Page 204 - From GMS to LTE

190 From GSM to LTE-Advanced Pro and 5G

two channels being used with SF = 2 and the other two with SF = 4. In terms of frame
length, 10 milliseconds are used for the E‐DPDCH by default with 2‐millisecond frames
being standardized as optional.
The Enhanced Dedicated Physical Control Channel (E‐DPCCH) is used for physical
layer control information. For each E‐DPDCH frame, a control frame is sent on the
E‐DPCCH to the Node‐B, which, most importantly, contains the 7‐bit Traffic Format
Combination ID (TFCI). Only by analyzing the TFCI is the Node‐B able to decode the
MAC‐frame on the E‐DPDCH as the mobile device can choose the spreading factor and
coding of the frame from a set given to it by the Node‐B to adapt to the current signal
conditions and uplink‐user‐data buffer state. Furthermore, each frame on the E‐DPCCH
contains a 2‐bit Retransmission Sequence Number (RSN) to signal HARQ retransmis-
sions and the RV (see Section 3.10.2) of the frame. Finally, the control frame contains a
so‐called ‘Happy’ bit to indicate to the network if the maximum bandwidth currently
allocated to the mobile device is sufficient or if the mobile device would like the net-
work to increase it. While the spreading factor of the physical data channel is variable,
a constant spreading factor of 256 is used for the E‐DPCCH.
A number of existing channels which might also be used together with an E‐DCH are
shown in the middle and on the right of Figure 3.42. Usually, an E‐DCH is used together
with HSDPA HS‐DSCHs, which require a separate DPCCH to send control information
for downlink HARQ processes. To enable applications like voice and video telephony
during an E‐DCH session a mobile must also support simultaneous Release 99 dedi-
cated data and control channels in the uplink. This is necessary as these applications
require a fixed and constant bandwidth of 12.2 and 64 kbit/s, respectively. In total, an
E‐DCH‐capable mobile device must therefore be able to simultaneously encode the
data streams of at least five uplink channels. If multicode operation for the E‐DPDCH
is used, up to eight code channels are used in the uplink direction at once.
In the downlink direction, HSUPA additionally introduces two mandatory and one
optional channel to the other already numerous channels that have to be monitored in
the downlink direction. Figure 3.43 shows all channels that a mobile device has to
decode while having an E‐DCH assigned in the uplink direction, HSDPA channels in
the downlink direction and an additional DCH for a simultaneous voice or video ses-
sion via a circuit‐switched bearer.
While HSUPA only carries user data in the uplink direction, a number of control
channels in the downlink direction are nevertheless necessary. For the network to be
able to return acknowledgements for received uplink data frames to the mobile device,
the Enhanced HARQ Information Channel (E‐HICH) is introduced. The E‐HICH is a
dedicated channel, which means that the network needs to assign a separate E‐HICH to
each mobile device currently in E‐DCH state.
To dynamically assign and remove bandwidth to and from individual users quickly, a
shared channel called the Enhanced Access Grant Channel (E‐AGCH) is used by the
network, which must be monitored by all mobile devices in a cell. A fixed spreading
factor of 256 is used for this channel. Further details about how this channel is used to
issue grants (bandwidth) to the individual mobile devices are given in Section 3.11.3.
Finally, the network can also assign an Enhanced Relative Grant Channel (E‐RGCH)
to individual mobile devices to increase or decrease an initial grant that was given on
the E‐AGCH. The E‐RGCH is again a dedicated channel, which means that the network
has to assign a separate E‐RGCH to every active E‐DCH mobile device. The E‐RGCH is