7.7.1 Introduction
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.
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