Carrier aggregation using unlicensed frequency bands. A primary carrier for communication between a base station (BS) and a wireless user equipment (UE) device may be configured according to a first wireless communication technology. The primary carrier may be on a licensed frequency band. A secondary carrier for communication between the BS and the UE may also be configured according to a first wireless communication technology. The secondary carrier may be on an unlicensed frequency band. Channel conditions for each of multiple possible channels of the unlicensed frequency band may be assessed according to a second wireless communication technology as part of configuring the secondary carrier.