Binary Phase Shift Keying BPSK Modulation Demodulation Computer Science Essay

Binary Phase Shift Keying BPSK Modulation Demodulation Computer Science Essay This experiment is based on the Binary Phase Shift Keying (BPSK) modulation/demodulation technique. The aim of the experiment is to gain familiarity with the components of a simple data transmission system, gain experience using an experimental communication system and studying its performance under the influence of white noise and also, to compare experimental results with theoretical deductions. Bandpass modulation, of which BPSK is a type, is a process whereby, a sinusoid usually called a carrier wave, is modulated or have its characteristics changed by a digital pulse baseband signal in other to enable wireless based transmission. In BPSK modulation, the phase of the carrier waveform is shifted to either 0Â ° or 180Â ° by the modulating data signal. To effectively model the transmission channel, the AWGN generator is used which adds the effect of noise to the signal at the receiver in other to properly characterise what obtains in real systems. SNR measurements are taken after the noise is added before the receiver and results of each stage of the experiment are presented. 2.0 RESULTS AND DISCUSSION The results obtained from the experiment and brief discussions are now presented. 2.1 The frequency of the waveform was measured to be 1.493kHz 2.2 The amplitude of the waveform was measured to be 3.608V 2.3 C:Documents and SettingsAGEBNIGADesktopLAB RESULTSPart 2.bmp Fig. 1: Square Waveform from NE555 timer circuit. The timer circuit produces a sequence of ones and zeros which together with the resistors and capacitor, produces a square waveform. It can be observed that the square top and bottom are not perfectly straight but with ripples, this is due to the resonance effect presented by the capacitor. Also, the rising pattern of the top is due to the voltage rise time in the capacitor. 2.4 The frequency of the message sequence is measured to be 374Hz 2.5 C:Documents and SettingsAGEBNIGALocal SettingsTemporary Internet FilesContent.WordPart 4 5.bmp Fig. 2: M essage sequence at the output of the frequency divider. The SN74LS74 integrated circuit implements a second order frequency divider, 2n (n=2). Hence the frequency of the timer circuit is divided by four. Hence, this is also evident in the frequency of the message sequence in 2.4 above. 2.6 The cut-off frequency of the 2nd order Butterworth low pass filter is given by; The cut-off frequency is the frequency at which the magnitude of the transfer function drops to 0.7071 of its maximum value which represents the point at which the power in the circuit is 3dB less than the maximum value. 2.7 The frequency of the sinusoid at the output of the filter was measured to be 1.328kHz. 2.8 C:Documents and SettingsAGEBNIGALocal SettingsTemporary Internet FilesContent.Wordpart 8.jpg Fig. 3: Output of the first and second Butterworth LPF. A B A – Output of first filter; B – Output of second filter The Butterworth lowpass filter is used to generate the sinusoidal carrier required for the baseband signal. The Butterworth filter has a gentle roll-off, has no ripple in the pass or stop band hence, it has a monotonic response. To maximise the smoothness of the sinusoid, we use two of such filters in series.