What is transmission?

The transmission is a process of transmitting or sending an analog or digital signal via a channel or medium, The medium can be wired or wireless.

Types of Transmission

 Two types:

  1. Baseband transmission
  2. Carrier Modulation/Passband transmission

 


Baseband Transmission

Baseband - frequency band of the original  message signal from the source or input transducer. Most baseband signals (audio, video) contain significant low-frequency component .A baseband bandwidth is equal to the highest frequency of a signal or system. Baseband transmission refers to transmitting the signal directly, without any modification to the spectral content. Baseband signals have overlapping band - results  in severe interference if sharing a channel. Cannot be effectively transmitted over the wireless channel . Can be transmitted with copper or coaxial    cable for dedicated transmitter-receiver pair.



Carrier Modulation/Passband Transmission

Baseband signal is modified with high frequency carrier(s). The amplitude, frequency or phase of the carrier(s) is varied according to amplitude of baseband signal, which is called modulation. Modulation shifts the baseband signal to a Modulation .Modulation is a process where some characteristic of a high frequency carrier wave is varied in accordance with the amplitude of an information-bearing signal





Significance of Modulation

1. To decrease antenna height:

For transmitting a signal, the antenna height must be of the order of a wavelength (e.g., λ, λ/2 and λ/4).

For a signal of 1 Hz (λ=3*10^8 m), antenna height has to be 75,000 Km (considering λ/4). If the same signal is modulated to some high frequency, say 100 MHZ (λ = 3 m), required antenna height is 0.8522 m.


2) To increase the operating range:

Signal requires more energy to travel longer distance Planck's formula: E = hf, h = Planck’s constant, f = frequency of the signal As the modulation increases the frequency content of a signal, it indirectly increases the energy of the signal to enable it to travel long distance .





3. For efficient utilization of frequency spectrum:

 Use of the available frequency bands .Simultaneous transmission of multiple users






4. To suite the channel requirement:

Say a channel that essentially acts like a bandpass filter. In such a case, modulation allows to send a signal by shifting the baseband signal to the bandpass frequency range.

Types of Modulation

1. Analog/Continuous Wave Modulation: Analog baseband signal using analog carrier (bandpass channel)
  1.  Amplitude Modulation (AM)
  2.  Angle Modulation
i) Frequency Modulation (FM)ii) Phase Modulation (PM)

 2. Digital Modulation: Digital bit stream using analog carrier (bandpass channel)

  •  Amplitude Shift Keying (ASK)
  •  Frequency Shift Keying (FSK)
  •  Phase Shift Keying (PSK)
3. Pulse Modulation: Analog narrowband signal using pulse (wideband baseband channel).
  1.  Pulse Amplitude Modulation (PAM)
  2.  Pulse Time Modulation (PTM)
i) Pulse Width Modulation (PWM) / Pulse Duration Modulation (PDM)
ii) Pulse Position Modulation (PPM)

 

3. Pulse Code Modulation (PCM)
4. Delta Modulation (DM)

Amplitude Modulation (AM) Family

Double‐sideband modulation (DSB)

 Double‐sideband modulation with carrier (DSB‐WC) / AM

 Double‐sideband suppressed‐carrier transmission (DSB‐SC)

 Double‐sideband reduced carrier transmission (DSB‐RC)

Single‐sideband modulation (SSB)

 SSB with carrier (SSB‐WC)

 SSB suppressed carrier modulation (SSB‐SC)

Vestigial sideband modulation (VSB)

Quadrature amplitude modulation (QAM)

Amplitude Modulation (AM)

Amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating (message) signal.



Advantages and Disadvantages of AM

Advantages:

I. 1t is simple to implement

2. It can be demodulated using a circuit consisting of very few components.

3.AM receivers are very cheap as no specialized components are needed.


Disadvantages

1. An amplitude modulation signal is not efficient in terms of its power usage.

2. It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency.

3. An amplitude modulation signal is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.


Applications of Amplitude Modulation

1. Used to carry message signals in early telephone lines.

2. Used to transmit Morse code using radio and other communication systems.

3.Used in Navy and Aviation for communications as AM signals can travel longer distances.

4. Widely used in amateur radio.

Expression for Amplitude Modulation

Amplitude modulation requires a high frequency constant carrier and a low frequency modulation signal.

   A sinewave carrier is of the from , $$ e_c = {E_c Sin(w_ct)}$$

  A sinewave modulation signal is of the form $$ e_m = {E_m Sin(w_mt)}$$

The overall signal can be describe by:  $$ e_am = {E_c+ E_m Sin(w_mt))sin(w_ct)}$$

The carrier amplitude is normalized to one and the am equation is written as:

$$ e_am = {E_c(1+m_am Sin(w_mt))sin(w_ct)}$$

The expression is simply written as,

$$ e= {E_c+ m Sin(w_mt))sin(w_ct)}$$

 Here,  m of $$ m_am= {Modulation Index }$$

$$m_am= {E_m \over E_c}$$ $$Where 0< m_am <1$$


Modulation index

The ratio between the amplitude of the modulating signal and the carrier is defined as the modulation index, m

Mathematically,

m=Em/Ec

Where, Em=peak amplitude of information signal

             Ec=peak value of the constant unmodulated carrier sine wave


Modulation index for AM

$$Vm = {  Vmax - Vmin \over 2}$$
$$Vc = {  Vmax - Vm}$$
$$ = Vmax - {Vmax - Vmin \over 2}.$$
$$ =  {2Vmax - Vmax + Vmin \over 2}.$$
$$ = {  Vmax - Vmin \over 2}$$
So, The modulation index $$  m =  {Vm \over Vc}.$$
$$=> Vm ={ {  Vmax - Vmin \over 2} \over  {  Vmax + Vmin \over 2} }$$
$$=> Vm ={ {  Vmax - Vmin } \over  {  Vmax + Vmin} }$$




Different modulating value of AM



Disadvantage of over modulated signals

1. For full-carrier AM, m must be in the range from 0 to 1. 

2. Over modulation creates distortion in the demodulated signal and may result in the signal occupying a larger bandwidth than normal.

3. Again, with increasing frequency, amplitude decrease. Since spectrum space is tightly controlled by law, over modulation of an AM transmitter is actually illegal, and means must be provided to prevent it. As result we won’t get correct signal in the receiver.