FFR-based Network MIMO with Regular Tri-Sector Frequency Partition 93

In this section, we combine network MIMO with FFR and the regular tri-sector frequency partition. As mentioned before, the FFR method partitions the whole frequency band into different zones. Compared to the conventional omni-cell with universal frequency reuse factor of one, the tri-sector cellular system combined with FFR can significantly reduce the interference, while fully utilizing the frequency band at each cell. As an example in Fig.

6.4, when a user of cell 0 utilize one of RU fB1,n (n = 1, . . . , N) in frequency band fB1, the interference comes from 7 cells {4, 5, 12, 13, 14, 15, 16} under the assumption of perfect 120^◦ sector antenna rather than being interfered by all 18 cells in the omni-directional case.

Here, the question is how we can further improve the SINR on top of FFR? The solution proposed in this chapter is to further incorporate the network MIMO technique with FFR.

From Fig. 6.4, we find that the seven interfering sources consists of two critical interferers coming from first-tier neighboring cells 4 and 5 and five weaker second-tier interferers due

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Figure 6.4: Interference example for three-cell FFR-based network MIMO with regular tri-sector frequency partition (consider cell 0).

to higher path loss. Therefore, we use the network MIMO technique to cancel the two most severe interference. Instead of coordinating huge number of cells, we propose a coordination scheme with only three cells. We define those coordinated cells as a group shown in Fig. 6.3.

For the arbitrary group Gi, we label the three cells as Cell^G_aⁱ, Cell^G_bⁱ, and Cell^G_cⁱ, respectively.

For the three-cell coordination, we can apply network MIMO transmission to each subband fBp,n for p = 1, 2, 3 and n = 1, . . . , N . Under the assumption of perfect sector directional

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Slot one: group with cells 4 and 5 Primary band fB1

Slot two: group with cells 1 and 6 Primary band fB2

Slot three: group with cells 2 and 3 Primary band fB3

Figure 6.5: Example of cells regrouping and partner selection for cell 0.

where (x) denotes the corresponding served user in cell x. We eliminate the interference caused by h_(0),4 and h_(0),5 (from cells 4 and 5) by network MIMO. As a result, the two most severe interference is canceled through a small-sized (3×3) matrix computation. Importantly, this small-sized cooperation scheme can be applied to all the cells within the entire service area. For example in Fig. 6.4, at certain time slot we have many cooperated groups among the 19-cell layout: cells {0, 4, 5}, {8, 2, 1}, {10, 11, 3}, and {18, 6, 17}. Not only the middle cell (cell 0) is coordinated with its neighboring cells, but the cells in the outer layer are also coordinated simultaneously.

Cells Regrouping and Partner Selection

In this section, we propose a cell regrouping and partner selection scheme to address the service fairness issue caused by the network MIMO systems with the regular tri-sector fre-quency partition. From Fig. 6.4, among arbitrary group Gi, the cell labelled as “Cell^G_aⁱ” can achieve free intra-group interference within the whole subband f_B1 = {f_B1,1, . . . , f_B1,N} under network MIMO systems. However, subbands fB2 and fB3 are still affected by two first-tier cells and five second-tier cells. We therefore define f_B1 as the primary band of Cell^G_aⁱ for group Gi. Similarly, we define fB2 and fB3 as the primary band of Cell^G_bⁱ and

Cell^G_cⁱ, respectively. Therefore, the cell users served by different subbands among a group can have different signal quality. Note that this service fairness issue is different from the unbalanced signal quality issue mentioned in Section 6.3.3.

The service fairness issue can be resolved by the proposed regrouping and partner selec-tion scheme. Assume that cell 0 is grouped with cells 4 and 5 with primary band f_B1 at certain time slot one, as shown in Fig. 6.5. Cell 0 will regroup with cells 1 and 6 at the next time slot two by counterclockwise rotating way to reselect coordinated partner. After this regroup, the primary band of cell 0 becomes f_B2. Similarly, cell 0 regroups with cells 2 and 3 at time slot three by counterclockwise reselecting coordinated partner again and the corresponding primary band becomes f_B3. In this way, all cells will simultaneously “rotate”

and regroup with two new neighboring cells at a new time slot, where “rotate” means the coordinated partner reselection procedure. Take the three cells {0, 4, 5} an example. Those cells form a group at time slot one. At time slot two, cell 0’s regroup set is now {1, 0, 6}, cell 4’s regroup set is {3, 12, 4}, and cell 5’s regroup set is {5, 14, 15}. Similarly, cell 0’s regroup becomes {2, 3, 0}, cell 4’s regroup set is {4, 13, 14}, and cell 5’s regroup set is {6, 5, 16} at time slot three. Each cell has the chance to cooperate with neighboring six cells in order and each sector has the opportunity to become the primary band under this kind of TDMA-based regrouping and partner selection scheme. Similar concept can be also applied to frequency division multiple access (FDMA) manner by further partitioning each outer subband f_Bp into three disjoint subsets. Then each subset of f_Bp regroups with different neighboring cells.

6.4.3 FFR-based Network MIMO with Rearranged Tri-Sector Frequency Partition

Cell Planning for 120-degree Tri-Sector Architecture

In this section, we propose another multi-cellular architecture with rearranged tri-sector fre-quency partitions among cells. As shown in Fig. 6.6, there are totally three kinds of frefre-quency partitions. After this rearranged tri-sector frequency partition among a multi-cellular sys-tem, a cell coordinates with six neighboring cells to form three individually network MIMO groups for each subband. For example, cell 0 coordinates with cells 1 and 2 for subband fB1, with cells 3 and 4 for subband f_B2, and with cells 5 and 6 for subband f_B3. In other words,

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Figure 6.6: Interference example for three-cell FFR-based network MIMO with rearranged tri-sector frequency partition (consider cell 0).

we perform three-cell network MIMO transmission in each subband.

From Fig. 6.6, when a user of cell 0 unitize one of RU f_B1,n (n = 1, . . . , N) in frequency band fB1, the interference sources are cells {1, 2, 11, 12, 14, 16, 17} under the assumption of perfect 120^◦ sector antenna. Similarly we use the network MIMO technique to cancel the two most severe interfering sources. That is, the two first-tier interference from cells 1 and 2 (the dotted area in Fig. 6.6). The channel matrix for the group {0, 1, 2} in f_B1,n is

For the users in cell 0, we now eliminate the interference caused by h_(0),1 and h_(0),2 (from cells 1 and 2) by network MIMO through a small-sized (3 × 3) matrix computation. Ideally,

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Figure 6.7: Interference example for three-cell FFR-based network MIMO with rearranged tri-sector frequency partition and 60^◦ cell sectoring (consider cell 0)

there are only five second-tier interferers for each subband. Similar the network MIMO with regular tri-sector frequency partition, this cooperation can also be applied to all cells not only for a particular cooperated group. However, there is no service fairness issue for network MIMO systems with rearranged tri-sector frequency partition. As a result, we avoid cells regrouping to ease the complexity in deploying the network MIMO.

Cell Planning for 60-degree Tri-Sector Architecture

To address the effects of cell planning with different sectorization technique, we further de-sign the three-cell network MIMO systems with three 60^◦ cell sectoring. Here we consider rearranged tri-sector frequency partition to avoid the cells regrouping procedures. Figure 6.7 shows the example of the proposed rearranged 60^◦ tri-sector network MIMO systems.

In this design, each cell has three hexagon-shaped sectors with different frequency assign-ment. The cells with 60^◦ and 120^◦ sectors are also called as the clover-leaf-shaped cells and diamond-shaped cells, respectively [36, 37]. Generally, this clover-leaf-shaped cells can match sector contour better than the cells with diamond-shaped sectors [36]. Addition-ally, the cells with 60^◦ sectoring can use spectrum more efficiently [37]. Similar to the case of 120^◦ sectoring, each cell under 60^◦ sectoring performs three individually three-cell net-work MIMO coordinations with its six neighboring cells for each frequency partition. The two first-tier interferers (the dotted area in Fig. 6.7) are cancelled through network MIMO transmissions. The three-cell coordination structure can also be implemented for all the cells.

Note that actual cell sectorization cannot be perfect by using directional antenna pattern (6.1). The other cells will also affect the received signal quality of cell 0. However, by taking advantage of sectoring and FFR, the impact caused by the side-lobe and back-lobe transmissions (the grid areas in Fig. 6.4, Fig. 6.6 and Fig. 6.7) is not significant compared to the omni-cell with universal frequency reuse factor of one. Finally, we summarize the features of the proposed FFR-based three-cell network MIMO systems: (i) using a small coordination size M_c = 3 to perform low complexity network MIMO systems, (ii) reducing the effects of IGI via the combination of FFR and directional antennas, (iii) avoiding the unbalanced signal quality issue caused by larger coordination size M_c.

6.5 Numerical Results

In this section, we show the SINR performances of the proposed FFR-based three-cell net-work MIMO systems. Consider a multi-cellular system with base station-to-base station distance R_B2B = 2 km. The interference-free SNR at cell edge is Γ = 18 dB for 120^◦ sectorized cells. The same values of base station-to-base station distance and Γ (for same transmission power comparison) are used for the clover-leaf-shaped cells. The standard de-viation of shadowing is 8 dB and the path loss exponent µ = 4. Mobile users are uniformly distributed within each sector/cell. The channel response between any user-and-cell pair is represented by (6.3), where the angle-depend antenna pattern is considered. We don’t con-sider the effect of inner region here and set the inner distance be zero. In the next section,

−200 −10 0 10 20 30 40 0.1

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Received SINR (dB)

CDF

Omni−cell with reuse one

ZF−DPC network MIMO in rearranged frequency partition

ZF−DPC network MIMO in regular frequency partition Sectoring (1/3) FFR

Figure 6.8: Comparison of received SINR for conventional 120^◦ tri-sector (1/3) FFR cellular systems and three-cell ZF-DPC network MIMO systems with regular and rearranged tri-sector frequency partitions.

we will discuss how to design the uncoordinated inner region for FFR-based multi-cellular broadband system.