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Performance of RSCP-triggered and Ec/No-triggered inter-frequency
handover criteria for UTRA
Conference Paper · February 2002
DOI: 10.1109/VTC.2002.1002576 · Source: IEEE Xplore
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Performance of RSCP-Triggered and Ed"-Triggered
Inter-Frequency Handover Criteria for UTRA
Wang Yingl, Gong Fei', B a n g Ping', Wang Hai2
'Wireless Technologies Innovation Laboratory, Beijing University of Posts and Telecommunications,
P.O.Box 92, BUPT, Beijing 100876, China,
2 .
Encsson (China) Company Ltd.,
9A, Hanwei Plaza, N0.7 Guanghua Lu, Cbaoyang District, Beijing 100004, China,
Fax:++86-10-62283627
Tel: ++86-10-62283622-211
E-mail: wanm+M2ilbuDt.edu.cn
Abstract: This paper compares two different handover
criteria in WCDMA through simulations. The simulations
are carried out in a WCDMA HCS system where the
hexagonal macro and the Manhattan-like micro layer use
different frequencies. Two possible handover triggering
schemes Le. CPICH RSCP and CPICH EcINo, are compared
under the same simulation environments and various cell
loads. The handover performance are observed in terms of
system efficiency including signalling and QoS such as call
dropping probability and call blocking probability.
I INTRODUCTION
Hierarchically layered cell structures based on pico,
micro, and macro cells, partly or in some cases even fully
overlapping, will be vital for third-generation mobile
systems. Since the same frequency allocated to the different
layers in the HCS could result in severe self-interference and
make the WCDMA unstable, different frequencies have to
be assigned to different layers in order to avoid power
control problems and excessive interference[l]. So in thirdgeneration mobile systems, one of possible types of
Handover is FDD inter-frequency hard handover[2].
Inter-frequency handover presupposes that a mobile
station is able to monitor pilot channels and to transmit and
receive signaIs quasi-simultaneously on two different
frequencies. This can be accomplished by using a costly
second radio transceiver in a CDMA mobile station or using
compressed mode technique. In order to limit the complexity
of a UE and reduce the cost, compressed mode is used to
perform inter-frequency measurement. However, this will
introduce new problems. The disadvantage of using
compressed mode is the performance degradation of the
network because of the power rising[31[4][51.
What we focus in this report is the criteria to trigger an
inter-frequency handover. Currently, some parameters used
to trigger handover are as follows[6]:
1
CPICH RSCP: Received Signal Code Power, the
received power on one code measured on the Primary
CPICH.
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CPICH EcINo: The received energy per chip divided
by the power density in the band. Measurement shall be
performed on the Primary CPICH.
SIR: Signal to Interference Ratio. The SIR shall be
measured on DPCCH after RL combination.
There have been some research results about handover
criteria[7][8][9][10]. According to [8], the SIR measurement
needs significantly longer averaging than the EdNo
measurement. This ~ h l r a l l ycauses significant delay to the
bandover preparation process meaning that in case SIR on
CPICH is used for the handover evaluation, the actual
bandover is delayed. On the other hand if the averaging
period is not lengthened for the SIR measurement, the
probability of the correct evaluation of neighbor cell
strengths becomes poorer and thereby undesirable. Since we
do not see any practical reason why the SIR should he added
as a measurement quantity for the handover evaluation, after
long time discussions within the 3GPP TSG-RAN WG4[9],
the SIR is not listed as a handover criterion in the 3GPP
technical specifications. So in this report the focus is laid on
the comparison between RSCP and E m o .
The structure of the report is described as follows.
Section U analyzes two different handover criteria
theoretically. Section III describes Inter-frequency handover
model which involves models for compressed mode
operation, inter-frequency bandover execution and handover
delay. Simulation results are presented in section lV
followed by the conclusions in sectionv .
.
II EcMo VS. RSCP
[lo] shows that in case of inter-frequency handover the
EcINo provides load sharing between different carrier
frequencies, while the RSCP does not take the load situation.
RSCP and EcINo are defined as follows:
RSCP =
VTC 2002
where
is the effectively received total wide-band power after
receiver filtering, and P,cp,,, is the received CPICH power,
and
'/O
and P
'fi
is the total wide-band received power
fi
from frequency f, and
respectively, and ACIRi is
adjacent channel interference power ratio between frequency
f,and ,and P' is the power of thermal noise.
fi
cell bo*
SCII borda
"0)
(RSCP)
Assuming that the traffic load on frequency 1 is larger
than the traffic load on frequency 2, consequently the
received power from frequency 1 is larger than the received
power from frequency 2
P,/, > P,rl, and moreover
c,>
-<l--
e,!
.
1
ACIR
Thus, we have
r
1
i
.
Figure 1. General inter-frequency hard handover in
WCDMA
Supposing that cell 1 and cell 2 are using frequency 1
and 2 respectively(shown as figure I), then
(E, / N o ) ,= -e@,.*,
e<-,
From the above equation, we can conclude that the
WNo-based handover relieves the traffic load on the
heavily loaded frequency. So it naturally provide a loadsharing feature.
However, it does not consider the impact of compressed
mode. Moving mobiles between the cell layers frequently
generates a lot of interference due to compressed mode and
requires a great amount of control signalling. Especially
when the load is heavy in a cell, too much inter-frequency
handover will result in power rising and system may be
unstable.
+PN
INTER-FREQUENCY HANDOVER MODEL
As the core of the research issue is the handover
between cell layers on different frequencies, models for
compressed mode operation and inter-kequency handover
(including bandover delays) are needed. The model can be
simple but should capture the cost of compressed mode in
terms of increased transmission power in both uplink and
downlink.
If only frequency 1 and frequency 2 are. used in the
WCDMA system, then
0-7803-7484-3102/$17.00 02002 IEEE
a. Models for compressed mode operation
Compressed mode is a mechanism that produces an idle
period for a UE to perform an inter-frequency measurement.
When in compressed mode, the information normally
transmitted during one 10 ms frame is compressed in time.
Then the user is able to make inter-frequency measurement
in the idle period and receive data in the rest period of a
compressed mode (figure 2).
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VTC 2002
Therefore, in OUT simulation, once 20 frames are
compressed successfully and handover conditions are met, a
hard handover request is sent to RNC by MS. This request
will be dealt with by CAC(Cal1 Admission Control) function
located in RNC. If it is accepted, the old link will be broken
and the new link will be established. Otherwise, the MS still
remains its old link and changes nothing.
Figure 2: Compressed mode transmission
In compressed mode, there are many changes in system.
Target SIR shall be adjusted in compressed mode. And the
power control procedure in compressed mode holds
additional features, which aims to recover as rapidly as
possible a SIR close to the target SIR after each transmission
gap. Two mechanisms are provided to achieve this goal. The
first is the Power Resume Mode (PRM), which sets the UE's
initial power after a transmission gap. In our simulation we
take a value equal to 61m.which shall be equal to the most
recently computed value of 6i. 6i shall be updated according
to the following recursive relations:
6,= 0.937S6,., -0.9687STPC-cmdiA,
c. Handover delay
Handover delay can be divided into handover decision
delay and handover execution delay. Handover decision
delay is the time between when the user should handover
and when the decision is made to handover, and handover
execution delay is the time between when the decision is
made to handover and when the corresponding handover is
completed.
For inter-frequency handover, handover decision delay
includes the time used to perform compressed mode. At least
20 compressed frames are needed to finish measurement,
that is, the minimum decision time of inter-frequency
handover is: T=20*10ms=200ms.
The handover execution delay is assumed to be zero in
our simulations.
= 6,
(8)
6i-1 is the value of 6i computcd for the previous slot. The
value of&-1 shall be i n i t i a l i d to zero.
The second mechanism is Power Control ModepCM),
which controls the power control step size and algorithm for
interpreting TPC commands for a number of slots after each
transmission gap.
Compressed mode may bc rriggercd periodically or when
something such as link quality degradation or load balancing
happens.
In periodic triggered method. all the candidate mobiles
trigger the compressed modc pcriodically. To minimise the
interference rising. MSs perform compressed mode in turn.
In event-triggered method. whahcr or not the compressed
mode can be triggered is dmided by a certain criteria. For
the sake of simplicity, RSCP is adopted as triggering
criteria.
b. Inter-frequency handover execution
As we know inter-frequency handover in WCDMA
FDD systems is hard handover, which should comply with
"break before make". According to [ll], hard handover
initiated by the network is normally performed by the
procedure "Physical channel reconfiguration". If the UE
does not succeed to establish the connection to the other
radio access system, it shall resume the old connection using
the resources used before and transmit the HANDOVER
FAILURE message. When the transmission of the
FAILURE message has been confirmed by RLC, the
procedure ends.
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SIMULATION RESULTS
The difference between the !&'"-based
and the RSCPbased inter-frequency handover is studied in a WCDMA
HCS (Hierarchical Cell Structure) system where the
hexagonal macro and the Manhattan-like micro layer use
different frequencies.
Simulations for the Edo-based and the RSCP-based
inter-frequency handover are done provided that cell
selection and sofl handover criteria are both based on
CPICH RSCP. Moreover, it is assumed that compressed
mode is triggered periodically. In periodic triggered method,
all the candidate mobiles trigger the compressed mode
periodically. To minimise the interference rising, MSs
perform Compressed mode in tum.
According to 1121, which discusses criteria for triggering
compressed mode, three parameters are needed to determine
the measurement period. They are the number of users
allowed to be in one compressed frame at the Same time per
cell, the number of compressed frames that one
measurement takes and the number of users allowed to be in
compressed frame in every cell, which are denoted by ' M ,
'T', 'N'respectively.
In our simulation, we set these parameters as follows:
M=2, only two users in the same cell can perform
compressed mode at the same time;
* T=20, at least 20 compressed frames are needed to
finish measurement.
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VTC 2002
.
N=the number of all users in the cell. All of the user
are allowed to perform compressed mode.
Thus, we can get the measurement period, which can be
regarded as the handover decision delay.
Measurement period =rNIM].T=rN/21.20
(9)
Where the function [Xirounds the elements of X to the
nearest integers greater than or equal to X.
If a cell has 30 users, the measurement period will be
(30/2)*20*10ms=3s.
To access the performance of the different handover
criteria, comparisons are made under various cell loads. Fig.
3 shows the call blocking probabilities of EcNo-based and
the RSCP-based inter-frequency handover scheme,
respectively, against the offered load per cell. And the hard
handover thresholds are set OdB and 3dB, respectively. In
the figure, the blocking probability of the Ec/No-based
scheme is a little bit lower than that of the RSCP-based
inter-frequency handover scheme.
Fig. 4 shows the call dropping probabilities for Ec/Nobased and the RSCP-based inter-frequency handover
scheme, respectively, against the offered load per cell. And
the hard handover thresholds are set OdB and 3dE3,
respectively. It can be seen that RSCP-based inter-frequency
handover scheme outperforms much better than the EdNobased scheme, especially when the cell loads are heavy.
To evaluate system performance roundly, we should
consider both dropping probability and blocking probability
together, so we define a new parameter Pms:
PQd = 3kd
In above equation,
+ opdmp
Figure 3. Blocking probability of EclNo-based and the
RSCP-based inter-frequency handover
(10)
ekkis blocking probability, Pd,opis
dropping probability, W is a weight factor. Generally, we
have w = 10.
Therefore, fig. 5 gives us the overall performance
comparison. Simulation results show that RSCP based interfrequency handover scheme outperforms the EcfNo based
scheme. And when hard handover threshold equals to 3dB,
the system performance is better.
It is known that not only the QoS but also the signalling
load of a system should be taken into consideration. Fig. 6
gives the number of inter-frequency handover requests per
second and f i g . 7 gives the probability of successful interlayer handover executions for different criteria under various
cell loads. To some extent, they can indicate whether the
signalling load is heavy or not.
From figure 6 , we can see that E m o causes more interfrequency handover requests than RSCP,which accords with
our analysis results in section 11. This naturally increases
the signaling load and results in performance degradation.
0-7803-7484-3/021$17.00 02002 lEEE
20
M
40
50
eo
70
80
DO
OFFERED LOAD PER CELL
ID0
110
I20
Figure 4. Dropping probability of EcNo-based and the
RSCP-based inter-frequency handover
In figure 7, we find an interesting phenomenon that the
probability of successful hard handover of Ec/No-based
scheme is much greater than that of RSCP-based scheme.
This is related with call admission control algorithm, which
is based on interference information[l31. CPICH EdNo
criteria includes interference information in each layer, so
inter-layer bandover request based on E c N o is apt to be
accepted by CAC algorithm. While CPICH RSCP only
indicates the pilot signal strength, doesn't include the
interference information. Thus results in lower successful
inter-layer handover probability.
VCONCLUSION
RSCP and EcNo triggering schemes are compared under
the same simulation environments and various cell loads.
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VTC 2002
The handover performance are observed in terms of system
efficiency including signalling and QoS such as call
dropping probability and call blocking probability.
Simulation results show that EcMo can trigger more hard
handover requests than RSCP. But RSCP based interfrequency handover scheme outperforms the EcMo based
scheme. And generally when hard handover threshold equals
to 3dB, the system performance is better.
ACKNOWLEDGEMENTS
This work is financed by Ericsson company. Their
support is gratefully acknowledged. And the authors are
indebted to Wang Weidong, Shang Dan and Shen Uichun
for many useful discussions.
a 021
21
IO
’
40
50
60
70
(10
OFFEREDLOADPERCELL
m
100
(10
1
$20
Figure 7. Probability of successful inter-layer handover
REFERENCE
ol
20
,
30
.
40
,
,
,
,
,
,
I
100
110
110
,
(10
10
IO
00
0FFERE.D LOAD PER CELL
$0
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[3] 3GPP TSG RAN, “Physical channels and mapping of
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[4] 3GPP TSG RAN, “Multiplexing and channel coding
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[5] 3GPP TSG RAN, “Physical layer procedures O D ) ” ,
3G TS 25.214 V3.3.0 (2000-06)
[6] 3GPP TSG RAN, “Physical layer - Measurements
Figure 5. QoS probability of EdNo-based and the RSCPbased inter-frequency bandover
,
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,
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,
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(TDD)”,
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[9] 3GPP TSG RAN WG4 Tdoc 00/264, “Further
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2n
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[13]Wang Ying, B a n g Jingmei et al, ‘‘Call Admission
Control in Hierarchical Cell Structure”, VTC 2002,
spring.
320
Figure 6. The number of inter-layer handover requests per
second
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