In a previous post, Understanding Radio Impairments on OFDM Transmitter Performance, we addressed the effect of physical radio impairments on OFDM transmitted signal quality. We also related the error vector magnitude (EVM) to DC offset, IQ gain, and phase imbalance in terms of carrier leakage ratio and unwanted side band rejection in a closed form. Carrier leakage and IQ imbalance can be seen as constellation offset and distortion respectively. For instance, 8-PSK modulation accuracy as used in EDGE is usually quantified in terms of origin offset where -35 dB is required for the pico BTS class.
IQ Gain Imbalance
As far as IQ imbalance is concerned, let’s consider the simple case of a gain imbalance. Here, we assume the ideal subcarrier
is being affected by gain imbalance β as
where Ck,m is a symbol within a constellation for the kth subcarrier and mth OFDM symbol.
Rearranging the second equation back into a complex exponential form, we get
Note that the second part of the error term is at the frequency of the –kth subcarrier. In other words, the IQ gain imbalance will result in a subcarrier interfered with by its frequency mirror. This is simply an unwanted side band. If the symbols are uncorrelated, which is the case for data symbols, the interference is random in nature which causes the IQ gain imbalance to distort the constellation.
DC offset, IQ gain, and phase imbalance calibration are important radio routines upon which radio conformance is highly dependent. Automated calibration can be achieved using internal feedback mechanisms that are readily available in most state-of-the-art transceivers. However, better performance can be achieved if external virtual feedback is exploited via RF signal leakage, taking into account the external transmitter stages (such as pre-driver amplifiers). Automatic calibration routines have been implemented in Radio420X to seamlessly achieve ‑45dBc typical LO leakage and unwanted side band rejection at any frequency between 300 MHz and 3000 MHz.
Calibration Routines Video
You can see live calibration routines captured in this video. The carrier is set at 1982.5 MHz wherein the sidebands are expected at +/-500 KHz offsets.