The present invention relates to methods, a communication network node and a user equipment for detecting collision of physical cell identities in a communication network system comprising radio base stations each serving at least one cell through which user equipments are moving. Reference signals comprising reference symbols corresponding to a physical cell identity are sent from said radio base stations to said user equipments over a radio interface. Transmission gaps of reference symbols are inserted in the reference signal in order to perform a physical cell identity collision test. The user equipments are arranged to detect if any other radio base station is sending reference symbols corresponding to the same physical cell identity during the transmission gaps. Thereby a physical cell identity collision is detected. The reference symbol transmission gaps are fractionally distributed within said reference signal such that at least some of said reference symbols are unaffected by said reference symbol transmission gaps.