Mobile Telecommunication Networks
Access Methods

In each cell it must be ensured that the connection between one base station and a higher number of mobile stations in each period of time might be established and realized. For this purpose the methods of the multiple access are used.

A dedicated frequency band for a given radio system can be accessed by one of the following methods:

- FDMA divides the allocated frequency band into the subchannels and then assigns these subchannels to each communication channel
- TDMA creates in a particular frequency subchannel the sequence of time slots and then its individual slots allocates to each communication channel on the principle of time multiplex
- CDMA processes data sequence on each side of the transmitting channel by coding process through unique coding policy, which is deliberately different from the coding policy of all the other channels. Thus, the signals of each channel can be transmitted in the same frequency band, which is without time resolution. The communication channels are distinguished from each other on the receiving side on the basis of a unique coding policy, which was used for the coding at side of transmitting channel
- OFDMA is a combination of time and frequency division multiple accesses. The available resources are split into subcarriers in a frequency domain and also into several time intervals in time domain. Then individual users have assigned not only one or several subcarriers but also a time interval for communication.

Multiple access methods

Above mentioned basic access methods are very often combined in practical applications (for example FDMA and TDMA).

The assigned frequency bands can be accessed in either Time Division Duplex (TDD) or Frequency Division Duplex (FDD) way. In case of TDD, the transmission in downlink (transmission to user) and in uplink (transmission from user) directions are separated in time domain but, the same frequency is assigned to both transmission direction. This means that the frequency is better utilized. On the other hand, the issue here is the high requirements on synchronization. In case of FDD, both transmission directions are separated in frequency, but the transmission/reception is simultaneous. The main disadvantage of this approach is that two antennas, one for receiving and one for transmitting, are necessary to be used. In most of countries, FDD duplex is utilized for its less complex implementation