Definitions

Wideband Code Division Multiple Access (W-CDMA) ay isang klase ng CDMA channeling algorithm na may apat na beses mas malawak sa sistemang ginagamit ngaun sa Estados Unidos.

Overview

In January 1998, European Telecommunications Standards Institute (ETSI) decided to choose the W-CDMA to be the multiple access techniques for the third generation mobile telephone system. For a mobile communication system, a key parameter is the system capacity. A number of methods  to increase  system capacity in a W-CDMA network  are discussed. This tutorial presents results from different antenna pattern parameters and sectorization techniques for optimizing the maximum number of active subscribers.

Topics

1. Cell Site loading

2. Three-Sector Cell site Configuration

3. Six-Sector Cell site Configuration

4. Capacity Limitations due to Orthogonality  and Antenna performance

Cell Site loading

Given the limit on Spectrum Resources, efficient optimization techniques are needed for realizing cost-efficient ways to maximize the capacity in a W-CDMA network. The capacity is essentially interference limited by the overall interference level in the coverage area of each site. Given this simulation, antenna characteristics such as sector orientation, number of sector per site, horizontal beamwidth, and front-to-back ratio can be studied. The influence of the radio environment, can also give rise to different Orthogonality factors.

The signal to interference ratio typically limits the probability of having service in a dense cell site. The roll off of a service probability for voice traffic is quite quick - from 160 to 180 users per cell in a typical three-sector configuration compared to about 90 users for omni site configurations.

Three-Sector Cell Site Configurations

With the given set of simulation conditions, the practical number of users per W-CDMA channel is between 170 and 175. Depending on the antenna characteristics, the number of actual users that can talk on a given sector face will change.

The typical shape for cell site footprints is a hexagonal shape. For such a shape, the typical number of sectors per cell is one, three or six. When using three or six sectors, there are two standard orientations. These orientation are shown below.

Typical 3-sector Cell Site Configuration 

Typical 3-Sector Cell site configuration rotated by 30 degrees

 Using these two different types of three-sector cell site configurations, the optimum horizontal half power beamwidth is predicted.

Six Sector Cell Site Configurations

To further increase the capacity at each cell site with out increasing the number of W-CDMA carriers, a common practice is to increase the number of sector faces at the desired cell sites. A common six-sector configuration can be see as below;

 As demonstrated , the rotational orientation of the sites and the number of sectors per site require different beamwidth antennas for optimal capacity performance. The choice of the proper antenna beamwidth is a compromise between the interference in the network by having antennas with too broad a beamwidth and the coverage holes induced by having antennas with too narrow a beamwidth. For all the capacity predictions, the overhead for the pilot was assumed to be 5 percent, and the number of sectors involved in a soft handoff was limited to three.

Capacity Limitations due to Orthogonality & Antenna performance

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 networks. Although the receiver will provide some additional diversity in a high multipath environment, the multipath is detrimental to the Orthogonality of the W-CDMA channel. 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.