26-04-2010, 11:15 AM
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A PROPOSED DISTRIBUTED CALL ADMISSION CONTROL SCHEME FOR MOBILE MICRO-CELLULAR NETWORKS
ABSTRACT
CAC scheme is a critical component to be considered for satisfying diverse Quality of service (QoS) requirements needed for future mobile networks. An attempt is made to suggest a new call admission control scheme in a distributed manner, considering the load conditions in all neighboring cells. The proposed admission policy is simple enough that the decision can be made in real time and hence suitable for micro-cellular networks, where frequent hand-offs takes place. KEYWORDS: Hand-offs, Call Admission Control, Quality of service
Presented BY
M.THIRUVISAGAN
M.UDAYKIRAN FINAL YEAR ECE MAILAM ENGG.COLLEGE
INTRODUCTION
Due to the limitations of radio spectrum, future wireless systems will use micro or Pico cellular architectures in order to provide higher capacity which may result in frequent hand-off. During hand-offs, the cell visiting a new call can be dropped due to insufficient resources leading to congestion state. It is the responsibility of the network to reserve the resource required for each admitted call. This critical job is taken by the call admission controller, whose function is to minimize hand-off dropping rate and as well increase the efficiency of the system by admitting more number of new users. This work has been investigated by many researchers. However, all their works used trunk reservations with constant holding times, which don't suit to realistic case
In our scheme, the call admission decision is taken in a distributed manner, for a non-uniform load conditions. Further more, for the sake of clarity we have analyzed the performance for uniform load conditions. Finally a comparison is also carried out to show the effectiveness of our proposed CAC scheme.
NETWORK MODEL
We consider a 2D homogeneous cellular array as shown in the Fig.1, which strictly follow fixed channel allocation scheme (FCA).
C1
C6
C2
CO
C5
C3
C4
Fig.1 2 Dimensional cellular array
Let X denotes the new rate to any radio cell, n & h denotes the call departure rate and hand-off rate respectively. Let N be the number of channel allocated to each cell. Considering a test mobile in radio cell C0 , we assume that the test mobile remains in the same cell with probability Ps and that it
hand-off to Ci where i=1,2 6,with equal probability Pm/6 during the estimation time 'T'. It is
also assumed that a call hand-offs more than once during T is negligible. Analysis is carried out for uniform load in cells and as well as for non-uniform load conditions.
CALL ADMISSION DECISION
Unlike, the trunk reservation schemes, the call admission is taken in a distributed manner by the periodic exchange of information among the base stations regarding the current load conditions. Let PQOS be the highest tolerable hand-off dropping probability in wireless networks for all the calls of the same type. The call admission controller of such system must fulfill two critical requirements namely.
1. By admitting the new call, the QoS of existing calls in the system must be maintained.
2. The system must provide the newly admitted call with its desired QoS.
Hence, a new call is admitted to cell C0, at time 'to', if and only if the above requirements are satisfied.
TRAFFIC ANALYSIS
Let the number of calls in a cell C0 at time to be 'k' and the probability distribution of the number of calls in Co at time to+Tis denoted by Pn0; t0+The (k).Then the probability of 'J' calls (out of k) moving to the neighboring cell and the remaining I calls follows a Gaussian distribution as suggested in [ ].
Pn0,t0+T (k) * G^Ps + ("1 + "2 + "3 + n4 + n5 + n6)Pm^(uPs(1-Ps) + (n + u + u + «4 + u + /6) (1)
Hence the overload probability P0; is given by
Pn
~> T w< W
X Pu0,f0 +T (k )
N +1
(2)
0
^ N - (u 0 Ps + (u1 + u 2 + u 3 + u 4 + u 5 + u 6) Pm / 6 ^
Where Q (.) is the integral over the tail of Gaussian distribution.
For a given PQOS; there exists a value 'a' where PQOS = Q (a) and we have the following relation.
( 6 ^ N - u0 Ps - Z ui
\i=1 J
Pm/6 - a
"0Ps (1 - Ps ) + Z (1 - Pm / 6) / 6) = 0
i=1
(3)
Solving for 'n0',one can find the admission threshold 'n00' which satisfies the first requirement .Similarly ,the number of new call user to be admitted can be calculated. Based on which, the overload probability is given by
u 0 + «1 + u 2 + u 3 + u 4 + u 5 + u 6
P0 = Z Pu, 10 + T (k) (4)
N +1
Where
Pu,t0 +T (k) = G (u.Ps + (E (u) + u0) Pm /6 + XT ,J u1Ps (1 - Ps) + (E (u) + u0) Pm (1 - Pm / 6) / 6 + XT (5)
The number of new call admitted to cell C1, such that the second admission requirement can be satisfied is found using Q (.).
SIMULATION RESULTS:
Simulations are performed for both uniform and non-uniform load conditions for N=20 &50.The proposed scheme is based on Ps and Pm ,which incorporate the average speed of mobiles and the coverage area of the mobiles. Our scheme has got the lower hand-off dropping probability even under heavy load Conditions.
FOR UNIFORM TRAFFIC LOAD:
CONCLUSION:
QoS provisioning in micro/Pico cellular networks necessitates control of wireless network resources. These resources need to be allocated in such a way that when a wireless terminal hands-off from one access point to another, network resources are available with an acceptably high probability. Our proposed CAC scheme is simple enough that the admission decision can be made in real time and without much computational effort.
REFERENCES
[1].S.T.S.Chia, "Mixed cell architecture and handover" in IEEE Colloquium 'Mobile Communication in the year 2000 ', no 139, U.K.June 1992.
[2].S.S.Rappaport, "Models for call hand-off schemes in cellular communication networks", in 3rd WINLAB Workshop: Third Generation Wireless Information Networks, Workshop Record Apr.28
1992.
[3].M.Naghshineh and A.S.Acampora "An architecture and methodology for mobile executed cell hand-off in wireless ATM networks", IEEE J.Selected Areas Communication, vol.12, no .8, Oct 1994.
[4]. M.Naghshineh and Mischa Schwartz "Distributed Call Admission Control in Mobile /Wireless networks", IEEE Journal on Selected Areas in Communication Vol 14.N0.4 May 1996.
