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

A user equipment (UE) for performing random access in a mobile communication system, the UE comprising:

• a receiver configured to receive system information indicating a group of identification(ID)s;
• a signal generator configured to:
• select an first ID from among the group of the IDs, and
• generate a first uplink signal corresponding to the selected first ID for random access;
• a downlink signal processor configured to:
• after transmitting the first uplink signal, wait for a predetermined delay duration without checking a downlink channel,
• after waiting for the predetermined delay duration, check the downlink channel during a valid period, and
• determine whether a downlink signal responding to the first uplink signal is received in the valid period, the downlink signal comprising a second ID and an UE-ID;
• and
• a transceiver configured to:
• transmit the generated first uplink signal to a Node B, and
• transmit 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 the 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, the method 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);
• after the receiving of the first uplink signal, waiting for a predetermined delay duration without transmitting a downlink signal responding to the first uplink signal;
• after the waiting for the predetermined delay duration, 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 the predetermined delay duration starting from transmission of the first uplink signal has terminated, and
• wherein data is transmitted and received after the transmitting of the downlink signal during the valid period.

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

• a transceiver configured to:
• transmit system information indicating a group of identification(ID)s,
• receive 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), after receiving the first uplink signal, wait for a predetermined delay duration without transmitting a downlink signal responding to the first uplink signal,
• after waiting for the predetermined delay duration, transmit 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
• receive a second uplink signal using the UE-ID, if the second ID is equal to the first ID;
• and
• a controller configured to control the transceiver to transmit the downlink signal in the valid period,
• wherein the valid period starts when the predetermined delay duration starting from transmission of the first uplink signal has terminated, and
• wherein data is transmitted and received after transmitting the downlink signal during the valid period.