ll transmission power is considerably higher than the transmission power of pico cell. UE selects the cell based on the downlink power which means that UE is much closer to pico cell before making cell reselection. Therefore, the uplink reception may be better via pico cell while the downlink reception works better from the macro cell. The studied HetNet scenario is shown in Figure 7.32 with four pico cells under each macro cell. The CoMP performance depends on the accuracy of the channel estimation. Demodulation reference signals are utilized for the channel estimation. eNodeB in the same CoMP set needs to estimate the channel of more than one UE transmitted in the same subband. In order to distinguish channels from different UEs, it is important to provide orthogonality of reference signals. Release 10 improves the channel estimation by providing inter-cell orthogonality reference signals but the uplink CoMP benefits can be obtained also for legacy Release 8 and 9 UEs. LTE Small Cell Optimization HetNet scenario with four pico cells under each macro cell Figure 7.32 HetNet scenario The CoMP gains in uplink with joint processing are shown in Figure 7.33. The gains are higher for the cell edge users than for the average users, which is easy to understand since the UE transmission from the cell edge is more likely to be received by more than one cell. It is also beneficial to get high gains at the cell edge, since the cell edge users are the ones suffering from lowest data rates. The cell edge gains are on average two times more than the average gains. The intra-site CoMP between macro eNodeB sectors can provide quite nice gains: 19% for the average data rate and 36% for the cell edge data rate. The intra-site CoMP does not require any changes to the network architecture since it can be implemented within single eNodeB. The CoMP gains increase to 32% on the average and to 54% at the cell edge when up to nine cells are included into CoMP set from different eNodeBs. We can note that CoMP gains in uplink Average Cell