To cope with the increasing demand of wireless communication services multi-carrier systems are being used. Radio resources are very limited and efficient usages of these resources are inevitable to get optimum performance of the system. Paging channel is a low-bandwidth channel and one of the most important channels on which system performance depends significantly. Therefore it is vulnerable to even moderate overloads. In this paper, an efficient paging algorithm, Concurrent Search, is proposed for efficient use of paging channel in Multi- carrier CDMA system instead of existing sequential searching algorithm. It is shown by the simulation that the paging performance in proposed algorithm is far better than the existing system.
In order to deliver services to a Mobile Station (MS) the cellular network must be able to track MS in the network. The forward-link communication channels are used to page and transmit system overhead messages to the MS is called paging channel [1].
Efficient utilization of limited resources is required to support the growing demands of personal communication services. To utilize the paging channel efficiently, many researchers have been studying different techniques. One of the concepts is dividing a location area into paging zones is described in [8] and [9]. Another research has proposed to increase paging efficiency-MS will be paged on the last registered cell and subsequently to other cells in the location area if necessary [4]. A mobility tracking scheme that combines a movement-based location update policy with a selective paging scheme is proposed in [5].
In CDMA system, an MS accesses the system on a particular carrier frequency [10]. A single carrier frequency occupies 1.25MHz bandwidth contains seven paging channels. When a carrier reaches its capacity limit, it is desirable to increase the system capacity. One way of achieving this is to increase the number of carriers to greater than one. When more than one carrier is used then, the system is referred to as multi-carrier system.
In existing multi carrier system, an MS is capable of tuning to only one of the carrier frequencies at any instant of time. Because of this, the MS is only listening to one of the carrier frequency paging channels at a time. In order to page an MS, the paging message must be sent on the paging channels of all of the carrier frequencies to ensure that it is sent on the paging channel the MS is listening to. To do this the paging message must be duplicated for each of the paging channels on which the message is to be sent [10]. The existing system can’t search more than seven users simultaneously. This results in an inefficient utilization of paging channel recourses.
In this paper, a concurrent search algorithm is proposed which can be used to search more than seven users simultaneously in multiple carriers. The concurrent search algorithm has been used for paging in cells [6]. But the novelty of our paper is the adoption of concurrent search algorithm in multiple carrier CDMA system rather then cells. The system performance has been analyzed by using absorbing Markov chain and Erlang C formula.
This paper focused concurrent search algorithm based on the problem of sequentially locating a number of users. Suppose there is n number of carriers in the network and k mobile users to be located. In existing multicarrier CDMA system a straight forward sequential paging scheme has been used to locate user i among k number of users. We assume that the user must exists in the network so n 1 j 1 j) P(i, i {1,2, …k}. When 1 ≤ c ≤ n where P(i,j) is the probability of existence of user i in j th carrier. Whereas F (i,c) is the indicator of finding user and that is F(i,c) {0,1} if the i th mobile user has located at the c th carrier then F(i,c) = 1 elsewhere F(i,c) = 0. To clarify the existing problem we use simplify algorithm of sequential search.
Step 0: Initially F(i,c) = 0 where 1 ≤ i ≤ k and 1 ≤ c ≤ n
Step 1: [flood the message] for all 1 ≤ c ≤ n Copy and send the paging message for user i
This technique requires k*n paging channels by using concurrent paging scheme it is possible to reduce average use of paging channels.
Consider an example to demonstrate the effectiveness of concurrent search scheme. Suppose that there are only two carriers in the system with same number of users. Among them two users: X and Y are to be found. Assume that the system has no previous knowledge about the location of the users: each user can exist in any of the two carriers with probability 0.5. Using sequential paging, the system would page and locate user X to the both carriers in the first attempt. Then, the system would page and locate user Y to the both carriers in the second attempt. Thus total of (2+2) four pages would be required for finding two users.
On the other hand, if the system is to page in the first attempt user X in carrier 1 and for user Y in carrier 2 simultaneously, then there is 50% probability that any of the two users would be found in the first attempt. If a user Y is not found at first attempt, the system would page for the user in the “other” carrier in the second attempt. Thus, (2+1) 3 paging messages are needed for the two users. On the average, 1.5 pages would be required per user, a saving of 25% over the sequential paging.
The proposed concurrently searching algorithm for locating i th user in n carriers is described here. To minimize the paging cost, a methodology is described in [7], it is stated that the higher probability of finding users must be paged before the lower probability. This is also true for proposed carrier concept. From Fig. 1 and Fig. 2, it is visualized that if the priori
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