7.4. Distribution of channels in a cellular cellular network of the GSM standard.

When allocating radio-frequency channels in a cellular network, two approaches are possible [7.1]:

- fixed distribution;

- dynamic distribution.

With a fixed distribution of channels, a certain subset of them is permanently assigned to certain cells, while this subset of channels is reused in cells separated by a reuse interval. That is, the channels are fixed within the clusters and then they are repeated from cluster to cluster within the common service area. Therefore, with a deterministic distribution, the number of channels is determined as follows:

a) the total number of channels allocated to the entire cellular system within the cluster service area, which, in turn, will be determined as the ratio of the frequency band of the whole Afs system (for the GSM Afs = 25 MHz standard) to the channel spacing fs (fs = 0.2 MHz): nsysk = (A / 5 / fs) -1 = 124 channels;

b) the number of channels per cell (with the number of cells in the cluster NcJc = 7): ps = nsysk / Nck = = 124/7 = 17.7 "18 channels. Separating the two channels for the control system (signaling), we take the value of ps "20 channels in a cell;

c) the number of channels allocated for the entire service area consisting of 7 clusters: nz = Ixrisysk = 868 channels, i.e. it is necessary to have 7 main lines providing a total of 868 channels. This number of channels provides a choice of a free channel when the mobile station moves from a subset of channels in the corresponding cells and clusters. At the same time, if the mobile station does not cross the service cell, then the estimated load for the subset of channels is equal to the product of the intensity of the new calls A by the average call duration (t, -) (channel busy time). This condition remains approximately fair if the product of the average call duration and the average speed of the mobile station VMS is small compared to the cell size: (t) (Vms) ^^ max.

For example, when the speed of the mobile station (Vms) = 1 km / h and the average duration of a call, (t, -) = 3 min (i.e. 3/60 = 0.05 hours), their product will be:

1 km / h • 0.05 hour = 0.05 km “20 km, with (Vms) = 100 km / h • 0.05 hour = 5 km“ 20 km. When crossing the cell border and moving the mobile station to a new frequency allocated by the new BTS base station, the following factors should be taken into account: 1) the average call time (t, -) in each cell decreases, since the call originated within the neighboring cell frees the channel prematurely; 2) the effective intensity of incoming calls increases by the amount of mobile stations crossing the cell boundaries, since a call arriving at a new cell requires for its service a channel in a new cell, just like the service request originated within this new cell. Some conversations are forced by subscribers to stop prematurely due to the fact that they are entering a new cell of service, in which there may be no free channels. It also reduces the average talk time.

Dynamic distribution of channels in a cellular communication system assumes that any channel can be used in any cell service. Channels are distributed at the same time to service calls, taking into account the state of the system, based on a defined channel allocation strategy, which should optimize some system parameters taking into account the restriction on channel reuse. A search to select a channel for a specific cell at a certain point in time includes enumerating all channels allocated to the system, and finding a free channel or multiple channels for an MS in a given cell. All unused channels located at a distance from the considered cell are considered free if they are smaller than allowed by the given ratio q = D / R. If such a channel cannot be found, then it is impossible to service the call in this cell at this time. Channel searches can be carried out in some regular fashion among all selected channels or randomly. If more than one channel is available, the channel must be selected in accordance with some optimization strategy. This strategy can be based on criteria such as the search order, the weighted distance to the remaining cells using the same channel, the number of uses of the channel with the minimum D / R, or even random selection. Some strategies and their main characteristics are described in the work.

[7.1]. Currently, the GSM standard from the switching center implements automatic channel selection using dynamic channel allocation.