l change in how UEs are distributed in the network as a large number of UEs become concentrated within a small geographical area. In some cases a large mass event can be visible even in network level statistics as indicated by point 4 in Figure 13.31. Figure 13.32 shows the impact on the average reported CQI during the mass event on 24 cells covering the main event area. The downlink data volume per cell is almost quadrupled during the peak mass event hour compared to the data volume per cell during the normal hours. This traffic increase causes additional inter-cell interference and a big reduction in average reported CQI and therefore in spectral efficiency. The average CQI decrease during the mass event hours follows roughly the same slope depending on the downlink data volume as during the normal hours indicating that the inter-cell interference conditions have not changed during the mass event hours. The network spectral efficiency can be improved (CQI increased) by improving the dom- inance areas of the cells. One example is shown in Figure 13.33 where the improvement in spectral efficiency (CQI) is achieved by physical layer optimization. The physical layer optimization included antenna downtilt adjustments (limit the cell coverage spillage), antenna direction adjustments to cover only the areas with most of the people and downlink transmis- sion power decrease (can only be done in case of no coverage limitations). In the case above the physical layer optimization improved the spectral efficiency SO much that ~50% more downlink data volume per cell could be delivered with the same CQI compared to previous year. Capacity Optimization Average CQI year 1 Average CQI year 2 Linear (average CQI year 1) Linear (average CQI year 2) ==-0.0035x+10.17 = -0.009x + 10.867 Average DL data volume (MB) per cell per 15 min Figure 13.33 Spectral efficiency improvement by physical layer optimization The actual spectral efficiency in terms of bit/s/Hz per cell can be calculated based on the active cell throughput statistics 