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Chapter: Digital Communication - Digital Modulation Scheme

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Generation and Coherent Detection of BPSK Signals

To generate the BPSK signal, we build on the fact that the BPSK signal is a special case of DSB-SC modulation. Specifically, we use a product modulator consisting of two components.

GENERATION AND COHERENT DETECTION OF BPSK SIGNALS

 

(i) Generation

 

To generate the BPSK signal, we build on the fact that the BPSK signal is a special case of DSB-SC modulation. Specifically, we use a product modulator consisting of two components.

 

(i) Non-return-to-zero level encoder, whereby the input binary data sequence is encoded inpolar form with symbols 1 and 0 represented by the constant-amplitude.

 

(ii)   Product modulator, which multiplies the level encoded binary wave by the sinusoidalcarrier of amplitude to produce the BPSK signal. The timing pulses used to generate the level encoded binary wave and the sinusoidal carrier wave are usually, but not necessarily, extracted from a common master clock.

 

(ii)   Detection

 

To detect the original binary sequence of 1s and 0s, the BPSK signal at the channel output is applied to a receiver that consists of four sections

 

(a)       Product modulator, which is also supplied with a locally generated reference signal thatis a replica of the carrier wave

 

(b)       Low-pass filter, designed to remove the double-frequency components of the product modulator output (i.e., the components centered on ) and pass the zero-frequency components.

 

(c)       Sampler, which uniformly samples the output of the low-pass filter at where; the localclock governing the operation of the sampler issynchronized with the clock responsible for bit-timing in the transmitter.

 

(d)       Decision-making device, which compares the sampled value of the low-pass filtersoutput to an externally supplied threshold, every seconds. If the threshold is exceeded, the device decides in favor of symbol 1; otherwise, it decides in favor of symbol 0. levels.

 




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