1) Coherent Detector Single Side Band (SSB) Modulation Generation of SSB Signals 1) Frequency Discrimination Method

Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

1) Coherent Detector

H.W. Find the output of the envelope detector in time domain and frequency domain. H.W. Find the output of the coherent detector in time domain and frequency domain.

Single Side Band (SSB) Modulation

Since each pair of sidebands (i.e., upper or lower) of the DSB signal contains the complete information of the original signal, so we will transmit only the upper or the lower pair of sidebands. This type of modulation is called single sideband (SSB) modulation.

Generation of SSB Signals

1) Frequency Discrimination Method

36 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

2) Phase Shift Method

∓() = ()cos () ± ()sin ()

37 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri where () is that signal obtained by shifting the phase of () by at each frequency. The difference represents the upper sideband (USB) while the sum represents the lower sideband (LSB). () is called the quadrature function of () and is found by using the Hilbert transform of ().

Hilbert Transform Let () ⇔ (), () ⇔ ()

So we have () = −()() Then taking the inverse Fourier transform of () to find (). Another way to find () is as follows () ⇔ 1 1 () () = () ∗ = − Demodulation of SSB Signals 1) Synchronous Detector

H.W. Show that an SSB signal can be demodulated by the synchronous detector by a) Sketching the spectrum of the signal at each point. b) The time domain expression of the signals at each point.

38 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

2) Phase Shift SSB Detector

H.W. Show that an SSB signal can be demodulated by the phase shift SSB detector by the time domain expression of the signals at each point.

Vestigial SideBand Modulation Vestigial Side Band (VSB) modulation is a compromise between SSB and DSB modulations. In this modulation scheme, one sideband is passed almost completely whereas just a trace, or vestige, of the other sideband is retained. The typical bandwidth required to transmit a VSB signal is about 1.25 that of SSB. VSB is used for transmission of the video signal in commercial television broadcasting.

Generation of VSB Signals

A VSB signal can be generated by passing a DSB signal through a sideband shaping filter (or vestigial filter)

39 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

Demodulation of VSB Signals For VSB signals, () can be recovered by synchronous or coherent demodulation.

This determines the requirements of the frequency response (). For distortionless recovery of (), it is required that

( + ) + ( − ) = || ≤

40 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri where is the maximum frequency of (). By letting the constant equals to

2() then

( − ) − () = −[( + ) − ()] which shows that () will have antisymmetry about the carrier frequency.

The demodulation of VSB signal is as shown

41 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

Quadrature Amplitude Modulation A block diagram of the Quadrature Amplitude Modulation (QAM) is shown below. The transmitted signals () consists of the sum of these two product modulator outputs, as given by

() = () cos() + () sin() where () is the in-phase component of the multiplexed band-pass signal () and

() as its quadrature component.

The signal at the output of the upper multiplier is:

2() cos() = [() + () cos(2) + () sin(2)] The signal at the output of the lower multiplier is:

2() sin() = [() − () sin(2) + () sin(2)] If we suppress the high-frequency components by low-pass filters, then at the output of the top detector (), while the output of the bottom detector is ().

42 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

Frequency Division Multiplexing Multiplexing is a technique where by several message signals are combined into a composite signal for transmission over a common channel. To transmit a number of these signals over the same channel, the signals must be kept apart so that they do not interfere with each other, and thus they can be separated at the receiver end. There are two multiplexing techniques: Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). In FDM the signals are separated in frequency, where as in TDM the signal are separated in time. The FDM scheme is illustrated as follows

Applications of Amplitude Modulation Both DSB and SSB techniques are widely used in communication. SSB signals are still used in some two-way radios. Two-way SSB communication is used in marine applications, in the military, and radio amateurs. DSB signals are used in FM and TV broadcasting to transmit two-channel stereo signals and to transmit the color information for a TV picture.

43 Communication Systems I Second Year Communication Engineering Department Assist. Prof. Dr. Atheer Alaa Sabri

An unusual form of AM is that used in TV broadcasting. A TV signal consists of the picture (video) signal and the audio signal, which have different carrier frequencies. The audio carrier is frequency-modulated, but the video information amplitude- modulates the picture carrier. The picture carrier is transmitted, but one sideband is partially suppressed. Video information typically contains frequencies as high as 4.2 MHz. A fully amplitude-modulated TV signal would then occupy 2(4.2) = 8.4 MHz. This is an excessive amount of bandwidth that is wasteful of spectrum space because not all of it is required to reliably transmit a TV signal. To reduce the bandwidth to the 6 MHz maximum allowed by the FCC for TV signals, a portion of the lower sideband of the TV signal is suppressed, leaving only a small part, or vestige, of the lower sideband. This arrangement, known as a vestigial sideband (VSB) signal, as illustrated below. Video signals above 0.75 MHz (750 kHz) are suppressed in the lower (vestigial) sideband, and all video frequencies are transmitted in the upper sideband. The newer high-definition or digital TV also uses VSB but with multilevel digital modulation called VSB.