Page 142 - From GMS to LTE

128 From GSM to LTE-Advanced Pro and 5G

Spreading, e.g., with OVSF SF = 8–512 Scrambling

User data
RF-modulator

Datarate: Chiprate: Chiprate:
e.g. 64 kbit/s 3.84 MChips/s 3.84 MChips/s
(+channel
coding)

Figure 3.8 Spreading and scrambling.
Another reason for using a unique scrambling code per mobile device in the uplink
direction is signal propagation delays. As different users are at different distances from
a base station the signals take a different amount of time to arrive. In the GSM radio
network this was solved by controlling the timing advance. The use of a timing advance,
however, is not possible in the UMTS radio network because of the soft handover state,
(Section 3.7.1) in which the mobile device communicates with several base stations at
the same time. As the mobile device is at a different distance from each base station it
communicates with simultaneously, it is not possible to synchronize the mobile device
to all base stations because of the different signal propagation delays. Therefore, if no
scrambling code was used, the mathematical equation shown in Figure 3.5 would not
work anymore as the chips of the different senders would be out of phase with each
other and the result of the equation would change (Table 3.2).

Table 3.2 Spreading and scrambling in uplink and downlink directions.

Downlink Uplink
Spreading ● Addressing of different users ● Controls the individual datarate for each
● Controls the individual datarate user
for each user
Scrambling ● Ensures consistent spectral ● Ensures consistent spectral distribution
distribution ● Differentiates users
● Used by the mobile device to ● Removes the need for a timing advance by
differentiate base stations preserving the orthogonal nature of the
codes necessary for soft handover

3.3.4 UMTS Frequency and Cell Planning
As all cells in a UMTS radio network can use the same frequency, the frequency plan is
greatly simplified compared to a GSM radio access network. While it is of paramount
importance in a GSM system to ensure that neighboring cells use different frequencies,
it is quite the reverse in UMTS, as all neighboring stations use the same frequency. This
is possible because of the CDMA characteristics, as described in the previous para-
graphs. While a thorough and dynamic frequency plan is indispensable for GSM, no