In case of mobile communication, which is a form of wireless communication, the only restraint on communication is the bandwidth restraint which means we have a limited frequency range that we can use for communication. Hence, we must somehow, allow multiple users communicate in the same frequency range.
Multiple Access Techniques are ways to access a single channel by multiple users. They provide multiple access to the channel. A “channel” refers to a system resource allocated to a given mobile user enabling the user to establish communication with the network (other users). Based on the type of channel, we can use a particular multiple access technique for communication.
The types of channel and the corresponding multiple access techniques are listed below:
- Frequency Channels [FDMA – Frequency Division Multiple Access] – Frequency band divided into small frequency channels and different channels are allocated to different users – like in FM radio. Multiple users can transmit at the same time but on different frequency channels.
- Time-slot Within Frequency Bands [TDMA – Time Division Multiple Access] – Each user is allowed to transmit only in specified time-slots with a common frequency band. Multiple users can transmit at the same frequency band at different times.
- Distinct Codes [CDMA – Code Division Multiple Access] – Users may transmit at the same time using the same frequency band but using different codes so that we can decode to identify a particular user.
We often use a combination of TDMA+FDMA to achieve a greater number of multiple access channels as explained below:
Transmission using different frequency bands at the same time = 2 channels.
Similarly, Transmission using different time slots but same frequency band = 2 channels.
But transmission using two frequency bands and two time-slots = 4 channels.
The above as well as the major three multiple access schemes are explained in the figure below:
These multiple access techniques are used in the various mobile communication systems. The 1G analog AMPS system used FDMA. The 2G digital systems D-AMPS/IS-136 & GSM use TDMA+FDMA. And the CDMA/IS-95 system uses CDMA.
SEE ALSO Mathematical Modeling of Control Systems & Transfer Function Modeling
