Wireless Signal Multiplexing

Modulations of analog signals or digital data are concerned with a single input signal to be converted efficiently into other forms. In contrast, multiplexing is a collection of schemes that addresses the issue of transmitting multiple signals simultaneously in a wireless system in the hopes of maximizing the capacity of the system. The devices for multiplexing and demultiplexing are multiplexers and demultiplexers, respectively. If signals of the same frequency are spatially separated from each other such that no frequency overlapping occurs at any given place, then multiple signals of different frequencies can be transmitted and received without a problem. Radio stations are an excellent example of this spatial division multiplexing : AM and FM radio signals only cover the area in which the radio stations are located, and they cannot interfere with other radio signals on the same frequency in adjacent areas. Apart from spatial division multiplexing, three prominent schemes of multiplexing have been devised.


Frequency-Division Multiplexing
In frequency-division multiplexing (FDM), signals from a transmitter are modulated to a fixed frequency band centered at a carrier frequency (i.e., a channel). To avoid inference, these channels have to be separated by a sufficiently large gap (i.e., a guard band) in the frequency domain; hence, transmission and reception of signals in multiple channels can be performed simultaneously but independently. Analog cellular systems use FDM; in these systems, calls are separated by frequency.


Time-Division Multiplexing
Time -division multiplexing (TDM) allows multiple channels to occupy the same frequency band but in small alternating slices of time following a sequence known to both the transmitter and the receiver. Each channel makes full use of the bandwidth of the medium but only contributes a portion of the overall data rate. Coordination among the transmitters is necessary to prevent conflicting use of the frequency band. When applied to digital signals, TDM can be done on bit level, byte level, block level, or levels of larger quantities. GSM and D-AMPS both use TDM but in different ways. TDM and FDM can be combined to increase the robustness of the system. In this case, signals from a transmitter are modulated onto different carriers for a certain amount of time and jump to another carrier, effectively creating a �frequency-hopping� phenomenon.


Code-Division Multiplexing
Code -division multiplexing (CDM) makes better use of a frequency band than FDM and TDM. Signals from different transmitters are transmitted on the same frequency band at the same time but each has a code to uniquely identify itself. The orthogonal codes mathematically ensure that signals cannot interfere with each other at the receiver. CDM effectively converts the problem of limited frequency space into ample code space but adds the overhead of implementation complexity. The transmitter and receiver must be synchronized such that individual signals can be correctly received and decoded. Compared to FDM, CDM provides greater security against signal tapping because transmitted signals appear as noise if the receiver does not know the code. CDM is the underlying multiplexing scheme of orthogonal frequency-division multiplexing (OFDM). CDMA cellular systems use similar CDM schemes to provide multiple wireless communication channels access to the same frequency band. Another multiplexing scheme, wavelength-division multiplexing (WDM), is very common in optical networks using fiber as the transmission medium. It is actually FDM for fiber, which offers an extremely high bandwidth. In WDM, a fiber can be divided into a number of wavelengths (nanometers), each of which can be assigned to a transmission channel. Dense wavelength-division multiplexing (DWDM) systems support eight or more wavelengths. Because of their high data rate, WDM and DWDM are the predominant multiplexing schemes used by optical networks in the wired Internet backbone.

Source of Information :  Elsevier Wireless Networking Complete 2010