Performance of TD-LTE ICIC

Inter Cell Interference Coordination (ICIC) seems to have effective performance on cancelling the interferences concerned by the whole industry. The basic idea of ICIC is keeping the inter-cell interferences under control by radio resource management (RRM) methods. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells.

ICIC schedules each cell’s resources including time domain resource and transmitting power in the coordination mode. As per the way of resources coordination, ICIC methods can be divided into: fractional frequency reuse (FFR), soft frequency reuse (SFR) and the full frequency reuse.

• FFR
  All of the frequency resources are divided into 4 groups.
  All cell center users use the group 1 frequency fixedly, and the frequency reuse factor is 1.
  The remaining 3 frequency groups are allocated among the edge users to ensure the frequency orthogonality between the users from neighbor cells and the frequency reuse factor of 3.
• SFR
  All the frequency bands are divided into two groups, one is primary sub-carrier, and another is secondary sub-carriers.
  The primary sub-carrier is used in the whole cell, while the secondary sub-carrier is only used in the cell centre with smaller transmitting power.
  Neighbor cell’s primary sub-carriers are orthogonal, which suppresses the interference in the cell edge. The secondary sub-carriers can use the same frequency as distributed in cell center with small interference.
  SFR inherits parts of the frequency reuse advantages, while obviously enhances the frequency utilization efficiency using dynamic frequency reuse factor.
• Full Frequency Reuse
  All the frequency points are used in the whole cell and the frequency reuse factor is 1. The limitation of time-frequency resource and transmitting power is based on each RB.
   

Among the above 3 ICIC methods, SFR achieves the best performance in the spectrum efficiency and scheduling complexity. Therefore, SFR is the most preferred one.

SFR networkingis shown above. The users are divided into two categories, one is Cell Center User (CCU), and the other one is Cell Edge User (CEU). CCUs are the users distributed in the gray region of above figure, and CEUs are the users distributed in the above red, green and blue areas. CCU can use all the frequencypoints to communicate with the base station, while CEU must use corresponding specified frequency points to ensure orthogonality between different cells.

CEUs can be assigned a higher transmissionpower for the frequency reuse factor is greater than 1. The frequency points are not overlapped at the edges so the adjacent cell interference is small. CCU’s frequency reuse factor is 1; for the path loss is small and transmission power is low. Therefore the interference to the adjacent cells is not high either.

Suppose there are totally 10MHzfrequencyband. The whole band is divided into A, B, C groups for different CEUs. A includes 17RBs, B includes 17RB, and C includes 16RB. The transmission power per sub-carrier for CCU is half of that for CEU. The spectrum efficiency is shown in the following table with varying CEUpercentage.

It seems that cell average spectrum efficiency increases while edge spectrum decreases when the CEU ratio increased. The reason is that the increasing CEU ratio makes more users scheduling different resources with neighbor cell users, which reduces the inter-cell interference and increase the spectrum efficiency. As more users becoming CEU, the chance for edge users gain RB drops down. As a result, the spectrum efficiency is lower.

The system bandwidth is divided into three groups, as shown below. The Out Cell (OC) resources will be used for CEU scheduling firstly. CEUs are also allowedthe usage of non-OC resources, but the transmission power (PwRatio) is limited. If the OC resources are not used up by CEUs, the remaining OC resources and In Cell (IC) resources are scheduled to CCUs. The simulation result is shown in the following table.

It showsthe improvement of system spectral efficiency is not significant with ICIC. But the 5% CDF CEU spectrum efficiency is enhanced significantly.