2 6 Orthogonal frequency division multiplexing

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Frequency-division Multiplexing (FDM)


* It is an analog technique. * Frequency Division Multiplexing is a technique in which the available bandwidth of a single transmission medium is subdivided into several channels. * In the above diagram, a single transmission medium is subdivided into several frequency channels, and each frequency channel is given to different devices. Device 1 has a frequency channel of range from 1 to 5. * The input signals are translated into frequency bands by using modulation techniques, and they are combined by a multiplexer to form a composite signal. * The main aim of the FDM is to subdivide the available bandwidth into different frequency channels and allocate them to different devices. * Using the modulation technique, the input signals are transmitted into frequency bands and then combined to form a composite signal. * The carriers which are used for modulating the signals are known as sub-carriers. They are represented as f1,f2..fn. * FDM is mainly used in radio broadcasts and TV networks.Advantages Of FDM: * FDM is used for analog signals. * FDM process is very simple and easy modulation. * A Large number of signals can be sent through an FDM simultaneously. * It does not require any synchronization between sender and receiver.Disadvantages Of FDM: * FDM technique is used only when low-speed channels are required. * It suffers the problem of crosstalk. * A Large number of modulators are required. * It requires a high bandwidth channel.Applications Of FDM: * FDM is commonly used in TV networks. * It is used in FM and AM broadcasting. Each FM radio station has different frequencies, and they are multiplexed to form a composite signal. The multiplexed signal is transmitted in the air.* * *

Wavelength Division Multiplexing (WDM)


* Wavelength Division Multiplexing is same as FDM except that the optical signals are transmitted through the fibre optic cable. * WDM is used on fibre optics to increase the capacity of a single fibre. * It is used to utilize the high data rate capability of fibre optic cable. * It is an analog multiplexing technique. * Optical signals from different source are combined to form a wider band of light with the help of multiplexer. * At the receiving end, demultiplexer separates the signals to transmit them to their respective destinations. * Multiplexing and Demultiplexing can be achieved by using a prism. * Prism can perform a role of multiplexer by combining the various optical signals to form a composite signal, and the composite signal is transmitted through a fibre optical cable. * Prism also performs a reverse operation, i.e., demultiplexing the signal.* * *

Time Division Multiplexing


* It is a digital technique. * In Frequency Division Multiplexing Technique, all signals operate at the same time with different frequency, but in case of Time Division Multiplexing technique, all signals operate at the same frequency with different time. * In Time Division Multiplexing technique, the total time available in the channel is distributed among different users. Therefore, each user is allocated with different time interval known as a Time slot at which data is to be transmitted by the sender. * A user takes control of the channel for a fixed amount of time. * In Time Division Multiplexing technique, data is not transmitted simultaneously rather the data is transmitted one-by-one. * In TDM, the signal is transmitted in the form of frames. Frames contain a cycle of time slots in which each frame contains one or more slots dedicated to each user. * It can be used to multiplex both digital and analog signals but mainly used to multiplex digital signals.There are two types of TDM: * Synchronous TDM * Asynchronous TDM

Digital Multiplexing


The term digital represents the discrete bits of information. Hence theavailable data is in the form of frames or packets, which are discrete.Time Division Multiplexing (TDM)In TDM, the time frame is divided into slots. This technique is used totransmit a signal over a single communication channel, with allotting one slotfor each message. Of all the types of TDM, the main ones are Synchronous andAsynchronous TDM.Synchronous TDMIn Synchronous TDM, the input is connected to a frame. If there are ‘n’ numberof connections, then the frame is divided into ‘n’ time slots. One slot isallocated for each input line. In this technique, the sampling rate is commonto all signals and hence same clock input is given. The mux allocates the sameslot to each device at all times.Asynchronous TDMIn Asynchronous TDM, the sampling rate is different for each of the signalsand the clock signal is also not in common. If the allotted device, for atime-slot, transmits nothing and sits idle, then that slot is allotted toanother device, unlike synchronous.Published on 23-Jan-2019 12:09:40Multiplexing | IntechOpen

2.1. Frequency division multiplexing


Frequency division multiplexing [1, 2, 3, 4] is a networking technique whichcombines many signals into a single one and then transmitted the combinedsignal through a common communication channel. In the receiver side, theopposite process is carried out which is known as demultiplexing whichextracts the individual channel signals. Here the transmitter side performsmultiplexing, and the receiver side performs demultiplexing. In FDM the totalbandwidth available in a communication medium is divided into a series ofnonoverlapping frequency bands. Each of these bands is used to carry aseparate signal. In FDM all users use the same common channel at full time.But each of the users is allocated with different frequencies for transmissionfor avoiding the signal interference. Sometimes there is a possibility ofcross talk because all the users use the transmission medium at the same time.FDM is used for analog signal transmission. It does not need synchronizationbetween the transmitter and receiver. Here a large number of signals can betransmitted simultaneously. It suffers the problem of cross talk, andintermodulation distortion may take place.FDM is used in amplitude modulation (AM) and FM broadcasting, public telephonenetworks, and cable TV network systems. The allocation of frequency bands todifferent users is shown in Figure 2.

2.2. Wavelength division multiplexing


Fiber-optic communications require a different kind of multiplexer called awavelength division multiplexer (WAD) [2, 4]. It is an analog multiplexingtechnique. It is designed for high data rate capability fiber cable. In thistechnique the bandwidth of the communication channel should be greater thanthe combined bandwidth of the individual channels. Here signals are convertedto light signals; each light which has different wavelengths is transmittedthrough the same fiber cable. WDM transmission system divides the opticalfiber bandwidth into a number of nonoverlapping optical wavelengths; these arereferred to as WDM channels. WDM mixes all incoming signals having differentwavelengths and are transmitted over a common channel. A demultiplexer doesthe reverse operation and separates the wavelengths. This multiplexingmechanism provides a much higher available transmission capacity.Figure 3 shows the representation of WDM system that consists of bothmultiplexer and demultiplexer.

2.3. Frequency division multiplexing


FDM [1, 2, 3, 4] is based on sharing of the available bandwidth of acommunication channel among the signals to be transmitted. It is an analogmultiplexing technique that uses a single transmission medium which is dividedinto several frequency channels. Here the total bandwidth of the channel mustbe higher than the sum of the individual bandwidth. If the channels are closerto each other, then cross talk may occur; thus, it is necessary to implementchannel synchronization. For that purpose some bandwidth is allocated as guardband; these are unused channels placed between the successive transmissionchannels to avoid cross talk.For frequency division multiplexing if the input signal is digital, it must beconverted to analog before giving it as the input to the modulator.

2.4. Time division multiplexing


In time division multiplexing (TDM) [1, 2, 3, 4], all signals operate with thesame frequency at different times, i.e., it is a technique of transmittingseveral signals over a single communication channel by dividing the time frameinto equal slots. Here the signal transmitted can occupy the total bandwidthof the channel, and each signal will be transmitted in its specified timeperiod only. In TDM all signal operates at same frequency at different timeslots.Figure 8 shows the schematic diagram of implementation of TDM system. Fromthis it is clear that a circular ring has been split into eight equal segmentsand is completely separated from one another. It is also noted that there is amovable arm attached to the inner ring, and it slides over the eight segmentsover the ring. The eight segments are eight inputs, and the selector moves inclockwise direction from A to H; after completing one revolution, it startsagain. The output is taken from the inner ring that contains the signal fromonly one slot at a time.

2.4.1. Synchronous time division multiplexing


In synchronous TDM the slots are arranged in a round robin manner, i.e., ifthere are n sources, then a single frame consists of n time slots, and eachtime slot is dedicated to exactly one source for carrying data from thecorresponding input. Each source places its data to the link only when thecorresponding slot arrives. In synchronous TDM, if a device does not have datato send, then its time slots remain empty. The transmission of data withsynchronous TDM is shown in Figure 9.

2.4.2. Asynchronous time division multiplexing


In synchronous TDM if a particular terminal has no data to transmit at aparticular time period, the corresponding slot in a frame is wasted or anempty slot will be transmitted. Asynchronous TDM or statistical TDM is used toovercome this difficulty. It dynamically allocates the time slots on thedemand to separate input channels, thus saving the channel capacity. Here thetime slots are flexible, and the total capacity of input lines can be greaterthan the link capacity of the channel. In synchronous TDM if there are n inputlines, there must be n time slots, but in asynchronous TDM if we have n inputlines, then the frame may contain less than n slots. Here the number of slotsin a frame is based on a statistical analysis of the number of input lines.The transmission of data with asynchronous TDM is shown in Figure 10.

2.6. Orthogonal frequency division multiplexing


Orthogonal frequency division multiplexing (OFDM) [4, 5] is a multiplexingtechnique used in broadband communication system. It is a multicarriermodulation scheme. Now it is used in 4G broadband communication system andnext-generation systems. OFDM is popular in broadband wireless systems due toits resistance to multipath fading. OFDM has high data rate capability withreasonable computational complexity. OFDM divides a broadband channel intomultiple parallel narrowband subchannels, and each channel carries a low datarate stream of signals. Finally these signals are summing and then transmit asa high data rate stream. In an OFDM transmitter, the input signal bits aremapped into a bank of quadrature amplitude modulator which encodes these intocomplex symbols. This is fed to an inverse fast Fourier transform (IFFT) toensure the orthogonality of the subchannels. This output is converted intoparallel to serial, modulated into a carrier wave, and then transmitted intothe air. At the receiver the reverse process is performed for recovering theoriginal signal. The advantages of OFDM are that its low computationalcomplexity because OFDM may be viewed as a many slowly modulated narrowbandsignals rather than a rapidly modulated wideband signal.

5.2. Frequency shift keying


The principle of frequency shift keying is that the frequency of the carrierwave is modulated in accordance with the digital message signal, i.e., FSKsignal represents the binary data in the form of variations in the frequencyof the carrier signal. When an ON condition of digital pulse exists, thencarrier will be switched to one frequency, and when an OFF conditionencounters the carrier, it will be switched to another frequency, i.e., in“frequency shift keying,” the frequency of a sinusoidal carrier is shiftedbetween two discrete values (Figure 21).

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