Page 216 - From GMS to LTE

202 From GSM to LTE-Advanced Pro and 5G

A scenario where the one‐tunnel option is not applicable is international roaming.
Here, the SGSN has to be in the loop to count the traffic for inter‐operator billing pur-
poses. Another case where the one‐tunnel option cannot be used is when the SGSN is
asked by a prepaid system to monitor the traffic flow. This is only a small limitation,
however, since in practice it is also possible to perform prepaid billing via the GGSN.
Proprietary enhancements even aim to terminate the user data tunnel at the Node‐B,
bypassing the RNC as well. However, this has not found the widespread support in 3GPP.

3.13 HSPA Performance in Practice

After the introduction above of how UMTS works from a technical point of view, this
section now takes a look at which features are used in practice and how. Looking back
at early networks, voice calls are still handled the same way now as then. The way packet
data is transferred, however, has completely changed with HSPA. This section is there-
fore a snapshot of how the system performs in practice at the time of publication.

3.13.1 Throughput in Practice
In practice, the experienced throughput depends on a variety of different factors:

The maximum throughput capability of the mobile device.
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The sophistication of the receiver and antenna of the mobile device.
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The capability of the network.
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The radio conditions at the place of use. This includes the signal level received from
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a base station and the interference of neighbor cell transmissions on the same
frequency.
The bandwidth of the backhaul link between the base station and the rest of the
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network.
The number of other users in the cell who are actively exchanging data at the same time.
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With HSDPA category 24 and HSUPA category 6 devices, downlink speeds of 30
Mbit/s and uplink speeds of around 3–4 Mbit/s can be reached in practice under very
good signal conditions. But even under less favorable conditions, speeds of several
megabits per second can still be achieved. It can be observed that even small changes in
the position of the device or rotation of the antenna can have a significant impact on the
overall transmission speed. For example, by moving the device only by a few centime-
ters, throughput can easily change by several megabits per second. When using a 3G
USB stick indoors in stationary situations, it is often advisable to use a USB extension
cable and put the USB stick closer to a window to increase data‐transfer speeds. Also,
increasing the height of the location of the USB stick improves reception, especially
under very weak signal conditions. This way it is possible to reach speeds well over
1 Mbit/s, even if the device receives the network with a very low signal. Over time,
higher and higher theoretical peak datarates have been specified in the standards. This comes
at the expense of the size of the areas in which these speeds can be reached. A study
published in Bergman [24] shows that transmission modes requiring MIMO and
64‐QAM modulation can only be used close to base stations. In the majority of the
coverage area the same lower‐order modulation and coding schemes are used as before.