A method (600, 700, 900) in a digital unit connected with a plurality of distributed antennas including a first antenna, a second antenna and a third antenna, the method (600, 700, 900) comprising: causing transmitting (610, 730, 740, 750) a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource; obtaining a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna; and obtaining estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained receiver and transmitter side loop-back phase difference; characterized in that obtaining the receiver and transmitter side loop-back phase difference between the first and second antennas comprises: obtaining a first loop-back phase for the first antenna by comparing a phase for the third signal received at the first antenna and a phase for the first signal received at the third antenna; obtaining a second loop-back phase for the second antenna by comparing a phase for the third signal received at the second antenna and a phase for the second signal received at the third antenna; and obtaining the receiver and transmitter side loop-back phase difference by determining a difference between the first and second loop-back phases. causing transmitting (610, 730, 740, 750) a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource; obtaining a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna; and obtaining estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained receiver and transmitter side loop-back phase difference; characterized in that obtaining the receiver and transmitter side loop-back phase difference between the first and second antennas comprises: obtaining a first loop-back phase for the first antenna by comparing a phase for the third signal received at the first antenna and a phase for the first signal received at the third antenna; obtaining a second loop-back phase for the second antenna by comparing a phase for the third signal received at the second antenna and a phase for the second signal received at the third antenna; and obtaining the receiver and transmitter side loop-back phase difference by determining a difference between the first and second loop-back phases. obtaining a first loop-back phase for the first antenna by comparing a phase for the third signal received at the first antenna and a phase for the first signal received at the third antenna; obtaining a second loop-back phase for the second antenna by comparing a phase for the third signal received at the second antenna and a phase for the second signal received at the third antenna; and obtaining the receiver and transmitter side loop-back phase difference by determining a difference between the first and second loop-back phases.