In the WCDMA system, the capacity of the uplink is lower that of the downlink, because the BS has better receiving technologies than the MS, such as the antenna diversity and multi-subscriber detection. In the UMTS, the downlink capacity is considered more important than the uplink capacity because the asymmetrical traffic is closely related to download services. In the 3G, more consideration is given to the downlink capacity. The factors causing the differences between the capacity uplink and the downlink capacity are the orthogonal code and the BS transmit diversity. WCDMA system adopts long extended code to distinguish cells in the downlink and subscribers in the uplink.
7.7.2 Downlink Orthogonal Code
The downlink orthogonal code will affect the capacity, so it is considered to adopt irregular short code, which is orthogonal in the case of one path. Part of the orthogonality will disappear in case of multi-path, causing mutual interference among the subscribers in the cell. In the GSM system, there is no mutual interference in the same cell, because the time domain is orthogonal.
7.7.3 Link Budget
To estimate the maximum area of the cell, it is required to carry out RLB ( Radio link budget) calculation. In the RLB, it is necessary to consider such factors as antenna gain, cable loss, diversity gain and fading margin. The RLB calculation result is the maximum allowed transmission path loss, based on which the cell radius and the number of required sites may be obtained. Compared with the TDMA-based radio access system (like the GSM system), the WCDMA system has some special problems in the link budget, including interference margin, fast fading margin, transmit power increasing and soft handover gain.
7.7.4 Capacity and Coverage Analysis
If the maximum allowable path loss of a cell is known, it is easy to calculate the coverage area of the cell with the known transmission model. If the coverage area of the cell is known, it is required to select such sites as the channel elements, sector and carrier frequency and site density (cell radius) to configure, so as to satisfy the traffic requirements. The cell radius is also closely related to the number of access subscribers. Therefore, the coverage is correlative with the capacity, and the network operator should know the subscriber distribution and the growth trend, because it will directly affect the coverage. The network should be configured properly to meet the traffic requirements and reduce the network cost as much as possible. The number of carrier frequencies, number of sectors, cell load, number of subscribers and cell radius will affect the final result.
7.7.5 Soft Capacity
The number of required cells may be calculated with the available spectrum, subscribers amount forecast and traffic density information. The traffic density is indicated by erl. The calculation is conditional on the given congestion. If the congestion is caused by the hardware, then the result may be obtained in Table B. If the maximum capacity is caused by the interference, then the capacity is defined as soft capacity. For the system with soft capacity restriction, it cannot be calculated with Ireland Table. The total channel capacity is larger than the average number of the channels of each cell. The neighboring cells share a part of interference, so more traffic may be processed if the congestions are the same. If the interference from the neighboring cell is less, there will be more available channels in the middle cell.
If the cell has few channels, that is, there are high bit rate real-time subscribers, then the average load should be reduced to ensure low congestion. With the reduction of average load, there is additional capacity to be provided to the neighboring cell for use. This part of capacity is borrowed from the neighboring cell, so the interference sharing provides soft capacity. It is important for the high bit rate real-time subscribers to connect as shown in the figure.
The soft capacity depends on the transmission environment (i.e. network planning). The value of a is of great importance, and determined by the equipment radio resource management algorithm.
In the WVDMA system, all subscribers share the interference source in the space channel, so the analysis cannot be conducted separately. The mutual affects among subscribers result in the changes of transmit power, which in turn cause further changes and repeating mutual affects. Forecast processing repeats unless it is stable. In the WCDMA system, the uplink/downlink fast power control, soft handover/softer handover and orthogonal downlink channels will affect its performance. Unlike the GSM, the BS sensitivity is depending on the number of subscribers and the rate of subscribers. It is constant in the GSM.
The interference planning and capacity planning are even more important in the 3G.
7.7.6 Planning Conclusion
The WCDMA cell capacity (at full load) is inversely proportional to the cell coverage radius. Under certain I/C, reducing the cell radius can improve the cell capacity. Essentially, it is to offset the cell capacity loss caused by the system noise. Reducing the cell capacity can increase the coverage radius. Of course, the coverage area should be subject to the radio coverage conditions. In the cell planning, it is required to determine proper coverage radius to meet the cell capacity requirements.
The cell coverage requirements for the capacity in the urban and rural are different. In the urban, there are many hot areas and the unit area traffic demand is high. In such a case, to solve the capacity problem is a main task. Whereas, in the rural area, the traffic demand is low and the main task is to solve the coverage problem. The CDMA system features soft capacity and can satisfy this requirement.
The unit area capacity may be improved by the cell splitting and multi-sector. Such improvement may be realized easier in the CDMA cell, which dynamically changes the cell coverage by controlling the pilot transmit power.