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The Cellular Concept

RF bandwidth has always been the primary constraint in wireless systems; there is never too much. Efficiently using this precious resource involves what is called frequency reuse, in which a radio channel is allowed to be simultaneously used by multiple transmitters as long as they are sufficiently separated to avoid interference. The essential idea of cellular radio is to transmit at power levels sufficiently low so as to not interfere with the nearest location at which the same channel is reused.

In this way a physical (RF) channel can be used more than once in a given city. The greater the reuse distance, the lower the probability of interference. Likewise, the lower the power levels used in cells sharing a common channel, the lower the probability of interference. Thus, a combination of power control and frequency planning is used in cellular systems to prevent interference.

The unit area of RF coverage for cellular is called a cell. In each cell, a base station transmits from a fixed cell site location, which is often centrally located in the cell, to mobile stations or subscriber units. The base station and mobiles are allowed to use a subset of the RF channels available to the system. These channels cannot be reused in any potentially interfering cells.

Base stations are supported by and interconnected to each other and the public switched telephone network (PSTN) via mobile switching centers (MSCs), as depicted in Figure 1.0 The operation of AMPS systems has historically been based on intelligent MSCs controlling the operations of the base and mobile stations. Cellular mobility management is handled by home location registers (HLRs) and visiting location registers (VLRs), described in Section 2.7.7.


 

  
Figure 1.0: AMPS Architecture
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AMPS Architecture

 

 

Cellular system capacity or spectrum efficiency can be most easily and inexpensively increased by subdividing cells into smaller cells or by sectorizing the cells. Sectorization consists of dividing an omnidirectional (360 degree) view from the cell site into non-overlapping slices called sectors, which when combined provide the same coverage but are considered to be separate cells. This trend has continued with the creation of microcells, which are aimed at increasing capacity in areas of dense user populations . While cells typically range in size from two to twenty kilometers in diameter, microcells range from about a hundred meters to a kilometer in diameter.

The capacity gain provided by cellular systems is offset somewhat by loss of trunking efficiency, which is the queueing efficiency resulting from a large number of customers receiving service from a set of servers rather than proportionally assigning each customer to one of the servers. If a disproportionate number of mobile stations are simultaneously located in a single cell, a cellular system might actually end up supporting fewer users than a wide area radio system. Because relatively few of the users who are aggregated in the cell can receive service (due to the fact that only a subset of channels is available in the cell), the cellular system could appear ineffective. If the cell can only support m channels, the (m+1)st simultaneous user could be blocked from receiving service.

So there is a trade-off: an n-cell frequency reuse scheme, in which RF channels can be "reused" every n cells, provides better channel quality the larger the value of n (due to reduced opportunities for interference). However, an n-cell frequency reuse scheme allows only 1/n of the total number of channels to be available in each cell, which greatly increases the probability of blocking for a user trying to access the system. Sectorization is actually more reuse efficient in that a smaller number of cells are needed in the reuse pattern, each providing a larger fraction of the total frequency spectrum. Typical values for n are 7 for sectored cells (typically partitioned into three sectors, as in Figure 1.1 or 12 for omnidirectional (non-sectorized) cells.


 

  
Figure 1.1: Frequency Reuse
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Frequency Reuse

 

 

Frequency planning-the assignment of channels to cells-can be static or dynamic. A static assignment of channels to cells and sectors is referred to as fixed channel allocation or FCA. FCA has historically been used by cellular service providers in their frequency plans and results in each cell having a fixed capacity for serving mobiles. The maximum number of simultaneously-transmitting mobile stations is equal to the number of channels statically assigned to the cell.

A more recent technique for channel assignment is called dynamic channel assignment or DCA. With DCA there is no fixed association of channels to cells. Each of the channels available to a cluster of cells could be used in any cell or sector within the cluster as needed. DCA eliminates the need for up-front frequency planning and provides the ultimate flexibility for capacity. However, DCA requires processing and signalling to coordinate dynamic channel assignments and avoid interference. It is really frequency planning on the fly.