Chapter 6. Digital Modulation Techniques Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad 6.1.1 Why Modulation Modulation for Ease of Radiation: Requires antennas of EM wavelength. Modulation for Multiplexing To Overcome Equipment Limitations For Frequency Assignment To Reduce Noise and Interference Digital Communication NUML, Islamabad

Performance of Digital Modulation Scheme lowest bit error rates, performs well in multipath and fading conditions, occupies a minimum bandwidth, cost effective, and minimum circuit complexity. Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad Modulation Formats Modulation format Application MSK, GMSK GSM BPSK Cable modems, satellite QPSK Satellite, CDMA 8PSK Satellite 16QAM Microwave digital radio 32QAM Terrestrial microwave 64QAM Modems 256QAM Modems Digital Communication NUML, Islamabad

6.2 Common Digital Modulation Formats T: symbol duration A: symbol energy E = PT is energy of si(t) contained in symbol duration. Digital Communication NUML, Islamabad

Waveform Amplitude Coefficient General form Derivation: s(t) = Acosωt Or, s(t) = √2 Armscosωt = √(2 A2rms) cosωt A2rms represents average power P (normalized to 1 ohm). Therefore, s(t) = √2P cosωt Replacing P watts by E joules/T seconds, we get s(t) = √(2E/T) cosωt where, parameter E: symbol energy; T: symbol duration time Digital Communication NUML, Islamabad

6.3 Amplitude Shift Keying Multiplication Operation ASK(t) = c(t) ・d(t) Single-frequency Carrier: c = cosct Unipolar periodic Data: d(t)= 1/2 + (2/) X {cos0t –1/3 cos30t + 1/5 cos50t - ……} ASK(t) = 1/2 cosct + 2/{cosct cos0t –1/3 cosct cos30t + 1/5 cosct cos50t - ..} Using 2cosAcosB = cos(A – B) + cos(A + B) ASK(t) = 1/2 cosct + 1/{ cos(c - 0)t + cos(c + 0)t - 1/3 cos(c - 30)t – 1/3 cos(c + 30)t ……} Digital Communication NUML, Islamabad

 Digital Communication NUML, Islamabad c(t) fc + (fc + f0 ) 0 ~ f0 c(t) = cosct (Carrier) d(t) (Unipolar binary input data) c(t) fc + (fc + f0 ) 0 ~ f0 (Binary data output)  LPF PSTN VASK(t) BPF (a) ASK modulator ’d(t) f0 = Fundamental frequency component = 1/2 bit rate, rb, (Hz) ASK modulated signal m(t) cosct t fc fc + f0 fc - f0 fc + 3f0 fc - 3f0 2f0 6f0 Signal power Frequency (c) (b) Carrier cosct Modulating signal m(t) 1 0 1 1 0 0 0 1 Tb Digital Communication NUML, Islamabad

Signal Space Diagram for ASK Signals Vector (or phasor) schematic: It presents only the information bearing phasor positions, relative to one another.) Space signal diagrams for the ASK signals Digital Communication NUML, Islamabad

ASK Probability of Bit Error Pe = Q( ) Q: complementary error function. Eb: signal energy per binary symbol; and is equal to (A2/2)Tb. N0: noise power spectral density Eqn. can be written as Pe = Q( ) where, A: bit amplitude Tb: bit duration Digital Communication NUML, Islamabad

6.4 Frequency Shift Keying Frequency Shift: Difference between two frequencies. Bit rate and baud rate are equal. Digital Communication NUML, Islamabad

6.4.1 Generation of FSK Signal Summing Two ASK Outputs: [cos(2πf0t + θ)] s0 = s1 = [cos(2πf1t + θ)] Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad FSK Operation VFSK(t) = cos1t・d(t) + cos2t・’d(t) ’d(t) is complement of original data signal, d(t). Mathematically, ’d(t) = 1 - d(t). VFSK(t) = cos1t[1/2 + 2/(cos0t – 1/3cos30t + . . .)] + cos2t[1/2 - 2/(cos0t – 1/3cos30t + . . .)] VFSK(t) = 1/2cos1t + 1/[cos(1 – 0)t + cos(1 + 0)t - 1/3cos(1 - 30)t – 1/3cos(1 + 30)t + . . .] + 1/2cos2t + 1/[cos(2 – 0)t + cos(2 + 0)t – 1/3cos(2 - 30)t - 1/3cos(2 + 30)t + . . ] Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad FSK Bandwidth Carson’s Rule: BFSK= 2(B + Δf) B: baseband bandwidth Δf: frequency deviation In case of rectangular pulses, baseband bandwidth B = R (R is data rate), equation becomes BT = 2(R + Δf) Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad 300 bps FSK Modem Example Figure illustrates frequency assignments that are used for two types of FSK modem to provide a full-duplex (two-way simultaneous) 300 bps link between two DTEs. One set of frequency assignments is defined by EIA for Bell 103 modem and other by ITU-T for V.21 modem. In such modems, fundamental frequency component associated with each carrier is 75 Hz. Hence, frequency shift of 200 Hz allows 50 Hz between the two primary sidebands Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad Bandwidth Efficiency 1) Bandwidth Efficiency: For M-ary signals, bandwidth efficiency is ηB = (2log2M)/M Hz 2) Spectral Efficiency β = Rb/RT Rb: bit transmission rate RT: transmission bandwidth β shows “how much data rate is achieved per unit Hz of transmission bandwidth”. Example, spectral efficiency of binary ASK is 0.5 bps/Hz as its RT is double the bit transmission rate Rb. 3) Improving ASK Waveform Spectral Efficiency: Use higher values of M. Spectral efficiency of M-ary ASK waveform is β = (log2M)/2 bps/Hz Digital Communication NUML, Islamabad

Spectral Efficiency Bandwidth A V.92 modem for telephone network can transfer 56,000 bit/s downstream and 48,000 bit/s upstream over an analog telephone network. Due to filtering in telephone exchange, frequency range is limited to between 300 hertz and 3,400 hertz, corresponding to a bandwidth of 3400 − 300 = 3100 hertz. 1) Downstream spectral efficiency β = Rb/RT 56,000/3,100 = 18.1 (bit/s)/Hz downstream, Upstream spectrum efficiency β = Rb/RT 48,000/3,100 = 15.5 (bit/s)/Hz upstream. Digital Communication NUML, Islamabad

FSK Bit Error Probability FSK probability of a bit error Pe is given by Pe = Q ( ) Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad Binary FSK Modulator Oscillator freq. = f1 freq. = f2 Electronic Switch  Binary data input m(t) Control line FSK output Demerit – Large Sidelobes/Large TX Bandwidth: Abrupt switching results in relatively large sidelobes outside the main spectral band of the signal and, consequently, this method requires a large transmission bandwidth. Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad 6.4.2.2 Continuous-phase FSK binary 0 is transmitted as f0 binary 1 is transmitted as f1. FSK modulator (VCO) Binary input Analog output Digital Communication NUML, Islamabad

Digital Communication NUML, Islamabad 6.4.3 PLL FSK Receiver Voltage-controlled oscillator Phase comp. Amp Binary data output Analog FSK in dc error voltage  Digital Communication NUML, Islamabad