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Modulation of Signals International Journal of Research (IJR) e-ISSN: 2348-6848, p- ISSN: 2348-795X Volume 2, Issue 4, April 2015 Available at http://internationaljournalofresearch.org Modulation of Signals Akshay dutt (16195); Anosh Justin (16199) & Gautam saini (16211) Department of ECE Dronacharya College of Engineering Khentawas, Farrukh Nagar-123506 Gurgaon, Haryana Email- [email protected] ABSTRACT This paper addresses the technique of rapidly A modem (from modulator–demodulator) can updating the weights of an adaptive filter to perform both operations. cancel non-stationary interference. This method The aim of digital modulation is to transfer is used, for example, in radar systems that a digital bit stream over an receive coherent multipath interference, known analog bandpass channel, for example over as "hot clutter" or "terrain scattered jamming". the public switched telephone network (where We develop a framework for understanding how a bandpass filter limits the frequency range to this rapid weight updating (inadvertently) 300–3400 Hz), or over a limited radio frequency modulates signals. Two principal types band. of modulation are studied: random modulations induced by finite-length The aim of analog modulation is to transfer training sets and non- an analog baseband (or lowpass) signal, for randommodulations induced by the coherence of example an audio signal or TV signal, over an the interference. Our framework is then used to analog bandpass channel at a different frequency, assess the impact of these modulations on the for example over a limited radio frequency band overall performance of various well-known or a cable TV network channel. jammer multipath mitigation algorithms. Several Analog and digital modulation techniques for controlling this modulation are facilitate frequency division multiplexing (FDM), proposed. where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband KEYWORDS channels (several different carrier frequencies). modulation; signal modulation; pulse modulation; bandwidth; carrier signal The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a baseband INTRODUCTION channel, typically a non-filtered copper wire such In electronics and telecommunications, modulati as a serial bus or a wired local area network. on is the process of varying one or more The aim of pulse modulation methods is to properties of a periodic waveform, called transfer a narrowband analog signal, for example the carrier signal, with a modulating signal that a phone call over a wideband baseband channel typically contains information to be transmitted. or, in some of the schemes, as a bit stream over another digital transmission system. In telecommunications, modulation is the process of conveying a message signal, for example a In music synthesizers, modulation may be used to digital bit stream or an analog audio signal, inside synthesise waveforms with an extensive overtone another signal that can be physically transmitted. spectrum using a small number of oscillators. In Modulation of a sine waveform transforms this case the carrier frequency is typically in the a baseband message signal into a passband signal. same order or much lower than the modulating waveform. See for example frequency A modulator is a device that performs modulation synthesis or ring modulation modulation. synthesis A demodulator (sometimes detector or demod) is a device that performsdemodulation, the inverse of modulation. Available online:http://internationaljournalofresearch.org/ P a g e | 1038 International Journal of Research (IJR) e-ISSN: 2348-6848, p- ISSN: 2348-795X Volume 2, Issue 4, April 2015 Available at http://internationaljournalofresearch.org carrier signal are chosen from a finite number of ANALOG MODULATION M alternative symbols (themodulation alphabet). METHODS 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) Schematic of 4 baud (8 bit/s) data link containing Double-sideband modulation (DSB) arbitrarily chosen values. Double-sideband modulation with A simple example: A telephone line is designed carrier (DSB-WC) (used on the AM for transferring audible sounds, for example radio broadcasting band) tones, and not digital bits (zeros and ones). Double-sideband suppressed-carrier Computers may however communicate over a transmission (DSB-SC) telephone line by means of modems, which are Double-sideband reduced carrier representing the digital bits by tones, called transmission (DSB-RC) symbols. If there are four alternative symbols Single-sideband modulation (SSB, or (corresponding to a musical instrument that can SSB-AM) generate four different tones, one at a time), the SSB with carrier (SSB-WC) first symbol may represent the bit sequence 00, SSB suppressed carrier modulation the second 01, the third 10 and the fourth 11. If (SSB-SC) the modem plays a melody consisting of 1000 Vestigial sideband modulation (VSB, or tones per second, the symbol rate is 1000 VSB-AM) symbols/second, or baud. Since each tone (i.e., Quadrature amplitude symbol) represents a message consisting of two modulation (QAM) digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is Angle modulation, which is similar to the technique used by dialup modems approximately constant envelope as opposed to DSL modems. Frequency modulation (FM) (here the According to one definition of digital signal, the frequency of the carrier signal is varied modulated signal is a digital signal, and in accordance to the instantaneous according to another definition, the modulation is amplitude of the modulating signal) a form of digital-to-analog conversion. Most Phase modulation (PM) (here the phase textbooks would consider digital modulation shift of the carrier signal is varied in schemes as a form of digital transmission, accordance with the instantaneous synonymous to data transmission; very few amplitude of the modulating signal) would consider it as analog transmission. DIGITAL MODULATION FUNDAMENTAL METHODS DIGITAL MODULATION In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital METHODS modulation methods can be considered as digital- The most fundamental digital modulation to-analog conversion, and the techniques are based on keying: corresponding demodulation or detection as analog-to-digital conversion. The changes in the PSK (phase-shift keying): a finite number of phases are used. Available online:http://internationaljournalofresearch.org/ P a g e | 1039 International Journal of Research (IJR) e-ISSN: 2348-6848, p- ISSN: 2348-795X Volume 2, Issue 4, April 2015 Available at http://internationaljournalofresearch.org FSK (frequency-shift keying): a finite called equivalent lowpass signal or equivalent number of frequencies are used. baseband signal is a complex-valued ASK (amplitude-shift keying): a finite representation of the real-valued modulated number of amplitudes are used. physical signal (the so-called passband QAM (quadrature amplitude modulation): a signal or RF signal). finite number of at least two phases and at These are the general steps used by least two amplitudes are used. the modulator to transmit data: In QAM, an inphase signal (or I, with one example being a cosine waveform) and a 1. Group the incoming data bits into quadrature phase signal (or Q, with an example codewords, one for each symbol that will being a sine wave) are amplitude modulated with be transmitted. a finite number of amplitudes, and then summed. 2. Map the codewords to attributes, for It can be seen as a two-channel system, each example amplitudes of the I and Q channel using ASK. The resulting signal is signals (the equivalent low pass signal), equivalent to a combination of PSK and ASK. or frequency or phase values. In all of the above methods, each of these phases, 3. Adapt pulse shaping or some other frequencies or amplitudes are assigned a unique filtering to limit the bandwidth and form pattern of binary bits. Usually, each phase, the spectrum of the equivalent low pass frequency or amplitude encodes an equal number signal, typically using digital signal of bits. This number of bits comprises processing. the symbol that is represented by the particular 4. Perform digital to analog conversion phase, frequency or amplitude. (DAC) of the I and Q signals (since today all of the above is normally If the alphabet consists of alternative achieved using digital signal processing, symbols, each symbol represents a message DSP). consisting of N bits. If the symbol rate (also 5. Generate a high frequency sine carrier known as the baud rate) is symbols/second waveform, and perhaps also a cosine (or baud), the data rate is bit/second. quadrature component. Carry out the modulation, for example by multiplying For example, with an alphabet consisting of 16 the sine and cosine waveform with the I alternative symbols, each symbol represents 4 and Q signals, resulting in the equivalent bits. Thus, the data rate is four times the baud low pass signal being frequency shifted rate. to the modulated pass band signal or RF In the case of PSK, ASK or QAM, where the signal. Sometimes this is achieved using carrier frequency of the
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