what is multiple access

Modern telephone networks allow bandwidths in their channels that are much larger than those is needed for a digitalized telephone channel. Basically, a number of channels share a common transmission medium with the aim of reducing costs and complexity in the network. Multiplexing is defined as the process by which several signals from different channels share a channel with greater capacity. When the sharing is carried out with respect to a remote resource, such as a satellite, this is referred to as multiple access rather than multiplexing. There are various ways of performing this sharing:

1. FDM/FDMA (Frequency Division Multiplexing/Frequency Division Multiple Access): Assigns a portion of the total bandwidth to each of the channels.
2. TDM/TDMA (Time Division Multiplexing/Time Division Multiple Access): Assigns all of the transport capacity sequentially to each of the channels.
3. CDMA (Code Division Multiplexing Access): In certain circumstances it is possible to transmit multiple signals in the same frequency and at the same time, with the receiver being responsible for separating them. This technique has been used for years in military technology, and is based on extending the spectrum of the signal and reducing the transmission power.
4. PDMA (Polarization Division Multiple Access): Given that polarization can be maintained, the polarization direction can be used as a multiple access technique, although when there are many obstacles noise can make it unsuitable, which is why it is not usually used in indoor installations. Outside, however, it is widely used to increase transmission rates in installations that use microwaves.
5. SDMA (Space Division Multiple Access): With directional aerials, the same frequency can be re-used provided the alignment of the aerials is correctly adjusted. There is a great deal of interference but this system lets frequencies obtain a high degree of reusability.


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