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Position location strategies, applications (Part 3)

Posted: 07 Nov 2011     Print Version  Bookmark and Share

Keywords:wireless network  mobility  CDMA 

One important issue that needs to be addressed in a wireless network is mobility as it limits capacity. For example, in a cellular network with no mobility, we can establish a capacity criterion based on the Erlang-B formula. Channels will be occupied by users that contribute to traffic so that a cell with C channels will have a blocking probability given by the formula.

If we allow users to move, then some users from adjacent cells will hand off their calls to the cell of interest, producing a higher occupancy state in the cell and thus a higher blocking probability. This simple reasoning shows how mobility could limit capacity.

In the following sections, we introduce some aspects of mobility that need to be considered to balance the capacity–coverage trade-off that mobility causes.

Capacity, coverage issues
Demand for wireless services has increased, which in turn brings new issues to consider such as mobility management for service providers [13]. One of the major objectives of a telecommunications system is to offer a service of excellent quality based on user requirements. In order to evaluate how a network or a communication system performs, we need to define measures that quantify the effects of varying parameters such as demand and capacity. In wireless networks, we allow users to move from area to area, thereby causing handoffs in the network. A user requesting service for the first time from the network is considered a new call.

Two of the most important performance measures in wireless networks are the new call and the handoff blocking probabilities [34, 48]. The handoffs are a fundamental feature in cellular systems;their performance and efficiency strongly depend on the use of adequate algorithms. For cellular communication systems, to ensure mobility and capacity, to maintain the desired coverage areas, and to avoid problems of interference, it is necessary to correctly assign the calls to the corresponding service areas in the entire cell and in the entire network.

In a CDMA network, soft handoff has been modelled considering overlapping areas, such as in Kwon and Suang [42], Miranda-Guardiola and Vargas-Rosales [53], and Scaglione et al. [72]. Admission of handoffs is done using one of three criteria. The first is to consider handoffs and new call arrivals equally for occupancy of the channels, the second reserves channels to give priority to handoffs, and the third sends them to a queue if no channel is available. Several performance evaluation algorithms have been introduced for these handoff strategies (e.g., see [34] and [79]) for reservation and queueing strategies and in McMillan [48] for the reservation and no-reservation strategies.

In a simplistic way, we can see each i-th cell as a resource with Ci channels offered Poisson traffic for new calls with λi calls per time unit, with exponential channel residence times with mean 1/µi . This would translate to considering each cell as an M/M/Ci/Ci system with performance provided by the Erlang-B formula,

Equation (5.1)

But we need to see that each cell receives offered traffic due to handoffs from adjacent cells, as shown in figure 1. And since the success of the new call traffic depends directly on the available capacity of the cell to which it is being offered, and the handoff calls depend on the available capacity of the cell from which it comes, we can see a cellular network such as that in figure 1 as a Jackson-type network of queues [5] as shown in figure 2.

 wireless network

Figure 1: Traffic offered to cell in wireless network.

 Cellular network

Figure 2: Cellular network as Jackson network of queues.

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