1. TE4201-Communication Electronics 1 12. Digital Modulation Digital Coding and Digital Modulation Amplitude Modulation (AM,OOK,PRK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM) Quadrature Partial Response Signaling (QPRS)

2. TE4201-Communication Electronics 2 Quantization error is produced when PAM sample is converted to a digital codeword to form PCM Quantization noise is an additive noise in the same way as the white (thermal noise is additive in communication system 3-bit will have 8 Quantization levels At the time of a sample the level counted by the code is lower than actual signal level The difference is the quantization error which can be higher or lower than actual level that determines the polarity of the error (see following figure)

3. TE4201-Communication Electronics 3 2. Bipolar coding “Alternate Mark Inversion" (AMI) 1. There are numerous line codes that are specifically designed to not contain dc energy and thereby be unaffected by dc removal. 3. Hence the average voltage level is maintained at zero—to eliminate dc components in the signal spectrum. Since bipolar coding uses alternate polarity pulses for encoding logic 1's, it is also referred to as "alternate mark inversion" (AMI).* 2. Bipolar coding solves the dc wander problem by using three levels to encode binary data. Specifically, a logic 0 is encoded with zero voltage while a logic 1 is alternately encoded with positive and negative voltages

4. TE4201-Communication Electronics 4 Digital Modulation What is the difference between Digital line coding and Digital Modulation? Digital line coding such as RZ, NRZ coding, Bipolar coding, AMI coding and Binary N zero Substitution coding is devising various means of encoding digital information for trans­mission over wire-line (or fiber) transmission links. Choosing a line code depends upon the spectrum of the line code andthe spectrum of the line code and the available bandwidth (particularly at low frequencies),the available bandwidth (particularly at low frequencies), noise and interference levels,noise and interference levels, synchronization acquisition times,synchronization acquisition times, performance monitoring, andperformance monitoring, and implementation costs.implementation costs.

5. TE4201-Communication Electronics 5 Digital Modulation such as Amplitude Modulation(AM), Frequency Shift Keying(FSK), Quadrature Amplitude Modulation(QAM), impress the base-band information onto a carrier for radio transmission or to modulate the carrier. It emphasizes multilevel digital modulation Choosing a modulation technique depends upon getting high data rates within the rigidly defined bandwidth of a radio channel the available bandwidth (particularly at low frequencies), defined as Information Density getting high data rates within the rigidly defined bandwidth of a radio channel the available bandwidth (particularly at low frequencies), defined as Information Density performance monitoring, and performance monitoring, and implementation costs. implementation costs.

6. TE4201-Communication Electronics 6 Information Density Frequency Band (GHz) Channel BW (MHz) =BW Required Bit Rate (MBPS) =R Information Density =  (bps/Hz of BW) 23.66.1441.8 42073.73.7 63073.72.5 114073.71.8 where R = data rate in bits per second BW = bandwidth of the digital signal in hertz BW = bandwidth of the digital signal in hertz As a practical example, a representative high-speed data rate on a dial- up analog telephone line is 9600 bps (usually with 4 bits per signal interval and a 2400-Hz signaling rate). Since the usable bandwidth of a telephone channel is approximately 3 kHz, dial-up lines provide an information density of 3.2 bps/Hz. Correspondingly, a typical maximum data rate on leased analog lines is 19,200bps, implying an information density of 6.4 bps/Hz. What is the important parameter in Digital Modulation?

7. TE4201-Communication Electronics 7 What is the principle of Digital Modulation? modulated signal detected signal demodulated signal baseband signal transmitted signal 1. 1. 2. 1.In many cases the modulation process can be viewed as a special form of amplitude modulation by a non-return to zero (NRZ) line code baseband signal (or simply baseband logic level change the property of the carrier freq.) 2.Radio transmission has the necessity of strictly band-limiting the transmitted signals to prevent interference into other channels

8. TE4201-Communication Electronics 8 Amplitude modulation process is where a non-return to zero (NRZ) line code baseband signal varies the amplitude of the radio frequency carrier Unfortunately. the error performance of digital amplitude modulation in general, and envelope detection in particular, is inferior to other forms of digital modulation and detection. For this reason, amplitude modulation is used only where the cost of the receiver is a significant consideration. Amplitude Modulation Basic form of Amplitude modulation is by a two-level digital baseband signal varying the amplitude of the carrier  C

9. TE4201-Communication Electronics 9 Second form of Amplitude Modulation is using (a=1) 100% modulation where no carrier is produced for a logic 0. this form of amplitude modulation is often referred to as on/off keying, or "amplitude shift keying" (ASK). Amplitude Shift Keying (ASK) / On/Off Keying (OOK).

10. TE4201-Communication Electronics 10 It is known that the maximum use of transmitted power is achieved when one signal is the negative of the other. Thus, a second deficiency of amplitude modulation arises because a 0 signal is not the exact negative of a 1 signal. Then comes a third form of modulation which produces two identical signals except for a 180° phase reversal, sometimes referred to as phase reversal keying (PRK), or more often, two-level phase shift keying (2-PSK). Notice that PRK cannot be detected by envelope detection. Instead, a PRK signal must be detected by comparing if to a coherent carrier reference. Phase reversal keying (PRK),

11. TE4201-Communication Electronics 11 Frequency Shift Keying (FSK) AM is quite vulnerable to signal saturations that narrow the distance between amplitude levels. A common source of saturation in a radio system occurs in the output power amplifier of the transmitter. In most cases output amplifiers are operated at less than maximum power to eliminate saturation But in Angle modulated systems: frequency modulation (FM) or phase modulation (PM) use constant amplitude signals not adversely affected by signal saturations. Hence FM and PM can be transmitted at higher power levels than AM systems

12. TE4201-Communication Electronics 12 Minimum Shift Keying (MSK) Basically, MSK is binary FSK with the two signaling frequencies selected so that exactly 180° difference in phase shift exists between the two frequencies in one signal interval. In this manner MSK produces a maximum phase differential at the end of an interval using a minimum difference in signaling frequencies. Notice that there is exactly one-half cycle difference between a 1 signal and a 0 signal. Phase reversal keying (PRK)(=2-PSK) Amplitude of the carrier is ± Maximum Shift keying (MSK), Phase of the carrier is ± Compare MSK and PRK

13. TE4201-Communication Electronics 13 Phase Shift Keying (PSK) Already discussed one form of PSK when phase reversal keying (PRK) was introduced. PRK is more commonly referred to as 2-PSK, indicating that each signal interval uses one of two phases that are 180° apart to encode binary data. Multiple-phase Phase Shift Keying is also possible. 4-PSK (also called QPSK) and 8-PSK are the most common examples of multiple phase PSK. Phase shift keying is the most popular modulation technique primarily due to its constant envelope, insensitivity to level variations, and good error performance. A 2-PSK modulator can be implemented by merely inverting the carrier (multiplying by — 1) for a logic 0 and by not inverting for a logic 1.

14. TE4201-Communication Electronics 14 cc PSK Modulator Implementations 1. Synthesis of the desired waveforms using DSP 2. Generating multiple phases of a single carrier and selecting between phases depending on the data values. 4. Generating the PSK signals as a linear combination of quadrature signals. Phase is “0” or “180” in 2-PSK So that Amplitude of the carrier is ± 3. Using controlled delays selected through a switching arrangement lo provide the desired phase shifts.

15. TE4201-Communication Electronics 15 1 0 0 1 0 0 0 1 1 with 2-PSK modulation 11 10 01 00 10 with 4-PSK modulation 2-PSK and 4-PSK Modulation

16. TE4201-Communication Electronics 16 Notice that cos  and sin  are constants over a signaling interval, and hence represent coefficients for expressing cos(  c t +  ) as a linear combination of the signals cos  c t and sin  c t. Since cos  c t and sin  c t are 90  out of phase with respect to each other, they are orthogonal in a phasor diagram and hence are said to be "in quadrature." Quadrature Signal Representations In essence, cos  c t and sin  c t represent basis vectors in a two-dimensional phasor diagram. The cosine signal is usually referred to as the in-phase or I signal, and the sine signal is referred to as the out-of-phase or Q signal. Data values Quadrature Coefficients Composite Signal cos  c tsin  c t 01 0.707- 0.707 cos (  c t +  /4) 00- 0.707 cos (  c t + 3  /4) 10- 0.707 0.707 cos (  c t - 3  /4) 11 0.707 cos (  c t -  /4) Cos  c t + Sin  c t + =-0.707 sin  c t =-0.707 0.707 cos  c t= 0.707

17. TE4201-Communication Electronics 17 4-PSK Modulator / Demodulator 10 01 10 1 with 4-PSK modulation

18. TE4201-Communication Electronics 18 8-PSK Modulation 100 111 101 000 010 110 001 011 with 8-PSK modulation

19. TE4201-Communication Electronics 19 Data values Quadrature Coefficients Composite Signal cos  c tsin  c t 011 0.924- 0.383cos (  c t +  /8) 010 0.383- 0.924cos (  c t + 3  /8) 000- 0.383- 0.924cos (  c t + 5  /8) 001- 0.924- 0.383cos (  c t + 7  /8) 101- 0.924 0.383cos (  c t - 7  /8) 100- 0.383 0.924cos (  c t - 5  /8) 110 0.383 0.924cos (  c t - 3  /8) 111 0.924 0.383cos (  c t -  /8) cos  c t sin  c t The general expression for the distance between adjacent points in a multiphase PSK system is d = 2sin(  /N) where N is the number of phases d d 2-PSK = 2sin(  /2)=2

20. TE4201-Communication Electronics 20 8-PSK Demodulator / Detector

21. TE4201-Communication Electronics 21 Quadrature Amplitude Modulation (QAM) (QAM) can be viewed as an extension of multiphase PSK modulation wherein the two baseband signals are generated independently of each other. Thus two completely independent (quadrature) channels are established including the baseband coding and detection processes. Signal constellation of 16 QAM modulation The dots represent composite signal points the two baseband signals are generated independently of each other. Thus two completely independent (quadrature) channels (see multilevel signaling) are established including the baseband coding and detection processes. (see multilevel signaling)

22. TE4201-Communication Electronics 22 16-QAM Modulator/Demodulator

23. TE4201-Communication Electronics 23 Comparison of 16-QAM and 16-PSK signal sets four levels / 4-bit channel11 11 10 00 01 11100001 1000 01 01 four phases / 4-bit channel QAM signal shown in Figure 6.19 does not have a constant envelope. A constant envelope is maintained with PSK modulation by restricting the combination of levels on the quadrature channels. A QAM system does not restrict the combinations since the levels on each channel are selected independently.

24. TE4201-Communication Electronics 24 With large numbers of signal points, QAM systems always outperform PSK systems. The basic reason is that the distance between signal points in a PSK system is smaller than the distance between points in a comparable QAM system. Specifically, 16-QAM has a spectrum shape that is identical to 16-PSK, and 64-QAM has a spectrum shape identical to 64-PSK. Compare and contrast 16-QAM and 16-PSK PSK modulation has constant levels on the quadrature channels, and a QAM system has the levels are selected independently on each channel. 16-QAM16-PSK selected levelsconstant levels d d The general expression for the distance between adjacent signal points in a unit peak amplitude QAM system with L levels on each axis is The general expression for the distance between adjacent points in a multiphase PSK system where N is the number of phases is

25. TE4201-Communication Electronics 25 Solution:(a) with the same peak power level error performance will be due to distance between the signal points “d” as 16-PSK system and 16-QAM have identical signal points (same data rate) and so the same bandwidth. Example : 6.1(a) Determine the error performance of a 16-PSK system to a 16-QAM system with the same peak power level. (b) Determine the relative performance with respect to identical average power level. (c) Find the overall performance in dB between the two systems. Error performance in dB due to distance between the signal points will be

26. TE4201-Communication Electronics 26 (b) the relative performance with respect to identical average power level is found from peak power/average power ratio of both 16-PSK and 16-QAM systems. Power performance in dB at equal average power between the two systems will be (c) Overall performance in dB between the two systems will be

27. TE4201-Communication Electronics 27 Comparison of Various Digital Modulation Techniques Based on Equal Data Rates System Designation Information Density (bps/Hz) Signal-to-Noise Ratio for BER = 10 -6 (dB) Peak to Average ratio (dB) E 0 /N 0 on the channel SNR at decision CKT 2-PSK110.613.60.0 4-PSK,4-QAM210.613.60.0 QPR212.617.62.0 8-PSK31418.80.0 16-QAM414.520.52.55 16-QPR416.524.54.55 16-PSK418.324.30.0 32-QAM517.424.42.3 64-QAM626.6 3.68

28. TE4201-Communication Electronics 28 Quadrature Partial Response Signaling (QPRS) Another popular modulation technique is quadrature partial response signaling (QPRS). QPRS modulator is a QAM modulator followed by a narrow band-pass filter that "over-filters" the quadrature signals and produces controlled inter-symbol interference in each channel.

29. TE4201-Communication Electronics 29 The most common application of QPRS involves two levels on each channel before filtering and three levels afterward. (+1,-1)(+1,0,-1) (+1) (-1) Two-level input have two signal points on a single channel (+1) (-1) After filtering,the output has three levels making three signal points (0)

30. TE4201-Communication Electronics 30 In a 4-PSK system with partial response filtering will increase the information density (creating more signal points). As shown below, the effect of partial response filtering with two levels on each channel before filtering, has three levels on each channel afterward is to produce nine signal points from the original four. filtering

31. TE4201-Communication Electronics 31 In a similar manner, a 16-QAM partial response system, with four levels on each channel before filtering, has seven levels afterward, and 49 signal points in all.

32. TE4201-Communication Electronics 32 No filtering in QPSK Filtering in QPRS As discussed before, partial response filtering cuts the distance between signal points in half, indicating a 6-dB reduction (poor) in error performance. However, the noise bandwidth of a PRS receive filter is lower than the noise bandwidth of the corresponding full response system (because of band- limiting by filter) so that some of the error distance degradation is recovered.