EE3723 : Digital Communications Week 8-9: Bandpass Modulation MPSK MASK, OOK MFSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 1
In-phase and Quadrature (I&Q) Representation Any bandpass signal can also be represented as s( t) = x( t) cos( ω t ) y( t) sin( ω 0 0 t x(t) is a real-valued signal called In-phase (I) y(t) is a real-valued signal called Quadrature (Q) This is often a convenient form which Emphasizes the fact that two signals may be transmitted within the same bandwidth Closely parallels the physical implementation of the Tx/Rx ) 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 2
Concept of a constellation diagram 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 3
Digital Modulation Schemes Basic Digital Modulation Schemes: Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Amplitude Phase Keying (APK) For Binary signals (M = 2), we have Binary Amplitude Shift Keying (BASK) Binary Phase Shift Keying (BPSK) Binary Frequency Shift Keying (BFSK) For M > 2, many variations of the above techniques exit usually classified as M-ary Modulation/detection 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 4
Bandpass MOdulation and DEModulation 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 5
Overview of Modulation Schemes digital modulations, (a) PSK (b) FSK (c) ASK (d) ASK/PSK (APK) 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 6
Modulation Process Amplitude Shift Keying In Amplitude Shift Keying (ASK), the amplitude of the carrier is switched between two (or more) levels according to the digital data For BASK (also called ON-OFF Keying (OOK)), one and zero are represented by two amplitude levels A 1 and A 0 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 7
Amplitude Shift Keying Analytical Expression: s( t) = A cos( ω t), 0 i c t T where A i = peak amplitude 2 s( t) = Acos( ω t) = 2Arms cos( ω0t) = 2A cos( ω0 = 0 t 2E E 2 P cos( ω t) = cos( ω 0 T 0 t Hence, 2E i ( t) si ( t) = cos( ω 0 t), T 0 t T, i = 1,2,... where E T 2 = s ( t) dt, i = 0,2,... M 0 i ) rms 1 ) M 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 8
Amplitude Shift Keying Where for binary ASK (also known as ON OFF Keying (OOK)) s1( t) = Acm( t) cos( ω ct + φ ), 0 t T binary s0 ( t) = 0, 0 t T binary Mathematical ASK Signal Representation The complex envelope of an ASK signal is: g( t) = A m( t) c The magnitude and phase of an ASK signal are: A( t) = Acm( t), φ ( t) = 0 The in-phase and quadrature components are: x( t) = A m( t) y( t) = c 0, the quadrature component is wasted. 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 9 1 0
ASK, OOK, MASK The amplitude (or height) of the sine wave varies to transmit the ones and zeros One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation) 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 10
Implementation of binary ASK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 11
OOK and MASK OOK (On-OFF Key) 0 silence. Sensor networks: battery life, simple implementation MASK: multiple amplitude levels 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 12
Pro Con Pro, Con and Applications Simple implementation Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude Susceptible to sudden gain changes Inefficient modulation technique for data Applications On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical fiber Morse code Laser transmitters 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 13
Detectors for ASK: Coherent Receiver Coherent detection requires the phase information A coherent detector mixes the incoming signal with a locally generated carrier reference Multiplying the received signal r(t) by the receiver local oscillator (say A c cos(w c t)) yields a signal with a baseband component plus a component at 2f c Passing this signal through a low pass filter eliminates the high frequency component In practice an integrator is used as the LPF 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 14
The output of the LPF is sampled once per bit period This sample z(t) is applied to a decision rule z(t) is called the decision statistic Matched filter receiver of OOK signal A MF pair such as the root raised cosine filter can thus be used to shape the source and received baseband symbols In fact this is a very common approach in signal detection in most bandpass data modems 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 15
Noncoherent Receiver Does not require a phase reference at the receiver If we do not know the phase and frequency of the carrier, we can use a noncoherent receiver to recover ASK signal Envelope Detector: The simplest implementation of an envelope detector comprises a diode rectifier and smoothing filter 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 16
Frequency Shift Keying (FSK) In FSK, the instantaneous carrier frequency is switched between 2 or more levels according to the baseband digital data data bits select a carrier at one of two frequencies the data is encoded in the frequency In the past, FSK has been the most widely used form of digital modulation;why? Simple both to generate and detect Insensitive to amplitude fluctuations in the channel FSK conveys the data using distinct carrier frequencies to represent symbol states An important property of FSK is that the amplitude of the modulated wave is constant Waveform 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 17
Analytical Expression s 2 E s ( t ) = cos( it + ), i = 0,1,... M T 1 ω 2 φ i 3 s t θ ( ) [ ( ) ] i t = ω0t + ωd m τ dτ Analog form d f = ( t) = f0 + f m( t) i θ i d dt 1 General expression is 2 E s s i ( t ) = cos( 2πf 0t + 2πi ft ), i = 0,1,... M T s Where f = f i f i 1 1 f = f + i f and E = ke, T = i 0 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 18 s b s kt b
Frequency Shift Keying One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation) s ( t ) = ( 2πf t) ( 2πf t) Acos 1 Acos 2 binary 1 binary 0 Represent each logical value with another frequency (like FM) 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 19
Multiple Frequency-Shift Keying (MFSK) More than two frequencies are used More bandwidth efficient but more susceptible to error s i ( t ) A cos 2π f t = 1 i M i f i = f c + (2i 1 M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L L = number of bits per signal element 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 20
Phase Shift Keying One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation) s ( t ) = A A cos ( 2πf t) ( c cos 2 π f t +π ) c binary 1 binary 0 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 21
DBPSK, QPSK Differential BPSK 0 = same phase as last signal element 1 = 180º shift from last signal element Four Level: QPSK s ( t ) = π A cos 2 π f c t + 11 4 3π A cos 2 π f c t + 01 4 3π A cos 2 π f t 00 c 4 π A cos 2 π f t 10 c 4 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 22
QPSK Example 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 23
FSK Vs PSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 24
PSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 25
BPSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 26
BPSK Detector 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 27
M=4 QPSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 28
QPSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 29
QPSK Detection 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 30
QPSK Detection: Matched Filter 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 31
MPSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 32
MPSK 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 33
MPSK Detection 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 34
Matched filter detection 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 35
Properties of MPSK Constellation In time domain, all M signals have the same amplitudes. All M signals have the same energy. All M constellation points are the same distance from the origin -- they are equally spaced on a circle They differ in phase only (i.e. the data is encoded in the phase of the transmitted carrier). Each phase differs by 2p/M radians. 04-May-15 Muhammad Ali Jinnah University, Islamabad - Digital Communications - EE3723 36