DIGITAL MODULATION TECHNIQUES

Fundamental to all wireless communications is modulation, the process of impressing the data to be transmitted on the radio carrier. Most wireless transmissions today are digital, and with the limited spectrum available, the type of modulation is more critical than it has ever been.

The main goal of todays era is to squeeze modulation data as much  into the least amount of spectrum possible. That objective, known as spectral efficiency, measures how quickly data can be transmitted in an assigned bandwidth. The unit of measurement is bits per second per Hz (b/s/Hz). Multiple techniques have emerged to achieve and improve spectral efficiency.

Digital Signals: are signals with amplitudes that may take only a specific number of values.

Modulation: is changing one or more of the characteristics of a signal (known as the carrier signal) based on the value of another signal (known as the information or modulating signal) to produce a modulated signal. 

Amplitude Shift Keying (ASK) and Frequency Shift Keying (FSK)
There are three basic ways to modulate a sine wave radio carrier: modifying the amplitude, frequency, or phase. More sophisticated methods combine two or more of these variations to improve spectral efficiency. These basic modulation forms are still used today with digital signals.

Amplitude Shift Keying (ASK): Is a type of Amplitude Modulation which represents the binary data in the form of variations in the amplitude of a signal.

Frequency Shift Keying (FSK): Is the digital modulation technique in which the frequency of the carrier signal varies according to the discrete digital changes. FSK is a scheme of frequency modulation.

In this Bit 1 is transmitted by a signal of a particular Amplitude. To transmit Bit 0 we change the amplitude keeping the Frequency constant.

To provide better spectrum management and enhanced communications, modulation techniques grow ever-more complex. Yet these cutting-edge techniques are still derived from the three basic forms of digital modulation: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). For example, quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) build upon these formats to provide more efficient bandwidth usage and security

PSK matches ASK’s bandwidth efficiency while adding power efficiency. With the same signal-to-noise ratio (SNR), PSK will therefore have a lower bit error rate. With QPSK, the modulator produces a pair of sine carriers separated by 90 degrees. Each phase is modulated according to the binary data. This process results in four individual sine signals, with the final signal combining both phases. With every carrier phase equal to two bits of data, QPSK offers high spectral efficiency.

Amplitude phase shift keying (APSK) uses a smaller number of amplitude levels to surpass QAM. It combines ASK and PSK to make changes to the carrier wave’s amplitude and phase, thereby boosting the signal set. APSK boasts increased efficiency and output power.

Binary Phase Shift Keying (BPSK) And Quadrature Phase Shift Keying (QPSK)

A very popular digital modulation scheme, binary phase shift keying (BPSK), shifts the carrier sine wave 180° for each change in binary state . BPSK is coherent as the phase transitions occur at the zero crossing points. The proper demodulation of BPSK requires the signal to be compared to a sine carrier of the same phase. This involves carrier recovery and other complex circuitry. 

A simpler version is differential BPSK or DPSK, where the received bit phase is compared to the phase of the previous bit signal. BPSK is very spectrally efficient in that you can transmit at a data rate equal to the bandwidth or 1 bit/Hz.

In a popular variation of BPSK, quadrature PSK (QPSK), the modulator produces two sine carriers 90° apart. The binary data modulates each phase, producing four unique sine signals shifted by 45° from one another. The two phases are added together to produce the final signal. Each unique pair of bits generates a carrier with a different phase.

Binary Phase-Shift Keying(BPSK) :The simplest form of PSK is binary phase-shift keying (BPSK), where N = 1 and M = 2.Therefore, with BPSK, two phases (21 = 2) are possible for the carrier. One phase represents a logic 1, and the other phase represents a logic 0. As the input digital signal changes state (i.e., from a 1 to a 0 or from a 0 to a 1), the phase of the output carrier shifts between two angles that are separated by 180°. Hence, other names for BPSK are phase reversal keying (PRK) and bi phase modulation. BPSK is a form of square-wave modulation of a continuous wave (CW) signal.

QUADRATURE PHASE SHIFT KEYING (QPSK): This is the phase shift keying technique, in which the sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°. If this kind of techniques are further extended, PSK can be done by eight or sixteen values also, depending upon the requirement. The following figure represents the QPSK waveform for two bits input, which shows the modulated result for different instances of binary inputs.

Advantages of Digital Communication over Analog Communication

  Every system has its own advantages and disadvantages 

  Immunity to Noise (possibility of regenerating the original digital signal if signal power to noise power ratio (SNR) is relatively high by using of devices called repeaters along the path of transmission). 

  Efficient use of communication bandwidth (through use of techniques like compression). 

  Digital communication provides higher security (data encryption). 

  The ability to detect errors and correct them if necessary. 

  Design and manufacturing of electronics for digital communication systems is much easier and much cheaper than the design and manufacturing of electronics for analog communication systems.

DISADVANTAGES

  TDM digital transmission is not compatible with FDM.

  A digital system requires large bandwidth.

REFRENCES :

[1]. https://www.srecwarangal.ac.in/ece-downloads/DC%20Notes.pdf

[2]. https://www.google.com/imgres?imgurl=https%3A%2F%2Fpbs.twimg.com%2Fmedia%2FESRBF3mU0AAtobm.png&imgrefurl=https%3A%2F%2Ftwitter.com%2FEEWorld_Aimee%2Fmedia&tbnid=NQf_5xuQIm_yHM&vet=12ahUKEwjVndL6_5DuAhWHsUsFHbYAB0IQMygAegQIARAZ..i&docid=-rJf5tPd5NyTVM&w=659&h=601&itg=1&q=digital%20modulation&ved=2ahUKEwjVndL6_5DuAhWHsUsFHbYAB0IQMygAegQIARAZ

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[4]. https://www.electronicdesign.com/technologies/communications/article/21798737/understanding-modern-digital-modulation-techniques