Transport Channels

 In general, two types of transport channels
exist. These are common transport channels and dedicated transport
channels. Common transport channels may be applicable either to all
users in a cell or to one or more specific users. In the case when a common
transport channel is used to transmit information to all users, then
no specific addressing information is required. When a specific user needs
to be addressed by a common transport channel, then the user identification
is included in-band (within the message being sent). For example,
the Broadcast Channel (BCH), which is a common transport channel, is
used to transmit system information to all users in a cell and is not specific
to any given user. On the other hand, the Paging Channel, which is
also a common transport channel and which is used to page a specific
mobile, contains the identification of the user within the message being
transmitted.
As we describe the various types of channels supported, we will make
references to frames and slots. Basically, the various channels use a 10-ms
frame structure, which corresponds to 38,400 chips. Each frame is divided
into 15 slots, each with a length of 2,560 chips, as shown in Figure 6-9. The
content of each frame, and for that matter the content of each slot, is dependent
upon the type of channel in question.
The following common transport channels are defined:
■ The Random Access Channel (RACH) is used in the uplink when a
user wants to gain access to the network. It may also be used when a
user wants to send a small amount of data to the network. The
amount of data sent on the RACH is small—it lasts either 10 or 20 ms.
This is in accordance with the fact that the RACH is used primarily
for signaling related to initial system access. It must be possible for
the RACH to be heard at the base station from any user in the cell
coverage area—even from at the edge, at least when the RACH is used
for initial access to the network. Because, as we shall see in Chapter
12, “UMTS System Design,” the effective coverage area of a cell
decreases with the increasing bandwidth, it is necessary for the data
rate on the RACH to be quite low. The RACH is available to all users
in the cell. Consequently, the possibility of collision arises when
multiple users attempt to access the RACH. UTRAN includes
procedures at the physical layer for collision detection.
■ The Broadcast Channel (BCH) is used in the downlink to transmit
system information over the entire coverage area of a cell. For this
reason, it is sent with a relatively high power level. Moreover, the data
rate on the BCH is quite low compared to some other channels. When
sent over the air interface, the BCH information is sent at 30 Kbps,
including coding overhead.
■ The Paging Channel (PCH) is used in the downlink to page a given UE
when the network wants to initiate communication with a user. A page
for a given UE may be sent on a single cell or multiple cells, depending
on the location area/routing area configuration of the network. In a
given cell, the PCH must be heard over the whole cell area.
■ The Forward Access Channel (FACH) is used to send downlink control
information to one or more users in a cell. If, for example, a user
attempts to access the network on the RACH, then the response to the
access request will be sent on the FACH. The FACH can also be used to
send small amounts of packet data to a mobile. It is possible to have
more than one FACH in a cell. At least one FACH, however, must have
a sufficiently low data rate that all users in the cell can hear it.
■ The uplink Common Packet Channel (CPCH) is similar to the RACH
but can last for several frames. Thus, it enables a greater amount of
data to be sent than is allowed by the RACH. It can be used, for
example, when the terminal wants to send a single burst of data that
cannot be accommodated on the RACH. The CPCH is available to all
users in the cell. Consequently, the possibility of collision occurs when a
user attempts to access the CPCH. The UTRAN includes procedures at
the physical layer for minimizing the likelihood of collision and
detecting collision when it does occur.
■ The Downlink Shared Channel (DSCH) is used to carry dedicated user
data or control signaling to one or more users in a cell. It is similar to
the FACH, but does not have to be transmitted over the entire cell
area. Moreover, it supports higher data rates than the FACH and the
data rate on the DSCH can change on a frame-by-frame basis. The
DSCH is always associated with one or more downlink dedicated
channels described later.
Only a single dedicated transport channel type exists, known as the Dedicated
Channel (DCH). This is a channel that carries user data and is specific
to a single user. Although other channels can carry small amounts of
bursty user data, they are not designed for large amounts of data or for
extended data sessions. The DCH is used for those types of sessions.
For example, in a voice conversation, the coded voice uses the DCH. The
DCH exists in the uplink and the downlink and is mapped to the physical
channels DPDCH and DPCCH, previously described. In the uplink, at the
physical layer, the combination of frequency, the scrambling code, the channelization
code, and the phase is used to indicate a particular DPDCH or
DPCCH. In the downlink, the DCH is mapped to a Dedicated Physical
Channel (DPCH), which is identified in the downlink by a particular channelization
code. The downlink DPDCH and DPCCH are time multiplexed
onto the downlink DPCH. The data rate on a DCH can vary on a frame-byframe
basis.