In telecommunications and computer networks, multiplexing is a process where multiple analog signals or digital data are combined into a single signal through a common medium.
The multiplexed signal is transmitted through a communication channel, which can be a media for physical transmission. Multiplexing is divided into low-level communication channel capacity into several higher-level logic channels, one for each message or data stream. The reverse process, known as demultiplexing, can be extracted by the recipient of the original channel.
The device that performs multiplexing is called a multiplexer (MUX), and the device that performs the reverse process is called a De-Multiplexer (DEMUX).
Four main techniques are mainly used:
- Space Division Multiplexing (SDM)
- Frequency Division Multiple Access (FDMA)
- Time Division Multiple Access (TDMA)
- Code Division Multiple Access (CDMA)
Space Division Multiplexing means multiplexing on different transmitted information streams at the same time and the same frequency spectrum, but at different locations. This concept applies to cellular networks where space multiplexing is the basic postulate.
Frequency Division Multiple Access – FDMA, used in analog cellular systems, is organized in such a way that each user is assigned different frequencies, all of which can transmit at the same time, and each transmission of information takes place in a different part of the frequency band. Although analog FDMA systems are no longer common, and most analog systems are currently deactivated, the FDM technique is still very important because the distributed spectrum can be divided into individual channels, on which other techniques (digital) can be applied, which will allow multiple users to use the channel at the same time.
TDMA includes sequential groups of several bits or bytes from each individual input stream, one after the other and in such a way that they can be connected to a suitable receiver. If done fast enough, the receivers will not detect that someone on the circuit has been used to serve another logical communication route.
TDMA-related digital cell phones are more intelligent than those used with FDMA. For example, they have the ability to scan and, if the channel they are on encounters anomalies that will cause transmission errors, the phone may search for another available channel that provides better performance. The key benefits of TDMA systems are that they offer greater capacity and spectral efficiency than FDMA. In TDMA, everyone speaks on the same frequencies, but at different times. The users are experiencing their conversations as uninterrupted, although everyone actually gets very quick copies of his / her conversation.
Code Division Multiple Access is a versatile technique that is attracting a lot of attention today. In the extended spectrum, everyone uses the same frequency at the same time. Broad-spectrum multiplex technology provides the best in terms of supported user density, and this is possible because each conversation is uniquely encrypted. In the extended spectrum, one spectrum of the transmission band is available to all users. In Code Division Multiple Access , although everyone uses the same frequencies at the same time, each call is uniquely encoded, allowing the transmitter to select the right one from all the calls. This technique is used in 3G mobile systems as well as in WLAN networks and is the basis for most of the new wireless broadband solutions. In general, there are two main benefits to code division: greater interference resistance (and therefore greater reliability) and much better capacity.
OFDM is the latest broadband technique, and its main purpose is to solve distortion problems due to the expansion of multiple paths. OFDM has some key advantages over CDMA, which is used in many of today’s 3G cellular networks. OFDM overcomes most problems with FDMA and TDMA, including inefficient spectrum utilization and the need for TDMA synchronization. OFDM also offers improved bandwidth efficiency as it can support multiple users on the same transmission channel using different extension codes.