Page 206 - From GMS to LTE

192 From GSM to LTE-Advanced Pro and 5G

connection with the mobile device at the beginning. This is not part of the MAC‐es
layer but part of the RRC algorithm, which has to be enhanced for HSUPA as well.
As the RNC treats an E‐DCH channel like a DCH, the mobile device is in Cell‐DCH
state while an E‐DCH is assigned. While scheduling of the data is part of the
Node‐B’s job, overall control of the connection rests with the RNC. Thus, the RNC
can decide to release the E‐DCH to a mobile device after some period of inactivity
and put the mobile device into Cell‐FACH state. Therefore, HSUPA becomes part
of the Cell‐DCH state and thus part of the overall radio resource management as
described in Section 3.5.4.
One of the reasons for enhancing the dedicated connection principle to increase
uplink speeds instead of using a shared‐channel approach is that it enables the soft
handover principle to be used in the uplink direction. This is not possible with a
shared‐channel approach, which is used by HSDPA in the downlink direction,
because cells would have to be synchronized to assign the same timeslots to a user.
In practice, this would create a high signaling overhead in the network. By using
DCHs the timing between the different mobile devices that use the same cells in
soft handover state is no longer critical as they can send at the same time without
being synchronized. The only issue arising from sending at the same time is the
increased noise level in the cells. However, neighboring cells can minimize this by
instructing mobiles in soft handover state to decrease their transmission power via
the Relative Grant Channel (E‐RGCH) as further described below. Using soft
handover in the uplink direction might prove very beneficial as the mobile device’s
transmit power is much less than that of the Node‐B. Furthermore, there is a higher
probability that one of the cells can pick up the frame correctly and thus the mobile
device has to retransmit a frame only if all cells of the Active Set send a Negative
Acknowledge message for a frame. This in turn reduces the necessary transmission
power on the mobile device side and increases the overall capacity of the air
interface.
Another advantage of the dedicated approach is that mobile devices also do not have
to be synchronized within a single cell and thus do not have to wait for their turn to send
data. This further reduces RTD times.

3.11.3 E‐DCH Scheduling
If the decision is made by the RNC to assign an E‐DCH to the mobile device, the bearer
establishment or modification messaging is very similar to establishing a standard
DCH. During the E‐DCH establishment procedure, the RNC informs the mobile device
of the Transport Format Combination Set (TFCS) that can be used for the E‐DCH.
A TFCS is a list (set) of datarate combinations, coding schemes and puncturing patterns
for different transport channels that can be mapped on to the physical channel. In prac-
tice, at least two channels, a DTCH for user data and a DCCH for RRC messages, are
multiplexed over the same physical channel (E‐DPDCH). This is done in the same way
as for a standard dedicated channel. By using this list, the mobile device can later select
a suitable TFC for each frame depending on how much data is currently waiting in the
transmission buffer and the current signal conditions. By allowing the RNC to flexibly
assign a TFC set to each connection, it is possible to restrict the maximum speed on a
per‐subscriber basis based on the subscription parameters. During the E‐DCH setup