Saturday, December 17, 2011

Optimization Parameters and Targets | AUTOMATIC CELL CONFIGURATION

Optimization Parameters and Targets

The targets of the radio network optimization are mainly twofold. First target is to minimize the interference caused by the individual cells, while a sufficient coverage over the planning area is maintained. This is in general a trade-off and needs to be balanced, e.g., tilting down the antenna causes lower coverage, but also lower interference in neighboring cells and thus a potentially higher network capacity. Second target is the traffic distribution between cells. It is desirable to maintain similar cell loading of neighboring cells to minimize blocking probabilities and to maximize spare capacity for traffic fluctuations and a future traffic evolution.
The most effective parameter in network optimization is the antenna tilt. Antenna tilts need to be set such that the traffic within the “own” cell is served with maximum link gain, but at the same time the interference in neighboring cells is minimized. The possible tilt angles are typically restricted because of technical and civil engineering reasons. Especially in case of collocated sites with multiband antennas there might be strong restrictions on the possible tilt angles to be taken into account during optimization.
The transmitted pilot channel power and the other common channel powers, which are typically coupled by a fixed offset, are also vital parameters of network optimization. It needs to be assured that these channels are received with sufficient quality by all users in the serving cell. At the same time a minimization of the common channel powers yields significant capacity gains: Firstly, additional power becomes available for other (user traffic) channels, and secondly, the interference is reduced. The gains obtained from reducing the pilot power are often underestimated. It is important to note that in a capacity-limited WiMAX network (e.g., in urban areas) the reduction of pilot power levels by a certain factor also reduces the total transmit power of cells and as a consequence the cell loading by up to the same factor.
Optimization of azimuth angles of sectored sites is of great importance in particular in case of antennas with rather small horizontal beam-width (e.g., 65° vs. 90° in case of three-sectored sites). In this case the difference between antenna gains in direction of the main lobe and the half-angle between neighboring sectors is comparatively large, and cells of neighboring sites might need to be adjusted such that maximum coverage is achieved. It is observed that during optimization azimuth changes are in particular introduced to reduce coverage problems. For possible azimuth angles typically even stronger restrictions apply than for the tilt angles.
The antenna height is also often a degree of freedom for the optimization. Higher antennas can provide better coverage, but on the other hand also cause more interference in neighboring cells. Additional important parameters are the antenna type and the number of deployed sectors at a site. Both parameters are closely coupled, as a larger number of sectors also suggest the use of antenna pattern with smaller horizontal beam-width. The choice of sectorization is typically a trade-off between increased network capacity and higher monetary cost.
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