Dedicated Channels

The second class of downlink physical channel is the Dedicated CHannel (DCH). The
DCH is the mechanism through which specific user (handset) information (control +
data) is conveyed. The DCH is used in both the downlink and uplink, although the
channel format is different. The differences arise principally through the need to meet
specific hardware objectives in the Node B and the handset—for example, conformance
with EMC regulations, linearity/power trade-offs in the handset, handset complexity/
processing power minimization, and so on.
The DCH is a time multiplex of the Dedicated Physical Control CHannel (DPCCH)
and the Dedicated Physical Data CHannel (DPDCH), as shown in Figure 3.5. The
DPCH is transmitted in time-multiplex with control information. The spreading factor
of the physical channel (Pilot, TPC, and TFCI) may range from 512 to 4.

The number of bits in each field may vary, that is:
 Pilot: 2 to 16
 TPC: 2 to 16
 TFCI: 0 to 16
 Data 1: 0 to 248
 Data 2: 2 to 1000
Certain bit/field combinations will require the use of DTX to maintain slot structure/
timing.
Table 3.3 shows spreading factors against user data rate. Low-bit-rate users have
24/25 dB of spreading gain, highest-bit-rate users 2/3 dB. From column 5, it is seen
that the channel symbol rate can vary from 7.5 to 960 kbps. The dynamic range of the
downlink channel is therefore 128:1, that is, 21 dB. The rate can change every 10 ms. In
addition to spreading codes, scrambling codes are used on the downlink and uplink to
deliver additional selectivity.
In the uplink, user data (DPDCH) is multiplexed together with control information
(DPCCH) to form the uplink physical channel (DCH). Multiple DPDCH may be used
with a single DPCCH. The DPCCH has a fixed spreading ratio of 256 and the DPDCH
is variable (frame-by-frame), from 256 to 4 (see Table 3.4). Each DPCCH can contain
four fields: Pilot, Transport Format Combination Indicator (TFCI), Transmission Power
Control (TPC), and FeedBack Information (FBI). The FBI may consist of 0, 1, or 2 bits
included when closed-loop transmit diversity is used in the downlink. The slot may or
may not contain TFCI bits. The Pilot and TPC is always present, but the bit content
compensates for the absence or presence of FBI or TFCI bits.

It is the variability of DPDCH (single to multiple channels) that define the dynamic
range requirements of the transmitter PA, since multiple codes increase the peak-toaverage
ratio. From column 4, we see that the channel bit rate can vary from 15 to 960
kbps. The dynamic range of the channel is therefore 64:1—that is, 18 dB. The rate can
change every 10 ms.
There are two types of physical channel on the uplink: dedicated physical data channel
(DPDC) and dedicated physical control channel (DPCCH). The number of bits per
uplink time slot can vary from 10 to 640 bits, corresponding with a user data rate of 15
kbps, to 0.96 Mbps. The user data rate includes channel coding, so the actual user bit
rate may be 50 percent or even 25 percent of this rate.