Page 208 - From GMS to LTE

194 From GSM to LTE-Advanced Pro and 5G

E-HICH, E-RGCH E-HICH E-RGCH E-HICH E-AGCH + E-HICH, E-RGCH
E-HICH, E-RGCH

1 2 3 4 5

RNC

Figure 3.45 Serving E‐DCH cell, serving RLS and non‐serving RLS.

The serving E‐DCH cell and all other cells of the Node‐B that are part of the Active
●
Set of a connection (cells 3 and 4 in Figure 3.45) are part of the serving radio link set.
The commands sent over the E‐RGCH of these cells are identical and thus the mobile
device can combine the signals for decoding.
All other cells of the Active Set are part of the non‐serving radio link set (cells 1, 2 and
●
5 in Figure 3.45). The mobile device has to decode all E‐RGCHs of these cells sepa-
rately. Cells in the non‐serving RLS can only follow send, hold or down commands.
If an ‘up’ command is received from the serving RLS, the mobile device is allowed to
increase its transmission power only if at the same time no ‘down’ command is received
by one or more cells of the non‐serving RLS. In other words, if a ‘down’ command is
received by the mobile device from any of the cells, the mobile device has to immedi-
ately decrease its power output. This way, only the serving E‐DCH is able to increase or
decrease the power output of the mobile via the relative grant channels while all other
cells of the non‐serving RLS are only permitted to decrease the power level.
It should be noted that in a real environment it is unlikely that five cells as shown in
Figure 3.45 would be part of the Active Set of a connection, as the benefit of the soft
handover would be compromised by the excessive use of air interface and Iub link
resources. Thus, in a normal environment, it is the goal of radio engineering to have two
or at most three cells in the Active Set of a connection in soft handover state.
As has been shown, the Node‐B has quite a number of different pieces of information
to base its scheduling decision on. The standard, however, does not describe how these
pieces of information are used to ensure a certain QoS level for the different connec-
tions and leaves it to the network vendors to implement their own algorithms for this
purpose. Again, the standards encourage competition between different vendors, which
unfortunately increases the overall complexity of the solution.
To enable the use of the E‐DCH concept for real‐time applications like voice and
video over IP in the future, the standard contains an optional scheduling method, which