1. CHAPTER 4

2. OUTLINES 1. Digital Modulation Introduction Information capacity, Bits, Bit Rate, Baud, M- ary encoding ASK, FSK, PSK, QPSK, QAM 2. Digital Transmission Pulse modulation PCM Delta Modulation Line Codes 3. Multiplexing FDM, TDM, SDM

3. DIGITAL MODULATION Introduction Information capacity, Bits, Bit Rate, Baud, M-ary encoding ASK, FSK, PSK, QPSK, QAM PART 1

4. Introduction Digital modulation Is the transmittal of digitally modulated analog signals (carriers) between two or more points in a communications system. Can be propagated through Earth’s atmosphere and used in wireless communication system - digital radio 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 1.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)

11. Cont’d… Simplified block diagram of a digital modulation system

12. 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.

13. INFORMATION CAPACITY, BIT, BIT RATE, BAUD & M-ARY ENCODING Information capacity 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.

14. 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.

15. 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

16. Nyquist Sampling rate (revision!!!) f s is equal or greater than 2f m f s >= 2f m fs = minimum Nyquist sample rate (Hz) fm = maximum analog input frequency (Hz)

17. 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.

18. Cont’d… Each symbol represents n bits, and has M signal states, where M = 2n. This is called M-ary signaling.

19. Baud & Minimum BW Baud refers 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) t s = time of one signaling element @ symbol (seconds)

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

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

22. Amplitude Shift Keying (ASK) A binary information signal directly modulates the amplitude of an analog carrier. Sometimes called Digital Amplitude Modulation (DAM) Where v ask (t) = amplitude shift keying wave v m (t) = digital information signal (volt) A/2 = unmodulated carrier amplitude (volt) ω c = analog carrier radian frequency (rad/s)

23. Digital Amplitude Modulation

24. 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 (v m (t)) and the direction of the shift is determined by the polarity Where v fsk (t) = binary FSK waveform V c = peak anlog carrier amplitude (volt) f c = analog carrier center frequency (Hz) ∆f = peak shift in analog carrier frequency (Hz) v m (t) = binary input signal (volt)

25. FSK in the frequency domain

26. Cont’d…

27. FSK in the time domain

28. Truth table Binary InputFrequency Output 0Space (f s ) 1Mark (f m )

29. Phase Shift Keying (PSK)

30. BPSK Transmitter

31. BPSK Receiver

32. BPSK Bit value 0 – sine wave Bit value 1 – inverted sine wave Very simple PSK Low spectral efficiency Robust, used in satellite system

33. QPSK

34. 2 bits coded as one symbol Symbol determines shift of sine wave Needs less bandwidth compared to BPSK More complex

35. CONSTELLATION DIAGRAM Graphical representation of the complex envelope of each possible symbol state  The x-axis represents the in-phase component and the y-axis the quadrature component of the complex envelope  The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how easily a receiver can differentiate between them.

38. Combine amplitude and phase modulation It is possible to code n bits using one symbol 2 n discrete levels, n = 2 identical to QPSK BER increase with n, but less compared to PSK scheme. QAM

40. Amplitude and phase shift keying can be combined to transmit several bits per symbol.  Often referred to as linear as they require linear amplification.  More bandwidth-efficient, but more susceptible to noise. For M = 4, 16QAM has the largest distance between points, but requires very linear amplification. 16PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise. High level M-array schemes (such as 64-QAM) are very bandwidth-efficient but more susceptible to noise and require linear amplification

41. 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