Digital Modulation Techniques

Modulation is the Fundamental Process in Communication Systems, In Modulation Low-Frequency Message Signal Blends with High-Frequency Carrier Frequency to Enable Efficient Transfer of the Signal. In this article, we will be going through Digital Modulation Techniques, First we will start our Article by Defining Modulation, Then we will go through the Three types of Digital Modulation techniques with Their Wave, At Last we will conclude our Article With their Application, Advantages, Disadvantages and Some FAQs.

What is Modulation?

Modulation is the process of superimposing high-frequency carrier signals with low-frequency message signals resulting in a modulated wave. In modulation, one of the three parameters of the carrier wave is changed i.e. amplitude, frequency or phase according to the amplitude of the message signal at a given time instant. It is an important technique used to increase the range of communication, increase the signal-to-noise ratio, and decrease the size of the antenna.

There are mainly two types of modulation techniques which are

1. Analog Modulation
2. Digital Modulation

Analog Modulation : In Analog Modulation, both carrier and message signals are analogue waves.

Digital Modulation : In Digital Modulation, only the carrier signal is analogue and the message signal is in digital form.

Analog waves : Analog waves are those signals which can take an infinite value between two points.

Digital signals : Digital signals are binary values that can take either logic 0 as in zero volts or logic 1 as in any other voltage. There are no in-between voltage levels in the digital signal.

Types of Signals

What are Digital Modulation Techniques ?

There are mainly three types in Analog Modulation which are Amplitude Modulation, Frequency Modulation and Phase Modulation. Here the amplitude, frequency and phase of carrier wave changes with respect to amplitude of message signal. Whereas in Digital Modulation a process called as Shift Keying is used.

How do we transmit a bit stream?

Shift Keying means that the amplitude, frequency or phase of the carrier wave is shifted between two or more discrete values rather than varying continuously like Analog Modulation. Binary data requires two discrete levels of amplitude, frequency or phase for modulation called as Binary Shift Keying. A group of bits can be clubbed together to form M-ary Shift Keying.

There are mainly three types of Digital Modulation techniques. They are :

• Amplitude Shift Keying
• Frequency Shift Keying
• Phase Shift Keying

Amplitude Shift Keying

In Amplitude Shift Keying (ASK), each symbol in the message signal gives a unique amplitude to the carrier wave. There are two types of ASK, Binary and M-ary. In Binary ASK, logic 1 is associated with certain amplitude of carrier wave e.g. 12V and logic 0 is associated with different amplitude other than 12V e.g. 0V. In M-ary ASK, a group of log₂M bits are considered together rather than 1 bit at a time and the amplitude level is associated with this group of bits.

For example, in 16-ary ASK, a group of 4 bits are considered and are given a respective amplitude. Since there are 16 possible 4 bit binary numbers (2⁴), 16 different amplitude levels are required for modulation. If all such amplitudes are created using a single carrier wave, then it is called as coherent ASK. If multiple carrier wave each with different amplitudes are used for modulation then it is called as non-coherent ASK.

Amplitude Shift Keying Waveform

Block Diagram of Amplitude Shift Keying

Given Below is the Block Diagram of Amplitude Shift Keying

Block Diagram of Amplitude Shift Keying

The analog message signal is converted to digital signal using Analog to Digital Converter. This digital signal is then passed to a multiplier which takes two inputs. A sine wave with high frequency is considered as carrier signal and is multiplied with the digital signal. When symbol present in the digital signal m(t) gets multiplied with the carrier Asin(2πft+p) it results in m(t)Asin(2πft+p).

When m(t) is high, the carrier wave is passed as it is. But when m(t) is logic 0, then the result of multiplication is 0. Hence ASK wave is generated. However this ASK wave contains abrupt changes in amplitude which causes unnecessary high bandwidth usage. Hence this signal is passed through Band Pass Filter which limits the bandwidth usage.

For demodulating, the ASK wave is passed through a multiplier again where the carrier wave is multiplied again which results in m(t)Asin²(2πft+p). This signal is passed through Low Pass Filter where the original digital message is received. This digital signal is converted to analog wave using Digital to Analog Converter.

Frequency Shift Keying

In Frequency Shift Keying (FSK), each symbol in the message signal gives a unique frequency to the carrier wave. There are two types of FSK, Binary and M-ary. In Binary FSK, logic 1 is associated with certain frequency of carrier wave e.g. 50MHz and logic 0 is associated with different frequency other than 50MHz e.g. 25MHz. In M-ary FSK, a group of log₂M bits are considered together rather than 1 bit at a time and the frequency is associated with this group of bits.

For example, in 16-ary FSK, a group of 4 bits are considered and are given a respective frequency. Since there are 16 possible 4 bit binary numbers (2⁴), 16 different frequencies are required for modulation. If all such frequencies are created using a single carrier wave, then it is called as coherent FSK. If multiple carrier wave each with different frequencies are used for modulation then it is called as non-coherent FSK.

Frequency Shift Keying Waveform

Block Diagram of Frequency Shift Keying

Given Below is the Block Diagram of Frequency Shift Keying

Block Diagram of Frequency Shift Keying

The analog message signal is converted to digital signal using Analog to Digital Converter. This digital signal is then passed to two multipliers which takes two inputs each. A sine wave with frequency f₁ is considered as carrier signal for logic 1 and a sine wave with frequency f₂ is considered as carrier signal for logic 0. These carrier waves are multiplied with the digital message signal. When logic 1 present in the digital signal gets multiplied with the carrier Asin(2πf₁t+p) it results in Asin(2πf₁t+p) only since the other multiplier gets logic 0 as input since it is passed through a NOT gate.

When logic 0 present in the digital signal gets multiplied with the carrier Asin(2πf₂t+p) it results in Asin(2πf₂t+p) only since the multiplier gets logic 1 as input since it is passed through a NOT gate. Both this signals are added to form FSK wave A[sin(2πf₁t+p)+sin(2πf₂t+p)]. However this FSK wave contains abrupt changes in frequency which causes unnecessary high bandwidth usage. Hence this signal is passed through Band Pass Filter which limits the bandwidth usage.

For demodulating, the FSK wave is passed through two multipliers again where their respective carrier waves are multiplied again. This signal is passed through two Band Pass Filters out of which the top BPF allows f₁ frequency to pass if logic is 1 and the bottom allows f₂ frequency to pass if logic is 0. The output of both BPF is compared with each other where the output of the comparator is high if output of BPF₁ is greater than output of BPF₂ and is low if output of BPF₂ is greater than output of BPF₁. Hence a digital signal is received at the output of the comparator. This digital signal is converted to analog wave using Digital to Analog Converter.

Phase Shift Keying

In Phase Shift Keying (PSK), each symbol in the message signal gives a unique phase shift to the carrier wave. There are two types of PSK, Binary and M-ary. In Binary PSK, logic 1 is associated with certain phase shift of carrier wave e.g. 90^° and logic 0 is associated with different phase shift other than 90^° e.g. 0^°. In M-ary PSK, a group of log₂M bits are considered together rather than 1 bit at a time and the phase shift is associated with this group of bits.

For example, in 16-ary PSK, a group of 4 bits are considered and are given a respective phase shift. Since there are 16 possible 4 bit binary numbers (2⁴), 16 different phase shifts are required for modulation. If all such phase shifts are created using a single carrier wave, then it is called as coherent PSK. If multiple carrier wave each with different phase shifts are used for modulation then it is called as non-coherent PSK.

Phase Shift Keying Waveform

Block Diagram of Phase Shift Keying

Given Below is the Block Diagram of Phase Shift Keying

Block Diagram of Phase Shift Keying

The analog message signal is converted to digital signal using Analog to Digital Converter. This digital signal is then passed to two multipliers which takes two inputs each. A sine wave with phase shift p₁ is considered as carrier signal for logic 1 and a sine wave with phase shift p₂ is considered as carrier signal for logic 0. These carrier waves are multiplied with the digital message signal. When logic 1 present in the digital signal gets multiplied with the carrier Asin(2πft+p₁) it results in Asin(2πft+p₁) only since the other multiplier gets logic 0 as input since it is passed through a NOT gate.

When logic 0 present in the digital signal gets multiplied with the carrier Asin(2πft+p₂) it results in Asin(2πft+p₂) only since the multiplier gets logic 1 as input since it is passed through a NOT gate. Both this signals are added to form PSK wave A[sin(2πft+p₁)+sin(2πft+p₂)]. However this PSK wave contains abrupt changes in phases which causes unnecessary high bandwidth usage. Hence this signal is passed through Band Pass Filter which limits the bandwidth usage.

For demodulating, the PSK wave is passed through two multipliers again where their respective carrier waves are multiplied again. This signal is passed through two Band Pass Filters out of which the top BPF allows the signal with phase shift p₁ to pass if logic is 1 and the bottom allows the signal with phase shift p₂ to pass if logic is 0. The output of both BPF is compared with each other where the output of the comparator is high if output of BPF1 is greater than output of BPF2 and is low if output of BPF2 is greater than output of BPF1. Hence a digital signal is received at the output of the comparator. This digital signal is converted to analog wave using Digital to Analog Converter.

M-ary Encoding

It Involves transmitting of more than two bits simultaneously on the same signal that can help to save bandwidth by efficiently utilizing the available frequency spectrum.

• M-ary ASK (Amplitude Shift Keying):It is also known as M-ASK or M-FSK,In this multiple amplitude levels are used to represent several different combinations of bits.Each amplitude level represents a specific pattern of bits which allows transmission of multiple bits in each signaling interval.
• M-ary FSK (Frequency Shift Keying):In this the carrier frequency is shifted to represent different symbols.It is Similar to M-ary ASK as it allows transmission of multiple bits per symbol by using multiple frequency shifts.
• M-ary PSK (Phase Shift Keying):It involves shifting the phase of the carrier wave to represent different symbols.By changing the phase of the carrier wave multiple symbols can be represented each corresponding to a unique bit pattern.

Applications of Digital Modulation Techniques

• Military Communication Systems, where security and accuracy of the signal plays a crucial role. Digital Modulation can provide confidential and error free communication.
• Mobile Communication Systems, where the number of users are daily increasing and Digital Modulation can provide high capacity and less interference. Long distance communication can be easily done with the help of Digital Modulation.
• Digital Broadcasting, Digital modulation techniques are used in digital broadcasting standards such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and Terrestrial Digital Multimedia Broadcasting (T-DMB) for transmitting audio, video, and data content efficiently.
• Radar Systems, In radar systems, digital modulation techniques like phase modulation (PM) and frequency modulation (FM) are used for transmitting radar pulses and modulating radar signals to detect and track objects accurately in various applications such as air traffic control, weather monitoring, and military surveillance.

Advantages of Digital Modulation Techniques

• High Immunity to noise: Since the modulated wave consist of finite collection of amplitude/frequency or phase shifts, changes occurred in above parameters due to noise, distortion and dispersion is less as compared to difference in amplitude/frequency or phase shift between two distinct symbols. For e.g. An Amplitude Shift Keyed wave will have higher noise tolerance since the interference cannot bring 0V i.e. logic 0 to 12V which is representing logic 1. Hence the demodulated wave at the receiver highly represents the input message signal applied at the transmitter.
• High Security: Since the message signal is digital in Digital Modulation, encryption techniques can be employed to improve authenticity, confidentiality and integrity of data. After encrypting the digital message we can proceed with modulation process for transmitting the message. Such features are not available in Analog Modulation as message signal is of analog type.
• Efficient Usage of Bandwidth: By using compression techniques we can reduce the number of bits of the message signal without affecting the data content in it. Hence if we compress the message signal and modulate it then it will take less bandwidth as compared to modulating the original message signal.
• High Accuracy of Data: With the help of error detection and error correction techniques we can find the presence of errors in the demodulated wave at the receiver side and correct it accordingly to get the original message signal.
• High Capacity: We can use Time Division Multiple Access (TDMA) instead of Frequency Division Multiple Access (FDMA) in case of Digital Modulation as frequency or spectrum is limited and it is not possible to assign certain frequency per user as number of users is quite high compared to available channels. Hence capacity can be increased by letting the users access the channel for certain time period one after the other.

Disadvantages of Digital Modulation Techniques

• Additional Circuitry: Analog to Digital Converter (ADC) must be connected before modulating the signal to convert analog message signal to digital signal at the transmitter side and Digital to Analog Converter (DAC) must be connected after demodulating the signal to convert digital signal back to analog wave. This can increase the cost of communication system.
• Synchronization: For proper detection and demodulation of received signal synchronization is required. This requires oscillator circuitry which must be present at both transmitter and receiver.
• High Power Consumption: While Shift Keying Modulation may consume less power than Analog Modulation, it still consumes more power as compared to Pulse Modulation techniques such as PAM, PWM and PPM where the message signal is analog but the carrier is rectangular wave.

Conclusion

Digital Modulation techniques offer numerous advantages such as high immunity to noise, enhanced security through encryption, efficient bandwidth utilization, and high data accuracy. Despite requiring additional circuitry and power consumption, the benefits outweigh the drawbacks, making Digital Modulation important in modern communication systems. Its applications is used in military communication, mobile networks, broadcasting, radar systems, and more, contributing to the advancement and reliability of global communication networks.

Digital Modulation Techniques – FAQs

Can we combine two or more modulation techniques?

It is possible to combine ASK, FSK and PSK. Combining different modulation techniques increases the number of symbols available which in turn increases the bit rate, because increasing the number of symbols increases the number of bits per symbol.

Can message signal be analog signal and carrier wave digital in Digital Modulation?

Yes, one type of Digital Modulation is Pulse Modulation where the message signal is analog and the carrier is digital whose amplitude, width and position is changed with respect to amplitude of message signal.

Can we use Frequency Division Multiple Access (FDMA) in Digital Modulation?

FDMA can be used with both Analog and Digital Modulation techniques. However FDMA is predominantly used with Analog Modulation since it can support TDMA and out of FDMA and TDMA, TDMA is commercially viable and has high spectral efficiency. Hence TDMA is used in Digital Modulation.