cell quality experienced by the UE. The useof higher layer time domain filtering also ensures certaindegree of stability in RSRQ measurement in this scenario.

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0 5 10 15 20 25 30-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0RSRQ

t [s]

dB

Starting BS, high load

Starting BS, low load

Target BS, high load

Target BS, low load

Figure 6. RSRQ variation in asynchronous network

2) Mean packet loss rateFigure 7 shows the mean voice packet loss rate for all the

five inter-frequency handover criteria in an asynchronous

network deployment scenario.

The results illustrate that all five criteria tend to fulfill thetarget voice packet loss rate of 1%. However criteria # 2 and #5 lead to relatively lower packet loss rate. This is because in

both of these criteria the HO decision is based on at least thecomparison of RSRQ between the serving and the target cells.This in turns ensure that the voice packet loss rate remain well

below the desired limit.

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5Mean Voice Packet Loss Rate

%

Criteria #1

Criteria #2

Criteria #3

Criteria #4

Criteria #5

Figure 7. Mean packet loss rate in asynchronous network

3) Mean number of handoversFigure 8 illustrates the normalized mean number of

handovers for the five inter-frequency handover criteria in anasynchronous network deployment scenario.

The figure shows that although criteria # 4 lead to thelowest number of handovers but the voice packet loss rate washighest. This is because the unnecessary avoidance or delay inthe handover increases the packet loss rate. As expressedearlier, the most desired handover criterion should ensure lowernumber of handovers as well as lower packet loss rate. Thus, interms of the overall performance, the handover criterion # 5

(bases on both RSRP and RSRQ) also appears to be the bestcompromise in an asynchronous scenario.

0 1 2 3 4 5 60

10

20

30

40

50

60

70

80

90

100Relative Mean Number of IF HO

%

Criteria #1

Criteria #2

Criteria #3

Criteria #4

Criteria #5

Figure 8. Mean number of handovers in asynchronous network

V. CONCLUSIONSSystem simulation results comparing several inter-

frequency handover evaluation criteria are shown in this paper.The two main performance measures are: mean voice packetloss rate and mean number of handovers. It has been shownthat the IF handover scheme entirely based on RSRP leads tothe worst overall performance compared to all other schemes,which involve RSRQ. Overall the handover criterion # 5 using

both RSRP and RSRQ is observed to show the bestperformance. This is because the combined criterion (#5)ensures that after handover both signal quality and the pilotstrength in the target cell are within the desired limit.

The simulated scenario is comprised of 2 E-UTRA carrierfrequencies, which are deployment in a regular hexagonal basestation sites. In more challenging downlink limited scenarios,

which are often encountered in high rise buildings in practicaldeployments, the RSRQ based handover schemes are likely to

provide even more gain in terms of reduced number of IFhandovers. Furthermore in such scenarios RSRP-only based IFhandover would cause even higher packet loss rate since worsedownlink quality is likely to be experienced due to the delay inthe handover process.

REFERENCES

[1] E. Dahlman, S. Parkvall, J. Skold, Johan, P. Beming, 3G Evolution:HSPA and LTE for Mobile Broadband. Academic Press Inc, 2008.

[2] 3GPP TS 36.331, Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification.

[3] 3GPP TS 36.214, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer - Measurements.[4] 3GPP TS 25.215, Physical layer Measurements (FDD).[5] 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-

UTRA); Physical Channels and Modulations.

[6] 3GPP TS 36.133, Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for support of radio resource management.

[7] 3GPP TR 25.943, Deployment aspects, version 4.1.0.