Interference Floor Shaping. As mentioned above, the distances to strong transmitters as seen from an individual UE might in some cases be very large, when distant transmitters have almost line-of-sight channels to an UE. Countering such effects by forming cooperation areas over 7 or more sites seems to be unrealistic and ineffective. Therefore, these far-off interferers will generate an interference floor that may significantly limit the potential CoMP gains. This is illustrated by the 2-user rate region in Figure 5.4. Assuming an interference limited scenario with a very low noise floor, the rate region without cooperation will be limited by the inter-cell interference (blue area). The optimum rate region with CoMP is (or might be) just limited by the maximum modulation and coding schemes (MCS) for example 64QAM9/10 for LTE. This assumes that that the interference from other clusters is below the noise floor. In case there is a noise floor generated by the out-of- cluster cells, the rate region will be reduced significantly to the red area. (A similar effect would happen in case of a significant noise floor, but this would violate our main assumption of being in an interference-limited scenario.) Interference floor shaping as such is difficult to achieve in conventional networks as there is typically a trade-off between reduced interference at the outside of the cell versus signal quality within the cell, leaving relatively little room for optimization. From the beginning of cellular radio systems, antenna tilting has been used to localize signal power to the vicinity of the base stations. It is commonly used with good results for network planning [WLS+00].16 16 For the LTE Advanced case 1, increasing and optimizing the antenna tilts for all eNBs from 7 to 15 degree provided a substantial gain in spectral efficiency helping to fulfil the target requirements of e.g.
