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Chapter 4

Chapter 4. 4.1 : Digital Modulation. 4.2 : Digital Transmission. 4.3 : Multiple Access Methods. 4.1 Digital Modulation. Outlines Introduction Information capacity, Bits, Bit Rate, Baud, M-ary Encoding Digital Modulation Techniques - ASK, FSK, PSK, QAM.

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Chapter 4

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  1. Chapter 4 4.1 : Digital Modulation 4.2 : Digital Transmission 4.3 : Multiple Access Methods

  2. 4.1 Digital Modulation Outlines • Introduction • Information capacity, Bits, Bit Rate, Baud, M-ary Encoding • Digital Modulation Techniques - ASK, FSK, PSK, QAM

  3. Introduction to Digital Modulation • Digital modulation • Is the transmittal of digitally modulated analog signals between to or more points in a communications system. • Can be propagated through Earth’s atmosphere and used in wireless communication system - digital radio. • Offer several outstanding advantages over traditional analog system. • Ease of processing • Ease of multiplexing • Noise immunity

  4. Cont’d... • Applications: • Low speed voice band data comm. modems • High speed data transmission systems • Digital microwave & satellite comm. systems • PCS (personal communication systems) telephone

  5. Why digital modulation? • The modulation of digital signals with analogue carriers allows an improvement in signal to noise ratio as compared to analogue modulating schemes.

  6. Important Criteria • High spectral efficiency 2. High power efficiency 3. Robust to multipath 4. Low cost and ease of implementation 5. Low carrier-to-co channel interference ratio 6. Low out-of-band radiation

  7. Cont’d… 7. Constant or near constant envelop 8. Bandwidth Efficiency • Ability to accommodate data within a limited bandwidth • Tradeoff between data rate and pulse width 9. Power Efficiency • To preserve the fidelity of the digital message at low power levels. • Can increase noise immunity by increasing signal power

  8. Forms of Digital Modulation • If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK) • If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)

  9. Cont’d… • If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK) • If both the amplitude and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

  10. Cont’d...

  11. Example 1 For the digital message 1101 1100 1010, sketch the waveform for the following: a. ASK b. FSK c. PSK d. QAM

  12. Block Diagram Simplified block diagram of a digital modulation system

  13. Cont’d… • Precoder performs level conversion & encodes incoming data into group of bits that modulate an analog carrier. • Modulated carrier filtered, amplified & transmitted through transmission medium to Rx. • In Rx, the incoming signals filtered, amplified & applied to the demodulator and decoder circuits which extracts the original source information from modulated carrier.

  14. Information Capacity, Bits, Bit Rate, Baud, M-ary Encoding • Information capacity, Bits & Bit Rate • Represents the number of independent symbols that can be carried through a system in a given unit of time. • Basic digital symbol is the binary digit or bit. • Express the information capacity as a bit rate.

  15. Hartley’s Law Where I = information capacity (bps) B = bandwidth (Hz) t = transmission time (s) From the equation, Information capacity is a linear function of bandwidth and transmission time and directly proportional to both.

  16. Shannon’s Formula Where I = information capacity (bps) B = bandwidth (Hz) = signal to noise power ratio (unitless) The higher S/N the better the performance and the higher the information capacity

  17. Example 2 By using the Shannon’s Formula, calculate the information capacity if S/N = 30 dB and B = 2.7 kHz.

  18. Nyquist Sampling Rate • fs is equal or greater than 2fm fs >= 2fm fs = minimum Nyquist sample rate (Hz) fm = maximum analog input frequency (Hz)

  19. Example 3 Determine the Nyquist sample rate for a maximum analog input frequency 7.5 kHz.

  20. M-ary Encoding • It is often advantageous to encode at a level higher than binary where there are more then two conditions possible. • The number of bits necessary to produce a given number of conditions is expressed mathematically as Where N = number of bits necessary M = number of conditions, level or combinations possible with N bits.

  21. Cont’d… • Each symbol represents n bits, and has M signal states, where M = 2N.

  22. Example 4 Find the number of voltage levels which can represent an analog signal with a. 8 bits per sample b. 12 bits per sample

  23. Baud & Minimum BW • Baudrefers to the rate of change of a signal on the transmission medium after encoding and modulation have occurred. Where baud = symbol rate (symbol per second) ts = time of one signaling element @ symbol (seconds)

  24. Cont’d… • Minimum Bandwidth • Using multilevel signaling, the Nyquist formulation for channel capacity Where fb= channel capacity (bps) B = minimum Nyquist bandwidth (Hz) M = number of discrete signal or voltage levels

  25. Cont’d… For B necessary to pass M-ary digitally modulated carriers Where N is the number of bits encoded into each signaling element.

  26. Digital Modulation Techniques • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK) • Quadrature Amplitude Modulation (QAM)

  27. Amplitude Shift Keying (ASK) • A binary information signal directly modulates the amplitude of an analog carrier. • Sometimes calledDigital Amplitude Modulation (DAM) Where vask (t) = amplitude shift keying wave vm(t) = digital information signal (volt) A/2 = unmodulated carrier amplitude (volt) ωc = analog carrier radian frequency (rad/s)

  28. Cont’d... Digital Amplitude Modulation

  29. Frequency Shift Keying (FSK) • Called as Binary Frequency Shift Keying (BFSK) • The phase shift in carrier frequency (∆f) is proportional to the amplitude of the binary input signal (vm(t)) and the direction of the shift is determined by the polarity Where vfsk(t) = binary FSK waveform Vc = peak anlog carrier amplitude (volt) fc = analog carrier center frequency (Hz) ∆f = peak shift in analog carrier frequency (Hz) vm(t) = binary input signal (volt)

  30. Cont’d...

  31. Phase Shift Keying (PSK) • Another form of angle-modulated, constant amplitude digital modulation. • Binary digital signal input & limited number of output phases possible. • M-ary digital modulation scheme with the number of output phases defined by M. • The simplest PSK is Binary Phase-Shift Keying (BPSK) • N= 1, M=2 • Two phases possible for carrier with one phase for logic 1 and another phase for logic 0 • The output carrier shifts between two angles separated by 180°

  32. Cont’d... a) Truth Table b) Phasor Diagram c) Constellation Diagram

  33. Cont’d... BPSK Transmitter

  34. Cont’d... BPSK Receiver

  35. Bandwidth Efficiency • Used to compare the performance of one digital modulation technique to another. Bη = Transmission bit rate (bps) Minimum bandwidth (Hz)

  36. Example 5 For 16-PSK system, operating with an information bit rate of 32 kbps, determine: a. Baud b. Minimum bandwidth c. Bandwidth efficiency

  37. CONCLUSION • To decide which modulation method should be used , we need to make considerations of a) Bandwidth b) Speed of Modulation c) Complexity of Hardware

  38. End of Part 1

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