MOB Subject 5 –Access methods Static and dynamic access techniques

Master of Computer Science 1 - MOB Mobile Internet and Surrounding

1/12 Baey, Fladenmuller – Subject 5

MOB Subject 5 –Access methods

Static and dynamic access techniques

1. Static access techniques

1.1 Space-division multiple-access (SDMA)

When one seeks to cover a broad territory by a whole of cells of the same dimension, one

considers that the traffic is uniformly distributed, that the propagation law is the same and that the

power of all the equipment is constant. The same number of carrier will have thus to be allocated to

each basic station.

One will often use a hexagonal paving to cut out the zone has to cover, the hexagon being the

polygon nearest to the circle which allows paving the plan. One calls cluster, a set of adjacent cells

which uses the unit of carriers one and only one time. This cluster is repeated on all surface has to

cover.

Arithmetic and geometrical considerations allow showing that a cluster having a given number

of carrier is optimal if it is regular. For a hexagonal paving, the size K of cluster verifies the relation:

K = i

2

+ ij + j

2

(1)

with i and j entries positive natural or null.

The paving of the zone has to cover is realized of iterative manner in leaving of a cluster given

of K cells. From one cells of this cluster, one moves vertically i cells to one of its sides, then j cells

in a direction forming an angle of +60 degrees with the initial direction. The arrival point defines

then the "homologous" cell of adjacent cluster. By repeating this process for the set of cells of the

cluster chosen at the origin while keeping these same directions, one defines an adjacent cluster.

One repeats then this process for the five other possible directions (other dimensions of the cell) to

find the set of adjacent clusters. In proceeding secondly little by little on the cluster, one then builds

a paving of the cover zone.

Each cell of a cluster is seen allocated a subset of the carrier which is own for it. The homologous

cells of all the cluster use the same carrier. The minimal distance between two cells (homologous)

using the same carrier is called reuse distance. The size K of the cluster also constitutes the reuse

factor of the carrier.

1. What is the principle of the reuse of frequencies in the domain of the cellular networks?

In a cellular network, the space is divided into cells, each served by a base station (i.e a cell is

a portion of the territory covered by a base station).

It was affecting to each cell (i.e at each base station), a number of carriers of the total

bandwidth available based on estimated traffic in this cell.

It is possible to reuse one even at the same carrier in different cells if they are sufficiently

remote. The reuse of frequencies thus allows an operator has to cover an unlimited

geographic area by using a frequency band width limited.

2. Discuss the advantages and disadvantages to use of cells of small size, in comparing with

cells of big size in the cellular networks.

Advantages:

- greater reuse of frequencies: optimization of bandwidth;

- increase battery life because the power of emission decreases;