14. Modulation

Modulation is the process of shifting a frequency signal to a higher frequency band suitable for transmission. The modulating (original) signal is modulated onto the carrier signal. The following diagram is an illustration of the basic modulation process. Note that the actual process may vary based on the modulation technique used.

Basic diagram workflow for Amplitude Modulation

The original signal is recovered at the receiving end through the demodulation process. The demodulation technique often mirrors the modulation technique in reverse; if the modulation technique involves mapping various voltage values to its corresponding frequency values (e.g in the case of frequency modulation where 2V could be represented by 5kHz and 3V to be presented by 10kHz), then the demodulation process will involve deriving the values of the original signal based on the different frequency values of the signal. (10 kHz -> 3V and 5 kHz -> 2V).

a. Modulation techniques

While there are various techniques for modulation, these can be largely classified into two categories:

• Analog Modulation
• Digital Modulation

Digital modulation techniques are employed to transmit digital data across an analog channel. Aside from shifting the signal frequency band to a suitable transmission band, it also converts the digital signal to a suitable analog signal that is to be decoded at the receiving end.

Analog Modulation Techniques

Types of Modulation

In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques are:

Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

Frequency modulation (FM) – the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.

Phase modulation (PM) – the phase of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.

Comparison between the modulation techniques

	Advantages	Disadvantages
Amplitude Modulation	1) Simple and cheap to implement One advantage of adopting AM systems is that AM Systems is relatively simple to construct – fewer components are needed to actually build such a system. As such, it is actually cheaper to implement compared to FM or PM. 2) Less bandwidth needed As only one frequency for transmission is used for AM, the bandwidth being used is considerably less than FM. This translates into cheaper costs for AM.	1) High power requirement. As the extraction of the AM signal on the receiving side is dependent on the amplitude level of the receiving signal, it is important to amplify the voltage signal so that the effects of attenuation (a phenomenon where a signal gets smaller with distance) will be minimized. It is important to keep the signal at a significantly high voltage level compared to the noise(interfering signals) so that the noise can be negligible compared to the signal (i.e adding 1V noise to a 20V signal would be less pronounced than adding 1V to a 20V signal). 2) Poor noise immunity As the demodulation process is based on processing the amplitude level of the receiving signal to retrieve the original signal, AM is very susceptible to noise. Therefore, it is not very ideal for it to be transmitted over large distances or components with a known high noise factor. 3) Limited bandwidth range AM cannot be deployed at high frequencies. This is a huge limitation as the antenna for such systems has to be longer due to longer wavelength needed.
Frequency Modulation	1) Less susceptible to noise (See note on Poor Noise Immunity under Disadvantages of AM). With FM, the FM modulator converts the voltage variation in the modulating signal to a frequency variation. At the receiver, this frequency variation is converted back to a voltage variation. If the frequency variation is large, then the output voltage will also be large. The demodulation process being not dependent on amplitude level on signal essentially means that noise will not have so much of a large effect on FM systems. 2) No need to transmit at high power Since the demodulated output level is not dependent on the received FM level, there is no need to ensure a high level FM signal at the receiver. Hence, transmission power can be lower than the AM to cover the same area. Together with the advantage of constant power, this makes FM suitable for battery operated transmitted like cordless phones, wireless microphone and Walkie-Talkie. 3) Able to transmit at high frequencies FM is known for its application in VHF and UHF systems, and being able to transmit at high frequency lessens the need for a long antenna, as required length of antenna is directly correlated to the wavelength of the signal.	1) More bandwidth needed With FM, the carrier frequency is varied in response to the voltage level of the original signal. Therefore, a larger range of frequencies is actually sent across the transmission channel. This translates into higher costs. 2) Complexity of circuit Designing a FM system is considerably more complex compared to AM systems.
Phase Modulation	1) Not very susceptible to noise distortion As the modulation process is based on reading the different phases of the signals, it is not as susceptible to noise distortion as AM, which relies completely on reading the voltage level. However, noise can still affect the accuracy of demodulation, although to a lesser extent. 2) Less bandwidth needed As PM does not incur additional bandwidth in the same way FM does, it retains the AM advantage of lesser bandwidth costs.	1) Extremely complex to design The complexity of a phase modulator is extremely expensive. This is because the circuit needs to be sensitive enough to detect phase changes which can be very minute. 2) Usually associated with high error rate As it is difficult to design a perfect system to be able to detect phase variations accurately, PM systems often run into error rate problems.

b. Digital Modulation Techniques

Digital modulation is similar to analog modulation, but rather than being able to continuously change the amplitude, frequency, or phase of the carrier, there are only discrete values of these attributes that correspond to digital codes. In data communications, these discrete values generally take on the binary values of ‘0’ and ‘1’s.

It is important to note that since electrical signals are analog, conversion from the digital values to its analog counterpart has to be done to be able to transmit it across the channel.

Types of digital modulation techniques

As can be observed from the figure above, the digital bit 1 maps to a significantly higher sinusoidal waveform while the digital bit 0 maps to a smaller sinusoidal waveform. This is analogous to its counterpart of AM, with the main difference being that this signal can only take on a finite set of values. (as opposed to a continuous analog range; in this case, only 2 values can be used).

It is important to note that the digital bit stream is not restricted to mapping one bit at a time, it is possible to design a ASK system that takes in 2 bits at a time; in which case there could be four different values being interpreted (2² = 4 values).

In this illustrated example of FSK above, the digital bit ‘1’ maps to a higher frequency waveform and ‘0’ maps to a lower frequency waveform. FSK shares the same advantages as FM: lower amount of power needed for transmission, better noise immunity and ability to transmit at higher frequencies.

Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases; each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data.

Quadrature Amplitude Modulation

QAM combines ASK and PSK techniques to increase the number of bits evaluated at a time. Quadrature amplitude modulation (QAM) requires changing the phase and amplitude of a carrier sine wave. An example of how the mapping of the values to the analog waveforms can be found in the table below:

2nd bit	1st bit	Result
0	0	2V sin wave with phase angle of 0 degrees
0	1	2V sin wave with phase angle of 180 degrees
1	0	5V sin wave with phase angle of 0 degrees
1	1	5V sin wave with phase angle of 180 degrees.

QAM is usually used with 8-bit, 16-bit, 32-bit and 64-bit requirements.

Summary of Digital Modulation advantages and disadvantages

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