IEEE-International Conference on Signal processing, Communications and Networking
Madras Institute ofTechnology, Anna University Chennai India, Jan 4-6, 2008. pp401-404
Base Station Selection Strategy in Multihop Cellular
Networks: A New Approach
N. V. Marathe, U. B. Desai, S. N. Merchant
Electrical Engineering Department
Indian Institute of Technology, Bombay
Powai, Mumbai, INDIA 400 076
{ nvmarathe,ubdesai,merchant } @ ee.iitb.ac.in
Abstract- Multihop Cellular Networks (MCN) have been
proposed for quite a long time as of now. The major
improvements these networks offer over the conventional cellular
networks are increase in capacity and coverage area. However the
congestion near the base station (BS) happens to be the bottleneck
even in the approach of MCN. Therefore approach to reduce this
congestion is a potential area of further research in this field. Our
proposed architecture is one such attempt in this direction. In the
proposed scheme instead of a single cell covered by a particular
base station the entire coverage area of the cellular network
consisting of many cells is considered as a whole and an attempt
has been made to find out the direct links between the source and
the destination of a connection without approaching any of the
base stations. Since in MCN any mobile terminal (MT) can
communicate with another MT directly; much importance should
not be attached to the parent or associated base station. Rather
the any suitable base station should be the last hop in this network
only when an attempt to find out the direct links between the
source and the destination of a connection fails. This failure may
either because the path is too long or either of source and
destination is outside the network. Out of the cluster of parent BS
and six neighbouring BSs one base station is to be selected
depending upon the distance and the channel occupancy. This
approach distributes the traffic load almost uniformly over the
entire region and overcomes the problem of localized congestion
in an effective manner.
Keywords-multihop cellular network; base station selection;
decision matrix
I.
INTRODUCTION
With the ever-growing demand for the cellular services
increase in the capacity of the cellular network has remained
on the forefront for quite a long time. Even though there is a
natural solution to this effect viz. smaller cell size, this
solution is not economically feasible as more number of base
stations (BS) will require heavy infrastructural cost. It is
therefore important to work towards alternate solution.
Multihop Cellular is one such attempt in this direction. Some
work has already taken place in this context. There are two
approaches towards this direction. First approach [1], [2], [3]
is to install a new element in the setup called as 'Fixed Relay
Station' (FRS). These stations are supposed to act as a relay
between the mobile terminals (MT) and the BS. As the fixed
relay is having a functionality which is much less complex
than that of a base station, it was argued that it would be a cost
effective method to increase the capacity of the cellular
network. However a major shift from the conventional
architecture is taken in the second approach [4],[5] where the
architecture turns to be more towards ad hoc in nature by
using the MTs as relays.
Because of a shorter range of communication, naturally it
gives a scope for a wider channel reuse thereby resulting in the
increase in the capacity of the network. However there is a
limit to which the capacity can be increased by introduction of
multihop approach in the conventional cellular network. This
limitation is caused by the phenomenon called as base centnc
routing. Since the proportion of the intra-cell calls is much
less, majority of the calls are routed through the base station
only. This increases the traffic near the base station and
therefore the total number of inter-cell calls a cell can support
is limited by the total number of traffic channels with the
operator.
II.
PROPOSED SCHEME
With the introduction of the multihop concept, the pivotal
role of the base station is reduced to some extent and it
remains no more the next hop for all the links in the network
unlike in the conventional cellular network. Also there has to
be a major shift from the concept of cellular network. There
should not be any specific need now for every MT to get
associated to a particular base station. Instead the base station
role is now limited to that of a backhaul station. The overall
network now becomes more ad hoc in nature and the base
stations become the permanent infrastructure nodes, which
keep the network organized. These base stations with a
backhaul link and organized mechanism of communications
amongst themselves play a major role in the routing process to
provide a good quality of service. The network is considered
as a whole and the base stations are considered as backhaul
stations for communication outside the network. For the calls
in the network the route is found out between the source and
destination via other MTs without reaching the base station.
Calls in which either the source or destination is outside the
network or are at a larger distance are routed through the
suitable base station.
SYSTEm ARCHITECTURE
1.
We have considered a large region covering 127 hexagonal
cells. There is a base station at the center of each cell. All the
MTs in the cell are attached to the parent base station with the
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Base station selection strategy in multihop celluarnetwork:A New Approach
single hop control channels. However as far as the traffic
channels are concerned the multihop approach is adopted.
When a new call is generated first an attempt is made first to
discover a direct path between the source and destination if
both of them happen to be in the same or neighbouring cells. In
this case the call is routed via intermediate MTs and the base
station does not come in the route. However if such attempt of
finding out a direct path fails then the call must be routed
through the base station only.
With the introduction of multihop communication the
transmission range decreases enabling higher level of channel
reuse. Thus it is now possible that all the traffic channels can
now be used in every cell instead of a cluster of cells in case of
conventional single hop communication. This approach leads
to the increase in capacity of the network without increase in
the base station density.
However this multihop cellular approach also suffers from
a limitation that the total number of calls a cell can support is
limited by the total number of traffic channels because since
most of the calls are to be routed through the base station the
area near the base station becomes congested and in any case
the base station cannot support the calls more than the number
of traffic channels available.
When we consider a large region comprising of multiple
cells then it can be easily observed that the nature of traffic
over this region is location as well as time dependent. At any
given point of time one can find that there is congestion in few
cells whereas in some other cells the capacity is not being fully
utilized.
We would like to propose a new scheme in which we try to
exploit the fact that due to multihop nature of communication
the coupling of an MT is not rigid with the parent base station.
A decision matrix is formed for deciding the appropriate base
station to be selected for the newly arrived call. The parameters
considered are the distance of MT from various base stations in
the cluster of parent and neighbouring 6 base stations and the
traffic channel availability with these base stations.
Table I shows this matrix where R is the radius of a cell and
the distance of MT from different base stations in the cluster is
there in the heading row named as DIST. The heading column
named AVAL shows the traffic channel availability with the
base station. For a particular MT the values of DIST and
AVAL are found out for all the base stations in the cluster of 7
cells. Then that particular base station is selected for which the
matrix returns the minimum value. In case of a tie the
preference is given to the parent base station. If the tie is
between other base stations then one of them is selected
randomly.
TABLE I.
BASE STATION SELECTION DECISION MATRIX
R
1.5R
R-
1.5R-
>2R
12
15
17
19
20
2
6
9
11
14
18
3
4 or more
3
1
5
2
7
4
10
8
16
13
DIST
<0.5R 0.5R
2R
AVAL
1
IV. SIMULATION RESULTS
We have considered a large region covering 127 hexagonal
cells. All the calls generated are assumed to be inter-cell so that
each of the call requires connectivity to the base station. The
number of traffic channels available in each cell is taken to be
equal to 40. The average call duration is considered to be 180
seconds with an exponential distribution. The call arrival rate
distribution is considered to be Poisson distribution and the
mean call arrival rate is varied in between 60000 to 120000
calls per hour over the entire region for consideration of
different traffic conditions. The overall traffic in randomly
distributed over different cells. For the border cells the traffic
density is naturally lower as these areas usually belong to city
outskirts. Simulations are carried out in MATLAB for the
conventional multihop cellular system in which the calls
generated in a particular are routed through the parent base
station only and also for the proposed scheme where a decision
is made regarding the selection of the base station amongst a
cluster of 7 base stations based on a decision matrix consisting
of the distance of MT from the base station and the traffic
channel availability with the base station.
Graphs have been plotted for the overall blocking
probability in the entire region against call arrival rate for the
conventional multihop cellular approach as well as the
proposed scheme. Figure 1 shows these graphs.
Also the Table II shows call blocking probability in the
individual cells for the conventional multihop cellular approach
(PB1) as well as the proposed scheme (PB2) for a particular
value of call arrival rate. This value selected here is 96000 calls
per hour.
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IEEE-ICSCN Jan 2008
BLOCKING PROBABILITY IN VARIOUS CELLS
TABLE II.
Call Blocking Probabiity for Concention and Proposed Schemes
25
0
PB2
Cell
-
41+
_I _ + _ js ^
OsE;F - + -j+7;
8
9
Cell
PBR
No.
PB2
0.97
44
18.79
1.70
87
23.21
1.21
15.48
0
45
21.86
1.46
88
3.49
0.67
3
26.13
1.41
46
9.80
1.03
89
14.75
0
4
2.09
1.25
47
24.13
0.89
90
2.51
0
5
16.21
0.32
48
0
0
91
27.75
3.76
6
5.24
1.15
49
0.43
0
92
0
0
7
15.79
1.29
50
21.15
1.30
93
0
4.64
v /
8
13.62
0.69
51
20.06
1.26
94
0
1.42
/tS
9
0
3.67
52
12.24
0
95
0
0.72
8.24
1.29
53
29.28
0.00
96
0
0.49
11
15.81
0
54
0.47
0
97
3.38
0
12
23.46
0
55
15.74
0
98
0
1.97
.
10:1
PB2
27.49
~ Proposed Scheme
/
PB7
1
15
0
No.
2
Conve-ntional Method
.gf0
-U
PBR
.
20
n.
Cell
No.
/~
l10
l
12
Call Arrival Rate (No. of calls per hour) xn4
13
9.39
0
56
16.45
3.55
99
0
0
14
23.31
1.54
57
27.79
0.85
100
0
2.45
15
21.12
1.26
58
15.23
1.68
101
0
0.50
Figure 1. Call Blocking Probability for Conventional Method and the
Proposed Scheme
16
17
15.07
7.88
0.38
0.36
59
60
15.87
7.72
0.98
0
102
103
0
0.46
0
0
We can easily notice from the figure that the overall call
blocking probability shows a remarkable decline in the
proposed scheme over the conventional multihop cellular
approach.
From the table we can easily observe that the proposed
scheme tries to distribute the non-uniform traffic in different
cells uniformly over all the cells thereby the higher values of
call blocking probabilities in some cells in the conventional
multihop cellular approach are brought down to a very
moderate level in the proposed scheme.
18
19
20
21
20.12
6.37
10.37
23.53
0.62
0.37
0
61
62
63
4.21
3.77
15.06
12.08
0
0
1.23
0.41
104
105
106
0
0
0
0
5.63
3.70
22
18.67
0.87
65
0.42
0
108
0
0
23
24
25
18.51
10.04
2.14
0.30
0.84
0
66
67
68
29.46
9.23
11.46
0
0
0.72
109
110
111
0
0
0
0
9.26
1.50
26
1.95
0.43
69
14.33
1.75
112
0
0
2.76
64
107
0
0
27
0.41
0.38
70
1.81
0.86
113
0
0
28
19.05
1.25
71
0.87
0
114
2.08
0
29
20.19
0
72
9.79
0
115
0
1.16
30
21.70
0.27
73
7.81
0
116
0
2.42
31
7.72
0
74
11.03
1.45
117
0
1.01
32
0
3.80
75
2.61
0.44
118
0
1.73
33
20.77
1.15
76
17.28
0.28
119
0
0.64
34
0
0.43
77
20.06
0
120
0
0
35
8.30
0
78
23.81
0.30
121
0
0.52
36
0
0
79
10.58
0
122
0
1.41
37
0
0
80
0
1.36
123
0
0.88
38
13.21
0.72
81
16.43
0.67
124
2.08
0
39
8.61
0.81
82
20.23
1.32
125
0
0.49
40
7.22
0.36
83
8.76
0.69
126
0
0
41
19.09
0.29
84
13.83
0
127
0
3.90
42
14.03
0
85
8.03
0
43
12.05
0
86
6.30
0.86
TOT
11.00
0.85
403
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Base station selection strategy in multihop celluarnetwork:A New Approach
V.
DISCUSSIONS AND CONCLUSION
From the results it is very clear that our proposed scheme
provides a better performance over the conventional multihop
cellular approach. In the proposed scheme there is effective
utilization of the scarce wireless resources over the entire
region. Our proposed scheme tries to deviate more from
cellular network approach towards ad hoc network approach.
It is expected that with small modifications the proposed
architecture will become a potential next generation candidate
in multihop cellular communication.
[1]
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[3] Hongyi Wu, Chunming Qiao, Swadesh De, and Ozan Tonguz,
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[4] Y. -D. Lin and Y. -C. Hsu, "Multihop Cellular: A New Architecture for
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