Page 213 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 199

follows. The network instructs the mobile to listen not only to the HS‐SCCH but also to
all packets being transmitted on one of the HS‐DSCHs. The mobile device then attempts
to blindly decode all packets received on that shared channel. To make blind decoding
easier, packets which are not announced on a shared control channel can only have one
of four transmission formats (number of data bits) and are always modulated using
QPSK. These restrictions are not an issue for performance, since HS‐SCCH‐less opera-
tion is only intended for low‐bandwidth real‐time services.
The checksum of a packet is additionally used to identify the device for which the
packet is intended. This is done by using the mobile device’s MAC address as an input
parameter for the checksum algorithm in addition to the data bits. If the device can
decode a packet correctly and if it can reconstruct the checksum, it is the intended
recipient. If the checksum does not match then either the packet is intended for a dif-
ferent mobile device or a transmission error has occurred. In both cases, the packet is
discarded.
In case of a transmission error, the packet is automatically retransmitted since the
mobile device did not send an acknowledgement (HARQ ACK). Retransmissions are
announced on the shared control channel, which requires additional resources, but
should not happen frequently as most packets should be delivered properly on the first
attempt.
It should be noted at this point that at the time of publication, HS‐SCCH‐less opera-
tion is not used in networks, as IMS VoIP has not yet been deployed in 3G networks.

3.12.5 Enhanced Cell‐FACH and Cell/URA‐PCH States
The CPC features described above aim to reduce power consumption and signaling
overhead in HSPA Cell‐DCH state. The CPC measures therefore increase the number
of mobile devices that can be in Cell‐DCH state simultaneously and allow a mobile
device to remain in this state for a longer period of time even if there is little or no data
being transferred. Eventually, however, there is so little data transferred that it no longer
makes sense to keep the mobile in Cell‐DCH state, that is, there is no justification for
even the reduced signaling overhead and power consumption. In this case, the network
can put the connection into Cell‐FACH or even Cell‐PCH or URA‐PCH state to reduce
energy consumption even further. The downside is that a state change back into Cell‐
DCH state takes much longer and that little or no data can be transferred during the
state change. In Releases 7 and 8, the 3GPP standards were thus extended to also use the
HS‐DSCHs for these states as described in 3GPP TR 25.903 [21]. In practice, this is
done as follows:
Enhanced Cell‐FACH. In the standard Cell‐FACH state, the mobile device listens to
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the secondary common control physical channel in the downlink direction for incom-
ing RRC messages from the RNC and for user data (IP packets). With the Enhanced
Cell‐FACH feature, the network can instruct a mobile device to observe a high‐speed
downlink control channel or the shared data channel directly for incoming RRC mes-
sages from the RNC and for user data. The advantage of this approach is that in the
downlink direction, information can be sent much faster. This reduces latency and
speeds up the Cell‐FACH to Cell‐DCH state‐change procedure. Unlike in Cell‐DCH
state, no other uplink or downlink control channels are used. In the uplink direction,
data packets can be sent in two ways. The first method is to use the RACH as before