Method and apparatus for initiating communications on a shared channel in a mobile communications system

A user equipment for performing random access in a mobile communication system, comprising:

• a receiver for receiving system information indicating a group of identification(ID)s;
• a signal generator for selecting an first ID from among the group of the IDs, generating a first uplink signal corresponding to the selected first ID for random access;
• a downlink signal processor for determining whether a downlink signal responding to the first uplink signal is received in a valid period, the downlink signal comprising a second ID and an UE-ID;
• and
• a transceiver for transmitting the generated first uplink signal to a Node B and transmitting a second uplink signal using the UE-ID, if the downlink signal is received during the valid period and the second ID is equal to the first ID,
• wherein the valid period starts when a predetermined delay duration starting from transmission of the first uplink signal has terminated.

A method for performing random access in a Node B of a mobile communication system, comprising:

• transmitting system information indicating a group of identification(ID)s;
• receiving a first uplink signal corresponding to a first ID selected from among the group of the IDs for random access from a User Equipment (UE);
• transmitting a downlink signal responding to the first uplink signal in a valid period, the downlink signal comprising a second ID and an UE-ID;
• and
• receiving a second uplink signal using the UE-ID, if the second ID is equal to the first ID,
• wherein the valid period starts when a predetermined delay duration starting from transmission of the first uplink signal has terminated.

A Node B for performing random access in a mobile communication system, comprising:

• a transceiver for transmitting system information indicating a group of identification(ID)s, for receiving a first uplink signal corresponding to a first ID selected from among the group of the IDs for random access from a User Equipment (UE), for transmitting a downlink signal responding to the first uplink signal in a valid period, the downlink signal comprising a second ID and an UE-ID, and for receiving a second uplink signal using the UE-ID, if the second ID is equal to the first ID;
• and
• a controller for controlling the second transmitter to transmit the downlink signal in the valid period,
• wherein the valid period starts when a predetermined delay duration starting from transmission of the first uplink signal has terminated.