EE 3220: Digital Communication Dr. Hassan Yousif Ahmed Department of Electrical Engineering College of Engineering at Wadi Aldwasser Slman bin Abdulaziz University 1 Dr Hassan Yousif Lec-6: Digital Modulation

2 of 82 Linear Modulation Techniques: Modulation is the process of facilitating of transfer information over medium Digital modulation can be broadly classified as: 1.Linear (change Amplitude or phase) 2.Non linear modulation techniques (change frequency). Linear Modulation Techniques: The amplitude /phase of the transmitted signal s(t), varies linearly with the modulating digital signal, m(t). These are bandwidth efficient (because it doesn’t change frequency) and hence are very attractive for use in wireless communication systems where there is an increasing demand to accommodate more and more users within a limited spectrum.

3 of 30 Linear Modulation schemes have very good spectral efficiency, However, they must be transmitted using linear RF amplifiers which have poor power efficiency. Pros & Cons

4 of 30 Note “Phase modulation” can be regarded as “amplitude” modulation because it can really change “envelope”; Thus both of them belong to “linear modulation”!

Bandpass Channels Bandpass channels pass a range of frequencies around some center frequency f c –Radio channels, telephone & DSL modems Digital modulators embed information into waveform with frequencies passed by bandpass channel Sinusoid of frequency f c is centered in middle of bandpass channel Modulators embed information into a sinusoid f c – W c /2 f c 0 f c + W c /2

Information T 2T2T 3T3T 4T4T5T5T 6T6T Amplitude Shift Keying +1 Frequency Shift Keying 0 T 2T2T 3T3T 4T4T5T5T 6T6T t t Amplitude Modulation and Frequency Modulation Map bits into amplitude of sinusoid: “1” send sinusoid; “0” no sinusoid Demodulator looks for signal vs. no signal Map bits into frequency: “1” send frequency f c +  ; “0” send frequency f c -  Demodulator looks for power around f c +  or f c - 

Amplitude Shift Keying (ASK) Pulse shaping can be employed to remove spectral spreading ASK demonstrates poor performance, as it is heavily affected by noise, fading, and interference Baseband Data ASK modulated signal Acos(  t) 7

Frequency Shift Keying (FSK) Example: The ITU-T V.21 modem standard uses FSK FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states Baseband Data BFSK modulated signal where f 0 =Acos(  c -  )t and f 1 =Acos(  c +  )t f0f0 f0f0 f1f1 f1f1 8

Phase Modulation Map bits into phase of sinusoid: –“1” send A cos(2  ft), i.e. phase is 0 –“0” send A cos(2  ft+  ), i.e. phase is  Equivalent to multiplying cos(2  ft) by +A or -A –“1” send A cos(2  ft), i.e. multiply by 1 –“0” send A cos(2  ft+  ) = - A cos(2  ft), i.e. multiply by -1 We will focus on phase modulation +1 Phase Shift Keying 0 T 2T2T 3T3T 4T4T5T5T 6T6T t Information

Modulate cos(2  f c t) by multiplying by A k for T seconds: AkAk x cos(2  f c t) Y i (t) = A k cos(2  f c t) Transmitted signal during kth interval Demodulate (recover A k ) by multiplying by 2cos(2  f c t) for T seconds and lowpass filtering (smoothing): x 2cos(2  f c t) 2A k cos 2 (2  f c t) = A k {1 + cos(2  2f c t)} Lowpass Filter (Smoother) X i (t) Y i (t) = A k cos(2  f c t) Received signal during kth interval Modulator & Demodulator

A -A 0 T 2T 3T 4T5T 6T Information Baseband Signal Modulated Signal x(t) +A -A 0 T 2T 3T 4T5T 6T Example of Modulation A cos(2  ft)-A cos(2  ft)

Recovered Information Baseband signal discernable after smoothing After multiplication at receiver x(t) cos(2  f c t) +A -A 0 T 2T 3T 4T5T 6T +A -A 0 T 2T 3T 4T5T 6T Example of Demodulation A {1 + cos(4  ft)} -A {1 + cos(4  ft)}

Signaling rate and Transmission Bandwidth Fact from modulation theory: Baseband signal x(t) with bandwidth B Hz If then B f c +B f f f c -B fcfc Modulated signal x(t)cos(2  f c t) has bandwidth 2B Hz If bandpass channel has bandwidth W c Hz, Then baseband channel has W c /2 Hz available, so modulation system supports W c /2 x 2 = W c pulses/second That is, W c pulses/second per W c Hz = 1 pulse/Hz Recall baseband transmission system supports 2 pulses/Hz

AkAk x cos(2  f c t) Y i (t) = A k cos(2  f c t) BkBk x sin(2  f c t) Y q (t) = B k sin(2  f c t) + Y(t) Y i (t) and Y q (t) both occupy the bandpass channel QAM sends 2 pulses/Hz Quadrature Amplitude Modulation (QAM) QAM uses two-dimensional signaling –A k modulates in-phase cos(2  f c t) –B k modulates quadrature phase cos(2  f c t +  /4) = sin(2  f c t) –Transmit sum of inphase & quadrature phase components Transmitted Signal

QAM Demodulation Y(t) x 2cos(2  f c t) 2cos 2 (2  f c t)+2B k cos(2  f c t)sin(2  f c t) = A k {1 + cos(4  f c t)}+B k {0 + sin(4  f c t)} Lowpass filter (smoother) AkAk 2B k sin 2 (2  f c t)+2A k cos(2  f c t)sin(2  f c t) = B k {1 - cos(4  f c t)}+A k {0 + sin(4  f c t)} x 2sin(2  f c t) BkBk Lowpass filter (smoother) smoothed to zero

Signal Constellations Each pair (A k, B k ) defines a point in the plane Signal constellation set of signaling points 4 possible points per T sec. 2 bits / pulse AkAk BkBk 16 possible points per T sec. 4 bits / pulse AkAk BkBk (A, A) (A,-A) (-A,-A) (-A,A)

Differential Modulation In the transmitter, each symbol is modulated relative to the previous symbol and modulating signal, for instance in BPSK 0 = no change,1 = In the receiver, the current symbol is demodulated using the previous symbol as a reference. The previous symbol serves as an estimate of the channel. A no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous symbol. 17

18 of 30 Let {d k } denote the differentially encoded sequence with this added reference bit. We now introduce the following definitions in the generation of this sequence: If the incoming binary symbol b k is 1, leave the symbol d k unchanged with respect to the previous bit. If the incoming binary symbol b k is 0, change the symbol d k with respect to the previous bit. DPSK

20 of 30 to send symbol 0, we advance the phase of the current signal waveform by 180 degrees, to send symbol 1, we leave the phase of the current signal waveform unchanged. Generation of DPSK: The differential encoding process at the transmitter input starts with an arbitrary first bit, serving as reference. DPSK

21 of 30 Differential Phase Shift Keying (DPSK): DPSK is a non coherent form of phase shift keying which avoids the need for a coherent reference signal at the receiver. Advantage: Non coherent receivers are easy and cheap to build, hence widely used in wireless communications. DPSK eliminates the need for a coherent reference signal at the receiver by combining two basic operations at the transmitter:

Pulse Carrier Carrier: A train of identical pulses regularly spaced in time 22

Pulse-Amplitude Modulation (PAM) Modulation in which the amplitude of pulses is varied in accordance with the modulating signal. Used e.g. in telephone switching equipment such as a private branch exchange (PBX) 23

Pulse-Duration Modulation (PDM) Modulation in which the duration of pulses is varied in accordance with the modulating signal. Deprecated synonyms: pulse-length modulation, pulse-width modulation. Used e.g. in telephone switching equipment such as a private branch exchange (PBX) 24