Method and apparatus for accessing an uplink random access channel in a singular carrier frequency division multiple access system
A method and apparatus for accessing a contention-based uplink random access channel (RACH) in a single carrier frequency division multiple access (SC-FDMA) system are disclosed. A wireless transmit/receive unit (WTRU) randomly selects a RACH subchannel and a signature among a plurality of available RACH subchannels and signatures. The WTRU transmits a preamble using the selected signature via the selected RACH subchannel at a predetermined or computed transmission power. A base station monitors the RACH to detect the preamble and sends an acquisition indicator (AI) to the WTRU when a signature is detected on the RACH. When receiving a positive acknowledgement, the WTRU sends a message part to the base station. If receiving a negative acknowledgement or no response, the WTRU retransmits the preamble.
1. A wireless transmit/receive unit (WTRU) comprising:
a processor and a transmitter configured to:
transmit a random access preamble, at a first power level as a single carrier frequency division multiple access (SC-FDMA) signal;
on condition that a response to the random access preamble is not detected by the WTRU:
determine whether a retransmission limit has been reached, and
retransmit the random access preamble at a second power level on condition that the retransmission limit has not been reached; and
on condition that the response to the random access preamble is received by the WTRU:
obtain a timing advance, a power command and a resource allocation from the received response, and
transmit a random access message including an identity of the WTRU as a SC-FDMA signal having a timing derived at least from the timing advance, having a transmission power level derived at least from the power command and utilizing resources derived from at least the resource allocation.
2. The WTRU of claim 1 , wherein the processor is configured to transmit the random access preamble at a power offset.
3. The WTRU of claim 1 , wherein the processor is configured to determine whether the retransmission limit has been reached based on a number of times the random access preamble has been transmitted.
4. The WTRU of claim 1 , wherein the resource allocation indicates resource blocks or subcarriers to transmit the random access message in a subsequent subframe.
5. The WTRU of claim 4 , wherein the indicated subcarriers are consecutive subcarriers.
6. The WTRU of claim 1 , wherein the transmitter is configured to transmit the random access preamble having a duration which is variable in size.
7. The WTRU of claim 1 , wherein the processor is configured to set the random access preamble power level using open loop power control and on a condition that the response to the random access preamble is received, set subsequent transmission power levels using closed loop power control.
8. A method comprising:
transmitting a random access preamble, at a first power level as a single carrier frequency division multiple access (SC-FDMA) signal;
on condition that a response to the random access preamble is not detected by the WTRU:
determining whether a retransmission limit has been reached, and
retransmitting the random access preamble at a second power level on condition that the retransmission limit has not been reached; and
on condition that the response to the random access preamble is received by the WTRU:
obtaining a timing advance, a power command and a resource allocation from the received response, and
transmitting a random access message including an identity of the WTRU as a SC-FDMA signal having a timing derived at least from the timing advance, having a
transmission power level derived at least from the power command and utilizing resources derived from at least the resource allocation.
9. The method of claim 8 , wherein the transmitting of the random access preamble includes transmitting the random access preamble at a power offset.
10. The method of claim 8 , wherein the determining of whether the retransmission limit has been reached is based on a number of times the random access preamble has been transmitted.
11. The method of claim 8 , wherein the resource allocation indicates resource blocks or subcarriers to transmit the random access message in a subsequent subframe.
12. The method of claim 11 , wherein the indicated subcarriers are consecutive subcarriers.
13. The method of claim 8 , wherein the transmitting of the random access preamble includes transmitting the random access preamble having a duration which is variable in size.
14. The method of claim 8 , further comprising:
initially setting the random access preamble power level using open loop power control; and
on a condition that the response to the random access preamble is received, subsequently setting transmission power levels using closed loop power control.