An arrangement for determining clock offset between at least a first and second radio unit, the arrangement comprising at least a first radio unit, a second radio unit, and at least one processor, the arrangement being configured to:
- a. perform first two-way transmissions between at least one pair of radio units using a first signal comprising a selected first frequency, wherein said transmissions are sent as broadcasts and received at at least one non-transmitting radio unit to obtain the at least one pair of radio units,
- b. determine first phase information regarding the first signals received at the radio units,
- c. determine a first phase difference, for each pair of radio units, as a difference between the first phase information determined for each radio unit in the pair of radio units,
- d. perform second or subsequent two-way transmissions between the at least one pair of radio units using a second or subsequent signal comprising a selected second frequency or subsequent frequency,
- e. determine second or subsequent phase information regarding the second or subsequent signals received at the radio units,
- f. determine a second or subsequent phase difference, for each pair of radio units, as a difference between the second or subsequent phase information determined for each radio unit in the pair of radio units,
- g. determine a difference between the first phase difference and the second or subsequent phase difference, or a difference between the determined phase difference at the highest or lowest signal frequency and a subsequent phase difference,
- h. determine at least one clock offset variable, for each pair of radio units, being indicative of an approximated clock offset between the radio units in the pair of radio units, based on a difference determined at step g,
- i. determine an estimated maximum error in the determined clock offset variable based at least on a maximum error of the first phase difference and a maximum error of the second or subsequent phase difference,
- j. determine if the maximum error of the clock offset variable allows the clock offset to be unambiguously determined, by determining a set of a set of possible clock offset values obtained through variation of the clock offset corresponding to variations of integer numbers of half cycle periods at the first or subsequent frequency, said set of possible clock offset values being limited by the estimated maximum error in the determined clock offset variable,
- k. repeat the steps d-j using a subsequent selected frequency that differs from the first frequency by an amount that is more than the difference between the first frequency and the second or previously used frequency, if it is determined that the clock offset cannot be unambiguously determined.