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University of Palestine College of Information Technology

University of Palestine College of Information Technology. Relationship between data rate and bandwidth Assignment. Prepared By Manar abd elrahman. Supervised By. Eng. Nael alian. Figure 5.1 Digital-to-analog modulation.

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University of Palestine College of Information Technology

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  1. University of Palestine College of Information Technology Relationship between data rate and bandwidth Assignment Prepared By Manarabdelrahman Supervised By Eng. Nael alian

  2. Figure 5.1Digital-to-analog modulation • Modulation of binary data or digital-to-analog modulation is the process of changing one of the characteristics of an analog signal based on the information in a digital signal (0s and 1s) • Telephone wires carry analog signals. • When we vary any one of these characteristics [amplitude or frequency or phase], we create a different version of that wave. • Three mechanisms for modulating digital data into an analog signal: amplitude shift keying (ASK), frequency shift keying (FSK) , and phase shift keying (PSK).

  3. Figure 5.4Relationship between baud rate and bandwidth in ASK • Bandwidth of a signal is the total range of frequencies occupied by that signal. • When we decompose an ASK-modulated signal, we get a spectrum of many simple frequencies. • Most significant ones are those between fc - Nbaud/2 and fc + Nbaud/2 with the carrier frequency fc at the middle. • Bandwidth BW = (1 + d)* Nbaud • BW is bandwidth, Nbaud is baud rate, and d is a factor related to the modulation process (with a minimum value of 0). • Although there is only one carrier frequency, the process of modulation produces a complex signal that is a combination of many simple signals, each with a different frequency.

  4. Figure 5.6FSK • Frequency of the carrier signal is varied to represent binary 1 or 0. • Frequency of the signal during each bit duration is constant, and its value depends on the bit (0 or 1); both peak amplitude and phase remain constant. • FSK avoids most of the problems from noise. • Because the receiving device is looking for specific frequency changes over a given number of periods, it can ignore voltage spikes.

  5. Figure 5.7Relationship between baud rate and bandwidth in FSK • Although FSK shifts between two carrier frequencies, it is easier to analyse as two co-existing frequencies. • FSK spectrum is a combination of two ASK spectra centered on fc0 and fc1. The bandwidth required for FSK transmission is equal to the baud rate of the signal plus the frequency shift (difference between the two carrier frequencies): BW = fc1 – fc0 + Nbaud • Although there are only two carrier frequencies, the process of modulation produces a composite signal that is a combination of many simple signals, each with a different frequency.

  6. Figure 5.14The 4-QAM and 8-QAM constellations • PSK is limited by the ability of the equipment to distinguish small differences in phase. • Why not combine PSK, FSK and ASK? • Bandwidth limitations make combinations of FSK with other changes practically useless. • Combining ASK and PSK, we could have x variations in phase and y variations in amplitude, giving us x times y possible variations and the corresponding number of bits per variation. Quadrature amplitude modulation does that. • Quadrature amplitude modulation is a combination of ASK and PSK so that a maximum contrast between each signal unit (bit, dibit, tribit, and so on) is achieved. • In 4-QAM and 8-QAM, number of amplitude shifts is fewer than the number of phase shifts. Because amplitude changes are susceptible to noise and require greater shift differences than do phase changes, the number of phase shifts used by a QAM system is always larger than the number of amplitude shifts.

  7. Figure 5.1616-QAM constellations • The first example, 3 amplitudes and 12 phases, handles noise best because of a greater ratio of phase shift to amplitude. It is ITU-T recommendation. • The second example, four amplitudes and eight phases, is the OSI recommendation. • It is to be noted that every intersection of phase and amplitude is utilized. • In fact, 4 times 8 should allow for 32 possible variations. But by using only one-half of those possibilities, the measurable differences between shifts are increased and greater signal readability is ensured. In addition, several QAM designs link specific amplitudes with specific phases. This means that even with the noise problems associated with amplitude shifting, the meaning of a shift can be recovered from phase information. • QAM has an advantage over ASK as is its less susceptibility to noise. • Minimum bandwidth required for QAM transmission is the same as that required for ASK and PSK transmission. QAM has the same advantages as PSK over ASK.

  8. Figure 5.18Telephone line bandwidth • Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. • All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility. • Data signals require a higher degree of accuracy to ensure integrity. For safety’s sake, therefore, the edges of the bandwidth range are not used for data communications. • We can say that the signal bandwidth must be smaller than the cable bandwidth. The effective bandwidth of a bandwidth line being used for data transmission is 2400 Hz, covering the range from 600 to 3000 Hz. • Today some telephone lines are capable of handling more bandwidth than traditional lines. • A telephone line has a bandwidth of almost 2400 Hz for data transmission.

  9. Figure 5.19Modulation/demodulation • Bandwidth defines a baseband nature, which means we need to modulate if we want to use this bandwidth for data transmission. Devices that were traditionally used to do so are called modems. • Modem stands for modulator/demodulator • Modulator creates a band-pass analog signal from binary data. • Demodulator recovers the binary data from the modulated signal. • The computer on left sends binary data to the modulator portion of the modem; the data is sent as an analog signal on the telephones lines. The modem on the right receives the analog signal, demodulates it through its demodulator, and delivers data to the computer on the right. The communication can be bidirectional, which means the computer on the right can also send data to the computer on the left using the same modulation/demodulation processes.

  10. Figure 5.26Amplitude modulation • Bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal and covers a range centered on the carrier frequency BWt = 2 x BWm. • Bandwidth of audio signal (speech and music) is usually 5 KHz. Therefore, an AM radio station needs a minimum bandwidth of 10 KHz. Federal Communications Commission (FCC) allows 10 KHz for each AM station. • AM stations are allowed carrier frequencies anywhere between 530 and 1700 KHz. Each station’s carrier frequency must be separated from those on either side of it by at least 10 KHZ to avoid interference.

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