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Multiplexing in Communication System

Dec, 20 2020, 12:31 pm [IST]
Multiplexing in Communication System

Multiplexing

Multiplexing is not only a primary mechanism in communication systems but also in normal life. Multiplication describes how many users can share a medium with minimum are no interference. One example from your everyday life is Highways with several lanes. Several users use the same medium with probably no interference.

Space division multiplexing

For wireless communication, multiplexing can be carried out in four dimensions: space, time, frequency, and code. In this field, the task of multiplexing is to assign space, time, frequency, and code to each communication channel with a point of interference and a peak of medium utilization. The word communication channel here only indicates an Association of senders and receivers that need to exchange data.

In wireless transmission, space division multiplexing separate standard for each communication from with a far enough gap between senders. Using space-division multiplexing, obvious obstacles arise if two or more channels were established within the same space, for example, many radio stations want to broadcast in the same city. Then, one of the multiplexing systems must be applied (frequency, time, or code division multiplexing).

Frequency division multiplexing

Frequency division multiplexing (FDM) describes schemes to subdivide the frequency dimension into several non-overlapping frequency bands. Each channel k1 is now allotted its own frequency band as indicated. Senders using a certain frequency band can use this band continuously. Again, Guard spaces are needed to avoid frequency band overlapping also called adjacent channel interference.

This scheme is applied for radio stations within the same religion, radio station has its one frequency. This really simple multiplication scheme does not require Complex coordination between sender and receiver, the receiver only has to tune into the particular sender.

However, this scheme also exhibits disadvantages. While radio stations broadcast 24 hours a day, mobile communication typically takes place for only a few minutes a day. Specifying a separate frequency for each possible communication scenario would thus be a large waste of frequency sources. Additionally, the fixed assignment of a frequency to a sender executes this came very inflexible and restricts the number of senders.

Time-division multiplexing

A more flexible multiplexing scheme for typical Mobile Communications is time-division multiplexing. Here a channel k1 is given the whole bandwidth for a certain amount of time, that is, All senders use the same frequency but at different points in time. Again, guard spaces, which now represent time gaps, how to separate the different periods when the sender uses the medium.

Additionally, for a receiver tunning into a sender now does not involve adjusting the frequency but listening to accurately the right point in time. However, this scheme is very flexible as one can specify more sending time to send us a huge load and only less sending time to send us a light load.

A disadvantage of this scheme is again necessary coordination between different senders. No one has to control the sequence of frequencies and the time of changing to another frequency. Two senders will interfere as soon as they select the same frequency at the same time. However, if the frequency change is fast enough, the periods of interference may be so small that, depending on the coding of data into signals, a receiver can still recover the original data.

Code division multiplexing

While SDM and FDM are well known from the early days of radio transmission and TDM is used in connection with many applications, code division multiplexing (CDM) is a new scheme in commercial communication systems. First used in military applications due to its inherent security features, it is now also introduced in many civil wireless transmission scenarios. Separation is now achieved by assigning the channel its own code, guard faces are realized by using codes with the necessary distance in code space, e.g., orthogonal codes.

The main Advantage of CDM for wireless transmission is the protection against interference and tapping. Different codes have to be assigned, but code space is huge compared to the frequency space and, thus, assigning individual codes to each sender typically does not cause problems.

The main disadvantage of this scheme is the relatively high complexity of the receiver. A receiver has to know the code and must separate the channel with user data from the background noise composed of other signals and environmental noise.


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