The capacity of the W–CDMA network is also limited to the amount of self-induced interference by the mobiles using the same sector face. This self-induced interference is the disruption of the orthogonality of the codes by the multipath in the wireless network. Although the fingers in the rake receiver will provide some additional diversity in a high multipath environment, the multipath is detrimental to the orthogonality of the W–CDMA channel. For the previous results, an orthogonality factor of 0.4 is used. A study of maximum orthogonality and no orthogonality in relation to antenna beamwidth can be seen in Figure 8 and Figure 9.

Figure 8. Capacity Predictions for a Typical Three-Sector Cell Site Configuration
Rotated by 30 Degrees with a Maximum Level of Orthogonality
Figure 9. Capacity Predictions for a Typical Three-Sector Cell Site Configuration
Rotated by 30 Degrees with No Measurable Amount of Orthogonality

Here, the optimal beamwidth of the antenna is quite invariant to the level of orthogonality in the network. Over the complete range of orthogonality, the optimal beamwidth varies from 75 to 80 degrees.

The previous two figures show that the orthogonality of the W–CDMA channel significantly impacts the network capacity. Another contributor to the degradation of the W–CDMA channel is the radiation from the antennas in the backward direction. All of the results presented previously are for antennas with a front-to-side ratio of 30 dB. If the radiation from the base station antenna is increased in the backward direction in the form of a single back lobe, the degradation of the capacity can be studied. The results of this numerical study are shown in Figure 10. The different lines are for the back-lobe beamwidths 60, 30, and 10 degrees.

Figure 10. Capacity Predictions for a Typical Three-Sector Cell Site Configuration
Rotated by 30 Degrees for Different Back-Lobe Beamwidths

The results in Figure 10 show that as the total energy in the backward directed lobe is increased by either increasing the beamwidth of the back lobe or by decreasing its front-to-back ratio, the capacity of the W–CDMA network diminishes. In general, the capacity of the W–CDMA network can be made optimized by choosing antennas that have the proper beamwidth for the given environment and cell site configuration.