The Random Access Channel (RACH) is defined by 3GPP as an
uplink, contention based, common transport channel. It is
typically used for random access transmissions and the transport
of control information such as to register the terminal to the
network after power-up, to perform an location update after
moving from one location area to another, or to initiate a call
by setting up a connection from the User Equipment (UE) to the
Node-B. In current UMTS standard, RACH can also be used to send
small amount of data on the uplink, which is quite different
from the approach taken by 3GPP. Due to the orthogonal uplink in
LTE a specific resource must be reserved to avoid interference
to other users in the cell. In LTE no other information in
addition to information provided by the preamble ID can be send.
This keeps that non-orthogonal resource at a minimum.