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Lecturer: Siriwhaddhanah Pongpadpinit PhD(Lon) MIET

Understanding modulation in communication systems, exploring CW modulation, amplitude, angle modulation, pulse modulation, and spectral overlap.

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Lecturer: Siriwhaddhanah Pongpadpinit PhD(Lon) MIET

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  1. TS2240 Communication Signals and SystemsLecture 4: Applications to Communication Systems: Modulation Techniques Lecturer: Siriwhaddhanah Pongpadpinit PhD(Lon) MIET

  2. Modulation Modulation is the process which • shifts the range of frequencies contained in the message signal into another frequency range which suitable for transmission over the channel. • performs a corresponding shift back to original frequency range after reception of the modulated signal. • some characteristic of a carrier wave is varied in accordance with the message signal. TS2240 Communication Signals and Systems (Lecture 4)

  3. Type of Modulation The two most commonly used forms of carrier are: • Sinusoidal wave which is classified as continuous-wave (CW) modulation. • Periodic pulse train which is classified as pulse modulation. TS2240 Communication Signals and Systems (Lecture 4)

  4. Continuous-wave (CW) modulation • Consider the sinusoidal carrier wave c(t) = Accos((t)) (1) where Acand (t) are the carrier amplitude and angle of the sinusoidal wave. • CW modulation can be classified into 2 subclasses which are: • Amplitude Modulation (AM) which the carrier amplitude is varied with the message signal, as illustrated in Figure 1. • Angle modulation which the angle of the carrier is varied with the message signal, as illustrated in Figure 1. TS2240 Communication Signals and Systems (Lecture 4)

  5. CW modulation (2) Carrier Wave Modulating signal Amplitude Modulation Angle Modulation Figure 1 illustration of Amplitude and Angle modulation TS2240 Communication Signals and Systems (Lecture 4)

  6. CW modulation (3) Four types of amplitude modulation are: • Full amplitude modulation • Double sideband suppressed carrier modulation • Single sideband modulation • Vestigial sideband modulation TS2240 Communication Signals and Systems (Lecture 4)

  7. CW modulation (4) • In order to describe the angle modulation (t) , the notion of instantaneous radian frequency i(t) is introduced. The relationship between (t) and i(t) can be written as: (2) (3) substitute (3) in (1) we have c(t) = Accos(c(t)+) (4) hence TS2240 Communication Signals and Systems (Lecture 4)

  8. CW modulation (5) • When the instantaneous radian frequency i(t) is varied in accordance with a message signal denoted by m(t), we have i(t) = c + kfm(t) (5) where kf is the frequency sensitivity factor of the modulator. • By substituting (5) into (3), we get (6) TS2240 Communication Signals and Systems (Lecture 4)

  9. CW modulation (6) • The result of angle modulation is known as frequency Modulation (FM) and may be written as (7) where kf and m() are the frequency sensitivity factor of the modulation and message signal respectively. In (7) Ac is maintained constant. • The angle (t) can be written in term of phase sensitivity factor of the modulation kp. Hence the Phase Modulation (PM) is defined by (8) TS2240 Communication Signals and Systems (Lecture 4)

  10. Pulse Modulation • Consider a carrier wave (9) where T is the period and p(t) is a pulse of relatively short duration when compares with T Train of rectangular pulse Sinusoidal modulating signal Pulse amplitude modulation signal Figure 2 Pulse amplitude modulation TS2240 Communication Signals and Systems (Lecture 4)

  11. Full Amplitude Modulation • Consider a sinusoidal carrier wave c(t) = Accos(ct) (10) • Let m(t) be a message signal of interest, the Amplitude Modulation can be defined using s(t) = Ac[1 + kam(t)]cos(ct) (11) where ka is the amplitude sensitivity factor of modulation TS2240 Communication Signals and Systems (Lecture 4)

  12. Full Amplitude Modulation (2) • Percentage of Modulation From (11), the envelope of the AM s(t) can be defined as (12) Two cases arise depending on the magnitude of kam(t) as illustrated in Figure 3: • Under modulation • Over modulation TS2240 Communication Signals and Systems (Lecture 4)

  13. Full Amplitude Modulation (3) Message signal Undermodulation case kam(t)  1 Overrmodulation case kam(t) > 1 Figure 3 Amplitude modulation TS2240 Communication Signals and Systems (Lecture 4)

  14. Full Amplitude Modulation (4) • Generation of AM Wave (11) may rewrite as s(t) = ka[m(t) + B]Accos(ct) (13) where the constant B (which equal to 1/ka) represents a bias that is added to the message signal m(t) before modulation Figure 4 Block diagram of AM generator of (13) TS2240 Communication Signals and Systems (Lecture 4)

  15. Full Amplitude Modulation (5) Figure 5 Illustration of carrier and message in AM Source: http://en.wikipedia.org/wiki/Amplitude_modulation TS2240 Communication Signals and Systems (Lecture 4)

  16. Full Amplitude Modulation (6) • Frequency description of the AM wave can be obtained by taking the Fourier transform of both sides of (11) as (14) where S(j) and M(j) are the Fourier transform of s(t) and m(t) respectively Figure 6 Frequency spectrum of AM wave TS2240 Communication Signals and Systems (Lecture 4)

  17. Spectral Overlap • The spectral overlap is occurred if: • Movement of the lower sideband into the negative frequency range • Movement of the image of the lower sideband into the positive frequency range. • In order to avoid the spectral overlap problem, the following condition needs to be fulfilled Figure 7 illustration of spectral overlap TS2240 Communication Signals and Systems (Lecture 4)

  18. Double Sideband- Suppressed Carrier Modulation • A Double Sideband-Suppressed Carrier (DSB-SC) modulation signal can be described as a function of time as (15) Unlike full AM that independently transmit both carrier wave and message signal, DSB-SC modulates a message by suppressing the carrier wave. As a result, the modulated signal is proportional to the product of the carrier wave and the message signal. TS2240 Communication Signals and Systems (Lecture 4)

  19. Double Sideband- Suppressed Carrier Modulation (2) • The frequency domain description of the DSB-SC can be obtained using Fourier transform of (15) as (16) Figure 8 Frequency spectrum of DSB-SC TS2240 Communication Signals and Systems (Lecture 4)

  20. Single Sideband (SSB) Modulation • In the SSB modulation • The transmission power is reduced by 50% when compares with modulated signal with carrier wave transmission as the carrier wave is not transmitted. • The transmission power is further reduced by 50% when compares with DSB-SC because only one sideband is transmitted. • The required bandwidth is reduced by half when compare with full AM and DSB-SC. TS2240 Communication Signals and Systems (Lecture 4)

  21. Frequency Modulation • FM is a so called angle modulation scheme, it was inspired by phase modulation but has proved to be more useful partly for its ease of generation and decoding. The main advantages of FM over AM are: • Improved signal to noise ratio (about 25dB) w.r.t. to man made interference. • Smaller geographical interference between neighbouring stations. • Less radiated power. • Well defined service areas for given transmitter power. • Disadvantages of FM: • Much more Bandwidth (as much as 20 times as much). • More complicated receiver and transmitter. TS2240 Communication Signals and Systems (Lecture 4)

  22. Frequency Modulation (2) In this scheme the frequency of the modulating signal is changed in proportion to the message signal m(t) . Thus the signal that is transmitted is of the form (17) TS2240 Communication Signals and Systems (Lecture 4)

  23. Pulse-Amplitude Modulation (PAM) • The PAM signal s(t) for a message signal m(t) is defined as (18) where Ts is the sampling period, m[n] is the value of the message signal m(t) at time t = nTs, and h(t) is rectangular pulse of unit amplitude and duration T0. h(t) can be defined as (19) TS2240 Communication Signals and Systems (Lecture 4)

  24. PAM (2) • The message signal m(t) can be defined in term of impulse-sampled as (20) PAM signal may be expressed using convolution (21) TS2240 Communication Signals and Systems (Lecture 4)

  25. PAM (3) • By taking Fourier transform on both side of eq.(21), the frequency description is (22) The impulse sampled in the frequency description can be defined as (23) By substituting eq.(23) into eq.(22), we have (24) TS2240 Communication Signals and Systems (Lecture 4)

  26. PAM (4) • The frequency description of a rectangular pulse in eq.(19) can be defined using Fourier transform as (25) TS2240 Communication Signals and Systems (Lecture 4)

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