Design Implementation and Performance Analysis of M-QAM and PAM-n Signalling Schemes

Abstract

Two circuits have been simulated in the Optisystem 20 software. One circuit has M-QAM and the other has PAM-n. Both the circuits have been compared on the basis of Bits per symbol, signal power, Noise power, etc. The total power and noise power have been calculated in the electrical power meter visualizer. In both the circuits, PRBS generator is used to generate the sequence of the bits. In M-QAM, return to zero pulse generator is used and in PAM-n not return to zero pulse generator is used. The full width pulse is also known as NRZ (not return to zero) and half width pulse is also known as RZ (return to zero). The main objective of the simulation is to get perfect Eye diagram in the Eye Diagram Anaylzer. The parametric comparison of different types of QAM have also be done on the basis of number of samples, sequence length, samples per bit and output power in dBm and comparison is done by state of art. In the final result, it will be concluded that M-QAM performs much better than the PAM-n as it is capable of transferring higher data rates. The different types of M-QAM and PAM-n eye diagrams have been plotted like 16-QAM, 64-QAM and PAM-3.

Introduction

I. INTRODUCTION

The M-QAM basically means M-Ary Quadrature Amplitude modulation. This signalling scheme have the specification of both ASK (Amplitude Shift Keying) and PSK (Phase Shift Keying). These schemes are basically used to transmit binary sequence through free space. The message signal switches between 2 voltage levels correspondingly one of the parameters of carrier signal switches between 2 possible values. Therefore, these schemes are called as ‘switching or keying’. The binary values 1 and 0 are represented by  presence of carrier and  lack of the carrier respectively, in ASK also referred to as ON-OFF keying. In PSK,   binary 1 is represented with actual carrier and binary 0 is represented with 180 degrees phase shift of carrier.

PAM-n basically means Pulse Amplitude Modulation. It is     one of the 3 basic types of pulse modulation in which the    pulse amplitude varies with the modulating message signal.

This modulation scheme is used in digital transmission. It is suitable for Time Division Multiplexing. It has many applications like it is used in television, Ethernet, etc.

The bandwidth for M-QAM= 2Rb/log2M (For full width rectangular pulses only).

If sinc pulses are used for M-QAM, then bandwidth = Rb/log2M.

II. OBJECTIVES

1. Design of M-QAM in the Optisystem 20 software.
2. Design of PAM-n in the Optisystem 20 software.
3. Implementation and performance analysis of M-QAM and PAM-n.
4. Comparison is done by state of art.

III. MATERIAL AND METHODS

The M-QAM has bit rate of 10e+006 bit/s with 64 samples/ bit and sequence length of 2048 bits. In the M-QAM circuit, the PRBS generator is connected with the Fork 1*2. The Fork divides the signal into two parts. One part is connected with QAM sequence generator which generates the sequence of the QAM pulses and the other part is connected with the RZ pulse generator which produces return to zero pulses for its digital transmission. Further, QAM sequence generator is connected with 2 M-Ary pulse generator which has a gain of 1. Both are connected with Electrical Constellation Visualizer which shows the constellation diagram of various M-QAM.  Further the signal gets modulated at Quadrature modulator which is having 550MHz frequency and then the signal is demodulated at the Quadrature demodulator which is having cut of frequency of 10MHz.

With the demodulator the signal goes into two different eye diagram analyzers and the eye diagram of the circuit can be analyzed. Then, the signal goes straight into the M-ary threshold detector in which there are Threshold amplitudes and the output amplitudes with the delay compensation of 0 secs. Then, the signal moves to the QAM sequence decoder which is responsible for decoding the bits per symbol. Different bits per symbol are used to obtain the graph of M-QAM like for M=16, the bits/symbol is 4 and for M=64, the bits/symbol is 6, etc. The signal goes further to the RZ pulse generator and finally to the oscilloscope Visualizer. The different oscilloscope Visualizer are connected with different parts of the devices in the circuit to see graph between amplitude and time.

Now, in the PAM-n circuit, there are two PRBS generators which are joined with NRZ pulse generator. The NRZ pulse generator is further connected with directly modulated laser measured which is having 850nm frequency and power 0dBm. The bit rate is 25e+009 bits/s. The samples/bit is 64 and sequence length is of 128 bits. Then, the signal goes to the power combiner 2*1 where it gets combine and moves seamlessly to the optical Fiber CWDM with attenuation of 0.5 dB/km and 1 km length. The optical time domain Visualizers and optical spectrum analyzers are connected with the optical Fiber CWDM. Further, signal goes to the optical attenuator where attenuation gets 0db. Then, the signal moves to the PIN photodiode where optical signal gets converted into an electrical signal. Then, the signal moves to the low pass Bessel filter having c.o.f  = 0.75*symbol rate Hz. The Electrical power meter visualizer is connected with the Bessel filter. The band limited signal moves to the 3R regenerator where signal strength gets amplified and finally moves to the Eye Diagram Analyzer.

Conclusion

M-QAM performs much better than the PAM-n as QAM constitute more bits/ carrier. For example- 64-QAM maps 6 bits/carrier,16-QAM maps 4bits/carrier and 8-QAM maps 2 bits/carrier. The 4-QAM is also called QPSK i.e Quadrature Phase Shift Keying. As we increase the M- QAM numerology, it depicts the ability to have more data with the number of symbols being the same. However, the variation of noise in the PAM-n system is less which means probability of error is also less. In the further research work, there will be proper comparison of M-Ary PSK and M-Ary QAM. For M value greater than 8, M-Ary QAM is better than M-ARY PSK as probability of error will be very high in M-ARY PSK and for M value less than 8 M-Ary PSK is better.

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Copyright

Copyright © 2023 Vishwanath Wadhwa, Dr. Mansi Jhamb. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.