The Orthogonal Frequency Division Multiple Access OFDMA and Orthogonal Frequency Division Multiplexing OFDM techniques involve transmitting so called map messages in order to inform user terminals which slots are allocated to them. The large size of the map messages may constitute a problem in the system, and thus the present solution intends to alleviate this problem. The idea is to use semi-static allocation maps, wherein a location, size, and modulation and coding method of allocated slots are predefined for a connection. Then, the present solution allows defining validity information indicating which of the predefined allocations are to be used for a current frame.