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

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

Digital Modulation Carrier signal: Ac cos (2pfct + θ) Modulation: m(t) Modulated signal: Ac (t) cos (2pfc(t) t + θ(t))  m(t); discrete Vary Vary amplitude frequency & phase Variations are discrete!!!!! Binary OOK BPSK DPSK FSK Multilevel QPSK MPSK QAM

5-9 Binary Modulated Bandpass signaling: 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.

Binary bandpass signaling techniques 1 1 1 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

On-Off Keying (OOK) / Amplitude Shift Keying (ASK) Key/ Switch Carrier Cos(2fct) Message m(t) OOK output Acm(t)Cos(2fct) The complex envelope is The OOK signal is represented by 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

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

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

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

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

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

Binary Phase Shift Keying (BPSK) Generation: Carrier:Cos(2fct) Message: m(t) BPSK output AcCos(2fct+Dpm(t)) -90 Phase shift Message 1 0 1 0 1 0 1 Unipolar Modulation m(t) Bipolar Modulation m(t) s(t) BPSK output

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

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

Frequency Shift Keying (FSK) Continuous FSK Discontinuous FSK Discontinuous Phase FSK: Switching between two different oscillators. Cos(2f1t) Message: m(t) FSK output AcCos(2f1t+1) or AcCos(2f2t+2) Osc. f2 Osc. f1 Cos(2f2t) The discontinuous-phase FSK signal is represented by for t during a binary ‘1’ signal for t during a binary ‘0’ signal This FSK is not used often.

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

Frequency Shift Keying (FSK) Message 1 0 1 0 1 0 1 Unipolar Modulation m(t) Bipolar Modulation m(t) s(t) FSK output (Discontinuous) FSK output (Continuous) s(t) Mark(binary 1) frequency: f1 Space(binary 0) frequency: f2

Application of FSK – PC MODEM Computer FSK modem (Originate) Center (Answer) Digital data PSTN Dial up phone line f1 = 1270Hz f2 = 1070Hz f1 = 2225Hz f2 = 2025Hz 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.

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

Detection of FSK FSK signal can be detected both coherently and incoherently.