Page 207 - Antennas for Base Stations in Wireless Communications

180 Chapter Five

Figure 5.2b is another example of the W-CDMA system, which is the
3G network technology for multimedia cellular phone communication.
This technology has been used in Freedom of Mobile Multimedia Access
(FOMA) for the 3G services being offered by NTT DoCoMo since 2001.
In a W-CDMA system, the data at the same frequency channel can be
transmitted simultaneously. In this scheme, all users in the individual
cell employ the same frequency channel. Therefore, frequency planning
is unnecessary. System capacity decreases, however, due to interference
because the system shares the same transmission medium for frequency
and time domains for all users, including those in other cells. To further
increase the system capacity in IMT-2000, reducing the area of inter-
ference, namely reducing the overlapping area between adjacent cells,
is necessary. This results in the reduction of the Diversity Hand Over
(DHO) and of interference with other cells. As a result, base station
antennas are designed to have narrow beamwidths to fit the shapes of
cells with reduced overlapping areas.

5.1.2.2 Sectoring for CDMA System All mobile communication networks
operate with very limited frequency resources. Therefore, frequency
resources must be effectively used to ensure that all subscribers are able
to receive a consistent quality of service (QoS). Increasing the number
of sectors in each cell is an effective way to increase frequency usage
efficiency. The multiple sectors per cell scheme in mobile communica-
tion networks has been proposed to increase frequency usage efficiency
instead of the single sector per cell scheme. Using multiple sectors per
cell is also conducive to increasing W-CDMA system capacity and data
traffic per subscriber. Usually, three-sector or six-sector per cell structures
are used in mobile communication networks, as shown in Figure 5.3. As
mentioned previously, the number of sectors per cell is determined by
capacity and traffic.

Cell
Sector

(a) 3-sectors per cell (b) 6-sectors per cell
Figure 5.3 Sector structure