25-08-2012, 02:33 PM
Digital Modulation
Bandpass modulation.docx (Size: 289.75 KB / Downloads: 27)
Bandpass modulation and demodulation
Digital communication systems have one of the most exciting areas within electrical engineering because of the ever growing demand for data communication, and because digital transmission offers data processing options not available with analog transmission. The principal feature of a digital communication system is that during a finite interval of time, it sends a waveform from a finite set of possible waveforms, in contrast to an analog communication system, which sends a waveform from an infinite variety of waveform shapes with theoretically infinite resolution. In a digital communication system, the objective at the receiver is not to reproduce a transmitted waveform with precision; instead, the objective is to determine from a noise-perturbed signal which waveform from the finite set of waveforms was not sent by the transmitter.
Digital modulation is the process by which digital symbols are transformed into waveforms that are compatible with the characteristics of the channel. In the case of bandpass modulation the shaped pulses modulate a sinusoid called a carrier wave. Bandpass modulation provides important benefits in signal transmission. Some of the needs for bandpass modulations are:- ease of radiation, multiplexing, overcome equipment limitations (ease with which the filters can be built depend on the signal location in the frequency domain), frequency assignment, reduce noise and interference.
Quadrature phase-shift keying
QPSK is a method for transmitting digital information across an analog channel in which both cosine and sine carrier wave are varied in phase, keeping amplitude and frequency constant. In this modulation technique, two bits are transmitted in a single modulation symbol, resulting in four different symbols. The phase of the carrier takes one of the four possible values, such as0,π/(2,),π,3π/2 where each phase corresponds to a unique symbol.
The block diagram of a typical QPSK transmitter is shown in the figure below. The input bit stream db(t) is split into two bit streams, the in-phase and qudrature streams. These two bit strams are then separately modulated by two carriers, which are in phase quadrature. Each modulated signal is a BPSK signal and is summed to produce a QPSK. QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal.
Bandpass modulation.docx (Size: 289.75 KB / Downloads: 27)
Bandpass modulation and demodulation
Digital communication systems have one of the most exciting areas within electrical engineering because of the ever growing demand for data communication, and because digital transmission offers data processing options not available with analog transmission. The principal feature of a digital communication system is that during a finite interval of time, it sends a waveform from a finite set of possible waveforms, in contrast to an analog communication system, which sends a waveform from an infinite variety of waveform shapes with theoretically infinite resolution. In a digital communication system, the objective at the receiver is not to reproduce a transmitted waveform with precision; instead, the objective is to determine from a noise-perturbed signal which waveform from the finite set of waveforms was not sent by the transmitter.
Digital modulation is the process by which digital symbols are transformed into waveforms that are compatible with the characteristics of the channel. In the case of bandpass modulation the shaped pulses modulate a sinusoid called a carrier wave. Bandpass modulation provides important benefits in signal transmission. Some of the needs for bandpass modulations are:- ease of radiation, multiplexing, overcome equipment limitations (ease with which the filters can be built depend on the signal location in the frequency domain), frequency assignment, reduce noise and interference.
Quadrature phase-shift keying
QPSK is a method for transmitting digital information across an analog channel in which both cosine and sine carrier wave are varied in phase, keeping amplitude and frequency constant. In this modulation technique, two bits are transmitted in a single modulation symbol, resulting in four different symbols. The phase of the carrier takes one of the four possible values, such as0,π/(2,),π,3π/2 where each phase corresponds to a unique symbol.
The block diagram of a typical QPSK transmitter is shown in the figure below. The input bit stream db(t) is split into two bit streams, the in-phase and qudrature streams. These two bit strams are then separately modulated by two carriers, which are in phase quadrature. Each modulated signal is a BPSK signal and is summed to produce a QPSK. QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal.