The present invention relates to a method in a network control node of a wireless communication system, said wireless communication system using a system frequency bandwidth for downlink transmissions from said network control node to one or more mobile stations of said wireless communication system; said method comprising the steps of: allocating K number of sets of physical resource units arbitrarily chosen within said system frequency bandwidth for downlink transmissions, wherein K is an integer equal to or larger than 1 and the number of physical resource units of each set is selected from a finite set of allowed values; jointly encoding the frequency positions of the K number of sets of physical resource units into a first unique combinatorial index q; and signalling said first combinatorial index q in the downlink.