Page 217 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 203

However, subscribers in those areas can still benefit from higher datarates close to the
base station. As subscribers close to base stations can transmit and receive data faster,
more time is available to send data to other subscribers. In other words, the overall
throughput that can be reached in the cell increases, which in turn benefits all subscrib-
ers in the cell.
Another significant impact on individual speeds and overall cell throughput is the
development of advanced receivers in the mobile devices. Advanced HSPA devices use
reception (RX) diversity, that is, two antennas and two receiver chains, significantly
enhancing the device’s ability to receive a data stream. Even more advanced HSPA
receivers that have found their way into today’s devices use interference cancellation
algorithms that are able to distinguish between noise created by neighboring base sta-
tions transmitting on the same frequency and the signal received from the serving base
stations. This is especially beneficial at the cell edge.

3.13.2 Radio Resource State Management
From a throughput point of view an ideal mobile device would always be instantly
reachable and always ready to transfer data whenever something is put into the trans-
mission buffer from higher protocol layers. The downside of this is high power con-
sumption which, as will be shown in the next section, would drain the battery within
only a few hours. Therefore, a compromise is necessary.
In practice today, mobile devices are either in RRC idle state when not transferring
any data or in Cell‐PCH or URA‐PCH state. In these states, the mobile only listens to
the PCH and only reestablishes a physical connection when it receives a Paging message
or when new data packets arrive from applications running on the device. The time it
takes to switch from RRC idle to Cell‐DCH on the high‐speed shared channel is around
2.5 seconds and is the major source of delay noticeable to a user when surfing the web.
In Cell‐PCH and URA‐PCH states, the return to the fast Cell‐DCH state takes only
around 0.7 seconds as the logical connection to the RNC remains in place despite the
physical bearer no longer being present. As a consequence, no complex connection
request, authentication and activation of ciphering procedures are required. In practice,
it can be observed that most networks have adopted this approach, which is especially
beneficial for users due to the noticeably shorter delay when they click on a link on a
web page before a new page is loaded. While in the Cell‐DCH state, the mobile device
can transmit and receive at any time and round‐trip packet delay is in the order of
60–85 milliseconds with a category 24 HSDPA and category 6 HSUPA device. As Cell‐
DCH state requires a significant amount of power on the mobile device’s side even if no
data is transferred, most UMTS networks move the connection to a more power‐
conserving state after an inactivity time of only a few seconds. Typical inactivity timer
values range between 5 and 10 seconds. After the inactivity timer has expired, many
networks then assign Cell‐FACH state to the device. In this state, only a narrow down-
link channel is observed and no control or channel feedback information is sent or
received from the network. As the channel is relatively narrow, RTD times are around
300 milliseconds. If there is renewed data traffic beyond a network‐configurable threshold,
the connection is set into Cell‐DCH state again.
If no further data traffic occurs, the network will set the connection to an even more
power‐conserving state after a further 10–15 seconds. In practice, this is either the idle