UMTS Networks : Architecture, Mobility and Services

108 UMTS Networks
5.2.4 Cell capacity
In WCDMA technology, all users share the common physical resource: a frequency
band in 5-MHz slices. All users of the WCDMA TRX coexist on the frequency band at
the same time and different transactions are recognised using spreading codes. In the
first UMTS systems, the UTRAN used to employ the WCDMA-FDD variant. In this
variant, the Uu interface’s transmission directions use separate frequency bands. One of
the most interesting questions (and one of the most confusing) for people is the capacity
of the WCDMA TRX. In the Global System for Mobile Communications (GSM),
TRX capacity calculation is a very straightforward procedure, but because the radio
interface in WCDMA is handled differently and system capacity is limited by variable
factors, the capacity of the WCDMA TRX is not very easy to determine.
Let us now outline a rough theoretical estimate of WCDMA TRX capacity, based on
radio conditions. In order to simplify the issue, we must make some assumptions:
. All subscribers in the TRX coverage area are equally distributed so that they are
situated at equal distances from the TRX antenna.
. The power level they use is the same and, thus, the interference they cause is at the
same level.
. Subscribers in the TRX area use the same base band bit rate (i.e., the same symbol
rates).
Under these circumstances a value called ‘‘processing gain’’ ðG pÞ can be defined. G p is a
relative indicator that provides information about the relationship between the whole
bandwidth available ðB RFÞ and the base band bit rate ðB InformationÞ:
BRF
Gp ¼
BInformation
There is another way to express G p by using chip and data rates:
Gp ¼
Chip rate
Data rate
As a result both ways (when expressed in dB values) show an improvement in the Signal
to Noise (S/N) ratio between the received signal and the output of the receiver.
Later on, we can see Gp is actually the same as the Spreading Factor (SF). Note that
the base band bit rate discussed here is the one achieved after rate matching. In this
process the original (user) bit rate is adjusted to the bearer bit rate. Bearer bit rates are
fixed (e.g., 30 kb/s, 60 kb/s, 120 kb/s, 240 kb/s, 480 kb/s and 960 kb/s). The system chip
rate is constant: 3.84 Mchip/s (38,400,000 chip/s). Hence, as an example a bearer having
a bit rate of 30 kb/s will have an SF of 128:
Gp ¼ 38,400,000
30,000
¼ 128 ¼ SF