c-m))) (((c-m,))) (((c),, Macro Small Macro Small Macro Small Stand-alone BTS Small BTS connected to macro Small BTS = RF heada Low transport Shared PDCP in macro BTS Heavy transport requirements Inter-site carrier aggregation requirements = direct fiber Figure 5.3 Small cell network architecture options LTE Small Cell Optimization Table 5.1 Comparison of all-in-one small base station and radio head Radio frequency head All-in-one base station (RF head) Transport requirements Low requirements. Transport can Tough requirements. Direct use radio, copper or fibre fibre needed in practice Feature parity between macro Yes possible, but requires extra Yes, comes naturally with the and small cells planning in development common baseband Mobility between macro and Inter-site handovers visible also Intra-site handovers are not small cells to the packet core visible to the packet core Coordinated Multipoint Joint Processing not possible but Yes, all CoMP options (CoMP) Dynamic Point Selection is possible between macro possible and small cells has simultaneous connection to both macro and small cells. The low latency functionalities are located in the small cell including Layer 1, Media Access Control (MAC) and Radio Link Control (RLC) while Packet Data Convergence Protocol (PDCP) is located in the macro cell. The X2 interface between macro and small cells has clearly more relaxed requirements than CPRI/OBSAI interface. There is also other intermediate functionality split alternative between these options. For example, part of layer 1 can be integrated with RF, like Fast Fourier Transformation (FFT) while Turbo decoding can be centralized. The RF head solution has a number of benefits since it is a straight forward extension of the macro cell. On the other hand, the interface between macro cell and RF heads needs very high capacity. These two options are compared in Table 5.1. The RF head requires high capacity for the CPRI/OBSAI interface typically beyond 1 Gbps, and very low latency. The all-in-one base station has relaxe