Handoff Failure management in Wireless Networks
Rahim Kamran
Shahid Bahonar university of Kerman, [email protected]
Abstract: During the past few years, advances in mobile
communication theory have enabled the development of different
wireless technologies. Next-Generation Wireless Systems (NGWS)
integrate different wireless systems, each of which is optimized for
some specific services and coverage area to provide ubiquitous
communications to the mobile users. One of the most important
problems in wireless networks is handoff which occurs during
movement of MT (mobile terminal) from one cell to another. In
this paper, we extend the handoff minimization method proposed
in reference [1] for real state that wireless cells having overlap on
each other. Based on the information of false handoff probability ,
we analyze effects of different parameters such as rate of
overlapping, signaling time delay, speed of MT(mobile terminal)
and other parameters on probability of false hand off and hand off
initiation probability.
Keywords: NGWS (Next Generation wireless Systems),
Handoff, False handoff probability, handoff initiation
1. Introduction
Mobility is the most important feature of a wireless
cellular communication system. A cell is the radio area
covered by a transmitting station or a Base Station (BS). All
Mobile Terminals (MTs) within that area are connected and
serviced by the BS. Mobility management contains two
components:
location
management
and
handoff
management .Location management helps to track the
locations of mobile users between consecutive
communications. But handoff management process keeps
its connection active even when it moves from one base
station (BS) to another. When a mobile user crosses the cell
boundary or the quality of the wireless link is unacceptable,
then the handoff process is initiated. Handoffs are broadly
classified into two categories—hard and soft handoffs.
Usually, the hard handoff can be further divided into two
different types—intra- and inter cell hand offs. The soft
hand off can also be divided into two different types—multi
way soft handoffs and softer handoffs. From the other point
of view handoff is divided into two categories: horizontal
handoff and vertical handoff [2,3,4,5].
Recently different approaches have been proposed to
enhance the performance of handoff in next generation
heterogeneous wireless networks. Some proposed new
algorithms or new protocols.
A novel mobility
management system is proposed in [11] for vertical handoff
between WWAN (Wireless Wide Area Network) and
WLAN (Wireless Local Area Network). The system
integrates a connection manager (CM) that intelligently
detects the changes in wireless network and a virtual
connectivity manager (VCM) maintains connectivity using
end-to-end principle. In [12] signal to interference ratio
between old base-station and neighboring base-stations are
calculated to make the handoff initiation decision for next
generation wireless system or 4G networks. In [13], the
authors propose a handoff algorithm in which the received
pilot signal strength is typically averaged to diminish the
undesirable effect of the fast fading component.
As the averaging process can substantially alter the
characteristics of path loss and shadowing components,
causing increased handoff delay. In [14], a handoff
algorithm using multi-level thresholds is proposed. The
performance results obtained, shows that an 8- level
threshold algorithm operates better than a single threshold
algorithm in terms of forced termination and call blocking
probabilities. In [1] a minimization method for handoff
failure probability in next generation wireless networks is
proposed.
The structure of this paper is as follows. In section 2
handoff failure probability minimization method proposed
in [1] is described briefly. Section 3 includes our extension
to method in [1] for real state and also analysis for effect of
different parameters on minimization of handoff. Finally
the paper will be concluded in section4.
2.
Hand off minimization method [1]
Ideally, the area covered by a cell is a circle, with the BS
being at the center. Thus, actually cells are not hexagonal.
Hexagon fitted the planed area nicely and hexagon is the
greatest area in the circle with respect to any other shape.
The cell is therefore approximated to a regular hexagon and
side of the hexagon is the common cord of two adjacent
cells.
It is suppose two cells overlap each other in such a
manner that the common cord between two adjacent
circular cells also becomes the common side of the regular
hexagons, when the cells are considered to be hexagons.
However, two cells may overlap in such a way that there is
some overlapping hexagonal portion between them. Such
type of structure is given below in Fig. 1.
(NBS). This is because RSS of NBS is greater than RSS of
OBS to the right side of 𝐴′𝐴"𝐵"𝐵′. Once the MT reaches
the boundary of the circular cell (real) then the MT
discovers that it may enter into the coverage area. MT is
suppose to move from its current BS (old BS), to the future
BS (new BS). By using the following definitions for
notations in Fig.1.
𝑆𝑡ℎ : The RSS threshold value to initiate the handoff.
𝑆𝑚𝑖𝑛 : The MT’s minimum RSS value to communicate
successfully between an MT and BS.
OBS: The old BS.
NBS: The new BS.
a: The cell size served by a BS (i.e., the length of each
side of hexagonal cells).
P: The point when the MT’s RSS from the OBS drops
below𝑆𝑡ℎ . On the point ‘P’ the MT understands that it is on
the overlapping position.
d: The distance from the hexagonal cell boundary to the
point ‘P’.
θ: The motion direction of MT from point ‘P’ to handoff
to NBS.
L: Distance between side of hexagon and common cord
of two hexagons.
β: Any direction of MT when at time‘t’ it covered the
distance ‘x’ from the point ‘P’.
From the figure, we have:
𝐴𝐵 = 𝑎 = 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑐𝑖𝑟𝑐𝑙𝑒
= 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑖𝑑𝑒 𝑜𝑓 ℎ𝑒𝑥𝑎𝑔𝑜𝑛
𝑂𝑄 = √3 𝑎⁄2
𝑃𝑄 = 𝑑 = 𝑂𝑃 − 𝑂𝑄 = 𝑎 − (√3 𝑎⁄2) =
(2−√3)
2
a
QR = L (assumption)
When MT crosses the line 𝐴′ 𝐵′ , then handoff will occur
𝑃𝑅 = 𝑃𝑄 + 𝑄𝑅 = 𝑑 + 𝐿 = (2a − √3a + 2L) / 2
𝐴′𝐴" = 𝐿𝑡𝑎𝑛30 = 𝐿/√3
𝐴′ 𝑅 = (√3𝑎 + 2𝐿)/2√3
Fig. 1: Supposed situation for analysis of the handoff
process
Here in Fig. 1, AB is the side of regular hexagonal cell
served by the Old BS (OBS). But 𝐴′𝐴"𝐵"𝐵′ is the common
cord of the two adjoining cells, one served by the OBS and
the other by the NBS. When a Mobile Terminal (MT)
crosses 𝐴′𝐴"𝐵"𝐵′
X = ((2a − √3a + 2L) sec β) / 2
t = ((2a − √3a + 2L) sec β) / 2 v , v is velocity of MT
𝑡𝑎𝑛𝜃1 = A′ R/PR = (√3a + 2L) /√3 (2a − √3a + 2L)
If the MT is moves with a speed ‘V’ that is assumed to
be uniformly distributed in [𝑉𝑚𝑖𝑛 , 𝑉𝑚𝑎𝑥 ]. So the probability
density function (pdf) of ‘V’ is given by
𝑓𝑣 (𝑣) =
1
(1)
𝑉𝑚𝑎𝑥 −𝑉𝑚𝑖𝑛
When the MT is located at point P, it is assumed that it
can move in any direction with equal probability, i.e., the
probability density function of MT’s direction θ is
1
fθ (θ) =
π > 𝜃 > −𝜋
(2)
2π
PL′ =
∂Pa
∂L
=
∂
∂L
1
√3a+2L
π
√3(2a−√3a+2L)
(4√3−12)a
{1 − tan−1 [
]} =
3(2a−√3a+2L)2 +(√3a+2L)2
(6)
As a>0 , so PL′ > 0 and it can be concluded that
probability of Pa doesn’t have minimum value for L>0
and it increases with increase of L. if we set L=0, we have
7
(Pa ). = = 0.5833 .Relationship between Pa and L is
12
shown in Fig.2.
It is clear that the need for handoff to NBS arises only if
MT’s direction of motion from P is in the range
√3a+2L
θ1 = tan−1 (
√3(2a−√3a+2L)
)
(3)
Therefore using (2), the probability of false handoff
initiation is
θ
Pa = 1 − ∫−θ1 fθ (θ) = 1 −
1
1
π
θ1
tan
=
π
−1
[
√3a+2L
]
√3(2a−√3a+2L)
(4)
So we can say false handoff initiation is independent of d
but it is dependent on L.
It is shown in [1] that false handoff probability can be
calculated from equation (5).
𝑝𝑓 =
1
𝜃1
2𝑎−√3𝑎
𝑐𝑜𝑠 −1 [
2𝑣𝜏
]
(5)
In which 𝑝𝑓 is false handoff probability, 'a' is cell size, v
is the speed of MT and 𝜏 is handoff latency. Detailed
calculations can be seen in [1].
3. Our extension to method proposed in [1]
In minimization method proposed in [1] - as mentioned
by authors- all calculations are conducted assuming L=0. t
means that the cells don’t have overlapping on each other.
So in this paper we analyze performance of proposed
method with assumption L ≠ 0
3.1
Relationship between false handoff initiation
probability and rate of overlapping(L)
As is shown in equation (4), if we supposeL ≠ 0,
increase of L leads to increase of false hand off initiation
probability(Pa ). Similariliy, decrease of L, results in
decrease of (Pa ). it can be proved as follows.
If (4) is differentiated with respect to L, we have
Fig. 2: Relationship between 𝑃𝑎 𝑎𝑛𝑑 𝐿
As is shown in fig.2, for a particular value of L, Pa
depends on cell size (a), i.e. probability of false hand off
initiation decreases with increase of cell size.
3.2
Relationship between false hand off initiation
and cell size (a)
If we differentiate equation (4) with respect to parameter
‘a’(cell size), we have:
Pa′ =
∂Pa
∂a
=
∂
∂a
1
√3a+2L
π
√3(2a−√3a+2L)
(−4√3)L
{1 − tan−1 [
]} =
3(2a−√3a+2L)2 +(√3a+2L)2
(7)
As L> 0 so Pa′ < 0 and we coclude that probability of
false hand off initiation decreases when ‘a’ increases. This
relationship is shown in Fig.3.
Fig4: Relationship between 𝑃𝑓 𝑎𝑛𝑑 𝐿
Fig.3: Relationship between 𝑃𝑎 𝑎𝑛𝑑 𝑎
We can see in Fig.3 that for a particular value of a(cell
size), Pa increases when L increases.
It should be mentioned that in Fig4, speed of MT (V)
and signaling delay(τ) are considered constant.
3.3
Relationship between false hand off probability
and rate of cells overlap (L)
3.4
Relationship between false hand off probability
and speed of mobile terminal (V)
From equation (5) it can be concluded that Pf is
proportional to speed of mobile terminal (v). This means
that when V increases, Pf increases too. This is because
when speed of MT increases, it requires less time to cross
coverage region of old base station to NBS(new base
station). Fig.5, 6 represent relationship between Pf and V for
inter and intra system. It should be mentioned that, in inter
system and intra system, signaling delay time is τ = 1.5s,
τ = 3s respectively.
As we mentioned before, in reference [1], it is assumed
L = 0 (cells don’t have overlap on each other). If it is
considered that cells have overlap, then in equation (12) stated in[1]- L appears and it can be written as follows
τ
P(t < 𝜏) = ∫0 ft (t) dt =
τ
∫(2a−√3a+2L)
2V
(2a+√3a+2L)
dt=
πt√(2Vt)2 −(2a+√3a+2L)2
1
θ1
2a−√3a+2L
cos −1 [
2Vτ
]
(8)
and consequently Pf (equation (5) ) changes to following
equation.
𝑝𝑓 =
1
𝜃1
2𝑎−√3𝑎+2𝐿
𝑐𝑜𝑠 −1 [
2𝑣𝜏
]
(9)
1
So, we see that Pf is proportional to . This means if L
L
increases then Pf will decrease. Fig.4 shows relationship
between Pf and L.
Fig.5: Relationship between 𝑃𝑓 𝑎𝑛𝑑 𝑉 for intra system
𝜏 = 1.5𝑠,a=500m
From Fig.7 we conclude for a certain value of a(cell size),
𝑃𝑓 decreases when L increases.
4. Conclusion
In this paper first we reviewed some methods of handoff
management in cellular communication. Then we described
handoff probability minimization method proposed in
reference [1] in details. We extended the method in
reference[1] with assuming cells have overlap on each
other(L ≠ 0) and analysed and draw curves for relationship
between two probabilities(false hand off initiation and
false hand off ) and different parameters such as L(rate of
cells overlap), cells size(a),speed of mobile terminal (V).
References
Fig.6: Relationship between 𝑃𝑓 𝑎𝑛𝑑 𝑉 for inter system
𝜏 = 3𝑠 , a=100m
By comparing fig.5 and Fig. 6, we find that for a
particular value of ‘L’, MT can have a higher speed in inter
system than intra system before false hand off occurs.
3.5
Relationship between false hand off probability
and cell size (a)
From equation (5) it is clear that Pf has a reverse
relationship with cell size and when cell size increase,
probability of false hand off decreases. Fig.7 shows
relationship between Pf and 'a'.
[1] Debabrata Sarddar, Tapas Jana, Souvik Kumar Saha, Joydeep
Banerjee ,Utpa Biswas, .K. Naskar1,” Minimization of Handoff
Failure Probability for Next-Generation Wireless Systems”,
International Journal of Next-Generation Networks (IJNGN) Vol.2,
No.2, pages 36-51, June 2010.
[2] Aggeliki Sgora, Student Member, IEEE, and Dimitrios D. Vergados,
Member, IEEE,” Handoff Prioritization and Decision Schemes in
Wireless Cellular Networks: a Survey”, IEEE COMMUNICATIONS
SURVEYS & TUTORIALS, VOL. 11, NO. 4, pages57-77 FOURTH
QUARTER 2009
[3] I.F. Akyildiz, J. Xie, and S. Mohanty, “A Survey on Mobility
Management in Next Generation All-IP Based Wireless Systems,”
IEEE Wireless Comm., vol. 11, no. 4, pp. 16-28, Aug. 2004
[4] Christian Makaya, Samuel Pierre, “Enhanced Fast Handoff Scheme
for Heterogeneous Wireless Network”. Computer Communications,
2008
[5] Debabrata Sarddar,” Reduction of Error in Handoff Initiation Time
Calculation for Next-GenerationWireless Systems, Debabrata
Sarddar et. al. / (IJCSE) International Journal on Computer Science
and EngineeringVol. 02, No. 06, 2010, 2047-2052
[6] D. Saha, A. Mukherjee, I. S. Misra, and M. Chakaraborty, “Mobility
Support in IP: A Survey of Related Protocols,” IEEE Network, vol.
8,no. 6, 2004, pp. 34-40
[7] W. Shen and Q.-A. Zeng, “A Novel Decision Strategy of Vertical
Handoff in Overlay Wireless Networks,” In the Proc. 5th IEEE
International Symposium on Network Computing and Applications
(NCA 06), Cambridge, MA, USA, July 2006, pp. 227-23
[8] I. Stojmenovic, “Handbook of Wireless and Mobile Computing,”
ISBN 0-471-41902-8, John Wiley & Sons, 2002
[9] I. F. Akyildiz, J. McNair, J. Ho, H. Uzunalioglu, and W. Wang,
“Mobility Management in Current and Future Communications
Networks,” IEEE Network, vol. 12, no. 4, 1998, pp. 39-49.
Fig.7 : Relationship between 𝑃𝑓 and a.
[10] A. E. Leu and B. L. Mark, “Modeling and Analysis of Fast Handoff
Algorithms for Microcellular Networks,” In the Proc. 10th IEEE
International Symposium on Modeling, Analysis and Simulation of
Computer and Telecommunication Systems (MASCOTS 2002), Fort
Worth, Texas, USA, October 2002, pp. 321-328
[11] J. Schiller, “Mobile Communications,” Addison-Wesley, London,
2003. ISBN: 0 321 12381 6
[12] Q. Zhang, C. Guo, Z. Guo and W. Zhu, Wireless and
NetworkingGroup, Microsoft Research Asia,” Efficient Mobility
Management forVertical Handoff between WWAN and
WLAN’’,IEEE,communications Magazine,2003
[13] S. BEN OUBIRA, M. FRIKHA, S. TABBANE,”Handoff
Management In 4G Networks”, IWCMC ’09, June 21-24, 2009,
Leipzig, Germany.Copyright 2009 ACM 978-1-60558-569-7/09/06
[14] Leu, A. E, Mark, B. L., "Local Averaging for Fast Handoffs in
Cellular
Networks"
IEEE
Transactions
On
Wireless
Communications, Vol. 6,No.3, March 2007, pp. 866 – 874