Communications and Network, 2013, 5, 344-347
http://dx.doi.org/10.4236/cn.2013.53B2063 Published Online September 2013 (http://www.scirp.org/journal/cn)
Analysis of Relay Deployment Based on Handover Outage
Probability in High Speed Scenarios
Liang Jing, Xiaojie Xu, Y afen g Wang
Beijing University of Posts and Telecommunications, Beijing, China
Received July, 2013
ABSTRACT
The LTE and LTE-Advanced systems are supposed to be the most popular cellular network in 4G networks. Relay
technology is one of the most preferred technologies proposed in LTE-Advanced standardization to improve the capac-
ity and coverage. This paper analyses the handover outage probability through relay deployment, and proposes some
proofs and suggestions for relay deployment to minimize the h andover outage probability in high speed scenarios.
Keywords: Outage Probability; Relay Deploy ment; Handover; High Speed Scenarios; LTE-advanced
1. Introduction
LTE-Advanced network, with one or more relays in one
cellular, may be used to cover some high speed scenarios
such as railway, highway and so on. As people are mov-
ing faster and faster via variou s vehicles, which brings up
some challenges to security and reliability of the network ,
handover is becoming an important issue in those high
speed scenarios.
Handover is usually based on sign als transmitted from
base stations (BS) or relay stations (RS) to mobile sta-
tions (MS). If MS finds out that the quality of signals
from neighbor BS or RS is better than that from home BS
or RS while other conditions are met, then MS releases
channels in home cell and begins to use channels in the
neighboring cell after a handover process. In high speed
scenarios, signals transmitted in LTE-Advanced network
not only experience shadowing and multi-path, but also
suffer Doppler shift due to high speed. As a result, some
solutions may be adopted to solve the problem. In previ-
ous work, handover algorithms are proposed to set dif-
ferent handover threshold [1-3] depending on different
speeds. As we know, handover often occurs in over-
lapped areas, which is far away from BS, an accurate
threshold can hardly be set to meet different speeds.
Relay technology can improve the coverage, enlarge
the network capacity and reduce the energy cost. Of
course, relay technology can also be used to assist hand-
over in LTE-Advanced network. In LTE-Advanced net-
work cells with relays, when MS decides to make hand-
over decisions, it not only measures signals from
neighboring BS and home BS, but also from RS. As a
result, the deployment of RS is very important for hand-
over. The position of RS may affect the outage probabil-
ity and the stability after h andover. In this paper, a novel
analysis of handover outage probability based on signal
strength in different relay deployments is made. Based on
the analysis results, conclusions about relay deployment
in high speed scenarios are made.
2. System Model
In cellular networks, there are two main deployments,
which are regional coverage and linear coverage respec-
tively. Regional coverage is usually applied in urban area,
where people in wireless communication needs are dis-
tributed in a large area. For those with wireless commu-
nication service demands in high speed such as railway,
linear coverage is preferred and should be applied. As
showed in Figure 1, the center of round cells is in the
same line with MS traveling along. Th e network showed
in Figure 1 has a frequency reuse factor of 3, and they
are 1
f
, 2
f
and 3
f
respectively.
2.1. Traditional Handover Scenario
(without Relay)
In th is sc enar io, whe n MS in cell travels along the line in
Figure 1. LTE-Advanced network deployment with reuse
factor of 3.
C
opyright © 2013 SciRes. CN

L. JING ET AL. 345
LTE-Advanced network, it constantly measures the sig-
nals from BS in cell (home BS) as well as from BS in
cell and cell (neighboring BS). As long as conditions for
handover are met, MS handovers are from cell to cell.
Without loss of generality, signals transmitted from
source to destination can be described as follows [4]:
()/1 410 xx
xx
SSr
  
0 x=i, j, k (1)
where is the ID number of home cell, and , k are
the ID numbers of neighboring cell.
ij
x
S is the power
received from BS in relative cell, S is the transmitted
power of the source.
x
r is the distance between the
source and the destination.
x

, which represents the
shadowing of signals received from the source, is a
Gaussian distributed random variable with a zero mean
and a standard deviation
x

.
x

is a constant that rep-
resent the Doppler shift effect of signals received from
BS or RS in corresponding cells. When MS moves faster,
x

is getting smaller.
x
G, representing path gain can be
defined as follows:
()/1 410 xx
xx
Gr
  
0
G


x=i, j, k (2)
The condition for handover in network without relay
can be described as follows[5]: when signals received
from home BS is at least smaller than that received
from neighboring BS, then the MS handover to the
neighboring BS. The MS handover probability is:

Pr ,
handoverj ijk
PGGG


 (3)
Let 0

 , since

is a Gaussian distributed ran-
dom variable with zero mean and a standard deviation
x

, Equation (3) become s[6 ] :
2
02
40
2
400
Pr ,
(10log(/)()/)
1
*(10log( / )()/)*2
j
handoverj ijk
iji ji
kjk jk
j
PGGGG
rr
rre d




 








 





(4)
After MS handovers to the target cell, it may be outag e
because of the co-channel interference and other inter-
ference. With the assumption that interfering signals re-
ceived only from BS in cell l that has the same frequency
as cell j, the co-channel interference ratio can be repre-
sented by:
()/10 ()/1
44
()/10 ()/10 44
*10 10
*10 10
jj jj
ll ll
jj
ll
Sr r
C
ISr r
 
 


 

0
(5)
where
j
r, l represent the distance between MS and
BS j or BS l, l
r

is a Gaussian distributed variable with
zero mean and a standard deviation l

.
j

, l

repre-
sent Doppler shift effect of signals received from BS in
cell j and in cell l, and
j

<0, l

<0. The outage prob-
ability after handover is given by:
__target cell outage
P
target_cell_outage C<
I



P=Pr


 (6)
where

is a predefined threshold. Let 0l


, then
Equation (6) becomes

2
02
l
target_cell_outage
1
4j40jl j
l -
-20
l
P
C
=Pr<=Pr I*>
+
C
I
r
=F(10log() +(-)/)
r
1
**ed
2



 














 (7)
2.2. LTE-Advanced Handover Scenario
(with relay in cells)
In this scenario, with relay in cells[7], when MS travels
along the line, it not only constantly measures signals
from home BS and neighboring BS, but also measures
signals from the RS that is deployed in cell j. In this
situation, the handover probability can be divided into
two parts: handover to th e BS in target cell and handover
to the RS in the target cell, which can be given by fol-
lows:
_
Pr, ,
Pr ,
*Pr
handover BS
jijkj
*Pr
RS
j
ij RS
handoverRS j
P
GG GGS
GGGGSS
PaG

 

k
S
G

 j 
 




,
*
k
k

 

(8)
_
Pr* ,
Pr* ,Pr
Pr, *Pr
handover RS
RS iRSRSj
RS iRSRSj
R
SiRSRS j
P
SSSS SS
SSSS SS
aG GaGaGG
  

  


 

k
S
S
G



__RS
(9)
Then the total handover probability in the scenario
with relay in cells is defined as follows:
_handover sumhandoverhandover BS
PPP

 (10)
In LTE-Advanced system, frequencies are orthogonal
and we assume that the two relay links are distinguished
in time division multiplex way. Thus the co-channel in-
terference ratio with BS can be represented as follows:
Pr
target_BS_outage target_cell_outage
C
PP
I


 (11)



L. JING ET AL.
346
Considering that the link between BS and RS in
LTE-Advanced system is wireless, so the outage prob-
ability of the relay access should also involve the BS-RS
link, whic h is given by:
arg __
__
__
1(Pr )*(Pr)
tet RS outage
BS to RS
RS RS
RSBS to RS
P
C
C
II



 








(12)
where
R
S
 is the a predefined threshold for RS, and
()/ 4
()/10 4
()/10 4
()/10 4
10
10
10
10
RS RS
ll
RS RS
ll
RSRS RS
RS l
RS
l
CSr
ISr
r
ar




 
 
 
 


10
(13)
The in Equation (13) represents the transmit p ower
ratio of RS and BS, that is:
a
R
S
S
aS  (14)
()/1
4
__ __
()/10 4
__ __
10
10
jj
ll
l
BS to RSBS to RS
BS to RSBSto RS
Cr
Ir




 

0
(15)
where __
l
B
StoRS
represents the distance between the BS
l and the RS and
r
R
S, and represent the distance be-
tween MS and RS or BS l.
rl r
3. Numerical Result
In order to show the impact of RS on handover in high-
speed scenarios, we run some simulations using Mathe-
matica and Matlab tools. First we need to take a look at
the scenario without RS in the cells.
As can be seen from Figure 2, the handover probabil-
ity increases as the distance between the BS and MS de-
creases, but the outage probability after the MS handover
to the target BS is opposite, it decreases as the distance
between the BS and MS decreases. Seen that handover
requests start to increase from about 0.8~1 radius from
BS, yet the outage probability need to be optimized dur-
ing this range.
We know that relay technology can improve the sys-
tem capacity[8] and can also enhance the cell coverage,
but whether wireless relay technology in LTE-Advanced
system can make contribution to the handover in high
speed scenarios is still a question to be studied. In the
following, we will make a novel research on this ques-
tion based on some numerical results that has been de-
duced in previous analysis.
Compared with the result of Figure 4, there are some
differences in Figure 3. In this scenario, RS is deployed
in the line that is 0.8 radius away from the BS j. When
we calculate the outage probability after handover to RS,
0.20.4 0.6 0.811.2 1.4 1.6 1.82
0
0. 1
0. 2
0. 3
0. 4
0. 5
0. 6
0. 7
0. 8
0. 9
1Handover and outage probabi l i ty wi t ho ut RS
Distance between M S and target B S
P robabil i ty
Handover probabili t y withou t RS
Out age Probabilit y after handover
Figure 1. Handover and outage probability without RS.
0.2 0.40.6 0.811.2 1.41.6 1.82
0
0. 1
0. 2
0. 3
0. 4
0. 5
0. 6
0. 7
0. 8
0. 9
1Handover and outa ge probabi l ity wit h RS
Dis tance between M S and targe t B S
P robabi l i t y
Handover probabili ty wit h RS
Out age P robabi l ity aft er handover
Figure 2. Handover and outage probability with RS.
considering that the BS-RS link is wireless, we calculate
both the RS-MS link and the BS-RS link, which is giv en
by Equation (12).
The cells with RS in the boundary do make contribu-
tions to the handover outage probability seen from Fig-
ure 3, on one hand, it can enlarge the distance that MS is
more possible to start a handover, on the other hand, the
outage probability is significantly lower and gentler than
that without RS.
4. Conclusions
In LTE-Advanced systems, coverage in high speed sce-
nario may be a question to be further studied. Based on
the previous analysis and some numerical results, we can
see that proper relay deployment in the target cell can
improve the handover probability distance. For example,
in the comparison of Figure 2 and Figure 3, relay de-
ployed in 0.8 radiuses can enlarge about 0.4 radiuses at
the same handover probability (0.5).

L. JING ET AL.
347
At the same time, proper relay deployment can also
make the outage probability after handover rise much
more slowly as the distances increase, thus it can make
the handover more reliable and stable for those in high
speed who n e ed wirele ss commu n i c ation service s .
5. Acknowledgements
This paper is supported by Key project (2012ZX-
03001030-004).
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