Updating an uplink contention window size in a listen before talk process in a wireless communication system (100) such as LTE may be done by the eNB (101) if transport blocks contained in the starting subframe of a reference scheduled burst transmitted by a User Equipment (103) are successfully decoded at the eNB. Otherwise, the User equipment adjusts the contention window size depending on information supplied by the eNB. This information identifies the first subframe in the burst whose transport block the base station was able to successfully decode. Depending on whether the User Equipment was first transmitting before or on the identified subframe, the User Equipment can either increase or reset the contention window size.