Hierarchical Cellular Systems Research Articles

Channel assignment and layer selection in hierarchical cellular system with fuzzy control

We consider a hierarchical cellular system, which consists of a macrolayer and a microlayer. The macrocells accommodate fast mobile users. However, if we direct too many mobile users to the macrocells, the system capacity is low. On the other hand, the microcells are designed to increase capacity, but they cause a large number of handoffs. The aim is to maximize the system capacity while keeping the amount of handoff small. We minimize the handoff rate by a fuzzy layer selection algorithm, which makes use of the past cell dwell times of a user and the channel occupancy of the target cell. To maximize the capacity, we propose a distributed channel assignment algorithm to dynamically allocate the channels among the two layers. Exchange of information is allowed between neighboring macrocells. The state of channel assignment in a macrocell and its interfering cells are tabulated in a channel allocation table, which provides all information required in the integrated resource allocation scheme. The performance is evaluated by simulation and compared with the popular layer selection scheme known as the threshold method.

Handoff analysis of the hierarchical cellular system

As mobile telephone traffic demands increase in the future, interest in cellular systems with a hierarchical structure will grow. A hierarchical cellular system can provide a high system capacity, efficient channel utilization, and an inherent load-balancing capability. In this type of system, calls blocked in a microcell flow over to a macrocell which is superimposed on a number of microcells. When overflow calls which are accommodated in a macrocell cannot be returned to a microcell, the macrocell channels become saturated. Thus, we propose a hierarchical cellular system with an underflow scheme which permits overflow calls to return to microcells. We model this underflow scheme and analyze system performance.

A QoS-guaranteed fuzzy channel allocation controller for hierarchical cellular systems

This paper proposes a fuzzy channel allocation controller (FCAC) for hierarchical cellular systems. The FCAC mainly contains a fuzzy channel allocation processor (FCAP) which is designed to be in a two-layer architecture that consists of a fuzzy admission threshold estimator in the first layer and a fuzzy channel allocator in the second layer. The FCAP chooses the handoff failure probability, defined as the quality-of-service (QoS) index, and the resource availability as input linguistic variables for the fuzzy admission threshold estimator, where the Sugeno's (1985) position gradient-type reasoning method is applied to adaptively adjust the admission threshold for the fuzzy channel allocator. The FCAP takes the mobility of user, the channel utilization, and the resource availability as input variables for the fuzzy channel allocator so that the channel allocation is finally determined, further based upon the admission threshold. Simulation results show that FCAC can always guarantee the QoS requirement of handoff failure probability for all traffic loads. Also it improves the system utilization by 31.2% while it increases the handoff rate by 12.94 over the overflow channel allocation (OCA) scheme; it enhances the system utilization by 6% and still reduces the handoff rate by 6.746 as compared to the combined channel allocation (CCA) scheme, under a defined QoS constraint.

Analysis of a hierarchical cellular system with reneging and dropping for waiting new and handoff calls

We analyze a hierarchical cellular system with finite queues for new and handoff calls. Both the effect of the reneging of waiting new calls because of the callers' impatience and the effect of the dropping of queued handoff calls as the callers move out of the handoff area are considered, besides the effect of the guard channel scheme. We successfully solve the system by adopting the multidimensional Markovian chain and using the transition-probability matrix and the signal-flow graph to obtain the average new-call blocking probability, the forced termination probability, and the average waiting time of queued new and handoff calls. We further investigate how the design parameters of the buffer sizes and guard channel numbers in macrocell and microcells affect the performance of the hierarchical cellular system. The results show that provision of a buffering scheme and guard channel scheme can effectively reduce the new call blocking probability and the forced termination probability in the hierarchical cellular system, and the effectiveness is more significant in the macrocell than in the microcells.

Fuzzy layer selection method in hierarchical cellular systems

Hierarchical cellular systems are proposed to accommodate mobile users of different speeds. While the microlayer serves the slow users, the macrolayer serves the fast users. After a handoff request is initiated, we must make a decision determining whether it should go to the micro or macrocell. In this paper, this problem is tackled by making use of the past cell dwell times and occupancies in the target micro and macrocell. We want those mobile users with high velocity to join the macrolayer. It is also desirable to select the target cell which has lower occupancy in order to prevent call blocking. These heuristics are incorporated in a fuzzy layer selection (FLS) algorithm, which is devised in terms of fuzzy set and fuzzy logic. The performance is compared to the popular threshold method in the literature by simulation.

A combined channel assignment mechanism for hierarchical cellular systems

This paper proposes a combined channel assignment (CCA) mechanism for hierarchical cellular systems with overlaying macrocells and overlaid microcells. The proposed CCA mechanism combines overflow, underflow, and reversible schemes, where new or handoff calls having no available channel to use in the overlaid microcell can overflow to use free channels in the overlaying macrocell, handoff calls from a neighboring macrocell can underflow to use free channels in the overlaid microcell, and handoff attempts from a macrocell-only region to an overlaid microcell can be reversed to use free channels in the microcell. We apply the CCA mechanism in two different hierarchical cellular systems of Strip type and Manhattan type and compare the CCA with the overflow channel assignment (OCA) scheme. Simulation results show that the CCA mechanism outperforms the OCA scheme by once in forced termination probability, by several times in new call blocking probability, and by 4.7% in system utilization for a hierarchical cellular system, and the CCA mechanism is more suitable for the Manhattan type than for the Strip type.