Modulation and Quadrature Phase Shift

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  • Topic: Modulation, Phase-shift keying, Data transmission
  • Pages : 6 (948 words )
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  • Published : May 9, 2013
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Aim : To design an optical data communication network which uses Offset Quadrature Phase Shift Keying modulation technique.

oftSware used : Optisystem

Components used:

1) Psuedo Random Bit Sequence Generator

2) RZ Pulse Generator

3) Fork 1x2

4) Oscilloscope Visualiser

5) RF Spectrum Analyser

6) OQPSK Modulator

7) Quadrature De-modulator

8) M-ary Threshold Detector

9) Eye Diagram Analyzer

10) PSK Sequence Decoder

Components Description and their Specifications:

PRBS Generator :

In this project, the data is in the form of digital data i.e 0’s & 1’s. Hence in order to generate a digital sequence we do need a data generating source, hence we use a digital data source generating a random binary sequence known as pseudo random binary sequence generator. The bit sequence is designed to approximate the characteristics of random data.

Bit rate = 30375000 bits/sec

Order = 8

RZ Pulse Generator : Generates a Return to Zero (RZ) coded signal. It uses RZ line coding technique to generate electrical signal using binary pulses as it’s input.

Duty cycle is 0.5 bits. Rise time and Fall time both are equal to 0.05 bits.

Sample rate = 1.94 GHz

Fork 1x2 : Here in this project , the bit sequence is to be given to RZ pulse generator and OQPSK modulator. Fork 1x2 copies the input signal into two output signals. This tool allows us to duplicate component output ports.

Oscilloscope Visualizer: This visualizer allows us to calculate and display electrical signals in the time domain. It can display the signal amplitude and autocorrelation.

RF Spectrum Analyser : This visualizer allows us to calculate and display electrical signals in the frequency domain. It can display the signal intensity, power spectral density and phase. Power is measured in dBm and Resolution Bandwidth is 10 MHz.

OQPSK Modulator : Encodes and modulates a binary signal to an electrical signal using offset quadrature phase shift keying modulation (OQPSK)

Operating Frequency = 550 MHz

Amplitude = 1

Phase Offset = 45 deg

Sample rate = 1.94GHz

Quadrature De-Modulator : This is used for demodulating the quadrature(I and Q) components of the electrical signals.This component implements an analog demodulator using a carrier generator for Q and I quadrature components. It comprises of a low pass filter of cosine roll-off type.

Operating Frequency = 550 MHz

LPF cut-off freq. = 15 MHz

Roll-off factor = 0.2

M-ary Threshold Detector : It decodes the multi-level pulses of electrical signal to M-ary signal output. It compares the electrical signal at a user-defined decision instant with the threshold levels. This comparison generates an index which is used to generate the output amplitude.

Reference Bit Rate = 1.51875e+007 (bit rate/2)

Delay Compensation = 0 sec

Threshold Amplitudes = -0.5 and 0.5

Decision instants = 0.75 bits.

Eye Diagram Analyser :

In telecommunication, an eye diagram, is an oscilloscope display in which a digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. This analyser can calculate different metrics from the eye diagram, such as Q factor, eye opening, eye closure, extinction ratio, eye height, mask violation ratio, etc.

PSK Sequence Decoder : It generates the binary sequence according to the phase variations in the M-ary (I and Q) signals.

Bits per symbol =2

Phase offset = 45 deg

Problem statement :

Offset QPSK avoids non-linear refraction effects, reduces the intensity variations, limits the penalty due to non-linear phase modulation in the channel results in...
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