In GSM, the adjacent radio cells use different frequencies (re-use factor varying from 1/3 to 1/7 depending on the case) which involves dividing and distributing the frequencies allocated to an operator between several radio cells and therefore having less bandwidth (fewer active subscribers simultaneously) in each cell.
An important improvement of UMTS over GSM is the possibility of using the same radio frequencies in all radio cells, thanks to a new coding technique (WCDMA) and a scrambling code (scrambling code). ) to identify each cell. As a result, it allows to allocate a larger spectral width to each cell (the UMTS standard provides 5 MHz per radio cell) and thus to have in total a larger bandwidth and more flow in each cell.
Another difference is that UMTS is optimized for carrying data with variable data rates (videos, internet); there is no longer a bandwidth cut into fixed size channels (adapted to the voice), but a dynamic allocation of resources allowing variable and irregular data rates (Internet access).
A third difference with GSM concerns the spectral width assigned to each radio cell, the dominant UMTS standard (FDD) provides, for each cell, a fixed spectral width of 2x5 MHz (1 band for transmission, another for reception) , higher than in GSM (which uses a maximum of ten to twenty channels of 200 kHz per cell). A recent evolution of the standard called DC-HSPA + or DC-HSDPA, makes it possible to aggregate 2 frequency bands of 5 MHz in the download direction to increase the reception rate in a radio cell for the compatible terminals.
Organization of UMTS radio cubicles
In a UMTS network, in the homogeneous coverage areas (macrocells), one finds the same type of hexagonal checkers as that described above for the GSM. The same radio frequencies are generally used in all cells (this is mandatory for operators with only one 2x5 MHz duplex band).
The counterpart is that, despite the WCDMA coding and scrambling codes for identifying a cell, there is interference in the boundary areas of 2 adjoining cells and the result is much lower flow than at the center of the cell. radio cells; the change of cells of a mobile terminal (handover) is also more complex with the risk of micro-cuts of ongoing communications, especially in the "broadband" (HSDPA and HSPA +) variants of UMTS.
If the mobile operator has multiple 5 MHz bands Modify
For operators with more than one 5 MHz UMTS frequency sub-bands, there are three possible types of use:
Have several independent (carrier) radio cells superimposed on the same geographical area. This increases the overall bandwidth, but a mobile terminal can only connect to one cell.
Use the Dual-Cell or Dual-Carrier (DC) option of the HSPA + standard which allows two 5-MHz carriers to be aggregated, allowing a compatible smartphone to increase the peak bit rate by up to 42 Mb / s download. This option can be used in conjunction with 1st; some subscribers thus benefit from the connection to two carriers, others (because their terminal or their subscription does not allow them) connect only to one of the two carriers.
Reserve small cells or femtocells, one of the 5 MHz sub-bands belonging to the operator to reduce the load of the main cells.
