Page 205 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 191

optional, however, and depending on the solutions of the different network vendors
there might be networks in which this channel is not used. If not used, only the E‐
AGCH is used to control uplink access to the network. Note that although all channels
are called ‘enhanced’, none of them has a Release 99 predecessor.
Besides these three control channels, an E‐DCH mobile device must also be able to
decode a number of additional downlink channels simultaneously. As HSUPA is used
together with HSDPA, the mobile device also needs to be able to simultaneously decode
the HS‐DSCHs as well as up to four HS‐SCCHs. If a voice or video call is established
besides the high‐speed packet session, the network will add another two channels in the
downlink direction, as shown in Figure 3.43 on the right‐hand side. In total, an E‐DCH
mobile must, therefore, be capable of decoding 10–15 downlink channels at the same
time. If the mobile device is put into soft handover state by the network (see Section 3.7.1)
the number of simultaneous channels increases even further as some of these channels
are then broadcast via different cells of the mobile device’s Active Set.

3.11.2 The E‐DCH Protocol Stack and Functionality

To reduce the complexity of the overall solution, the E‐DCH concept introduces two
new layers called the MAC‐e and MAC‐es. Both layers are below the existing MAC‐d
layer. As shown in Figure 3.44, higher layers are not affected by the enhancements and
thus the required changes and enhancements for HSUPA in both the network and the
mobile devices are minimized.
While on the mobile device the MAC‐e/es layers are combined, the functionality is
split on the network side between the Node‐B and the RNC. The lower‐layer MAC‐e
functionality is implemented on the Node‐B in the network. It is responsible for schedul-
ing, which is further described below, and the retransmission (HARQ) of faulty frames.
The MAC‐es layer in the RNC on the other hand is responsible for recombining
frames received from different Node‐Bs in case an E‐DCH connection is in soft
handover state. Furthermore, the RNC is also responsible for setting up the E‐DCH

Scheduling
error handling (HARQ)
Soft handover

MAC-d MAC-d
MAC-es
MAC-e/es
MAC-e E-DCH FP E-DCH FP

PHY PHY TNL TNL

Terminal (UE) Node-B RNC
Figure 3.44 E‐DCH protocol stack.