A method in a radio network node, for transmission of downlink control channel signals in a downlink frequency carrier of a wireless communication system, wherein at least two Physical Resource Block "PRB" pairs configurable for the transmission of a downlink control channel have, in at least one subframe, different numbers of time-frequency resource elements available for the transmission of said downlink control channel, the method comprising: allocating at least one set S of PRB pairs; arranging time-frequency resource elements from said set S into an aggregation comprising at least one level; determining a set LS of aggregation levels supported for the transmission of a downlink control channel within the allocated set of PRB pairs, based on the number of time-frequency resource elements available for the transmission of a downlink control channel in PRB pairs of the set S; and transmitting the downlink control channel within said set S of PRB pairs; wherein for each possible time-frequency position of the downlink control channel, corresponding to the lowest aggregation level in at least one set LS of aggregation levels, within the allocated set S of PRB pairs, the supported set LS of aggregation levels is determined from the payload size and the number of time-frequency resource elements used in said position; wherein for a given Downlink Control Information DCI format, the method further comprises: verifying in each PRB pair of the PRB pair set S allocated for downlink control channel transmission, the possible positions of downlink control channel for aggregation level 1 and discarding the positions that yield a code rate larger than a given threshold and is characterized by using positions of the downlink control channel within the PRB pair set S that are discarded for aggregation level 1 for higher aggregation levels. allocating at least one set S of PRB pairs; arranging time-frequency resource elements from said set S into an aggregation comprising at least one level; determining a set LS of aggregation levels supported for the transmission of a downlink control channel within the allocated set of PRB pairs, based on the number of time-frequency resource elements available for the transmission of a downlink control channel in PRB pairs of the set S; and transmitting the downlink control channel within said set S of PRB pairs; wherein for each possible time-frequency position of the downlink control channel, corresponding to the lowest aggregation level in at least one set LS of aggregation levels, within the allocated set S of PRB pairs, the supported set LS of aggregation levels is determined from the payload size and the number of time-frequency resource elements used in said position; wherein for a given Downlink Control Information DCI format, the method further comprises: verifying in each PRB pair of the PRB pair set S allocated for downlink control channel transmission, the possible positions of downlink control channel for aggregation level 1 and discarding the positions that yield a code rate larger than a given threshold and is characterized by using positions of the downlink control channel within the PRB pair set S that are discarded for aggregation level 1 for higher aggregation levels.