Page 145 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 131

parameter settings of the operator. As in other areas, the UMTS standard does not
dictate a specific solution to these issues. Therefore, network vendors that have imple-
mented clever solutions can gain a competitive advantage.

3.3.6 Advantages of the UMTS Radio Network Compared to GSM
While in the previous paragraphs the basic properties and methods of the UMTS
Release 99 air interface have been introduced, the following paragraphs describe how it
helped to overcome the limitations of GPRS.
One of the main reasons for the long delay times of early GPRS implementations was
the constant reassignment of resources for bursty data traffic. UMTS Release 99 solved
this issue by assigning a dedicated channel not only for voice calls but for packet data
connections as well. The channel remained dedicated to the user for some time, even if
there was no data transfer. A downside of this approach was that the spreading code was
not available to other users. As only control information was sent over the established
channel during times of inactivity, the interference level for other users decreased. As a
result, some of the overall capacity of the cell was lost by keeping the spreading code
assigned to a dormant user. From a user’s point of view, the spreading code should only
be freed up for use by someone else if the session remained dormant for a prolonged
duration. Once the system decided to reassign the code to someone else, it also assigned
a higher spreading factor to the dormant user, of which a greater number existed per
cell. If the user resumed data transmission, there was no delay as a dedicated channel
still existed. If required, the bandwidth for the user could be increased again quite
quickly by assigning a code with a shorter spreading factor. The user, however, did not
have to wait for this as in the meantime data transfer was possible over the existing
channel.
In the uplink direction, the same methods were applied. It should be noted, however,
that while the user was assigned a code, the mobile device was constantly transmitting
in the uplink direction. The transmission power was lower while no user data was sent
but the mobile device kept sending power control and signal quality measurement
results to the network.
While this method of assigning resources was significantly superior to GPRS it soon
became apparent that it had its own limitations concerning maximum data‐transfer
rates that could be achieved and the number of simultaneous users that could be active
in a cell. Later releases of the standard have therefore changed this concept and have put
the logically dedicated downlink channel on a physically shared channel. This concept
is referred to as HSDPA and is described in the later part of the chapter.
In both UMTS Release 99 and HSDPA, if the user remains dormant for a longer time
period the network removes all resources on the air interface without cutting the logical
connection. This prevents further wastage of resources and also has a positive effect on
the overall operating time of a mobile device. The disadvantage of this approach is a
longer reaction time once the user wants to resume data transfer.
This is why it is beneficial to move the user into the Cell‐FACH (Forward Access
Channel) state after a longer period of inactivity. In this state, no control information is
sent from the mobile device to the network and no dedicated channel is assigned to the
connection anymore. The different connection states are described in more detail in
Section 3.5.4.