Sending email is a commonplace occurrence in our daily life. When you send an email to a friend in another city, it will firstly join up with other messages being transmitted in your city, and then get dropped off at the correct destination in the correct city. How do all of these messages get to join together and be transmitted without getting mixed up? This process is achieved through the use of multiplexing technology, which is a method that combines multiple analog message signals or digital data streams into one signal over a shared medium. Actually, multiplexing is widely used in many telecommunications applications. This article will introduce the following common types of multiplexing technology: space-division multiplexing (SDM), time-division multiplexing (TDM), frequency-division multiplexing (FDM), and code-division multiplexing (CDM).
SDM—SDM uses separate point-to-point electrical conductors for each transmitted channel. In SDM, multiple radio frequency (RF) signals occupy the same bandwidth (or carrier frequency), but in a different physical space. There are two forms of SDM: cross-polarization and orbital angular momentum (OAM). In cross-polarization, independent signals are transmitted on the same carrier frequency through the use of orthogonal electric field vectors, commonly referred to as horizontal and vertical. OAM operates by placing separate signals on multiple elements of a specially constructed helical antenna.
TDM—TDM is essentially the time-based switching of multiple analog or digital input signals. In TDM, each input signal (or data stream) is assigned a fixed-length time slot on a communication channel. Each sender transmits a block of data during its assigned time slot. The defect of TDM is that each sender has a reserved time slot in each cycle in spite that whether the sender is ready to transmit. This may result in empty slots and underutilization of the multiplexed communication channel (as shown in the following figure).
FDM—FDM is a technique that divides the total bandwidth available in acommunication medium into a series of non-overlapping frequency sub-bands, each of which is used to carry a separate signal. Thus, in FDM, each signal is assigned its own frequency range (or channel) within a larger frequency band. It is primarily used for analog transmissions. WDM is one kind of FDM.
CDM—CDM is a class of techniques where several channels simultaneously share the same frequency spectrum, and this spectral bandwidth is much higher than the bit rate or symbol rate. In CDM, signals from multiple senders are transmitted in an assigned frequency band. CDM increases the bandwidth needed for transmission, however, it is more secure than other types of multiplexing.
All in all, the usual goal of multiplexing is to enable signals to be transmitted more efficiently over a given communication channel rather than save bandwidth. It is hoped that multiplexing technology would offer significant gains in bandwidth efficiency.
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