Chapter 5 Digital Modulation Systems Binary Bandpass Signalling

Chapter 5 Digital Modulation Systems Ø Ø Binary Bandpass Signalling Techniques OOK BPSK FSK Huseyin Bilgekul EEE 461 Communication Systems II Department of Electrical and Electronic Engineering Eastern Mediterranean University EEE 461 1

Digital Modulation Pulse Modulation Schemes The basic idea is to use a pulse train as the carrier signal Keying Schemes Either the frequency or phase of a carrier signal is keyed in response to patterns of 1 s and 0 s. Passband PAM Modulation PWM modulation EEE 461 2

Digital Modulation Carrier signal: Ac cos (2 fct + θ) Modulation: m(t) Modulated signal: Ac (t) cos (2 fc(t) t + θ(t)) Vary amplitude Vary frequency & phase Variations are discrete!!!!! m(t); discrete Binary • OOK • BPSK • DPSK • FSK Multilevel • QPSK • MPSK • QAM EEE 461 3

5 -9 Binary Modulated Bandpass signaling: Ø The most common binary bandpass signaling techniques are: • On –Off keying (OOK), – OOK is also called amplitude shift keying (ASK), which consists of keying (switching) a carrier sinusoid on and off with a uni-polar binary signal. Morse code radio transmission is an example of this technique. OOK was one of the first modulation techniques to be used and precedes analog communication systems. • Binary Phase-Shift Keying (BPSK), – BPSK consists of shifting the phase of a sinusoidal carrier 0 or 180 with a unipolar binary signal. BPSK is equivalent to PM signaling with a digital waveform. • Frequency-Shift Keying (FSK), – FSK consists of shifting the frequency of a sinusoidal carrier from a mark frequency to a space frequency, according to the baseband digital signal. FSK is identical to modulating an FM carrier with a binary digital signal. EEE 461 4

Binary bandpass signaling techniques 1 0 1 0 Change in Phase Change in Freq Note: • Digitally modulated bandpass signals are generated by using the complex envelopes for AM, PM, FM or QM • Modulating signal m(t) is a digital signal given by binary or multilevel signals EEE 461 5

On-Off Keying (OOK) / Amplitude Shift Keying (ASK) Key/ Switch Carrier Cos(2 fct) OOK output Message m(t) Acm(t)Cos(2 fct) Ø The complex envelope is Ø The OOK signal is represented by Ø The PSD of this complex envelope is given by where m(t) has a peak value of So that s(t) has an average normalized power of EEE 461 6

On-Off Keying (OOK) 1 Message Unipolar Modulation m(t) Bipolar Modulation m(t) OOK signal 0 1 0 1 s (t) Tb – Bit period ; R – Bit rate EEE 461 7

Spectrum of On-Off Keying (OOK) Ø PSD of the bandpass waveform is given by Ø For OOK Ø Null-to-Null bandwidth is Ø The Transmission bandwidth is and absolute bandwidth is Where B is the basebandwidth Ø Using Raised cosine pulse shape the bandwidth is: EEE 461 8

Detection of OOK Ø Non-Coherent Detection OOK in Binary output Envelope Detector Ø Coherent Detection with Low-pass filter OOK in LPF Binary output EEE 461 9

Optimum Detection of OOK Ø For optimum detection (Lowest Bit Error Rate BER) of OOK product detection with MATCHED Filter processing is required. EEE 461 10

Binary Phase Shift Keying (BPSK) Ø The BPSK signal is represented by To make this problem simple let, Pilot carrier term Data term Ø The level of the pilot carrier term is set by the value of the Peak Deviation Δθ = Dp Ø The digital modulation index ‘h’ is defined as 2∆θ – maximum peak-to-peak deviation during time Ts Ø If Dp is small, then there is little power in data term & more in pilot term Ø To maximize performance (minimum probability of error) Optimum case : Optimum BPSK signal : EEE 461 11

Binary Phase Shift Keying (BPSK) Generation: Message: m(t) Carrier: Cos(2 fct) BPSK output Ac. Cos(2 fct+Dpm(t)) -90 Phase shift 1 Message Unipolar Modulation m(t) BPSK output 0 1 0 1 s(t) EEE 461 12

Spectrum of Binary Phase Shift Keying (BPSK) Ø The complex envelope Optimum BPSK is given by Ø The PSD for this complex envelope is given by Ø PSD of the bandpass waveform is given by Average normalized power of s(t) : 2 R = 2/Tb Null-to-Null BW PSD of optimum BPSK EEE 461 13

Binary Phase Shift Keying (BPSK) Power Spectral Density (PSD) of BPSK: ( Non Optimum BPSK) If Dp /2 Pilot exists fc 2 R = 2/Tb EEE 461 14

Frequency Shift Keying (FSK) Discontinuous FSK Continuous FSK Ø Discontinuous Phase FSK: Switching between two different oscillators. Osc. f 1 Osc. f 2 Cos(2 f 1 t) Message: m(t) FSK output Ac. Cos(2 f 1 t+ 1) or Ac. Cos(2 f 2 t+ 2) Cos(2 f 2 t) Ø The discontinuous-phase FSK signal is represented by for t during a binary ‘ 1’ signal for t during a binary ‘ 0’ signal Ø This FSK is not used often. EEE 461 15

Continous Phase (FSK) Ø Continuous FSK : Message: m(t) FSK output Frequency Modulator Carrier fc Ø The Continuous-phase FSK signal is represented by or for FSK Where m(t) is discontinuous (Digital) θ(t) is continuous ( Integration of m(t)) EEE 461 16

Frequency Shift Keying (FSK) 1 Message Unipolar Modulation m(t) Bipolar Modulation m(t) 0 1 0 1 s(t) FSK output (Discontinuous) FSK output (Continuous) s(t) Mark(binary 1) frequency: f 1 Space(binary 0) frequency: f 2 EEE 461 17

Application of FSK – PC MODEM Computer Digital data FSK modem (Originate) Dial up phone line f 1 = 2225 Hz f 2 = 2025 Hz PSTN Computer Center FSK modem (Answer) f 1 = 1270 Hz f 2 = 1070 Hz FSK modem with 300 Bps Historically FSK signalling was for telephone modems. Fast (28. 8 kb/s and 56 kb/s modems use QAM signalling. EEE 461 18

Bandwidth of FSK Ø The approximate bandwidth of FSK is given by CARSON’S Rule. Ø If Raised cosine-rolloff premodulation filter is used then, EEE 461 19

Detection of FSK Ø FSK signal can be detected both coherently and incoherently. EEE 461 20