Uplink DPDCH and DPCCH

 Figure 6-14 shows the structure
of the uplink DPCCH as used with the uplink DPDCH. The DPCCH is
transmitted in parallel with the DPDCH and the information in a given
DPCCH frame relates to the corresponding DPDCH frame.

The DPCCH always uses a spreading factor of 256.Thus, each slot (2,560
chips) corresponds to 10 bits of DPCCH information. These 10 bits are
divided into pilot bits, Transport Format Combination Indicator (TFCI) bits, Feedback Indicator (FBI) bits, and Transmit Power Control (TPC) bits.

The pilot information bits are used for channel estimation purposes and
include specific bit patterns for frame synchronization. The TFCI bits indicate
the bit rate and channel coding for the DPDCH. A single DPDCH can
carry multiple DCH transport channels.

If, for example, a user were invoking multiple simultaneous services, the
associated DCH transport channels could be multiplexed together on a single
DPDCH. In that case, the DPDCH is said to carry a Coded Composite
Transport Channel (CCTrCH). The TFCI is used to indicate the format of
each of the transport channels within the CCTrCH. The FBI bits are used
in conjunction with transmit diversity at the base station. WCDMA supports
downlink transmit diversity, whereby two antennas can be used for
downlink transmission. When transmit diversity is used, it is possible for
the power and/or phase on one transmit antenna to differ from that on the
other. The FBI bits are used in the uplink to instruct the base station to
change the power or phase difference associated with transmit diversity.
Finally, the TPC bits are used to command the base station to change the
transmit power when necessary.

The number of bits in each of the uplink DPCCH fields depends upon the
slot format for the DPCCH. A number of slot formats are possible, as shown
in Table 6-3.

As can be seen from Table 6-3, in some slot formats, the full 15 slots are
not used in every radio frame. The reason for less than 15 slots per frame is
because of the use of compressed mode. In compressed mode, gaps exist in
both the uplink and downlink transmissions. These gaps are included to
enable the UE to take measurements on other frequencies. By taking measurements
on other frequencies and reporting those measurements, the UE
enables the network to enable an inter-frequency handover either to
another UMTS frequency or perhaps an inter-system handover to a GSM
system.
Also, a number of different slot formats exist for the DPDCH, but these
simply reflect the different spreading factors that can be applied to the
DPDCH data. For example, a spreading factor (SF) of 256 for the uplink
DPDCH means 10 bits per slot, whereas a spreading factor of 4 means 640
bits per slot. 255