Chapter 3: Digital Modulation and Demodulation

Learning Objectives LO 3.1 – Analyze binary digital modulation techniques such as ASK, FSK and PSK. LO 3.2 – Describe M-ary digital modulation techniques including QPSK, Offset QPSK, π/4-QPSK, MSK, GMSK, and QAM. LO 3.3 – Understand coherent and non-coherent including carrier recovery methods. LO 3.4 – Develop performance-determining criteria in terms of bandwidth efficiency and error probabilities of various digital modulation techniques.

3.1 Binary Digital Modulation Techniques 3.1.1 Amplitude Shift Keying (ASK) 3.1.2 Frequency Shift Keying (FSK) 3.1.3 Phase Shift Keying (PSK) 3.1.4 Differential Binary Phase Shift Keying (DBPSK) 3.1.5 Differentially Encoded Phase Shift Keying (DEPSK)

Types of Digital Modulation Techniques Digital modulation involves the process of amplitude, frequency, or phase of an analog carrier signal as a function of two or more than two finite, discrete states. Basic Types ASK - The amplitude of the carrier signal is varied in proportion to the digital information signal FSK - The frequency of the carrier signal is varied in proportion to the digital information signal PSK - The phase angle of the carrier signal is varied in proportion to the digital information signal

3.1.1 Amplitude Shift Keying (ASK) Binary data sequence 1 Data Waveform UP-NRZ(L) Carrier signal, sin (2πft) ASK signal waveform Tb Figure 3.1.1 ASK Signal Waveform

ASK Bandwidth and PSD Waveform Amplitude f fc fc - fb fc + fb BWmax = 2fb Figure 3.1.2 ASK Signal Bandwidth f PSD of ASK fc - fb fc - 2fb fc fc + 2fb fc + fb Figure 3.1.3 PSD of ASK Signal

ASK Modulator Figure 3.1.4 Functional Block Schematic of ASK Modulator BPF Balanced Modulator Carrier signal, sin(2πfct) Band-limited ASK signal Binary ASK signal, vASK(t) 1 0 1 1 0 1 0 1 1 1 Binary digital data (UP-NRZ) vm(t) Figure 3.1.4 Functional Block Schematic of ASK Modulator

Coherent Binary ASK Detection Balance Modulator & Integrator Decision Device Carrier signal Received binary ASK signal 1 0 1 1 0 1 0 1 1 1 ASK Demodulated Output Preset threshold level Figure 3.1.5 Functional Block Schematic of Coherent ASK Detector

Non-Coherent Binary ASK Detection Envelope Detector (Rectifier + LPF) Decision Device Received binary ASK signal 1 0 1 1 0 1 0 1 1 1 Detected Output Preset threshold level Figure 3.1.6 Functional Block Schematic of Non-coherent Binary ASK Detector

3.1.2 Frequency Shift Keying (FSK) Data Waveform UP-NRZ (L) Carrier signal, sin (2πfct) BFSK signal waveform Tb f2 f1 Binary data sequence Figure 3.1.7 Binary FSK Signal Waveform

BFSK Bandwidth and PSD Waveform fc1 fc0 BBFSK = 4fb PSD of BFSK fc Figure 3.1.8 PSD of a Binary FSK Signal

Figure 3.1.9 Functional Block Schematic of FSK Modulator Polar NRZ Line coder Binary data sequence Carrier Oscillator 2 sin ωc1t fc1 Balanced modulator M0 Balanced modulator M1 Adder BFSK signal Carrier Oscillator 1 sin ωc0t fc0 10110101 Bit Invertor Figure 3.1.9 Functional Block Schematic of FSK Modulator

Coherent Binary FSK Detection BFSK signal Subtractor Detected output Balance modulator M1 Carrier Oscillator 1 cos ω1t + Balance modulator M2 Carrier Oscillator 2 cos ω2t Decision Device Preset threshold level Correlator 2 Correlator 1 - Figure 3.1.10 Functional Block Schematic of Coherent Binary FSK Detector

Non-coherent Binary FSK Detection BFSK signal Comparator Detected output Sampling switch 1 (at t = Tb) Envelope Detector BPF (fc0) Sampling switch 2 BPF (fc1) Figure 3.1.11 Functional Block Schematic of Non-coherent Binary FSK Detector

FSK Detection using PLL FSK signal input Detected output Amplifier Phase Comparator Voltage Controlled Oscillator dc error voltage dc PLL Figure 3.1.12 Functional Block Schematic FSK Demodulator using PLL

3.1.3 Phase Shift Keying (PSK) Binary data 1 Data Waveform UP-NRZ (L) Carrier signal, sin (2πfct) BFSK signal waveform Tb 0° 180° Phase shift Figure 3.1.13 Binary PSK Signal Waveform

Constellation Diagram of BPSK Singal Phase shift in carrier signal 0° Reference sin(2πfct) ±180° Binary level 0 Binary level 1 -sin(2πfct) cos(2πfct) -cos(2πfct) +90° -90° Table 3-1 Symbol Bits in symbol Phase shift in carrier signal s0 Binary level `0’ 180° or π radians S1 Binary level `1’ 0° or 0 radians Figure 3.1.14

PSD of a Binary PSK Signal fc1 fc0 BBFSK = 4fb PSD of BPSK fc Figure 3.1.15 PSD of a Binary PSK Signal

Figure 3.1.16 Functional Block Schematic of a PSK Modulator Bipolar NRZ Encoder Binary data sequence Balance modulator BPSK signal 10110101 Carrier Oscillator Carrier signal Figure 3.1.16 Functional Block Schematic of a PSK Modulator

Coherent Binary PSK Detection Detected output, d(t) Synchronous Demodulator (Multiplier) Recovered Carrier, sin(2πfct) Decision Device Correlator Bit Synchronizer Sampling instants Square-law device Carrier Recovery Circuit BPF (2fc) Frequency Divider (/2) Figure 3.1.17 Functional Block Schematic of a Coherent Binary PSK Detector

3.1.4 Differential Binary Phase Shift Keying (DBPSK) DBPSK is an alternate form of BPSK where the binary data information is contained in the difference between the phases of two successive signaling elements rather than the absolute phase. Figure 3.1.18 Differential Binary PSK Modulator An Encoder or Logic circuit Balance Modulator Carrier Oscillator Binary data sequence Delay Tb Bipolar NRZ line encoder DBPSK Signal

Non-Coherent DBPSK Detection An Encoder or Logic circuit DBPSK signal Delay Tb BPF (f1) Decision Device Preset threshold level Detected output Balance modulator Correlator Figure 3.1.19 Functional Block Schematic of Non-Coherent DBPSK Detector

Table 3.1.2 Comparison of BPSK and DBPSK S. No. Parameter BPSK DBPSK 1. Variable characteristics of analog carrier signal Phase 2. Maximum bandwidth 2 fb fb 3. Probability of error Low Higher than BPSK 4. Noise immunity Good Better than BPSK 5. Bit detection at receiver Based on single-bit interval Based on two successive bit intervals 6. Synchronous carrier at demodulator Required Not required 7. System complexity Moderate High

3.1.5 Differentially Encoded Phase Shift Keying (DEPSK) In DEPSK, the input sequence of binary data is modified such that the next bit depends upon the previous bit. Figure 3.1.20 DEPSK Signal in Time-Domain

3.2 M-ary Digital Modulation Techniques 3.2.1 Quadrature Phase Shift Keying (QPSK) 3.2.2 Offset QPSK (OQPSK) 3.2.3 π/4=Phase Shift Keying QPSK 3.2.4 Minimum Shift Keying (MSK) 3.2.5 Gaussian Minimum Shift Keying (GMSK) 3.2.6 Quadrature Amplitude Modulation (QAM)

3.2.1 Quadrature Phase Shift Keying (QPSK) QPSK - An M-ary constant-amplitude digital modulation scheme in which number of bits is two and number of signaling elements are four. SQPSK (t) =

Table 3.2.1 Symbols, Bits and Phase Shift Phase shift in QPSK Signal About QPSK Signal Table 3.2.1 Symbols, Bits and Phase Shift sin(2πfct) Symbol 11 -sin(2πfct) cos(2πfct) -cos(2πfct) Symbol 01 Symbol 10 Symbol 00 Symbol Binary input Phase shift in QPSK Signal s1 0 0 -135° or –3π/4 radians s2 0 1 -45° or –π/4 radians s3 1 0 +135° or 3π/4 radians s4 1 1 +45° or π/4 radians Figure 3.2.1 Constellation Diagram

Figure 3.2.2 PSD of a QPSK Signal -fc – fb/2 -fc fc -fc + fb/2 fc – fb/2 fc + fb/2 BQPSK = fb Figure 3.2.2 PSD of a QPSK Signal

Figure 3.2.3 Functional Block Schematic of QPSK Modulator

QPSK Signal in Time Domain Figure 3.2.4

Coherent QPSK Detection signal MUX Balance modulator, M1 Coherent Quadrature Carrier 1 Correlator 1 Coherent Quadrature Carrier 2 Balance modulator, M2 Correlator 2 Detected Output Raised to 4th power Carrier recovery circuit BPF (4fc) Coherent Quadrature carriers Divide by 4 Power Splitter Figure 3.2.5 Functional Block Schematic of Coherent QPSK Detector

Constellation Diagram 3.2.2 Offset QPSK (OQPSK) OQPSK – A modified form of QPSK where the bit waveform on the I an Q channels are offset in phase from each other by one-half of a bit interval. Symbol 11 Symbol 01 Symbol 10 Symbol 00 Figure 3.2.6 Constellation Diagram

OQPSK Modulator Binary NRZ encoder and 2-bit serial-to-parallel converter Binary input data stream fb bps vOQPSK (t) fb /2 bps π/2 Σ Delay Tb I(t) Q(t) Carrier Signal cos 2πfct + Balanced Modulator 1 Balanced Modulator 2 Figure 3.2.7 Functional Block Schematic of OQPSK Modulator

OQPSK Signal in Time Domain Figure 3.2.8

Constellation Diagram 3.2.3 π/4-Phase Shift QPSK π/4-QPSK – A modified form of QPSK which has carrier phase transitions that are restricted to ±π/4 and ±3π/4 radians. Q I (11) (01) (10) (00) Figure 3.2.9 Constellation Diagram

π/4-QPSK Signal in Time Domain Figure 3.2.10

Table 3.2.2 Comparison of QPSK, OQPSK, π/4-QPSK S. No. Parameter QPSK OQPSK π/4-QPSK 1. Maximum phase change ±180° ±90° ±135° 2. Amplitude variations at the instants of abrupt phase changes large small Medium 3. Simultaneous change of inphase and Quadrature phase Yes No 4. Offset between inphase and Quadrature phase Yes, by Tb seconds Yes, by Tb/2 seconds 5. Preferred method of demodulation Coherent Coherent or non-coherent 6. Minimum bandwidth fb 7. Symbol duration 2 Tb 8. Receiver design complexity No in case of non-coherent

3.2.4 Minimum Shift Keying (MSK) MSK - A special case of binary continuous phase FSK modulation technique in which the change in carrier frequency from symbol 0 to symbol 1 or vice versa is exactly equal to one-half the bit rate of input data signal. vMSK (t) =

About MSK Signal Figure 3.2.12 Figure 3.2.13 MSK Signal at 1200 baud PSD -fc – 3fb/4 -fc fc -fc + 3fb/4 fc – 3fb/4 fc + 3fb/4 BQPSK = 3fb/2 Figure 3.2.12 MSK Signal at 1200 baud for NRZ data 1 0 1 1 Figure 3.2.13 PSD of MSK Signal

3.2.5 Gaussian Minimum Shift Keying (GMSK) GMSK - A special case of MSK in which a pre-modulation low-pass Gaussian filter is used as a pulse shaping filter to reduce the bandwidth of the baseband signal before it is applied to MSK modulator. Bipolar NRZ Encoder GMSK signal Gaussian Filter 10110101 Frequency Modulator Carrier Oscillator cos wct Figure 3.2.16 GMSK Modulator using Frequency Modulator

GMSK Modulator using Phase Modulator NRZ data In-phase path GMSK signal Gaussian Filter converter Parallel To Serial Balance modulator, M1 Carrier Oscillator cos wct Adder + - Quadrature path Carrier Oscillator sin wct Figure 3.2.17 GMSK Modulator using Phase Modulator

GMSK Demodulator Figure 3.2.18 GMSK Demodulator In-phase signal, I(t) GMSK signal Balance modulator, M1 Carrier Oscillator cos wct Gaussian Filter Decision Device Quadrature-phase signal, Q(t) Balance modulator, M2 Carrier Oscillator sin wct Threshold level for binary 0 Threshold level for binary 1 Figure 3.2.18 GMSK Demodulator

3.2.6 Quadrature Amplitude Modulation (QAM) QAM - A form of digital modulation similar to PSK except the digital information is contained in both the amplitude and the phase of the modulated signal QAM is an efficient way to achieve high data rates with a narrowband channel by increasing the number of bits per symbol, and uses a combination of amplitude and phase modulation. QAM can either be considered a logical extension of QPSK or a combination of ASK and PSK.

QAM Modulator Figure 3.2.21 QAM Modulator vQAM (t) Σ Modulator 2-bit serial-to-parallel converter Binary input data stream fb bps vQAM (t) fb /2 bps Σ d1(t) d2(t) Carrier Signal cos 2πfct -π/2 sin 2πfct + Clock @ Ts D/A converter Modulator Figure 3.2.21 QAM Modulator

QAM Coherent Demodulator QAM signal Balance modulator, M1 Correlator 1 Balance modulator, M2 Correlator 2 Detected Output Raised to 4th power Carrier recovery circuit BPF (4fc) Divide by 4 Parallel To Serial converter A 90° phase shifter A/D A/D Figure 3.2.22 QAM Coherent Demodulator

3.3 Coherent and Non-Coherent Detection Coherent Detection - A synchronous detection in which the digital receiver is phase-locked to the carrier signal of the incoming digitally modulated signal. Non-Coherent Detection - A non-synchronous detection in which the digital receiver does not require locally-generated receiver carrier signal to be phase-locked with transmitter carrier signal.

Coherent Detection Method 1 M-ary PSK signal Multiplier Phase-locked loop (PLL) LPF VCO Mth power device Frequency Divider by M Phase-shift Network * * * * * M-reference output signals BPF Figure 3.3.1 Mth Power Loop Carrier Recovery

Coherent Detection Method 2 BPSK signal Phase discriminator Balance modulator, M1 negative feedback In-phase path Balance modulator, M2 Quadrature path Demodulated Binary data LPF 90° phase shifter VCO + Loop filter Figure 3.3.2 Costas-Loop Carrier Synchronization

3.4 Performance of Digital Modulation Techniques Bandwidth Efficiency – Number of bits that can be transmitted for each Hz of channel bandwidth, expressed in bps/Hz. Bit Error Rate (BER) – Ratio of total number of bits received in error and total number of bits received over a large session of information transmission.

BER Performance Comparison 10-2 10-3 10-4 10-5 -5 -2.5 2.5 5.0 7.5 10 Eb/N (dB) BER 10-1 0.5 Non coherent binary FSK Coherent binary FSK DPSK Coherent binary PSK Coherent QPSK Coherent MSK 12.5 Figure 3.4.1 BER versus Eb/NO curves for BFSK, M-ary PSK, and MSK

Table 3.4.6 Performance Comparison of PSK and MSK Digital Modulation Technique Spectral Efficiency Required SNR BPSK 1 bps/Hz 11.1 dB QPSK 2 bps/Hz 14.0 dB 16-PSK 4 bps/Hz 26.0 dB 2-MSK 10.6 dB 4-MSK 13.8 dB

Table 3.4.7 Spectral Efficiency of QPSK and GMSK Digital Modulation Technique Channel Bandwidth, KHz Data Rate, Kbps Spectral Efficiency, bps/Hz Application π/4-QPSK 30 48.6 1.62 USDC 25 42.0 1.68 JDC GMSK (BTb =0.3) 200 270.8 1.35 GSM 100 72.0 0.72 CT-2 (BTb =0.5) 1728 1572.0 0.67 DECT

Table 3.4.8 Applications of Digital Modulation Techniques S. No. Digital Modulation Technique Typical Application Areas 1. Frequency Shift Keying (FSK) Paging Services, Cordless Telephony 2. Binary Phase Shift Keying (BPSK) Telemetry 3. Quaternary Phase Shift Keying (QPSK) Cellular Telephony, Satellite, Digital Video Broadcasting 4. Octal Phase Shift Keying (8-PSK) Satellite Communications 5. 16- or 32-level Quadrature Amplitude Modulation (16-QAM or 32-QAM) Microwave Digital Radio Links, Digital Video Broadcasting 6. 64-level Quadrature Amplitude Modulation (64-QAM) Digital Video Broadcasting, Set Top Boxes, MMDS 7. Minimum Shift Keying (MSK) Cellular Telephony

About the Author T. L. Singal graduated from National Institute of Technology, Kurukshetra and post-graduated from Punjab Technical university in Electronics & Communication Engineering. He began his career with Avionics Design Bureau, HAL, Hyderabad in 1981 and worked on Radar Communication Systems. Then he led R&D group in a Telecom company and successfully developed Multi- Access VHF Wireless Communication Systems. He visited Germany during 1990-92. He executed international assignment as Senior Network Consultant with Flextronics Network Services, Texas, USA during 2000-02. He was associated with Nokia, AT&T, Cingular Wireless and Nortel Networks, for optimization of 2G/3G Cellular Networks in USA. Since 2003, he is in teaching profession in reputed engineering colleges in India. He has number of technical research papers published in the IEEE Proceedings, Journals, and International/National Conferences. He has authored three text-books `Wireless Communications (2010)’, `Analog & Digital Communications (2012)’, and `Digital Communication (2015)’ with internationally renowned publisher McGraw-Hill Education.

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