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In general, the unequal RF circuitry in the transmit and receive chains at the Base Station (BS) prevents the exploitation of the UpLink (UL) channel estimate for proper pre-equalization in Time Division Duplex (TDD) systems. To avoid additional transceiver hardware costs for matching networks, the idea of relative calibration was introduced to cope with the different effective channel impulse responses of UL and DownLink (DL) by means of signal processing. However, Multiple-Input Multiple-Output (MIMO) transmission in frequency-selective channels does not allow for classical frequency-domain calibration principles based on Total Least Squares (TLS) approaches. Consequently, more complex Structured Total Least Squares (STLS) problems must be solved.
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