29-08-2011, 10:17 AM
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1.0 Digital Modulation
Firstly, what do we mean by digital modulation? Typically the objective of a digital communication system is to transport digital data between two or more nodes.In radio
Communications this is usually achieved by adjusting a physical characteristic of a sinusoidal carrier, either the frequency, phase, amplitude or a combination thereof. This is performed in real systems with a modulator at the transmitting end to impose the physical change to the carrier and a demodulator at the receiving end to detect the resultant modulation on reception.
1.1 Block diagram of Digital Communication System
Figure 1.1 is the block diagram of a typical digital communication system. The message to be sent may be from an analog source (e.g., voice) or from a digital source (e.g., computer data). The analog-to-digital (AID) converter samples and quantizes the analog signal and represents the samples in digital form (bit 1 or 0). The source encoder accepts the digital signal and encodes it into a shorter digital signal. This is called source encoding, which reduces the redundancy hence the transmission speed. This in turn reduces the bandwidth requirement of the system. The channel encoder accepts the output digital signal of the source encoder and encodes it into a longer digital signal. Redundancy is deliberately added into the coded digital signal so that some of the errors caused by the noise or interference during transmission through the channel can be corrected at the receiver. Most often the transmission is in a high frequency passband, the modulator thus impresses the encoded digital symbols onto a carrier. Sometimes the transmission is in baseband, the modulator is a baseband modulator, also called formatter, which formats the encoded digital symbols into a waveform suitable for transmission. Usually there is a power amplifier following the modulator. For high-frequency transmission, modulation and demodulation are usually performed in the intermediate frequency (IF). If this is the case, a frequency up-converter is inserted between the modulator and the power amplifier. If the IF is too low compared with the carrier frequency, several stages of carrier frequency conversions are needed. For wireless systems an antenna is the final stage of the transmitter. The transmission medium is usually called the channel, where noise adds to the signal and fading and attenuation effects appear as a complex multiplicative factor on the signal. The term noise here is a wide-sense term which includes all kinds of random electrical disturbance from outside or from within the system. The channel alsousually has a limited frequency bandwidth so that it can be viewed as a filter. In the receiver, virtually the reverse signal processing happens. First the received weak signal is amplified (and down-converted if needed) and demodulated. Then the added redundancy is taken away by the channel decoder and the source decoder recovers the signal to its original form before being sent to the user. A digital-to analog(D/A) converter is needed for analog signals. The block diagram in Figure 1 is just a typical system configuration. A real system configuration could be more complicated. For a multiuser system, a multiplexing stage is inserted before modulator. For a multi-station system, a multiple access control stage is inserted before the transmitter. Other features like frequency spread and encryption can also be added into the system. A real system could be simpler too. Source coding and channel coding may not be needed in a simple system. In fact, only the modulator, channel, demodulator, and amplifiers are essential in all communication systems (with antennas for wireless systems). For the purpose of describing modulation and demodulation techniques and
Figure1.1: Block diagram of digital communication system
-analyzing their performance, the simplified system model shown in Figure 1.2 will be often used. This model excludes irrelevant blocks with regard to modulation so that relevant blocks stand out. However, recently developed modem techniques combine modulation and channel coding together. In these cases the channel encoder is part of the modulator and the channel decoder is part of the demodulator. From Figure 1.2, the received signal at the input of the demodulator can be expressed as
Where * denotes convolution. In Figure 1.2 the channel is described by three elements. The first is the channel filter. Because of the fact that the signal s (t) from the modulator must pass the transmitter, the channel (transmission medium) and the receiver before it can reach the demodulator, the channel filter therefore is a composite filter whose transfer function is
Figure1.2: Digital communication system model for modulation and demodulation
where HT( f ), Hc( f ), and HR( f) are the transfer function of the transmitter, the channel, and the receiver, respectively. Equivalently, the impulse response of the
Channel filter is
Where hT(t), hc(t), and hR(t) are the impulse responses of the transmitter, the channel, and the receiver, respectively. The second element is the factor A (t) which is generally complex. This factor represents fading in some types of channels, such as mobile radio channel. The third element is the additive noise and interference term n (t). We will discuss fading and noise in more detail in the next section. The channel model in Figure 1.2 is a general model. It may be simplified in some circumstances, as we will see in the next section.
1.2 Basic Modulation Methods
For long distance and wireless transmissions, bandpass modulation is usually used. Bandpass modulation is also called carrier modulation. A sequence of digital symbols is used to alter the parameters of a high-frequency sinusoidal signal called carrier. It is well known that a sinusoidal signal has three parameters: amplitude, frequency, and phase. Thus amplitude modulation, frequency modulation, and phase modulation are the three basic modulation methods in passband modulation. Figure 1.4 shows three basic binary carrier modulations. They are amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying(PSK). In ASK, the modulator puts out a burst of carrier for every symbol 1, and no signal for every symbol 0. This scheme is also called on-off keying (OOK). In a general ASK scheme, the amplitude for symbol 0 is not necessarily 0. In FSK, for symbol I a higher frequency burst is transmitted and for symbol 0 a lower frequency burst is
Figure1.3: Three basic bandpass modulation schemes
transmitted, or vice versa. In PSK, a symbol I is transmitted as a burst of carrier with 0 initial phase while a symbol 0 is transmitted as a burst of carrier with 180' initial phase. Based on these three basic schemes, a variety of modulation schemes can be derived from their combinations. For example, by combining two binary PSK (BPSK) signals with orthogonal carriers a new scheme called Quadrature phase shift keying (QPSK) can be generated. By modulating both amplitude and phase of the carrier, we can obtain a scheme called quadrature amplitude modulation (QAM), etc.
