Page 141 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 127

Table 3.1 Spreading factors and datarates.

Spreading factor Raw datarate User datarate
(downlink) (kbit/s) (kbit/s) Application

8 960 384 Packet data
16 480 128 Packet data
32 240 64 Packet data and video telephony
64 120 32 Packet data
128 60 12.2 Voice, packet data, location
updates, SMS
256 30 5.15 Voice

Table 3.1 shows the spreading factors in the downlink direction (from the Node‐B to
the mobile device) as they are used in a real system. The raw datarate is the number of
bits transferred per second. The user datarate results from the raw datarate after
removal of the extra bits that are used for channel coding, which is necessary for error
detection and correction, signaling data and channel control.

3.3.3 Scrambling in Uplink and Downlink Direction
Using OVSF codes, the datarate can be adapted for each user individually while still
allowing differentiation of the data streams with different speeds. Some of the OVSF
codes are quite uniform. C(256, 1), for example, is only comprised of chips with value ‘1’.
This creates a problem further down the processing chain, as the result of the modula-
tion of long sequences that never change their value would be a very uneven spectral
distribution. To counter this effect the chip stream that results from the spreading
process is scrambled. This is achieved by multiplying the chip stream, as shown in
Figure 3.8, with a pseudo random code called the scrambling code. The chip rate of 3.84
MChips/s is not changed by this process.
In the downlink direction the scrambling code is also used to enable the mobile device
to differentiate between base stations. This is necessary as all base stations of a network
transmit on the same frequency. In some cases mobile operators have bought a license
for more than a single UMTS frequency. However, this was done to increase the capac-
ity in densely populated areas and not as a means to make it easier for mobile devices to
distinguish between different base stations. The use of a unique scrambling code per
base station is also necessary to allow a base station to use the complete code tree
instead of sharing it with the neighboring cells. This means that in the downlink direc-
tion, capacity is mainly limited by the number of available codes from the code tree as
well as the interference of other base stations as experienced by the mobile device.
In the uplink direction, on the other hand, each mobile device is assigned its own
scrambling code. Therefore, each mobile device could theoretically use all codes of
the code tree. This means that in the uplink direction the system is not limited by the
number of codes but by the maximum transmitting power of the mobile device
and by the interference that is created by other mobile devices in the current and
neighboring cells.