  This paper deals with the calibration of Time Division Duplexing (TDD)
reciprocity in an Orthogonal Frequency Division Multiplexing (OFDM) based Cell
Free Massive MIMO system where the responses of the (Radio Frequency) RF chains
render the end to end channel non-reciprocal, even though the physical wireless
channel is reciprocal. We further address the non-availability of the uplink
channel estimates at locations other than pilot subcarriers and propose a
single-shot solution to estimate the downlink channel at all subcarriers from
the uplink channel at selected pilot subcarriers. We propose a cascade of two
Deep Neural Networks (DNN) to achieve the objective. The proposed method is
easily scalable and removes the need for relative reciprocity calibration based
on the cooperation of antennas, which usually introduces dependency in Cell
Free Massive MIMO systems.
