Lecture 6: Amplitude Modulation II EE 3770: Communication Systems AM Limitations AM Limitations DSB-SC Modulation SSB Modulation VSB Modulation Lecture 6 Amplitude Modulation II Amplitude modulation is the oldest method of performing modulation. Advantages: AM s biggest advantage is the ease with which it is generated and reversed AM system is relatively inexpensive to build Disadvantages: AM is wasteful of power The carrier wave c(t) is completely independent of the message signal; its transmission therefore is a waste of power AM is wasteful of bandwidth Due to symmetry, the upper and lower sidebands of an AM wave are uniquely related, and the amplitude and phase of either sideband can be uniquely determined from the other sideband Multiplexing Mojtaba Vaezi 6-1 Lecture 6: Amplitude Modulation II 6-2 How to make AM more efficient? 1. Remove the carrier AM Modifications Double sideband-suppressed carrier (DSB-SC) 2. Transmit only one sideband Single sideband (SSB) Vestigial sideband (VSB) What is the cost? Increased system complexity and cost DSB-SC Modulation Double sideband-suppressed carrier (DSB-SC) modulation consists of the product of the message signal and the carrier wave Considering the sinusoidal carrier wave c(t) = A c cos 2πf c t the DSB-SC wave thus can be described as follows: s(t) = A c m(t) cos 2πf c t S(f ) = A c 2 [M (f f c) + M (f + f c )] We trade off system complexity for improved utilization of communication resources. Lecture 6: Amplitude Modulation II 6-3 Lecture 6: Amplitude Modulation II 6-4
Check Yourself Ring Modulator How can we multiply (modulate) two waves? Question: Why did we have the carrier at all? Short answer: To avoid phase reversal! A DSB-SC modulated signal s(t) undergoes a phase reversal whenever the message signal crosses zero Thus, the envelope of a DSB-SC modulated signal is different from the message signal This means that simple demodulation using an envelope detection is not a viable option for DSB-SC modulation (a) Ring modulator (b) outer/inner diodes are on/off (c) outer/inner diodes are off/on Lecture 6: Amplitude Modulation II 6-5 Lecture 6: Amplitude Modulation II 6-6 Synchronous Demodulation Synchronous Demodulation How can we demodulate DSB-SC? Synchronous demodulation is also called coherent detection or Passing v(t) through a low pass filter yields Why coherent? The receiver (demodulator) must know the carrier signal s both frequency (f c ) and phase (φ). v(t) = s(t) A c cos(ω c t + φ) = A c m(t) cos ω c t A c cos(ω c t + φ) = 1 2 A ca cm(t) cos φ + 1 2 A ca cm(t) cos(2ω c t + φ) v o (t) = 1 2 A ca c cos φ m(t) If φ is constant (but φ π ) the detector s output is an undistorted 2 version of the transmitted message. If φ changes with time, the detector s output will changed randomly in time, which is not desired we need another receiver (see Costas Receiver) Lecture 6: Amplitude Modulation II 6-7 Lecture 6: Amplitude Modulation II 6-8
SSB Modulation DSB-SC improves the power efficiency (how?) but suffers from the spectral (bandwidth) efficiency Why DSB is not bandwidth efficient? Baseband message m(t) has a bandwidth of W while modulated message requires twice bandwidth (2W ) for transition SSB Modulation Single-sideband (SSB): transmit only one sideband (either the LSB or the USB) Question: Any bandwidth efficient solutions? 1. Single-sideband (SSB) modulation: which removes either the LSB or the USB, i.e., transmits over bandwidth W 2. Quadrature amplitude modulation (QAM): which sends two messages over bandwidth 2W Lecture 6: Amplitude Modulation II 6-9 Lecture 6: Amplitude Modulation II 6-10 SSB in Time Domain SSB in Time Domain The upper sideband (USB) is the output of filtering signal m(t) with an ideal low pass filter H (f ) = 1, 0 f W 0, otherwise The modulated signal can be constructed by m h (t) = m(t) 1 is the Hilbert transform of m(t) πt M h (f ) = M (f ) j sign(f) H (f ) = j sign(f) j = e π 2, f > 0 +j = e π 2, f < 0 m USB (t) = m(t) cos ω c t m h (t) sin ω c t and m USB (t) = m(t) cos ω c t + m h (t) sin ω c t m(t) is the original real message signal, and m h (t) = m(t) 1 πt is the Hilbert transform of m(t) Lecture 6: Amplitude Modulation II 6-11 Lecture 6: Amplitude Modulation II 6-12
Which Signals and Which SSB Sideband? Vestigial Sideband (VSB) Modulation - Motivation SSB works satisfactorily for message with an energy gap centered around zero frequency (e.g., speech signal) SSB relies on being able to filter out one sideband and keep the other For audio this is possible because the voice spectrum drops off below 300 Hz, allowing space for a transition band (see previous slide) This is not possible for other signals, like video, that have strong components at low frequencies (i.e., we will need an ideal filter) DSB-SC fits the characteristics of video but it is not bandwidth efficient Which SSB Sideband? Transmitter and receiver must agree on use of LSB vs. USB SSB is common for amateur radio Below 10 MHz: LSB Above 10 MHz: USB There are some exceptions To overcome these two practical limitations, we need a compromise method of modulation The solution is Vestigial Sideband (VSB) Modulation VSB is a compromise method of modulation that lies somewhere between SSB and DSB-SC in its spectral characteristics SSB is also common for shortwave radio Lecture 6: Amplitude Modulation II 6-13 Lecture 6: Amplitude Modulation II 6-14 Vestigial Sideband Modulation (VSB) Vestigial Sideband (VSB) Modulation: a small portion (a vestige) of the unneeded sideband is used. This reduces DC distortion. VSB in Commercial Systems VSB modulation with envelope detection is used to modulate image in analog TV signals. (The audio signal is modulated using FM.) Channel bandwidth for TV broadcasting in Northern America is 6MHz Thechoice of VSB for analog TV was influenced by Image signal has large BW with significant low frequency content The circuitry used for VSB is simple (envelope detection) than SBB VSB is also applied to digital signals for the same reasons B T = f v + W where f v is the vestige bandwidth and W is the message bandwidth. Typically, is 25% percent of W. VSB signals are generated using standard AM or DSB-SC modulation then passing modulated signal through a sideband shaping filter Demodulation uses either standard AM or DSB-SC demodulation, depending on whether or not a carrier tone is transmitted Lecture 6: Amplitude Modulation II 6-15 digital TV signal Amplitude response for VSB modulated Lecture 6: Amplitude Modulation II 6-16
Quadrature Amplitude Modulation (QAM) QAM Modulator/Demodulator DSB-SC modulates a message signal with bandwidth W to a transmitted signal with bandwidth 2W SSB reduces the transmitted bandwidth to W, but requires more complex modulator reduces SNR (for a fixed carrier amplitude) Quadrature amplitude modulation uses the 2W transmitter bandwidth to send two independent signals: QAM has the same spectral efficiency as SSB but it does not need sharp bandpass filters QAM is used in almost all digital communication methods, including telephone modems, cable TV, satellite TV Two real messages: m 1 (t) and m 2 (t) m 1 (t) is modulated on a cosine and m 2 (t) is modulated on a sine Note that we need a synchronous receiver, or the two channels will interfere Question: What happens with a 90 phase shift at the receiver? Lecture 6: Amplitude Modulation II 6-17 Lecture 6: Amplitude Modulation II 6-18 Amplitude Modulation Summary Frequency Division Multiplexing (FDM) Many different ways to encode information in amplitude AM DSB-SC AM SSB VSB QAM Common issues Synchronization Bandwidth Multiplexing is amother motivation for modulation Next: Encoding information in frequency or phase (FM, PM) Lecture 6: Amplitude Modulation II 6-19 Lecture 6: Amplitude Modulation II 6-20
Quadrature Carrier Multiplexing Quadrature amplitude modulation (QAM) is also called quadrature carrier is multiplexing. mulipex Lecture 6: Amplitude Modulation II 6-21