Physical Channels

 As mentioned, information from upper layers
is passed to the physical layer through a number of transport channels.
These transport channels are mapped to a number of physical channels on
the air interface. In general, a physical channel is identified by a specific
frequency, scrambling code, channelization code, duration, and, in the
uplink, phase. In addition to those physical channels that are mapped to or
from transport channels, a number of physical channels exist only for the
correct operation of the physical layer. Such channels are not visible to
higher layers. The following are the physical channels:
■ The Synchronization Channel (SCH) is transmitted by the base station
and is used by a UE during the cell search procedure. In order for a
UE to retrieve broadcast information sent from the base station, it
must first be properly synchronized with the base station. That
synchronization is the primary purpose of the SCH. The SCH contains
two subchannels—the primary SCH and the secondary SCH, as shown
in Figure 6-7. The primary SCH contains a specific 256-chip codeword,
known as the primary synchronization code (PSC), which is identical
in every cell. This specific codeword is created from a set of 16-bit chip
sequences as follows:
Let a  (1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1). Then the
primary SCH contains a sequence of (1  j)
(a, a, a, a, a, a, a,
a, a, a, a, a, a, a, a, a).
The secondary SCH is comprised of 16 codewords, each with a length of
256 chips. These 16 codewords are arranged into 64 different sequences
of length 15. In other words, a sequence is a set of 15 codewords in a
particular order and there are 64 such sequences. The 64 available
sequences are mapped to the 64 downlink primary scrambling code
groups. Thus, when a terminal receives a particular secondary SCH
sequence, it can identify the primary scrambling code group of the cell
in question. Since only eight primary scrambling codes are in each primary scrambling code group, the UE then has relatively few
primary scrambling codes to check before being able to decode
transmissions from the base station. The SCH is transmitted in
conjunction with the Primary Common Control Physical Channel
(Primary CCPCH) described later.
■ The Common Pilot Channel (CPICH) is a channel always transmitted
by the base station and is scrambled with the cell-specific primary
scrambling code. It uses a fixed spreading factor of 256, which equates
to 30 Kbps on the air interface.
An important function of the CPICH is in measurements by the
terminal for handover or cell reselection, as the measurements made by
the terminal are based on reception of the CPICH. Consequently,
manipulation of the transmitted power on the CPICH can be used to
steer terminals towards a given cell or away from a given cell.
For example, if the CPICH power transmitted on given cell is reduced,
the effect is to make the CPICH reception from neighboring cells
appear stronger, which may trigger a handover to a neighboring cell.
This can be useful for load-balancing in the RF network. It is possible
to have more than one CPICH in a given cell. The primary CPICH is
transmitted over the entire cell area. The secondary CPICH can be
transmitted over the whole cell area or can be restricted by
transmission on narrow-beam antennas to specific areas of the cell,
such as areas of high traffic. The channelization code for the Primary
CPICH is fixed to Cch,256,0. An arbitrary channelization code of SF 256
is used for the S-CPICH.
■ The Primary Common Control Physical Channel (Primary CCPCH) is
used on the downlink to carry the BCH transport channel. It operates
at a spreading factor of 256, equivalent to 30 Kbps on the air interface.
In fact, the actual rate is reduced to 27 Kbps on the air interface
because of the fact that the Primary CCPCH is time-multiplexed with
the SCH, as shown in Figure 6-10. For many of the channels on the air
interface, all of the chips in a slot are allocated to a particular physical
channel. The Primary CCPCH is an exception in that it shares every
slot with the SCH. The first 256 chips of each slot are used by the SCH.
The remaining 2,304 chips are used by the Primary CCPCH to carry
the BCH transport channel. The 2,304 chips allocated to the Primary
CCPCH correspond to 18 bits of primary CCPCH data. Moreover, the
18 bits include half-rate convolutional coding (to support forward error
correction) so that the actual data rate is approximately 13.5 Kbps.

■ The Secondary Common Control Physical Channel (Secondary
CCPCH) is used on the downlink to carry two common transport
channels—the FACH and the PCH. The FACH and the PCH can share
a single secondary CCPCH or each can have a secondary CCPCH of its
own. The secondary CCPCH carrying the PCH must be transmitted
over the whole cell area, which applies regardless of whether the
physical channel carries just the PCH or both PCH and FACH. If a
secondary CCPCH is used just for the FACH, then it does not
necessarily have to reach the whole cell coverage area.
■ The Physical Random Access Channel (PRACH) is used in the uplink
to carry the RACH transport channel. The uplink the PRACH has 15
access slots, each with a duration of 5,120 chips. These access slots are
arranged in different combinations, known as RACH subchannels, for
which certain scrambling codes and signatures are available. A given
UE may be allowed to use one or more RACH subchannels according
to the class of UE. The signatures and scrambling codes available for a
particular RACH subchannel are broadcast on the BCH transport
channel.