Terminology » IQ Imbalance: Causes, Effects, and Compensation

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This page delves into the world of IQ imbalance, covering aspects like gain/amplitude, phase, and DC offset. It also explores the effect of these imbalances on constellation diagrams and outlines various methods used for IQ imbalance compensation/correction.

Fig.1 IQ modulator

There are three primary types of IQ imbalance:

IQ amplitude imbalance

IQ phase imbalance

IQ DC offset

These imbalances are mainly introduced by the in-phase (I) and quadrature (Q) components present in Cartesian transmitters, such as those used in GMSK modulators as shown in the figure above. IQ imbalance tends to be less pronounced in homodyne architectures compared to heterodyne architectures.

IQ gain and phase imbalance are particularly noticeable in non-constant envelope or linear modulation schemes like QPSK and 16-QAM. These schemes rely on both amplitude and phase variations of the carrier waveform to carry information. Let’s examine the effect of IQ imbalance on a 16-QAM signal.

IQ gain imbalance

The I and Q components travel different paths in both the transmitter and receiver to achieve IQ-modulation and IQ-demodulation. The IQ mixer itself can introduce IQ imbalance due to its different branches. Furthermore, all other circuits through which the I and Q signals pass can also introduce gain and phase errors between these signals.

The effect of IQ gain imbalance on a 16 QAM signal is depicted in the figure above.

IQ phase imbalance

The effect of IQ phase imbalance on 16 QAM constellations is depicted in the figure above.

IQ DC offset

IQ DC offset arises due to differences in the DC bias applied to the I and Q signals. This IQ DC offset results in carrier leakage at the output of the modulator.

carrier leakage

Fig.4

Fig.5 depicts carrier leakage on modulated spectrum,

If the carrier leakage level is comparable to the modulated signal level, it will cause interference.

Various methods exist to compensate for these imbalances at both the transmitter and receiver. The most common approaches involve using known patterns or modulation types. Additionally, IQ DC offset is often compensated/corrected by adjusting the DC bias applied to the I and Q signals, leading to better carrier rejection at the output.

By applying IQ compensation, we can improve the error vector magnitude of the modulated signal, which helps in decoding the signal correctly and ultimately increases the Bit Error Rate (BER) of the system.

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