Digital Cellular Systems (CDMA)

Multiple Access Schemes

The multiple access schemes are used to provide resources for establishing calls. There are five multiple access schemes:

FDMA serves the calls with different frequency channels.

TDMA serves the calls with different time slots.

CDMA serves the calls with different code sequences.

PDMA (polarization division multiple access) serves the calls with different polarization (Not applied to mobile radio)

SDMA (space division multiple access) serves the calls by spot beam antennas.

In the cellular system, the first three multiple access schemes can be applied. In analog systems, only FDMA can be applied. In FDMA and TDMA, each frequency channel or each time slot is assigned to one call. During the call period, no other calls can share the same channel slot. CDMA presents the opposite case, because all traffic channels are sharing a single radio channel. However, this feature causes a strong signal received from a near-in mobile unit to mask the weak signal from a far-end mobile unit at the cell site.