In the first scheme NPS , a handoff call is blocked if no channel is immediately available in the target cell. In the second scheme QPS , the handoff call attempt is held in a queue until either a channel becomes available for it, or the received power level becomes lower than the receiver threshold level.
Non-priority scheme: In the non-priority scheme, the co-coordinating system handles a handoff call as same as an originating call i. This is the scheme employed by typical radio technologies . In this scheme a channel is allocated only when it is available either the call is originating call or a handoff call, otherwise the call is blocked. So, the handoff request should be given priority over the originating call request by adopting certain policy to serve the handoff calls.
Queuing priority scheme: Queuing of handoffs is more effective than two-threshold-level handoffs. The MTSO will queue the request of handoff call instead of rejecting them if the new cell sites are busy. A queuing priority scheme becomes effective only when the request for handoffs arrive at the MTSO in batches or bundles.
If handoff requests arrive at the MTSO uniformly, then the queuing scheme is not needed. Before showing the equations, let us define the parameters as follows:. In this queuing priority scheme, for mobile unit controlled handoff, when a mobile unit with an ongoing call enters a handoff area, it checks if there is a channel available with the new cell.
If not, this scheme requires that there be a way for the mobile unit to signal to the new cell site its desire for a handoff and for the new cell site to buffer the request in a waiting queue. The mobile unit continues to use the old cell site until a new channel becomes available. No queuing on either the originating calls or handoff calls: The blocking for either a originating call or a handoff call is.
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Queuing the originating calls: The blocking probability for originating call is. Now, from the Fig. However, queuing of originating calls, results in increased blocking probability on handoff calls.
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However queuing of handoff is more important than queuing for those originating calls on assigned voice channels because call drops upset the customer more than the call blocking. Queuing of handoffs is possible due to the fact that there is a finite time interval between the times the received signal level drops below the handoff threshold and the time the call is terminated due to insufficient signal level.
The delay time and the size of the queue is determined from the traffic pattern of the particular service area. It should be noted that queuing does not guarantee a zero probability of forced termination, since large delays will cause the received signal level to drop below the minimum required level to maintain communication and hence lead to forced termination.
Queuing the handoff calls: The blocking probability for handoff calls is. But with queuing of handoff calls only, blocking probability decreases from 5. Also adding queues in handoff calls does not affect the blocking probability of originating calls, which is shown in the Fig. Therefore, this scheme is based on the fact that, adjacent coverage areas in a mobile cellular network overlay.
Thus, there is a considerable area where either base station, called the handoff area, can handle a call. The time that a MU moves across the handoff area is referred to as the degradation interval.
If a channel on the new cell becomes available, then the handoff actually occurs. In our study, if there is no idle channel after the MU moves out of the handoff area i. In this scheme, when a channel is released, the MU first checks if the waiting queue is empty. If not, the released channel is assigned to a handoff call in the queue; otherwise the channel is assigned to the originating call.
Note that the performance of the queuing schemes depends on the time that a mobile unit stays in the handoff area and the time that a mobile unit and the network will maintain the call connection while the radio link down or unavailable.
The queuing priority scheme provides reduced blocking probability of handoff calls by prioritizing the handoff request. This scheme adds extra computational and implementation complexity compared with NPS to manage the waiting queues.
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However, it provides reduced blocking probability of handoff calls and improved spectrum utilization when integrated with the proposed dynamic channel assignment technique. Proposed channel assignment scheme: In the proposed dynamic channel assignment scheme, channel assignment is based on the prevailing traffic condition and demand of the resources in the cell. Therefore, this assignment scheme has the potential to achieve significantly improved bandwidth utilization when there are temporal or spatial traffic variations. The goal of the proposed DCA technique is that to provide the advantage of increased channel utilization and decreased probability of a blocked call by prioritizing the handoff request.
In this technique, a channel cannot be simultaneously used in all the cells whose centers are closer than the frequency reuse distance D. These cells constitute the belt of interfering cells. Therefore the constraint on the frequency reuse distance should be respected.
When a new call attempt occurs in a cell, the local network controller carries the DCA. This controller may be a mobile telephone switching system or a Base station, depending on a centralized or decentralized implementation of resources assignment algorithm. In this work, centralized resource assignment algorithm is adopted. The performance of the proposed DCA technique has been derived through simulations under the following assumptions. Every time a Mobile Subscriber MS with a call in progress crosses the cell border, an available channel must be assigned to it in the destination cell to avoid a forced call termination.
Performance analysis: The performance of proposed dynamic channel assignment technique is evaluated in terms of the parameters of interest, new call blocking probability P ba , handoff call blocking probability P bh and call dropping probability P ns and compared with the results obtained by an fixed channel assignment technique under the assumption of an equal traffic load in each cell of the network. Specific channel propagation conditions have not been considered.
It is only assumed that, according to the mobile environment envisaged, it is allowed to reuse a same distance. Therefore, if channel is allocated to cell x, it cannot be used in adjacent cells with x because of unacceptable co-channel interference levels. The performance of FCA technique has been derived by application of standard results of queuing theory under the following assumptions for the purpose of comparison with the proposed DCA technique. The blocking probability of handoff call using fixed channel assignment scheme is shown in Fig.
In macro cellular system, the cells have wider sizes and handoff procedure occurrence during the call lifetime is very low. Simulation of the DCA technique: The proposed technique is simulated using the following procedures:. The parameter P b derived from equation 16 , in the case of a classical FCA technique, assuming 10 channels per cells, is also shown for comparison purposes.
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The superior performance of the proposed DCA technique is evident from the Fig. In this case, the handoff calls are treated as same as the originating calls and the handoff calls are blocked immediately if no free channel is available, but the probability of blocking is decreased appreciably by prioritizing the handoff requests. The proposed DCA technique permits to achieve handoff-blocking probability P bh, lower than the blocking probability of originating call P ba by means of handoff prioritization and channel re-arrangement technique. FCA cannot obtain this advantage because call management in a cell is independent of what happens in the neighboring cells.
The proposed DCA technique attains good performance levels with low to medium loads and constitutes an easier-to-manage allocation algorithm, even in the decentralized version .
yuzu-washoku.com/components/2020-02-06/2357.php Figure 9 shows the blocking probability for originating calls non-prioritized handoff calls as a function of the cell intensity for the proposed DCA technique and the FCA technique. Previous results point out that the density of potential users handled by a channel allocation technique significantly increases when cell side decreases. The blocking probability of handoff calls are decreased by prioritizing the handoff request over originating calls in the proposed technique.
This is evident from Fig. As mobility increases, handoff-blocking probability, P bh decreases, P ns , the call completely served increases, which is evident from the Fig. This is because the advantages obtained by using the rearrangement technique and handoff prioritization in the DCA case are enhanced when mobility is high, as in micro cellular systems. The performance of the proposed dynamic channel assignment technique for handoff prioritization has been evaluated and compared with the fixed channel assignment scheme.
From the result, it is clear that the proposed dynamic channel assignment technique outperforms the fixed channel assignment technique and decreases the handoff blocking probability by giving priority to the handoff request by employing the queuing priority scheme. The influence of the mobility on blocking probability of originating calls and the handoff calls are evident from the result that as mobility increases P bh decrease.
The better performance of the proposed DCA technique increases the spectrum efficiency by allocating the channel on demand and prioritizing the handoff request. Beck, R. Panzer, Strategies for handover and dynamic control allocation in micro cellular mobile radio system.
Choi, S. Shin, Predictive and adaptive bandwidth reservation for handoffs in QoS-sensitive cellular networks. Rappaport, Traffic model and performance analysis for cellular mobile radio telephone systems with prioritized and non-prioritized handoff procedures.
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