## 2 Tri-Sector FFR-Based Frequency Partition  
ICIC technique aims at minimizing CCI by efficiently coordinating channel allocation in adjacent cells. On top of network MIMO, this section introduces two FFR-based ICIC frequency reuse schemes as shown in Figure 1. The spectrum is divided into the inner  
Figure 1: Regular and rearranged frequency planning for a tri-sector cellular system.  
frequency band \(f_{A}\) and outer frequency band \(f_{B}\), where \(f_{B}\) is further partitioned into three equal sub-bands (\(f_{B1}\), \(f_{B2}\) and \(f_{B3}\)) [12; 15]. The inner and outer frequency bands are for CCUs' and CEUs' exclusive use, respectively. The differentiation between CEU and CCU can be made based on the received signal-to-interference-plus-noise ratio (SINR) at the user equipment [17]. All the CCUs within a cell share the sub-channels in inner frequency band \(f_{A}\). However, we serve a CEU either on a specific sub-channel of a sub-band \(f_{Bi}\) or on an entire sub-band \(f_{Bi}\) for SU- and MU-network MIMO, respectively. Specifically, the inner band \(f_{A}\) adopts universal reuse factor with omni-directional antenna for CCUs; the outer bands \(f_{B}\) uses frequency reuse factor of three for CEUs in each sector.  
As mentioned in the previous section, we consider two frequency partitions (regular and rearranged) in Figure 1[12]. Traditionally, the regular frequency planning in Figure 1 is the FFR-based frequency partition for a tri-sector cellular system, where the sectors of different cells with the same outer sub-band have the same main-beam direction. Accordingly, with regular frequency planning, the contiguous cell-edge areas of neighboring cells are assigned to different orthogonal outer sub-bands \(\{f_{B1},f_{B2},f_{B3}\}\) to avoid CCI as shown in Figure 1a. On the contrary, with rearranged frequency planning, the sectors of different cells with the same outer sub-band have different main-beam direction as shown in Figure 1b, where contiguous cell-edge areas of neighboring cells are with the same outer sub-bands.  
In both SU- and MU-network MIMO, each CEU is served by a group of collaborative BSs. These BSs form a cooperative cell set (CCS) associated with that CEU. For convenience, let a BS sector \(n(q)\) denote the sector \(q\) of cell \(n\). For example, consider a certain CEU \(u\) located in the cell-edge area of sector \(0(a)\) for regular frequency planning in Figure 1a. For this CEU \(u\), we have the corresponding CCS \(C_{u}=\{0(a),1(b),2(c)\}\), which use a particular sub-channel selected from outer sub-band \(f_{B1}\) to collectively serve the CEU \(u\). This is SU-network MIMO transmission. As in frequency-division multiple access (FDMA), SU-network MIMO allocates each CEU a unique sub-channel from an outer sub-band and thus the cooperative BSs in a CCS result in no multi-user interference. As for the rearranged one in Figure 1b, the collaborative BSs in CCS \(\{0(a),1(b),2(c)\}\) turn to together serve a group of their CEUs using the sub-band \(f_{B2}\) (i.e., MU-network MIMO), i.e., all CEUs in this group share the entire sub-band \(f_{B2}\). As a result, the serious multi-user interference is inevitable. To control this, joint preprocessing among cooperative BSs is necessary in MU-network MIMO with rearranged frequency planning.