Page 198 - From GMS to LTE

184 From GSM to LTE-Advanced Pro and 5G

DCH with 384 kbit/s which uses a spreading factor of 8, the transmission is around
eight times faster. This is achieved by using 64‐QAM modulation instead of QPSK,
which increases the maximum speed threefold, and by reducing the number of error
detection and correction bits while signal conditions are favorable.
In practice, many factors influence how fast data can be sent to a mobile device. The
following list summarizes, once again, the main factors:

signal quality;
●
number of active HSDPA users in a cell;
●
number of established channels for voice and video telephony in the cell;
●
number of users that use a dedicated channel for data transmission in a cell;
●
mobile device category;
●
antenna and transceiver design in the mobile device;
●
sophistication of interference cancellation algorithms in the mobile device;
●
bandwidth of the connection of the Node‐B to the RNC;
●
interference generated by neighboring cells;
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achievable throughput in other parts of the network, as high datarates cannot be
●
sustained by all web servers or other end points.
Transmission speeds that can be reached with HSDPA also have an impact on other
parts of the mobile device. Apart from the increased processing power required in the
mobile device, the interface to an external device, such as a notebook, needs to be capa-
ble of handling data at these speeds. Suitable technologies today are USB and Wi‐Fi
(Wireless LAN). While USB is mostly used by external 3G USB sticks, 3G to Wi‐Fi
routers or the Wi‐Fi tethering functionality built into most smartphones today are ide-
ally suited to connect several devices over UMTS to the Internet. In principle, these
devices work in the same way as the multipurpose Wi‐Fi access points typically found
in private households today for DSL connectivity (see Figure 6.6 in Chapter 6). The
advantage of using Wi‐Fi for connectivity is that no special configuration beyond the
Wi‐Fi password is required in client devices.

3.10.5 Establishment and Release of an HSDPA Connection
To establish an HSDPA connection, an additional DCH is required to be able to send
data in the uplink direction as well. If the network detects that the mobile device is
HSDPA‐capable during the establishment of an RRC connection, it automatically allo-
cates the necessary resources during the setup of the connection as shown in Figure 3.41.
To establish an HSDPA connection, the S‐RNC informs the Node‐B that a new con-
nection is required and the Node‐B will configure the HS‐PDSCH accordingly. In a
further step, the RNC then reserves the necessary resources on the Iub interface
between itself and the Node‐B. Once this is done, the network is ready for the high‐
speed data transfer and informs the mobile device via an RRC Radio Bearer
Reconfiguration message that data will now be sent on the HS‐DSCH. Once data is
received by the RNC from the SGSN, flow control information is exchanged between
the Node‐B and the RNC. This ensures that the data buffer of the Node‐B is not flooded
as the RNC has no direct information on or control of how fast the incoming data can
be sent to the mobile device. When the Node‐B receives data for a user, it is then the
task of the HSDPA scheduler in the Node‐B to allocate resources on the air interface