Difference between constellation Diagram vs Eye Diagram

This page describes basics of constellation diagram and eye diagram and mentions difference between constellation diagram and eye diagram.

As we know in digital modulation, baseband is separated into in phase(I) and quadrature phase(Q) components. The combination of I and Q is known as baseband modulating signal. In vector modulation amplitude and phase will vary to generate symbols. After the vector modulation, modulating signal becomes modulated signal. This modulated signal can be represented in various formats or diagrams. The diagrams include constellation diagram, eye diagram, CCDF curve, power spectrum etc. Refer PHY measurements for more information.

Constellation Diagram

constellation diagram

It is also referred as IQ diagram. The constellation diagram represent all the possible modulated symbols which will be used by modulation technique to map the information bits. This different symbols are represented in the complex plane with their amplitude and phase informations. The figure-1 depicts QPSK constellation diagram.

As shown in the figure, two information bits make one symbol. For example, '01' represent 0.707*(-1+1*j). This has been represented in the constellation diagram as dot with magnitude 'M' and angle 'θ' values as expressed below.

Magnitude M = (I2 + Q2)0.5

Angle θ = tan-1 (Q/I)

In QPSK there will be four possible symbols for the information bits. All the four combinations are represented in the constellation diagram. Refer QPSK modulation and QAM modulation   types for more information. Error Vector Magnitude is derived from the constellation diagram. Refer EVM for more information.

Baseband Data Eye Diagram

eye diagram

In wireless digital communication, Eye pattern is referred as Eye diagram. This is obtained using oscilloscope. It is being generated when digital data is sampled and applied to the vertical input of CRO and data rate is applied as trigger for the horizontal sweep. Different modulation types will have different eye diagrams.

It is used to evaluate performance of baseband transmission in the presence of noisy channel environment. Open eye represent less distortion in the signal. Hence more open is the eye better is the received modulated signal. Various measurements can be achieved using eye diagram. Amplitude measurements include eye amplitude, eye height, eye SNR etc. Time measurements include eye delay, eye fall time and rise time, crossing time, peak to peak jitter, random jitter etc.

Following measurements can be interpreted using eye diagram.
• Eye opening - Additive noise in the signal.
• Eye Width - Timing synchronization and jitter effects
• Eye overshoot/undershoot - peak distortion due to interruptions in the signal path.

Related links

Error Vector Magnitude
Various impairments for baseband chain MATLAB code
AM-AM conversion
AM-PM conversion

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