wcdma handover optimization
TRANSCRIPT
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Design of O ptimum Parameters for Handover Initiation in WC DMA
Jongin Kim , Dong-hoi Kim**, Pyeong-jung Song**, Sehun Kim*
*Korea Advanced Institute of Science and Technology (KAIST)
373-1, Kusung dong, Yusung Gu, Taejon, 305-701, KOREA
**Electronics and Telecommunications Research Institute (ETRI)
161 Kajong-Dong, Yusong-Gu, Taejon, 305-350, KOREA
Email : [email protected]
Abstract
WCDMA handover algorithms employ signal
averaging, hysteresis and the Time-to-Trigger mechanism to
optimize the trade off between nu mber of unnecessary handover,
reported events(system load) and handover delay time. We
investigate optimal parameters for the WCDMA intra-.
frequency handover algorithm and the impact of each
parameter on the system performance. Number of reporting
events triggered for handover and handover delay are key
performance measures in this paper. The study shows various
tradeoffs between the parameters related to averaging,
hysteresis and Time-to-Trigger. We have also discovered that
the layer3 filter and Time-to-Trigger mechanism may cause
negative effects on each other in some cases and there are
optimum values, when used sim ultaneously.
1
INTRODUCTION
Design of handover initiation can be made to use several
measurements such as the received signal level from the
communicating and neighboring base stations, the path loss
to the base stations, and bit error rate. In general, hysteresis
and signal averaging is employed to enhance the performance
of handover(i.e. probability of unnecessary handover at the
expense of handover delay). Previous studies on handover
initiation have revered that there are trade offs between
handover delay and n umbe r of unnecessary handover.
Handover initiation criteria analyzed in literature are
mainly based on the length of averaging window, the
threshold level and the hysteresis margin. In addition,
WCDMA introduce the Time-to-Trigger mechanism to
reduce unnecessary signaling and ping pong effects. Also
averaging window is used to smooth out random signal
fluctuations and to make handover decisions to be based on
underlying trends and not instantaneous changes.
Soft handover is essential for intra frequency
in
WCDMA.
The active set i s defined as the set of base stations to which
the mobile users is simultaneously con nected. Soft hand over
involves active set update procedure which include signaling
of appropriate event triggered by the mobile based on the
measurement of the measurement quantity (i.e. Ec/Io, path
loss, etc). Frequent reporting may cause unnecessary
handover and signaling overload. On the other, if the
reporting is too seldom, it may increase the han dover delay.
WCDMA(3GPP) recommendation does not specify the
measurement and averaging interval be fixed or variable.
Actual physical layer measurement depends on the
implementation of the mobile unit. However, WCDMA
specifies the network controlled features to enhance the
performance, which include the hysteresis, Time-to-Trigger
and Layer3 filtering. A network controlled Layer3
fiItering(exponentia1 smoothing) provides same options as to
hysteresis and Time-to-Trigger to some extent, but give some
extra benefits which makes it possible to control the rate of
reporting, i.e. system loads. Therefore, it is our interest is to
investigate the impacts of each of the network controlled
elements, including Layer3 filter, hysteresis margin and
Time-to-Trigger, to handover performance after applying a
minimal physical layer measurement. Our goal is to optimize
the parameters for these handover mechanisms considering
various ttadeoff relations. By using an appropriate
combination of filter, hysteresis and Time-to-Trigger, it is
possible to fine tune the real time decisions to be optimal in
time and amplitude. Therefore, we can optimize parameters
related to hiandover decision.
3. SYSTEM DESCRIPTION
A. Meas urem ents and Signaling
In WCDMA system, the mobile station performs intra-
frequency measurement and sends measurement report to the
Radio Network Controller(RNC), where the final decision is
made about which cell to add or remove from the Active
Sets[2]. The intra frequency measurement is done on the
downlink P-CPICH [l]. Measurement quanti ty can be any
of'
the followings; Ec/Io, path loss and the Received Signal Code
Power[
11.
Consider a model for a network controlled handover
filtering (Signal averaging) shown in Figure 1 This model is
as recommended in 3CPP specification[ 11. Parameter 1 is
related to :shape of Layer3 filter provided by the network and
0-7803-7005-8/01/ 10.00
0
2001
IEEE
2768
parameter 2 is related to types of handover, i.e.
frequency, inter-frequen cy, etc and reporting criteria.
I
Paramters
2
P a m t e r s
1
Layer Layer 3
filtering iltering
-b
valuation
_
ofreporting
Figure 1. Model for handover measurements
intra
D
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Uu lub
Physical layer implementation (inputs A and Layer 1
filtering) is not constrained by the standard i.e. the model
does not state a specific samp ling rate or even if the sampling
is periodic or not. What the standard specifies is the
performance objectives and reporting rate at point B in the
model. Th e reporting rate is equal to the measuremen t period,
which is 200ms for intra-frequency measurement. The
performance objectives for the physical layer measurements
are specified in [3].
In addition , the Layer3 filtering
is
performed according to
the following exponential averaging formula to give more
accuracy.
The variables in the formula are defined as follows;
F,,
s the
updated filtered measurement result. F,+,
is
the old filtered
measurement result. M,, is the latest received measurement
result from physical layer measurements. If
a
is
set to
1
that
will mean no layer 3 filtering. Also, smaller will mean that
it
is
giving more weights to past samp les.
Hysteresis and Time-to-Trigger mechanism on the other
hand is important for reducing unnecessary signaling or
handover and they complement to averaging mechanism.
Evaluation of reporting criteria
is
based on the measurement
results (after appropriate filtering) using the hysteresis and
Time-to-Trigger mechanism. The reporting event 1A and
1
B
is
defined as;
-Meas_Sign>Best-Ss-Hyst-Add for AT: Event A
-Meas-Sign
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2:
Propagation Model
Channel Model
Standard Deviation of
I
-
0
1000
2 3 4
Distance
traveled by mobile
m)
Figure4. Model for handover measurements
128.1+37.6log (R)
ITU-Vehicular A
1OdB
As it can be seen, the basic Layerl filtering of 200ms in
our model does not completely average out the signal
fluctuation. Relationship between the accuracy and the
measurement distance is described in [5]. Our interest i s to
investigate the effects of the network controlled elements,
such as Layer3 filter, hysteresis margin and Time-to-Trigger,
on the handover performance after applying a minimal
Layerl filtering. The simulation parameters are listed below
and the channel model
i s
described in the following
subsection.
CPICH Power
Parameter Value
47dBm
Log-Normal fading
Decorrelation des tance 20m
I
H
yst -ADD lSdB,3dB,4SdB,6dB
I
Hyst -Drop 2SdB,5dB,7SdB,lOdB
Ti me-to-Tri gge r Oms,200ms,400ms
Measurement Pen od 200m s
Sampling interval
Layer3 filter coefficient
0.1-1
I
Table 1. Simulation parameters
A.
Propagation Model
Th e received signal at a mobile consists of three parts; path
loss, slow fading and fast fading (Rayleigh distributed).
Therefore, the received signal (in dB) as a function of m obile
distance
i s
given by,
r d ) =
K ,
-
K , log,, d) + v d ) + 201og,,[e d)]
The parameters K1 and K2 accounts for path loss, v(d) i s
the shadow fading process; zero mean, variance lOdB,
stationary Gaussian process. The shadowing process is
assumed to have the exponential correlation function
proposed by Gudmundson[6]. Decorrelation distance is
assumed tci be 20m in vehicular environments [4].
For the fast fading, we use ITU Vech icular A m odel[4].
Received s,ignal after filtering is then given by
~
d )
= K,
-
K,
l o g , , ( d ) +
V d )+
2010g l , [ e ( d ) ]
B. Performance Measures
Optimal handover is the trade off between the number of
unnecessai-y handover and the h andover d elay. Many
previous literatures have studied the properties of this trade
off for various parameters such as the hysteresis margin and
the length of averaging distance[8]
In W CDM A, the standards specify the measurem ent model
and the range of parameters like Layer3 filter coefficient,
hysteresis and Time-to-Trigger. But, the impacts of these
parameter:: and different choices for the values remain to be
clarified.
WCDMA use soft handover mechanism to enhance the
coverage and capacity of the network. Soft handover
mechanisrn involves active set update and removal as
described in the previous section. To many reporting events
will cause unnecessary active set updates and increase the
signaling load. On the other hand, infrequent reporting may
cause delay in handover. Optimal size
of
soft handover
depends on loading conditions and, etc. The size of soft
handover area can be also controlled by the system
parameters.
Number of reporting events triggered for handover and
handover delay are key performance measures in this paper.
In our simulation, the tradeoff between the number of
reporting events and average distance of active set
additioniremaval, averaged over 1000 runs, are investigated
with different hysteresis margins, Layer 3 filter coefficients
and Time-to-Trigger. Average distance
of
reporting event
1
A
i s the mean distance at which the active set addition for BS2
takes place.
5. SIMULATION RESULTS
Figure
5 .
shows the expected number of report ing e ventlA
for mobile traveling at speeds 50km/h and 120km/h with
various hysteresis, not using Time-to-Trigger. Number of
reporting events
i s
quite large when Time-to Trigger is not
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used. It can be observed that the layer3 filter can reduce the
number of reporting events significantly. Especially at low
mobile speed, it shows significant improvements. The effect
of the hysteresis is also shown in this figure. Figures
9.
and
IO show the mean distance at which the mobile sends the
reporting Even tIA for BS2 and eve ntlB for BS1,respectively.
It can be interpreted as the expected point where the mobile is
enterindleaving the
soft
handover area. This position
depends on the setting of hysteresis levels, but Layer3
filtering also has effects of delaying the distance of
enterindle aving the area. Similarly, as show in Figures
1 1
and 12, the Time-to-Trigger mechanism also delays the
reporting even ts. Th e gain of soft handover and optimum size
depends on many factors including the system loads and the
capacity.
Figure 6. represents the expected number of reporting
event
1A
with Time-to-Trigger of 200ms. It is interesting to
observe that the rate of increase of the number of reporting
event starts to slow down at some point as alpha is increased.
Further, in Figures 7 and 8, with 4 00ms Time-to-Trigger, the
number of reporting actually begin to decrease at some point
along alpha. This characteristic is explained as fo llows. First,
with no Time to Trigger, the smoother curve will obviously
give less reporting events since it has smaller variations. If
the Time-to-Trigger of 200ms and 400ms is used, two and
three adjacent samples are subsequently evaluated,
respectively. Exponential averaging induces correlations
between these samples. Correlated samples will be
undesirable than independent samples in extracting the
average value. Therefore, there exists a tradeoff between
obtaining the stable measurement results and getting
independent samples. Consequently, in setting Time-to-
Trigger and Layer3 filter constant, we may consider this to be
compromised.
.+
c
3 -
0
r
a
[r
c
-
2 1 2 -
1 1 -
n
1 0 -
60
I
50
t
5
40
0
3
0
g
20
a
10
Z
0
yst-Add=B, Hyst-D
rop= lO,
V=l20
krnh
0 . H yst-Pd d=3, Hyst-D rop=5, V=l2Okrn/h
yst-Ad
d=6,
Hyst-0
r o p=lO , V=SOkrn/h
Hys t_Nd=3 . Hyst_Drop=5,
V Okrn/h
I I
0 0 0 2 0 4 0 6 0 8 1 0 1 2
Layer3 Filter Coefficient(a)
Figure
5.
Average number
of
reporting eventlA
Time-to-Trigger = Oms)
5
4
-0 /J
0 0
0 2
0 4 0 6
0 8
1 0
Layer3 Filter Coefficient(a)
Figure
6.
Average number
of
reporting event 1A
Time-to-Trigger
=
200ms )
2
1 5
0
-e Hyst_Pdd=3, Hyst_Drop=5
0 Hysl-Pdd=l 5, Hyst_Orop=25
0 9
0 0 0 2 0 4 0 6
0 8
1 0 1 2
Layer3 Filter Coefficient(0)
Figure 7. Average number of reporting event 1A
V=120km/h, Time-to-Trigger = 400ms)
U
f
v
/
0
0
.- 0
Hyst_Pdd=3,
Hysl_Drop=5
yst-Add=l 5. Hyst_Drop=25
/
0 0
0 2 0 4 0 6 0 8
1 0 1 2
Layer3 Filter Coefftcient(0)
Figure8. Average number
of
reporting event 1A
V=50km/h, Time-to-Trigger
=
400ms)
277 1
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1600
\
1750
700
650
-
E
J 1600
1550
s
0
-c H yst -A d d 4 d B , Hy st-D rop=5dB
0
Hyst_Pdd=6dB, Hyat-Drap=lOdB
0.
2600
2500
-
E
8 2400
E
P
2300
-
...
i
2200
21 00
0
* 0 0 0 0 0 0
Hyst_Pdd=3, Hyst_Drop=5
0 Hyst_Pdd=6, Hyst-Drop=lO
0 0 0 2
0 4
0 6 0 8 1 0 1 2
Layer3 ilter soefficient(0)
Figure
I O .
M e a n d i s t a n c e
of
r e p or ti n g e v e n t l B(50km/h)
1820
1800
-
E
-
8
1780
E
0
1760
...
d
1740
1720
T
-4- Time-to-Trigger=Oms
0 Time-to-Tr igge~200ms
ime-to-Tr igge~400ms
\O
0 0
0 2
0 4 0 6
0 8
1 0 1 2
Layer3 ilter coefficient(a)
Figure 11. Mean distance
of
reporting eventlA
Hyst_Add=3,Hyst_Drop=5,
50krn/h)
2300
r I
2280
.-.
E
2269
E
2240
A
Ln
22zD
Time_to_Trigger=2SOms
o\
\
\
2200
0 0
0 2
0 4
0 6 O B
1 0 1 2
Layer3 filter coefficient(a)
Figure 12. Mean distance of reporting eventl B 50km/h)
Hyst-Add=3, Hyst_Drop=5, 50km/h)
6.
CONCLUSIONS
This study investigates the impacts of each of the network
controlled features (including Layer3 filter, hysteresis margin
and Tim e to Trigger) in handover initiation mechanism. The
study shows various tradeoffs between the parameters. It was
investigated in terms of the number of event reporting and
handover delay distance. The performances are also shown to
depend on the velocity of the mobile. We have also
discovered that the layer3 filter and Time-to-Trigger
mechanism may cause negative effects on each other in some
cases anti there is an optimum combination, when used
simultaneously. The results presented in this study may help
in understanding the behavior of the features related to
triggering of handover measurement reports and in extracting
optimum parameter values. Further, these results can be used
for developing more efficient handover algorithms.
REFEREN E
[I ] 3GPP T S 25.302 Ver 3.3.0 Services prvided by physical
layer, March, 2000
[2] 3GPP T S 25.331 V er 3.6.0 RRC protocol specification,
March,
2000.
[3] 3GPP T S 25.133 Ver 3.5.0 Requirements for Support of
Radi o Resource Management ,December, 2000
[4] ETSI TR 101 112 V3.2.0, Selection procedures for the
choice of radio transmission technologies of the UMTS,
April , 1998
McGrawHill , 1981
[ ]
C.Y.Lee, Mobile Communications Engineering
[6] M.Gudmunson, Correlation Model for Shadow Fading in
Mobile Radio Systems, Electronics Letter, Vol 27, no23,
pp 2145-2146, NOV 1991.
[7] R.Vijayan and J.M.Holtzman, A Model for Analyzing
Handoff Algorithms, IEEE Trans. On Vehicular
Technology, August 1993.
Cominunications M agazine, March 1996
[8]
Gregory P. Pollini, Trends in Handover Design, IEEE
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