The cost of achieving this higher spectral efficiency using higher order QAM is that the transmitter and receiver requires a higher signal to noise ratio (SNR) with the same level of error rate performance (relative to a baseline BPSK, QPSK and other systems). One of the dominant vectors on SNR degradation is I/Q image rejection (I/Q gains and phases imbalance).
There are a lot of factors that degrade the matching of gains and phases between I/Q signals: the instinct layout mismatch, the random mismatch of the devices, the different temperatures over the I/Q signal paths. IQ Calibration Techniques For CMOS Radio Transceivers describes a fully-analog compensation technique without baseband circuitry to control the calibration process. This book will use an 802.11g transceiver design as an example to give a detailed description on the I/Q gains and phases imbalance auto-calibration mechanism.
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