M-PSK Modulation

Uptill now, we have discussed the basic concepts of digital modulation techniques.Let’s move one step ahead!

In digital modulation two commonly used terms are Symbols n Modulation order(M) Symbol:

It is actually a combination of bits.For instance, So and S1 are two symbols.Where ,So=00,S1=11 etc

Modulation order-M:

It refers to the number of states used in modulation,or the number of symbols.If M=2 it means two distinct states are being used in modulation. States could be different frequencies, amplitudes, or phase angles.If M is represented in power of ‘2’, that shows the number of bits per symbol.

For M=4

There will be 4 symbols(S0, S1, S2, S3).4=2^2.Therefore ,each symbol will be represented by Two bits.S0=00,S1=01,S2=10,S3=11

Keeping in view the ‘M’ factor, let’s restate the modulations techniques as:

M-PSK,M-FSK,M-ASK, and M-QAM

M-PSK:

For M=2: It is called as BPSK,(where B stands for Binary).Number of symbols are two. S0 and S1.As 2=2^1, so each symbol is represented by a single bit, therefore we wil have, S0=0 and S1=1

For M=4: It is called as 4-PSK.Number of symbols are Four.S0,S1,S2,and S3.As 4= 2^2,so each symbol is a combination of two bits,so we will have,S0=00,s1=01,s2=10,s4=11

So on and so forth for M=8,16,32,64

As PSK is phase modulation ,therefore the number of states actually refer to distinct Phase angles.These phase angles are used to modulate the carrier.

For M=2 , we have 2 phases/states
Total angle is 360 degree

360/M =180 deg

So.’S0’ will b represented by 0 degree and ‘S1’ will be represented by 180(phase difference of 180 between symbols).That is, if 0 bit appears in signal, it will modulate the carrier wave with 0 degree, and 1 bit will modulate the carrier with 180 deg phase.

For M=4, we have 4 distinct phases

360/4 =90

So, Symbols will be represented as:

S0=0 deg,S1=90 deg,S2=135 deg,S3=180 deg

As we have the following symbols ,S0=00,s1=01,s2=10,s4=11.So, if 00 bits appear in signal, they will modulate the carrier wave with 0 degree,01 will modulate the carrier with 90 deg phase.10 will modulate the carrier with 135 deg n 11 will modulate it with 180 deg.

For M=8,we have 8 phases

360/8=45 deg

So,So=0deg,S1=45 deg,S2=90 deg,S3=135 deg,S4=180 deg,S5=225 deg,S6=270deg deg,S7=360 deg

Each symbol is represented by three bits here,

S0=000,S1=001,S2=010,S3=011,S4=100,S5=101,S6=110,S7=111

So, 000 bits will indicate 0 degree phase shift ,001 will represent 90 deg phase.010 will modulate the carrier with 135 deg ,0 11 will refer to 180 deg, n so on .

Similarly, for M-FSK, M frequencies are used to modulate a carrier and for M-ASK , M amplitude levels will be used.

18:04 | Posted in Communication Basics, Digital Modulation, M-PSK Modulation | Read More »

Introduction to QAM modulation

In previous post we started about basic digital modulation techniques,ASK,PSK and FSK.Let’s look at another digital modulation technique called as QAM [Quadrature Amplitude Modulation],which is actually a combination of ASK and PSK .

QAM is the combination of Amplitude modulation and Phase modulation. More technically, QAM is a system of modulation in which data or signal is transferred by modulating the amplitude of two separate carrier waves, mostly sinusoidal, which are out of phase by 90 degrees (sine and cosine). The modulated carriers are then summed, and the resulting signal is a combination of both phase shift keying (PSK) and amplitude shift keying (ASK).

Keep in mind that,the term Quadrature refers to their phase difference of 90 degrees. As Sine and Cosine are 90 degree out of phase.

[Any two periodic waveforms whose phase difference is 1/4 th of their output period are said to have a Quadrature phase relationship].As Sine n Cosine have time period of 360 degrees so, 1/4 th of 360 degree is 90 degree! :)

In next post we will talk about different orders of ASK,PSK n QAM modulation

13:58 | Posted in Communication Basics, Digital Modulation, QAM | Read More »

Digital Modulation with introduction to ASK, FSK and PSK

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. 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. We begin our discussion by three basic types of digital modulations:

ASK [Amplitude Shift Keying]
FSK [Frequency Shift Keying]
PSK [Phase Shift Keying]

All of these techniques vary the parameter of a sinosoid to represent the information we wish to transmit.

A sinusoid has three parameters that can be varied, these are amplitude ,phase and frequency.

In FSK we change the frequency in response to information. One particular frequency for ‘1’ and another frequency for ‘0’.

In PSK, we change the phase of carrier signal to indicate information.

In ASK amplitude of carrier is changed in response to information n all else is kept fixed. Bit ‘1’ is transmitted by a carrier of one particular amplitude.To transmit ‘0’ we change the amplitude