broadband communication - bt0046

8/6/2019 Broadband Communication - BT0046

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Name AAGJA KETANKUMAR DAHYABHAIRoll Number 510821891Learning Centre 02798Subject Management Information System

Assignment No BT0046-01Date of

Submission

23/10/2009

1. What is bandwidth?


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Bandwidth is the span of frequencies within the spectrum occupied by a signal and used by the sigor conveying information, For example voice has a band width of 3 to 4 kHz. Audio signal (speech a

music) has 15kHz. Whereas a video requires a bandwidth of 5 MHz. The bandwidth is an importparameter in communication and it depends on the type of signal or type of application, the amounnformation to be communicated and the time in which the information is to be communicated.convey more information in short time we need more bandwidth. The same quantity of information be sent in a longer period using less bandwidth. Similarly to convey voice signal we need bandwidth and to convey video it requires more bandwidth and so on.

n computer networking and computer science, digital bandwidth, network bandwidth or

bandwidth is a measure of available or consumed data communication resources expressed in bit/smultiples of it (kbit/s, Mbit/s etc).

Bandwidth may refer to bandwidth capacity or available bandwidth in bit/s, which typically mehe net bit rate, channel capacity or the maximum throughput of a logical or physical communicat

path in a digital communication system. For example, bandwidth test implies measuring the maximhroughput of a computer network. The reason for this usage is that according to Hartley's law,

maximum data rate of a physical communication link is proportional to its bandwidth in hertz, whicsometimes called frequency bandwidth, radio bandwidth or analog bandwidth, the last especiallycomputer networking literature.

Bandwidth may also refer to consumed bandwidth (bandwidth consumption), correspondingachieved throughput or goodput, i.e. average data rate of successful data transfer througcommunication path. This meaning is for example used in expressions such as bandwidth shapbandwidth management, bandwidth throttling, bandwidth cap, bandwidth allocation (for exambandwidth allocation protocol and dynamic bandwidth allocation), etc. An explanation to this usaghat digital bandwidth of a bit stream is proportional to the average consumed signal bandwidth

Hertz (the average spectral bandwidth of the analog signal representing the bit stream) durinstudied time interval.

Digital bandwidth may also refer to: average bit rate (ABR) after multimedia data compression (soucoding), defined as the total amount of data divided by the playback time.

What is the bandwidth of

A. Telephone signal

3 to 4 kHz

B. Commercial radio broad casting

15 kHz

C. TV signal.

5 MHz

2. Define and prove sampling theorem using frequency spectrum.


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Digital transmission of information and digital signal processing all require signals to first acquired" by a computer. One of the most amazing and useful results in electrical engineeringhat signals can be converted from a function of time into a sequence of numbers without error: We

convert the numbers back into the signal with (theoretically) no error. Harold Nyquist, a Laboratories engineer, first derived this result, known as the Sampling Theorem, in the 1920s. It founo real application back then. Claude Shannon, also at Bell Laboratories, revived the result ocomputers were made public after World War II.

Most of the times, the signals we have to communicate will be in the analog form, such as vosignals. As a first step in digitization, the analog signal is converted to a discrete time signal by process of sampling. While sampling, sufficient number of samples of the signal must be taken so the signal is completely represented in its samples. Also it should be possible to reconstruct the sigrom its samples. The number of samples to be taken depends on the maximum frequency in the c

of a low pass signal and for a band pass signal it depends on the bandwidth.

Sampling theorem provides complete information regarding the number of samples to be taken orms the basis of digitization of analog signals. In 1928 H. Nyquist showed that an analog signal can

perfectly reconstructed from its samples without any loss of original information, if it is sampled at lewice the rate of the maximum frequency component, or the bandwidth of the signal. This is cal

sampling theorem or Nyquist criteria of sampling. The rate of sampling, which is equal to twice

signal bandwidth, is called the Nyquist rate.

n other words we can state the sampling theorem as follows:Any signal which is continuous in time (analog) can be completely represented by its samples and

be recovered if the sampling frequency fs 2 fm . Where fs is the sampling frequency and fm is maximum frequency of the signal. This means that if the signal is of bandwidth 4000Hz Then it mbe sampled at least 8000 samples/sec (8000Hz) or greater so as to enable the reproduction of signal without distortion. Similarly if the signal has 5000Hz. as the maximum frequency component other components in the signal has frequencies less than 5000Hz.) then it must be sampled at a rat10,000Hz. or more. The sampling theorem is the fundamental principle of digital communicationcommunication sampling is done in principle by multiplying the given analog signal by a narrow purain whose frequency is equal to the sampling frequency. Let us consider the message signal be m

band limited to fm and the sampling signal S(t) be a narrow pulse train of frequency fs 1 Ts , whTs is the time period of the sampling signal. The message signal and the sampling signal are appliednputs to the multiplier and the multiplier output is the sampled signal y(t) = S(t)m(t). This producm(t) whenever there is a pulse in S(t) , and equal to zero otherwise. The figure 2.2.1 shows espective waveforms.


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n the figure if we look at the output of the multiplier, the amplitudes of the pulse trains used

sampling varies in accordance with the amplitude of the message signal. This is known as PuAmplitude Modulation (PAM). In the case of PAM signals the top of the pulse may follow the messasignal in which case it is called Natural sampling, or it can be made flat and if so it is called flat tsampling.

The sampling signal is periodic with periodTs , and has a pulse width of dt. Using Fourier seexpansion we can express this sampling signal S(t) as

Now the first term in the series represents the signal m (t) itself with a multiplication by a constant. Tsecond term is a product of m(t) and is a sinusoid of frequency 2fm = fs. This represents the douside band suppressed carrier signal with the carrier frequency at 2fm and has frequency componerom fm (=2fm-fm) to 3fm(=2fm+fm). Similarly the succeeding terms are double side band suppres

carrier signals with carrier frequencies at 4fm, 6fm, 8fm and so on, the harmonics of 2fm.


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The figure 2.2.2 shows the frequency spectrum of the signal m(t). A frequency spectrum is a plosignal amplitudes versus the frequency. For sinusoidal signals the spectrum will be what is called spectrum, since each signal is represented as a line at the corresponding frequency and the heighhe line represents the maximum amplitude of the signal. But for a band limited signal, that is signal

3. Explain the concept of Path Clearance.

n Practice the direct ray Path from the transmitting antenna to the receiving antenna has to pabove and below or by the side of elevated structures like buildings, trees and hills etc., which capable of reflecting the microwaves and thus providing a reflected wave. If these reflecting objeincluding ground also) are not sufficiently removed from the direct ray path the reflected wave woend to cancel the direct wave at the receiver. Therefore it is necessary to ensure that adequate P

Clearance exists.

Consider the reflection from a point Q located at a distance 'r' off the straight line TR and let its locatalong TR be specified by the distance dt and dr from the transmitter and receiver respectively as shon the fig. 1.3. The Path difference between the direct ray and reflected ray path is given by = TQR TR.


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4. Explain Tropospheric Forward Scatter Systems.

The prime advantage of tropospheric forward scatter systems, compared with the line-of-smicrowave system is that they provide reliable communication over distances upto 1000 Km or mwithout repeater stations. On the other hand, the large-range tropospheric-scatter systems require varge antennas and very high-power transmitters.

Characteristics of Tropospheric Forward Scatter Prapogation : Large transmission loss, hencehigh-gain, narro beam antennas for both transmitting and receiving. Scattering angle must be kas small as possible.


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Troposphere extends up to about 10 Km and max range is about 650 Km Stratosphere (region betweroposphere and ionosphere) max range is 1000 Km.

1. Scatter Loss :This is the loss in addition to the free-space loss in LOS transmission. It is statistican nature and subject to two types of time variations or fading.a) Fast fading, a short-term variation ( 1 to 5 min) of the instantaneous values (believed to be), duemulti-path effects. The signal is the resultant of a large no. of components of random and varyphases.b) Slow fading : Superimposed on fast fading is a slow fading resulting from the change of the of atmosphere. It is a long term variation of the medium hourly or 15 to 30 minutes interval.

c) Usable signals can be received consistently, even up to 650 Km or more provided that very laantennas, very high power Transmitter and sensitive Receivers are employed.d) Frequencies from 100 MHz to 10 GHz can be used. However freqs. below about 1 GHz are genermore effective for the longer paths.e) Bandwidths of 3 to 4 MHz sufficient for a TV Channel or several hundred telephone channels mayransmitted over distances upto about 350 Km.

2. Aperture Medium Coupling Loss: Since the signals arrive at the Receiver from an extenscattering volume, narrowing the antenna beam eventually reduced this common volume. Full antegain is not utilized. As a result, a relative antenna gain loss is incurred which is known as the apertmedium coupling loss.

They are characterized by large antennas 4 m to 40 m dia and by high-power Transmitters from 1 toKW. Because of the large size and high cost of the antennas, to employ the same antenna


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ransmission and reception, with diplexers being used to combine the Transmitter output and Receinput as shown in fig. RF filters are provided at the Transmitter output, to minimize the noise in eceived freq. band, and at the Receiver input, to reject the high-level signal at the transmitting freq

most systems the high- power Transmitter amplifier is driven by a low power FM exciter, with FDMhe telephone channels advantages of SSB : (a) the ability to operate satisfactorily at low levelseceived signal due to the absence of a thresholds effect ; (b) a saving of up to 10 to 1 in the freque

space required; (c) freedom from inter-modulation due to multi-path signals; (d) reductionnterference to other systems; (e) and economy in high-tension power consumption.Disadvantage is a high degree of amplitude linearity is essential in a single side band system.

Tropospheric scatter (or troposcatter) is the scattering of distant TV and FM radio stations by roposphere so that they travel farther than the line of sight. This effect sometimes allows receptio

stations up to a hundred miles away.

The phenomenon has been used to build communication links in a number of parts of the world. Labillboard antennas focus a high power radio beam at the troposphere mid-way between the transmiand receiver. A certain proportion of the signal is refracted and received at a similar antenna at eceiving station.

One such link operated between the North of Scotland, at Mormond Hill and the Shetland Isles.

US Army TRC-170 Tropo Scatter Microwave System
http://en.wikipedia.org/wiki/File:Tropo_Scatter_Microwave_System_Antenna.jpg


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The U.S. Army uses tactical tropospheric scatter systems developed by Raytheon for long communications. The systems come in two configurations, the original "heavy tropo", and a nelight tropo" configuration exist. The systems provide four multiplexed group channels and tr

encryption, and 16 or 32 local analog phone extensions.

The U.S. Marine Corps also uses the same device, albeit an older version.

5. Explain various light sources for Optical Fiber Communication.

Light emitting diodes (LED) and semiconductor injection lasers are the two light sources that commonly used for optical Fiber Communication. These sources are having directional light outpsmall size, modulation capability etc., which are well suited for optical fiber communication. The chobetween these two types depend on particular application and type of fiber used.

Light Emitting Diode

Principle: In certain material, if an electron gains energy it jumps to conduction band. Suchexcited electron can not stay there permanently. It comes back to the original state with in a short tknown as carrier life time. While doing so extra energy is released in the form of photon (light energSuch phenomenon is called spontaneous emission and is seen in direct band gap materials like GallArsenide (GaAs), Gallium Aluminum Arsenide (GaAlAs) etc. A p-n junction is formed of thsemiconductors. Under the external biasing, the electron-hole pairs created recombine radiativ

emitting light. The wave length of emission depends on the band gap energy of the material.changing the composition of the alloys, band gap energy and hence wavelength can be changed.

Structure:Double heterostructures (DH) are used. A junction formed by two different semiconductors but wsome common features is heterojunction. Typical DH is as follows. (fig. 3.9)

The recombination takes place in the GaAs layer which is called active area. Dimension is compatiblehe fiber. Light emitted is coupled to the fiber. Either surface emitting or edge emitting LED's is capa

of sourcing a few mW of optical power. LED's are available in 0.8 to 0.9 m band. (First optical windwith spectral width 25 to 40 nm. These LED's find use in short distance links or links operating at lodata rate. At higher wavelength, spectral width is large. Generally Lasers are used at second and th

optical window.

LaserPrinciple: When a photon gives its energy to electrons, electron jumps to higher energy stAbsorption). This radiatively recombines to give photon again (spontaneous emission). If electronhe higher energy band stimulated by a photon to come back to original state, stimulated emissakes place. If many electrons are there in conduction band (population inversion), then large num

of stimulated emission take place leading to a coherent narrow beam of light output. This is caLASING action. Semiconductor injection laser works on this principle.


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Structure: DH structure is used in the construction of laser. Lasing is achieved by providing opteedback in the active layer. For this dielectric cleaved mirrors (partially reflecting) are used at

ends. This leads to Febry- Perrot laser structure. There are other types of layers also.

Characteristics: Lasers are highly coherent. Light output is more than that of LED's. Lasers operatat around 1300 and 1550 nm are available with low spectral widths (less than 2 nm). Hence lasers better suited for step-index single mode fibers. Lasers are used for long distance and high data rnks.

broadband communication - bt0046

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