Modulation Techniques
P.Lakshmikantha Reddy 200905030.(M.Tech CSE) Modulation:
modulation is the process of conveying a message signal.(OR) is a process of converting one form of signal to another form of signal. If two devices want to communicate each other then they should use same type of signals. If they are different then we have to convert signals into another form. The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal. In the field of communication engineering, the information bearing signal is also known as the modulating signal. The modulating signal is a slowly varying signal - as opposed to the rapidly varying carrier frequency. A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator. A device that can do both operations is a modem. There are two types of Modulations. 1. Digital modulation. 2. Analog modulation. Digital Modulation : In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. There are three basic types of modulation methods for transmission of digital signals. These methods are based on the three attributes of a sinusoidal signal, amplitude, frequency and phase. The corresponding modulation methods are called Amplitude Shift Keying (ASK), Frequency shift keying (FSK),and Phase Shift Keying (PSK). There are two important factors that govern the effectiveness of a digital modulation technique: power efficiency and bandwidth efficiency (also called spectral efficiency). Amplitude Shift Keying : In ASK, the amplitude of the carrier is changed in response to information and all else is kept fixed. Bit 1 is transmitted by a carrier of one particular amplitude. To transmit 0, we change the amplitude keeping the frequency constant. On-Off Keyring is a special form of ASK where one of the amplitudes is zero. The amplitude modulated carrier signal can be written as :V(t)= d sin (2PI fct) where fc is the carrier frequency and d is the data bit variable. Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over optical fiber.
. Frequency Shift Keyring: In FSK the frequency of the carrier is shifted between two discrete values, one representing binary „1‟ and the other representing binary „0‟. The carrier amplitude does not change. FSK is relatively simple to implement. It is used extensively in low speed modems having bit rates below 1200 bps. The value of the FSK signal is given by V(t) = d sin(2Pift(t) + d sin(2Pifot) where f1 and f0 are the frequencies corresponding to binary „1‟ and „0‟ respectively and d is the data signal variable. Minimum-Shift Keying (MSK) - a spectrally efficient form of coherent FSK. Audio Frequency-Shift Frequency-Shift Keying Keying (AFSK) - digital data is represented represented by changes in the frequency of an audio tone. Multi-Frequency-Shift Keying (M-ary FSK or MFSK), Dual-Tone Multi-Frequency (DTMF), Continuous-Phase Frequency-Shift Keying (CPFSK) – are some of the other forms. Phase Shift Keying : This is the most efficient of the three modulation methods and is used for high bit rates. Inn PSK, phase of the carrier is modulated to represent the binary values. The simplest form of PSK is called as BPSK. In the simplest form of phase modulation, the carrier wave is systematically shifted 45, 135, 225, or 315 degrees at uniformly spaced intervals. Each phase shift transmits 2 bits of information. ASK is also combined combined with with PSK to create hybrid hybrid systems systems such as Quadrature Quadrature Amplitude Modulation(QAM) Modulation(QAM) where both the amplitude and the phase are changed at the same time. The most popular wireless LAN standard, IEEE 802.11b, uses a variety of different PSKs depending on the data-rate required. PSK is not susceptible to the noise degradation that affects ASK, nor to the band width limitations of FSK. Smaller variations in the signal can be detected by the receiver. Instead of using two variations of a signal, each representing one bit. We can go for a four variations and each phase represents two bits. This technique is called 4-PSK or Q-PSK. The pair of bits is called as dibit. By using this technique data can be transmited two times faster than 2-PSK.
Quadrature amplitude modulation (QAM) : QAM is a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers. Information can be defined in two forms, digital or analog. Analog signal is considered continuous. Its signal amplitude can take on any number of values between the signal maximum and minimum. Voice is analog and can take any number of volume levels between its “dynamic -range”. Digital devices convert analog voice to a digital signal by process of sampling and quantization. The analog signal is first sampled and then quantized in levels and then each level is converted to a binary number.
Analog Modulation : To transfer an analog baseband (or low-pass) signal, for example an audio signal or TV signal, over an analog pass-band channel, for example a limited radio frequency band or a cable TV network channel. In analog modulation, the modulation is applied continuously in response to the analog information signal. Amplitude Amplitude of a signal is the value of the signal at any point on the wave. It is equal to the vertical distance from a given point on the wave form to the horizontal axis. It can be measured in volts, amperes or watts depending on the type of signal. Frequency refers to the number of periods in one second. Frequency of a signal is its number of cycles per second. It is expressed in hertz (Hz). Phase describes the position of the signal relative to time zero. It is measured in degrees or radians. A phase shift of 360 degrees corresponds to a shift of a complete period, a phase shift of 180 degrees corresponds to a shift of half a period, a phase shift of 90 degrees corresponds to a shift of quarter a period. Analog modulation modulation can be done in three three ways: ways: 1) Amplitude Modulation. 2) Frequency Modulation. 3) Phase Modulation. Amplitude Modulation (AM): In AM transmission, the carrier signal is modulated m odulated so that its amplitude varies with the changing amplitudes of the modulating signal. The frequency and phase of the carrier remains the same, only amplitude changes to follow variations in the information. A simple form of AM often used for digital communications communications is on-off keying, a type of amplitude-shift keying by which binary data is represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse code, referred to as continuous wave (CW) operation.
Fig: Amplitude modulation
Frequency modulation (FM): In FM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level of the modulating signal. The peak amplitude and phase of the carrier signal will remain constant. FM conveys information over a carrier wave by varying its frequency. In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis.
FM General Equation: Let the carrier be xc(t) = Xc·cos (?ct),and the modulating signal be xm(t) = ?·sin (?mt) Then x(t) = Xc·cos [?ct + ?·sin (?mt)]
Fig: Frequency modulation
Phase Modulation (PM): PM is used in some systems as an alternative to frequency modulation. In PM transmission the phase of the carrier is modulated to follow the changing voltage level of the modulating signal. PM is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave. PM is not very widely used. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180° or -180°.
Fig : Phase modulation Differences between Analog and Digital Modulation : The main difference between analog and digital modulation is that while the changes occur in a continuous manner in analog modulation, they occur at discrete time intervals in digital modulation. Analogue modulation modulation (AM, (AM, PM, FM) combines combines a higher frequency frequency sinusoidal sinusoidal carrier with with a lower frequency signal carrying the message where as Digital modulation combines a high frequency sinusoidal carrier signal and a digital data stream to create a modulated wave that assumes a limited number of states.
References: 1. Data communications and Networking – Forzen. 2. http://en.wikipedia.org/wiki .