This page describes IQ imbalance such as gain/amplitude, phase and DC offset. It also mentions its effect on constellation and outlines various methods used for IQ imbalance compensation/correction.
Fig.1 IQ modulator
There are three main types of IQ imbalance viz. IQ amplitude imbalance, IQ phase imbalance and IQ DC offset. They are mainly introduced by in-phase (I) and quadrature (Q) components present in Cartesian transmitter used in GMSK modulator shown in the figure. IQ imbalance is less in homodyne architecture compare to heterodyne architecture.
Effect of IQ imbalance on constellation
IQ gain and phase imbalance is prominent in non constant envelope or linear modulation schemes such as QPSK, 16-QAM etc. Here information is carried over both amplitude and phase variation of the carrier waveform. We will see effect of IQ imbalance on 16-QAM signal here.
I and Q components take different paths both in transmitter and receiver to achieve IQ-modulation and IQ-demodulation. IQ mixer introduces IQ imbalance due to its different branches. All other circuits through which I and Q signals go through also introduce gain and phase error between these signals. The effect of IQ gain imbalance is depicted in fig. 2 on 16 QAM signal.
The effect of IQ phase imbalance is depicted in fig. 3 on 16 QAM constellations.
IQ DC offset results due to difference between DC bias applied to I and Q signals. This IQ DC offset results in carrier leakage at the output of modulator. The effect of IQ DC offset is depicted in fig. 4 and fig. 5 on constellation and spectrum respectively.
Fig.5 depicts carrier leakage on modulated spectrum, if its level is comparable to the modulated signal level then it will cause interference.
IQ imbalance compensation/correction
There are various methods available to compensate for these imbalances at the transmitter and receiver end. The most common are using known pattern/modulation type. Moreover IQ DC offset is compensated/corrected by adjusting DC bias applied to I and Q signals for better rejection of carrier at the output. By IQ compensation we can improve error vector magnitude of the modulated signal that helps in decoding the signal correctly and will increase the Bit Error Rate (BER) of the system.