A method for a user equipment (UE) to perform a random access to a network includes transmitting a random access preamble to the network; receiving control information via a downlink control channel from the network, wherein the control information includes downlink resource location information for a downlink shared channel and information for decoding the downlink shared channel; receiving a response to the random access preamble via the downlink shared channel from the network corresponding the control information, wherein the response has a variable size; and decoding the response based on the information for decoding the downlink shared channel.