Page 211 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 197

transmitted continuously, even during times of inactivity, to remain synchronized. This
way, the mobile device can resume uplink transmissions without delay whenever
required.
The control channel carries four parameters:

1) Transmit power control (TPC).
2) Pilot (used for channel estimation of the receiver).
3) TFCI.
4) Feedback indicator (FBI).
The pilot bits are always the same and allow the receiver to get a channel estimate
before decoding user data frames. While no user data frames are received, however, the
pilot bits are of little importance. What remains important is the TPC. The idea behind
the new slot format is to increase the number of bits to encode the TPC and decrease
the number of pilot bits while the uplink channel is idle. This way, additional redun-
dancy is added to the TPC field. As a consequence, the transmission power for the
control channel can be lowered without risking corruption of the information contained
in the TPC. Once user data transmission resumes, the standard slot format is used again
and the transmission power used for the control channel is increased again.

3.12.2 CQI Reporting Reduction and DTX and DRX
CQI Reporting Reduction
To make the best use of the current signal conditions in the downlink direction, the
mobile has to report to the network how well its transmissions are received. The quality
of the signal is reported to the network with the CQI alongside the user data in the
uplink direction. To reduce the transmit power of the mobile device while data is being
transferred in the uplink direction but not in the downlink direction, this feature
reduces the number of CQI reports.

UL HS‐DPCCH Gating (Gating = Switch‐Off)
When no data is being transmitted in either the uplink or the downlink direction, the
uplink control channel (UL DPCCH) for HSDPA is switched off. Periodically, it is
switched on for a short time to transmit bursts to the network to maintain synchroniza-
tion. This improves battery life for applications such as web browsing, lowers battery
consumption for VoIP and reduces the noise level in the network (i.e. allowing more
simultaneous VoIP users). Figure 3.46 shows the benefits of this approach.

F‐DPCH Gating
Mobile devices in HSDPA active mode always receive a Dedicated Physical Channel
(DPCH) in the downlink direction, in addition to high‐speed shared channels, which
carries power control information and layer 3 radio resource (RRC) messages, for
example, for handovers, channel modifications and so on. The Fractional DPCH feature
puts the RRC messages on the HSDPA shared channels and the mobile thus only has to
decode the power control information from the DPCH. At all other times the DPCH is
not used by the mobile (thus it is fractional). During these times, power control infor-
mation is transmitted for other mobiles using the same spreading code. This way, up to
10 mobile devices use the same spreading code for the dedicated physical channel but