  This letter shows that the TX and RX models commonly used in literature for
downlink (distributed) massive MIMO are inaccurate, leading also to inaccurate
conclusions. In particular, the Local Oscillator (LO) effect should be modeled
as $+\varphi$ in the transmitter chain and $-\varphi$ in the receiver chain,
i.e., different signs. A common misconception in literature is to use the same
sign for both chains. By correctly modeling TX and RX chain, one realizes that
the LO phases are included in the reciprocity calibration and whenever the LO
phases drift apart, a new reciprocity calibration becomes necessary (the same
applies to time drifts). Thus, free-running LOs and the commonly made
assumption of perfect reciprocity calibration (to enable blind DL channel
estimation) are both not that useful, as they would require too much
calibration overhead. Instead, the LOs at the base stations should be locked
and relative reciprocity calibration in combination with downlink demodulation
reference symbols should be employed.
