Random access in dynamic and shared spectrums

A method for performing random access transmission in a communication system by a wireless transmit/receive unit (WTRU), the method comprising:

• identifying a timing for sending a plurality of short random access channel (RACH) preambles, wherein the plurality of short RACH preambles comprise a first and a second short RACH preamble that each has a shorter time duration than eight hundred micro seconds;
• transmitting the first short RACH preamble of the plurality of short RACH preambles based on the identified timing;
• and
• transmitting the second short RACH preamble of the plurality of short RACH preambles, the second short RACH preamble being the same as the first short RACH preamble at least in length or in power.