How does multiplexing aid in networking

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The transmission medium is used to send the signal from sender to receiver. The medium can only have one signal at a time. Clearly, asynchronous TDM demands more processing capability at multiplexer and de-multiplexer, and it may cause a delay up to one cycle for buffering and aggregating the existing messages.

However, it is worthwhile since it yields higher spectral efficiency and saving valuable resources. As another advantage, using the preambles diminishes the need of synchronized clocks between multiplexer and de-multiplexer. Frequency division multiplexing FDM is a multiplexing technique which divides the available bandwidth into multiple sub-bands each of which is able to carry a signal. Therefore, FDM enables concurrent transmissions over a shared communication medium.

As another common use, FDM enables the system to send a huge amount of data through several segments transmitted over independent frequency sub-bands. Figure 4 reveals basic principles of the conventional FDM.

In the figure, signal of three independent sources are multiplexed to be sent over the medium. Each source has a flat signal with large width in time domain. The baseband signals are converted to well-separated bandpass signals by multiplexer. The signals are conveyed by carriers with different frequencies such that each signal is located within a non-overlapping sub-band and does not leak onto other signals.

The carrier frequencies spacing, and sub-band width are application-specific and depend on the available bandwidth. The multiplexed signal will be transmitted over the medium.

At the receiver side, de-multiplexer employs proper filters to extract the desired bandpass signals. The intended bandpass signals will be converted to baseband signals for further processes at destinations. Frequency division multiplexing. The multiplexer uses N equally spaced carrier frequencies.

The carrier frequency for the i th user is f ci. For further simplicity, consider that sources use the same pulses for data transmission. Signals from the i th source lies within the i th frequency sub-band centered around f ci.

Spectrum of multiplexed signals via FDM for a system with N sources. Conventional FDM allocates the available spectrum to the sources very generously, and the spectral efficiency is not among its concerns. With this motivation, OFDM has been introduced for efficient use of spectrum. OFDM is a multi-carrier modulation through which a data stream, like voice, video, or data, is distributed among multiple subcarriers separated closely and precisely.

In a simple OFDM-based system, the modulated samples over frequency domain, i. Since the basis of the transformation is unit vectors with equally angular separated in polar plane, the spectrum of OFDM signal is composed of N shifted sinc functions. Figure 6 illustrates the spectral basis of an OFDM signal with four orthogonal subcarriers. The subcarrier spacing, i. Therefore, at the moment of sampling from the center of subcarriers, no interference is experienced from others.

The time domain signal goes through another process called cyclic prefix CP insertion to be immune against multipath fading.

If the maximum delay in multipath environment does not exceed the time duration CP, the signal can be recovered perfectly. The baseband analog signal is then converted to a bandpass signal to be sent over the communication medium. Bandwidth requirement for an OFDM system using four orthogonally spaced subcarriers.

At the receiver side, the superimposition of the analog signals received from different paths within reception interval is sampled and converted to a baseband digital signal. Then, the CP added by transmitter is removed.

The resulting series of modulated samples is fed into demodulator, and the desired data is recovered. The popularity of this technique stems from following advantages which make it highly amenable for real world application. Resilience in frequency selective environments: OFDM decomposes whole available spectrum to several narrow channel in frequency domain. It is very likely that signals carried over a subcarrier experience a relatively flat channel although the channel may be frequency selective.

Resilience to inter-symbol interference ISI : single-carrier communication is vulnerable to ISI specially when the data rate grows. OFDM tackle this problem with dispatching signals over multiple sub-channels. Indeed, OFDM technique changes a transmission with high rate into multiple low-rate transmissions. In this manner, it increases the symbol duration and push the duration beyond maximum delay of the channels.

At the end of this section, example 1 reveals how OFDM technique combats the frequency selectivity of the channel. Resilience to narrow-band interference : narrow-band interference drastically diminishes the throughput of single-carrier systems either by blurring the reference signals for synchronization or corrupting the data. However, if the signal is transmitted using OFDM, only a portion of symbols is contaminated by interference. The erroneous parts caused by interference can be recovered with the aid of error correction codes and interleaving to isolate errors.

Spectral efficiency : comparing Figures 5 and 6 , it is clear that closely separated frequency sub-channels yields higher spectral efficiency for OFDM. Low-computational complexity : although OFDM technique is more complex than conventional FDM, it intrinsically demands low-computational capability since it possesses simple mathematical operations.

Therefore, ISI is imminent. Multi-carrier case : in this case, data rate of a subcarrier is 7. In spite of aforementioned advantages, the OFDM technique suffers from two main disadvantages. This effect emerges due to hardware impairment between transmitter and receiver local oscillators and cause inter-carrier interference ICI. Code division multiplexing CDM , also called spread-spectrum technique, is a multiplexing technique which has been widely implemented in third generation of wireless network.

It takes full advantage of the available spectrum. Through several concurrent transmissions over the spectrum, this technique has enhanced the capacity of network 18 times compared to first generation and 6 times compared to the second generation of wireless communication technologies. For transmitting multiple messages over the channel simultaneously, the multiplexer assigns a separate spreading code from a set of orthogonal pseudo-random sequences to each user.

The orthogonality of these sequences will help users to recover their desired signals from the multiplexed signals. Consider a system with 4 users as depicted in Figure 7. AP intends to send one bit to each user, say b i for user i.

These sequences are devised such that the inner product of two different sequences is zero, i. The inner product of a sequence with itself is M which is the number of users. Upon receiving the multiplexed sequence, user i recovers its desired information by multiplying its corresponding spreading sequence and received sequence and dividing the result by the length of sequences. The key component in CDM is the spreading code.

Walsh code is among the most popular sequences used for CDM. An example of CDM for a network with four users. This procedure results four orthogonal sequences as follows. Therefore, the multiplexed signal is:. And other users do the same procedure with their own sequences.

As seen in the example the Walsh code is used as spreading code. However, a variety of codes can be utilized for this purpose, which can be classified into two major categories. PN codes : pseudo-random noise code is a sequence of pulses which shows the appropriate features to be used in CDM. Although PN sequences look like noise, they can be exactly generated at both multiplexer and demultiplexer locally using a finite number of shift registers with a pre-defined initial state.

The finite length of linear shift registers makes these codes deterministic. A local sequence has a high correlation with itself, but almost zero correlation with other sequences or a time-shifted version of itself. In cryptography applications, to ensure security, using PN codes with very large period is a necessity.

However, this is not a strict requirement for CDM. Non-random orthogonal codes : this kind of codes is designed in a specific and predefined manner and has a special set for desired length while satisfying primary features required by CDM.

An instance of these codes is Walsh code shown in the previous example. As described, choosing a well-defined code plays a critical role in spreading process. Here an OR gate is used to combine these four output lines as a single output Figure Circuit symbol and selector switch pattern of four-in-one multiplexer.

The logic symbol and data selector of eight-in-one multiplexer is shown in Figure Logic symbol and switching pattern of eight-in-one multiplexer. If input is 2, then one data selector switch is needed; if input is 4, then two selector switches are needed; if input is 8, then three selector switches are needed; if input is 16, then four selector switches are needed; and so on.

Logic diagram of eight-in-one multiplexer. When the three selector switches are actively low, then the three inputs of the first AND gate become actively high because the selector outputs are NOTed and given to the first AND gate. At that time all other gates are in 0 output position. Demultiplexer [ 6 ] is a logic circuit that performs the reverse multiplexer function.

Demultiplexer receives signal from a single line serial input and transmits these information into multiple output lines and parallel output lines Figure Circuit diagram of one-to-eight demultiplexer.

In analog communication we use different modulation schemes like amplitude modulation, frequency modulation, phase modulation, etc. In digital communication we use the following schemes: Amplitude shift keying ASK. The principle of amplitude shift keying is that the amplitude of the carrier wave is modulated in accordance with the digital message signal, i.

The time period for which the carrier is present or absent depends on the time interval for which the unipolar pulses are present. Here the amplitude of carrier signal is varied to represent binary 1 and binary 0 data inputs, while the frequency and phase of the carrier signal remain constant. Voltage levels are left to designers of the modulation system Figure Amplitude shift keying.

The major advantage of ASK includes high bandwidth efficiency and simplicity in its design. In ASK the modulation and demodulation processes are comparatively inexpensive.

Its disadvantages include lower power efficiency, and it is very susceptible to noise interference. The principle of frequency shift keying is that the frequency of the carrier wave is modulated in accordance with the digital message signal, i. When an ON condition of digital pulse exists, then carrier will be switched to one frequency, and when an OFF condition encounters the carrier, it will be switched to another frequency, i.

Frequency shift keying. Its advantages include lower probability of error and provide high signal-to-noise ratio. It has higher immunity to noise due to constant envelope. Therefore the probability of error-free reception of data is high. It is not preferred for the high-speed modems because with increase in speed, the bit rate increases. The principle of phase shift keying is that the phase of the carrier wave is modulated in accordance with the digital message signal, i.

Here the carrier signal changes its phase depending on the nature of the transmitted bit, i. In two levels of PSK, the difference of phase shift is used between binary 1 and binary 0 Figure Phase shift keying.

The disadvantage of PSK includes lower bandwidth efficiency. The binary data is decoded by estimation of phase states of the signal. These detection and recovery algorithms are very complex.

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Edited by Somayeh Mohammady. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Abstract In any communication system that is either digital or analog, we need a communication channel for transmission. Keywords frequency division multiplexing time division multiplexing code division multiplexing wave length division multiplexing orthogonal frequency division multiplexing amplitude shift keying frequency shift keying phase shift keying.

Introduction Multiplexing is the process of transmission of information from more than one source into a single signal over a shared medium. Types of multiplexing Multiplexers are mainly classified as shown in Figure 1. Frequency division multiplexing Frequency division multiplexing [ 1 , 2 , 3 , 4 ] is a networking technique which combines many signals into a single one and then transmitted the combined signal through a common communication channel.

Wavelength division multiplexing Fiber-optic communications require a different kind of multiplexer called a wavelength division multiplexer WAD [ 2 , 4 ]. It is easier to reconfigure. Optical components are more reliable and provide higher bandwidth. Provide high security and faster access to new channel.

Low cost and easy system expansion. Simultaneous transmission of various signals. Disadvantages of WDM Scalability is a concern as optical line termination OLT ; optical line termination has to have transmitter array with one transmitter for each optical network unit ONU. The cost of the system increases with addition of optical components. Frequency division multiplexing FDM [ 1 , 2 , 3 , 4 ] is based on sharing of the available bandwidth of a communication channel among the signals to be transmitted.

Advantage of FDM multiplexing A large number of signals channels can be transmitted simultaneously. Demodulation of FDM is easy. Due to slow narrowband fading, only a single channel gets affected. Disadvantages of FDM The communication channel must have a very large bandwidth.

Intermodulation distortion takes place. A large number of modulators and filters are required. FDM suffers from the problem of cross talk. All the FDM channels get affected due to wideband fading. FDM is used in television broadcasting. First-generation cellular telephone also uses FDM. Time division multiplexing In time division multiplexing TDM [ 1 , 2 , 3 , 4 ], all signals operate with the same frequency at different times, i. Synchronous time division multiplexing In synchronous TDM the slots are arranged in a round robin manner, i.

Disadvantages The channel capacity cannot be fully utilized when some source do not want to send the data. It is very complex to implement. Asynchronous time division multiplexing In synchronous TDM if a particular terminal has no data to transmit at a particular time period, the corresponding slot in a frame is wasted or an empty slot will be transmitted.

Disadvantages Frames have different sizes. Code division multiplexing Code division multiplexing CDM [ 3 ] is a form of multiplexing in which the transmitter encodes the signal by using a unique chip code which is generated by a pseudorandom sequence generator.

In CDM more number of users can share the same bandwidth. It is scalable. It is well-matched with other cellular technologies. Interference is reduced because different code words are allocated to each user.

Efficient utilization of fixed frequency spectrum. Disadvantages The system is more complicated. As the number of users increases, the overall quality of services decreases. More complex system and primarily it is used in wireless transmission. Orthogonal frequency division multiplexing Orthogonal frequency division multiplexing OFDM [ 4 , 5 ] is a multiplexing technique used in broadband communication system. OFDM system uses pilot subcarriers to prevent the frequency and phase shift errors.

Four-in-one multiplexer The logic symbol and circuit for a four-input multiplexer are shown in Figure Eight-in-one multiplexer The logic symbol and data selector of eight-in-one multiplexer is shown in Figure Working When the three selector switches are actively low, then the three inputs of the first AND gate become actively high because the selector outputs are NOTed and given to the first AND gate.

Amplitude shift keying The principle of amplitude shift keying is that the amplitude of the carrier wave is modulated in accordance with the digital message signal, i. ASK advantages and disadvantages The major advantage of ASK includes high bandwidth efficiency and simplicity in its design. Application Used in our infrared remote controls Used in fiber optical transmitter and receiver.

Frequency shift keying The principle of frequency shift keying is that the frequency of the carrier wave is modulated in accordance with the digital message signal, i. Advantages and disadvantages of FSK Its advantages include lower probability of error and provide high signal-to-noise ratio. Application Many modems used FSK in telemetry systems.