Physical Channelss

The process for accessing the uplink begins with the transmission from
the terminal of a specific preamble sent on a specific access slot. This preamble
is 4,096 chips long and comprises 256 repetitions of a 16-chip signature.
The preamble is scrambled by one of 8,192 long scrambling codes.
These 8,192 scrambling codes are grouped into 512 groups of 16 codes. A
correspondence exists between a specific group of preamble scrambling
codes and the primary downlink scrambling code used in the cell.
Once the base station detects the preamble, it uses the Acquisition Indicator
Channel (AICH) to indicate to the UE that the preamble has been
detected and that the UE either is or is not allowed uplink access. The
AICH is also structured in slots, each of which is 5,120 chips long. Each slot
indicates a number of PRACH signatures and an indication for each as to
whether the UE is allowed access to the uplink. The UE checks the AICH
to see whether it has been granted access (as determined by checking for
the signature it has just used). Assuming that the UE has been granted
access, then it transmits the actual RACH message (10 ms or 20 ms duration)
on subsequent access slots.
■ The Physical Common Packet Channel (PCPCH) is used in the uplink
to carry the uplink CPCH transport channel. Given that the CPCH is
somewhat similar to the RACH, the process for using the PCPCH is
similar to that for using the PRACH. A preamble is first sent using a
specific signature. The terminal then waits for a response from the
base station on the Access Preamble-Acquisition Indicator Channel
(AP-AICH), similar to what is done on the AICH for an access attempt
on the PRACH.
When the response is received on the AP-AICH, however, the terminal
does not yet proceed to transmit the desired data. The reason is that
the CPCH can support longer durations of data than the RACH. Thus,
if there is a collision, a greater amount of data is lost. Therefore, the
terminal next sends a specific collision detection (CD) signature and
waits for this to be echoed back from the base station on the Collision
Detection/Channel Assignment-Indication Channel (CD/CA-ICH). At
this point, the terminal can send the CPCH data on the PCPCH. The
duration of the data transfer can last several 10-ms frames. The
spreading factor can take any value from 4 to 256.terminal’s use of a CPCH. This avoids access attempts from the mobile
that are doomed to fail.
■ The Physical Downlink Shared Channel (PDSCH) is used in the
downlink to carry the DSCH transport channel. Because the DSCH
transport channel can be shared among several users, the PDSCH has
a structure that enables it to be shared among users. A PDSCH has a
root channelization code and there may be multiple PDSCHs with
channelization codes at or below the root channelization code. These
various PDSCHs may be allocated to different UEs on a radio-frameby-
radio-frame basis.Within one radio frame, UTRAN may allocate
different PDSCHs under the same PDSCH root channelization code to
different UEs.Within the same radio frame, multiple parallel PDSCHs
with the same spreading factor may be allocated to a single user.
PDSCHs allocated to the same user on different radio frames may have
different spreading factors.
■ The Indicator Channels include the AICH, AP-AICH, and CD/CA-ICH
already mentioned. In addition, there is the Paging Indicators Channel
(PICH). The purpose of the PICH is to let a given terminal know when
it might expect a paging message on the PCH (carried on the secondary
PCPCH). When a user device registers with the network, it is assigned
to a paging group. These paging groups are indicated through the use
of paging indicators carried on the PICH. When a terminal is to be
paged on the PCH, a paging indicator corresponding to the paging
group in question is carried on the PICH. If a terminal decodes the
PICH and finds that its paging group is indicated, then at least one
terminal in its paging group is being paged, which means that the
terminal must decode the PCH (carried on the secondary PCPCH) to
determine if it is being paged. If a given terminal’s paging group is not
indicated on the PICH, then the terminal need not decode the PCH.
■ The DCH transport channel is mapped to the two physical channels—
DPDCH and DPCCH, as previously mentioned. The DPDCH carries
the actual user data and can have a variable spreading factor, whereas
the DPCCH carries control information.
The mapping between the transport channels and the physical channel
is shown in Figure 6-11.
As an option, the base station can support the CPCH Status Indication
Channel, which is used to indicate the current state of affairs for any
CPCH defined in the cell. By monitoring this channel, the terminal can
determine, in advance, if resources are available to support the