Lecture 52 : 5G MIMO transceiver chain - part I

[@arjunmenonkandanat6328]

I have a question - what is the practical reason for a channel not to have reciprocity ? I mean, isn't it violation of basic principles of physics?

[@reubenstephen]

A channel would not have reciprocity if the uplink and downlink are in different frequency bands, which is the case in Frequency Division Duplex (FDD) systems, as opposed to Time Division Duplex (TDD) systems.

[@reubenstephen]

Even for a TDD system, although the physical channel may be reciprocal, once the signal reaches the baseband processor, it has already gone through several hardware components, whose properties are not exactly equivalent at the transmitter and receiver. Hence a calibration is still necessary to assume reciprocity of the baseband equivalent channels even in a TDD system.

[@arjunmenonkandanat6328]

@@reubenstephen Thankyou for your input Reuben. It was helpful. Actually prof is explaining it in the next video of this playlist. The phase in which LO is rotating in the tx and rx side is the villain. There is no means we can synchronise both of them . Usually this mismatch is included in the umbrella of channel estimation. But this will create a problem when we assume reciprocity. Prof tells this very briefly in subsequent lectures.