Page 200 - From GMS to LTE

186 From GSM to LTE-Advanced Pro and 5G

one of the Node‐Bs of the Active Set. On the basis of the configuration of the network,
the mobile device then reports to the RNC if a different cell of the Active or Candidate
Sets would provide better signal quality than the current cell. The RNC can then decide
to redirect the data stream to a different cell. As the concept is different from UMTS
soft handover, the standards refer to this operation as cell change procedure.
Compared to the cell update procedure of (E)GPRS, the cell change procedure of
HSDPA is controlled by the network and not by the mobile device. As the mobile device
is already synchronized with the new cell, a cell change only leads to a short interruption
of the data transfer on the HS‐PDSCHs.
Depending on the relationship between the old and the new cell, there are several
different kinds of cell changes:
Intra Node‐B cell change. Old and new cell are controlled by the same Node‐B. This
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is the simplest version of the operation as data that is still available in the buffer of the
Node‐B can simply be sent over the new cell.
Inter Node‐B cell change. Old and new cells belong to different Node‐Bs. In this
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scenario, the RNC has to instruct the new Node‐B to allocate resources for the
HSDPA connection. This is done in a similar way to establishing a new connection, as
shown in Figure 3.41. User data that is still buffered in the old Node‐B is lost and has
to be retransmitted by the RLC layer, which is controlled in the RNC.
Cell change with Iur interface. If the old and new cells are under the control of
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different RNCs, the HSDPA connection has to be established over the Iur interface.
Cell change without Iur interface. If the old and new cells are under the control of
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different RNCs which are not connected via the Iur interface, an SRNS relocation has
to be performed, which also involves core network components (SGSN and possibly
also the MSC).
Old and new cells use different frequencies (interfrequency cell change). In this
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scenario additional steps are required in the mobile device to find cells on different
frequencies and to synchronize them before data transmission can resume.
Inter‐RAT cell change. If the subscriber leaves the UMTS coverage area completely,
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a cell change procedure from UMTS/HSDPA to GSM has also been specified. Similar
to the interfrequency cell change described above, HSDPA connections can use a
compressed mode similar to that of dedicated channels to allow the mobile device to
search for cells on other frequencies.
During all scenarios it is, of course, also possible that an additional voice or video call
is established. This further complicates the cell change/handover as this connection
also has to be maintained next to the data connection and handed over into a new cell.

3.11 High‐Speed Uplink Packet Access (HSUPA)

Owing to the emergence of peer‐to‐peer applications like multimedia calls, video
conferencing and social networking applications, the demand for uplink bandwidth is
continually increasing. Other applications, such as e‐mail with large file attachments or
large MMS messages, also benefit from higher uplink datarates. UMTS uplink speeds
were not enhanced until 3GPP Release 6. Hence, for a long time the uplink was still
limited to 64–128 kbit/s and to 384 kbit/s in some networks under ideal conditions,