Question

Explain why the need for Re-use distance.

Answer 1

Principles of cellular frequency reuse

In the cellular concept, frequencies allocated to the service are re-used in a regular pattern of areas, called 'cells', each covered by one base station. In mobile-telephone nets these cells are usually hexagonal. In radio broadcasting, a similar concept has been developed based on rhombic cells.

To ensure that the mutual interference between users remains below a harmful level, adjacent cells use different frequencies. In fact, a set of C different frequencies {f₁, ..., f_C} are used for each cluster of C adjacent cells. Cluster patterns and the corresponding frequencies are re-used in a regular pattern over the entire service area.

Reuse Distance:

second, data reuse is a main determinant in cache performance because all cache reuse comes from reuse of the same or adjacent data, regardless of the organization of cache. Therefore, reuse distance separates program-specific factors from machine-specific factors.

The closest distance between the centres of two cells using the same frequency (in different clusters) is determined by the choice of the cluster size C and the lay-out of the cell cluster. This distance is called the frequency 're-use' distance. It can be shown that the reuse distance r_u, normalised to the size of each hexagon, is

ru = SQRT{3 C}

For hexagonal cells, i.e., with 'honeycomb' cell lay-outs commonly used in mobile radio, possible cluster sizes are C = i² + ij + j², with integer i and j (C = 1, 3, 4, 7, 9, ...). Integers i and j determine the relative location of co-channel cells.

7-cell reuse with i = 2 and j =1.

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