A hybrid channel allocation algorithm with priority to handoff calls in mobile cellular networks
Introduction
The channel allocation problem deals with the allocation of channels to cells in a cellular network. Cells allow users to communicate via the available channels according to a specific channel assignment scheme. Channels within a wireless communication system typically consist of time slots, frequency bands and/or CDMA pseudo noise sequences, but in an abstract sense, they can represent any generic transmission resource. There are three major categories for assigning these channels to cells (or base-stations). They are fixed channel allocation (FCA), dynamic channel allocation (DCA), and hybrid channel allocation (HCA) which is a combination of the first two methods.
FCA systems allocate specific channels to specific cells. This allocation is static and can not be changed. For efficient operation, FCA systems typically allocate channels in a manner that maximizes frequency reuse. The problem with an FCA system is its inability to use the available channels efficiently in an uniform traffic load situation. Despite this problem, FCA method implementation is widespread level to date. DCA attempts to alleviate the problem mentioned for FCA systems when the traffic is non-uniform. In DCA systems, no set relationship exists between channels and cells. Instead, channels are part of a pool of resources. Whenever a channel is needed by a cell, the channel is allocated under the constraint that frequency reuse requirements can not be violated.
HCA is the third category of channel allocation methods that are hybrids of fixed and dynamic channel allocation algorithms. Several methods have been presented in this category and a great deal of comparison has been made with corresponding simulations and analyses [1], [3], [6], [14], [15].
In this paper, we will investigate and propose a novel hybrid channel allocation scheme for wireless cellular networks, where each cell of the network pre-assigns a fixed number of channels and in addition the network may approve the request of additional channels for both new calls and handoff calls if all pre-assigned channels are occupied. This approval depends on the type of new call or handoff call, as well as the number of channels in the cell. If a request is denied for the arrived new call, this call will be blocked and cleared from the system. However, if a request is denied for an arrived handoff call, this call may not be blocked immediately but may be put on hold in a buffer. The idea behind these schemes is to smoothly throttle both the new and the handoff call streams as the network traffic is building up to a threshold value [5]. Thus, when all pre-assigned channels are occupied and the network is approaching congestion, the admitted call stream becomes thinner [4], [5]. Due to the flexible choice of the call admission probabilities, these schemes can be made to be very general. Along with this general idea, several thinning schemes including the “new call thinning scheme” and “fractional guard channel scheme” have been studied in special cases [5]. Also, a scheme has been proposed in [12] which allocates portions of the channel capacity to individual traffic classes by monitoring the call arrival patterns periodically and updating the share of each class.
The rest of this paper is organized as follows: A detailed description of the hybrid allocation scheme in a network is presented in Section 2. Analytical results of the stationary distribution of the system and some key performance measures are derived in Section 3. Section 4 includes numerical analysis and a comparative study to support the results. Section 5 provides a summary of the proposed scheme and concluding remarks.
Section snippets
Description of the hybrid FCA/DCA scheme
We consider a hybrid fixed and dynamic channel allocation scheme for the cellular network system, where each network is assumed to be homogeneous [13], i.e., the underlying processes and parameters for all cells within the cellular networks are the same, so that all cells are statistically identical. The other assumptions and notations for this wireless mobile network are as follows:
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Each cell is assigned a number of nominal channels, M channels, while additional channels controlled by the
Theoretical analysis
It is well-known that the blocking probability is an important performance measure for the channel allocation problem and has been widely used in the related literature [8], [10], [11], [14]. In order to obtain the blocking probabilities for both the new and handoff calls of the proposed system, our approach is to derive the system’s steady-state probability and then use the theorem of total probability to obtain the blocking probabilities. Through this analysis, we will gain a better judgment
Numerical analysis and comparison
In this section, we present several numerical results based on the analytical formula obtained in Sections 3.3 New call and handoff call blocking probability, 3.4 Average number of borrowed channels and the flow time analysis of handoff calls. Further, we investigate the results of the proposed scheme in two special cases: (1) new calls are not allowed to borrow from the network, i.e., ; (2) no buffer reserved for handoff calls, i.e., L = 0. These two special cases are similar to
Conclusion
In this paper, we present a new general analytical model for a hybrid channel allocation scheme in wireless cellular networks. In the proposed hybrid allocation scheme, we first obtained the stationary distribution of each cell when there are i calls in the status of connections and j calls waiting in the buffer. We then derive new call and handoff call blocking probabilities, the average number of borrowed channels, and the average flow time of handoff calls. Finally, by comparing our new
Acknowledgements
The authors thank the referees for their careful reading of the original manuscript. Their comments and suggestions have led to a much better presentation of the paper.
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